Session R1 - Poster Session III.
POSTER session, Wednesday afternoon, March 05
Room Exhibit Hall 2/3, Austin Convention Center

[R1.001] Chemical Physics

[R1.002] On Planck

The production of Planckian shaped thermal emission spectra requires quantized energy levels. However, the nature of the physical species undergoing the transition between these energy levels has not yet been outlined. It is now stated that the transitions in Planckian Thermal Emission are performed by vibrating nuclei within the confines of an atomic lattice structure. It is impossible to obtain a Planckian shaped thermal emission profile if either the nuclei or the lattice is absent. Thus, gases are unable to produce Planckian shaped thermal emission spectra since they lack the structural constraint of an atomic lattice. Liquids possess fleeting lattices. As such, they can report Planckian shaped thermal emission spectra, but they report only an apparent temperature, not necessarily a real temperature. That is because their lattice are not sufficiently rigid. These observations are in fact the basis Kirchhoff’s law of thermal emission. If one desires to obtain a reliable temperature from a thermal emission profile the constraint set in Kirchhoff‘s law must be followed. Kirchhoff‘s graphite blackbodies acted by providing a nearly ideal rigid lattice. No lattice - no Planck.

[R1.003] Ensemble Dynamics with Quantum Forces

We present a new methodology for approximating the solutions of the time-dependent Schrödinger equation. Our approach is rooted in the de Broglie Bohm interpretation of the quantum theory in which the evolution of a quantum system is characterized by an ensemble of particle trajectories. The paths of these ``Bohmian'' particles are analogous to hydrodynamic trajectories and are determined by the presence of both classical and quantum forces in the system. The quantum force is due to the nonlocal interactions between particles and is related to the curvature of the quantum density. In the present study we invoke an expectation-maximization algorithm to approximate a functional form for the density of a finite ensemble of Bohmian particles. From this density information we then calculate a quantum force and propagate the system forward in time using a Verlet type integration. In what follows we will describe the details of this approach and present some numerical results.

[R1.004] Wavepacket Dynamics on Dynamically Adapting Grids: Application of the Equidistribution Principle

A moving grid approach to wavepacket dynamics is described that enables grid points to be used efficiently in regions where high resolution of the wavepacket is required. The grid movement is based on the principle of equidistribution and by using a grid smoothing technique the grid points trace a path that continuously adapt to reflect the dynamics of the wavepacket. The technique is robust and allows accurate computations to be obtained for long wavepacket propagation times. Results are presented for two systems: tunnelling dynamics in a double well potential and scattering of a wavepacket from a repulsive Eckart barrier.

[R1.005] Minimum Energy Structures of Ni, Au and NiAl Clusters: A Genetic Algorithm

The lowest energy structures of (Ni)_n, (Au)_n and (NiAl)_2n (with n up to 100) clusters were obtained through a genetic algorithm using the semi-empirical many-body Gupta potential to mimic the interatomic interaction. A variety of structure types are observed in all the three class of clusters, repeatedly appering the icosahedral structural motivs. Global minima are generally more difficult to find for bimetallic clusters than those for the pure element clusters.

Work supported by CONACyT-México under grant 35224-E.

[R1.006] Stability of Pyrimidine Nucleic Acid Bases with Respect to Intra- and Intermolecular Proton Transfer Reactions Induced by Excess Electrons

Chemically transformed nucleic acid bases are considered as sources of point mutations in genetic material. Our computational results and photoelectron spectra provide information about chemical transformations of pyrimidine bases induced by excess electrons. The isolated pyrimidine bases as well as their complexes with X (X= amino acid, carboxylic acid, or alcohol) have been studied with the B3LYP and MPW1K density functionals, as well as at the second order Moller-Plesset level of theory. The photoelectron spectra of some anionic complexes reveal broad features with maxima around 2 eV. These features cannot be associated with the anion of intact pyrimidine base solvated by X and indicate occurrence of chemical transformations. Our main findings are: (i) the excess electron attachment can induce a barrier-free proton transfer (BFPT) from X to the O8 atom of uracil or thymine, (ii) thymine in complexes with carboxylic acids is more resistant to BFPT than uracil, (iii) the instability of neutral rare tautomers of uracil or thymine can be significantly suppressed due to the interaction with zwitterionic amino acids.

[R1.007] Measurement of the lifetime and the electron transfer scavenging time of the p-state hydrated electron

Kinetics of the electron transfer scavenging reaction of the excited state hydrated electron in competition with non-reactive internal conversion is studied using femtosecond pump-probe spectroscopy. In pure water, transient absorption signal of the hydrated electron after optical excitation to the p-state has been assigned to internal conversion and solvation. In the presence of electron scavengers, however, the pump-probe data clearly exhibit a signature of the presence of an additional decay channel from the excited state due to electron transfer scavenging. Analysis of the pump-probe data with a multi-level kinetic model indicates that the time scale of scavenging upon optical excitation to the p-state is 300-400 fs. Based on spectral moment analysis on the non-equilibrium ground state hydrated electron, the scavenging after the optical excitation to the p-state occurs primarily from the p-state with insignificant contribution from the non-equilibrium ground state. This result suggests that the controversial lifetime of the p-state is several hundred femtoseconds rather than tens of femtoseconds.

[R1.008] First-Principles calculations of water adsorption at the defective anatase TiO2(101) surface

Water adsorption at TiO_2 surfaces is a crucial process in many applications of this material. The relative stability of the molecular vs. dissociated state has been recently clarified for rutile [1], where water dissociates exclusively at oxygen vacancies. An equally clear picture of water adsorption at the surface of anatase, which is more efficient in (photo)catalytic applications, is still lacking. Previous density functional calculations [2] have shown that the molecular state is always favored on the stoichiometric anatase (101) surface, irrespective of coverage. However, point defects, strongly affecting surface properties, can change this behavior [1]. We have studied the adsorption of water on a partially reduced anatase (101) surface by means of ab-initio molecular dynamics simulations. As in the rutile case, at the vacancy site dissociative adsorption, leading to formation of two bridging hydroxyls, is significantly more stable than the molecularly adsorbed state. The free energy barrier for dissociation has been determined to be \sim 440 K. [1] I.M. Brookes et~al., Phys. Rev. Lett. 87 (2001) 266103. [2] A. Vittadini et~al., Phys. Rev. Lett. 81 (1998) 2954.

[R1.009] Scattering of Methane by LiF(001)

A theory previously used to calculate the scattering spectra of acetylene from surfaces has been extended to treat the case of methane scattering from clean LiF(001). The theory uses a classical treatment of the exchange of rotational and translational energy between the molecule and the surface and the internal modes of the molecule are treated semiclassically. Results of calculations compare well with recent experimental measurements for the scattered angular distributions produced by well-defined incident beams, and also for the energy resolved spectra measured at fixed final angle. A number of predictive calculations are presented for quantities that could be measured in future experiments.

[R1.010] Photolysis and Radiolysis of Simple Ices

The Department of Physics and Astronomy at Northern Arizona University has recently completed the development of an ultra high vacuum analysis system to study the photolysis and radiolysis of ices. The investigation of these fundamental processes in ices have important applications in astrophysics, planetary astronomy and atmospheric physics. The system incorporates photoelectron spectroscopy, Fourier transform infrared spectroscopy and mass spectrometry as analytical techniques. The system also includes a closed cycle helium cryostat for the growth of the ice samples. Processing tools include x-ray and UV light sources and a low energy (up to several keV) ion source. The capabilities of this new system are described and XPS and FTIR results from several simple ices will be presented.

[R1.011] The Liquid Surface/Interface Spectrometer at ChemMatCARS Synchrotron Facility at the Advanced Photon Sources

In this poster we present experimental results obtained during the commissioning period of a new liquid surface x-ray spectrometer at the ChemMatCARS sector (Sector 15, undulator beamline, at the Advanced Photon Source, Argonne National Laboratory). The spectrometer was designed and developed for the investigation of interfacial phenomena and properties of a wide variety of liquid systems. These include studies of polarized liquid-liquid interfaces (Mark Schlossman, UIC), liquid metal-X interfaces (Stuart A Rice, U of Chicago, and Peter Pershan, Harvard, respectively), and Langmuir monolayers (Schlossman, UIC, and Ian Gentle, U of Queensland, Australia, respectively). These results demonstrate the initial capabilities and potential broad applications of this new instrument in the field of liquid surface/interface studies.

[R1.012] Anti-stokes photoluminescence of II-VI nanoparticles with different emitting states

Anti-stokes photoluminescence of II-VI nanoparticles with different emitting states

Birol Ozturk(a), Wei Chen(b), Yimg Wang(a), Nicholas Kotov (a) (a) Department of Chemistry, Oklahoma State University,Stillwater,OK 74078,USA (b) Nomadics Inc., 1024South Innovation Way, Stillwater, OK 74074, USA

Abstract

Anti-stokes photoluminescence (ASPL) in II-VI colloidal nanoparticles of CdTe and CdSe was studied in dispersions. The measurements showed that photoluminescence PL and ASPL were likely to originate from different although close-lying electronic states. Temperature dependence measurements between 10K and 300K showed that there is no thermal excitation step involved in ASPL emission. Emission intensity dependence on excitation intensity is linear which indicates deviation from the classical two-photon absorption mechanism. The ASPL excitation scheme involving a long-lived intermediate state is discussed.

[R1.013] Hot Electron Cooling Dynamics of Nanolithographcally Prepared Au Arrays

The relaxation dynamics of Au arrays prepared via nanosphere lithography were investigated1 using femtosecond pump-probe spectroscopy. The hot electron relaxation dynamics of arrays of truncated tetrahedra shaped 90 nm gold nanoparticles on glass, 50nm gold thin film, and gold nanodots in solution are compared. The initial relaxation (due to the electron-phonon relaxation) of the Au array is longer (2.9 ps) than the relaxation of the Au thin film (1.5 ps). The thin film system exhibited single component decay, while the nanodot solution displayed an additional longer decay due to the phonon-phonon relaxation or heat dissipation which is in the 80-100 ps range for the nanodots. The relaxation dynamics of the array particles are also investigated as a function of solvent interaction using toluene, pyridine, and water. The relaxation times show a dependence on the propensity and ability of the solvent molecules to interact with the particles. This is explained in the context of the differences in the relative rate of heat dissipation compared to the rate of electron-phonon relaxation in the different systems 1) S.Link, D.J Hathcock, B. Nikoobakht, M.A. El-Sayed, “Medium Effect on the Electron Cooling Dynamics in Gold Nanorods and Truncated Tetrahedra” Submitted to Advanced Materials

[R1.014] Probing the Mechanism of Excited-State Evolution in a Transition Metal Complex

Femtosecond electronic transient absorption spectroscopy is being used to explore excited-state relaxation in simple transition metal complexes. Cr(acac)_3 (where acac is acetylacetonate) presents a typical case, where the lowest-energy absorption feature does not correspond to the lowest-energy excited state of the molecule. Following ~100 fs excitation into this ^4T ligand-field state, we observe the formation of the lower energy ^2E state with a time constant of ~1 ps. These dynamics represent in principal a combination of vibrational relaxation on the ^4T potential energy surface, intersystem crossing to the ^2E, and vibrational relaxation once the ^2E is formed. Conventional photophysical models would hold that these decay processes are well-separated in time, with vibrational cooling being significantly faster than intersystem crossing. However, it is becoming increasingly clear that excited-state evolution in inorganic systems does not adhere to this picture. Results from variable-pump/ variable-probe wavelength measurements will be presented which are helping to differentiate these various processes in Cr(acac)_3 and other related Cr(III) systems.

[R1.015] Preparation and characterization of Carbon-Polyvinylchloride composites

Composite materials have a big importance in many areas of science and technology because of their special properties. In this work, carbon-polyvinylchloride (C-PVC) composite was made by dispersing carbon powder in a PVC matrix. The physical and chemical properties of C-PVC composite allow us to use it as amperometric sensor of organic, inorganic substances or metallic compounds in a very low level of concentration. Morphological and structural characterization by SEM, EDS, AFM, TEM, and XR of C-PVC composite have shown that the composite works with a large effective surface area. Some physiological applications are discussed.

[R1.016] DSCF Study of Block Copolymers at Sheared Polymer Blend Interfaces

The dynamic self-consistent field (DSCF) theory, originally formulated for unentangled polymers, has been recently extended to entangled chains. The DSCF theory couples the time evolution of chain conformations, volume fractions and momenta, based on local conservation laws. A modification of the lattice random walk formalism of Scheutjens and Fleer is used to generate anisotropic chain conformations under flow. Here we present a DSCF study of the interfacial properties, such as velocity slip, interfacial viscosity and chain stretching, and explore how they are affected by the presence of block copolymer. We have studied both unentangled and entangled regimes and compared with experiments and other models when applicable.

[R1.017] The correlation length of an eight-arm polystyrene in methylcyclohexane near the critical point

The turbidity of eight-arm polystyrene in methylcyclohexane has been measured and used to determine the correlation length amplitude \xi_0. The turbidity in this system was determined from the measured ratio of the transmitted to incident light intensities over three decades in reduced temperature. Using Ornstein-Zernike theory, we are able to fit the turbidity data and determine that \xi_0 = 0.604 \pm 0.010 nm for this branched polystyrene with a total molecular weight of 74,000. This value of \xi_0 is less than that reported in the literature for a linear polystyrene of the same molecular weight in methylcyclohexane. Support for this research was prvoided by NSF-REU 9987850 and NASA grant NAG8-1433.

[R1.018] Photophysics of Conjugated Polymers Aligned in a Nematic Liquid Crystal Host

It is of interest to develop new kinds of anisotropic optical materials. To this end, we have studied the orientation and the photophysics of a non-linear dye, N’,N’-Bis (2,6 dimethylphenyl)-3,4,9,10, perylenetetracarboxylic dimide (PERY), and a conjugated polymer, poly[2-(2’ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH-PPV), in a low molecular weight nematic liquid crystal matrix, 4-n-pentyl-4’-cyanobiphenyl (5CB). The dye was incorporated into the liquid crystal using the ‘guest-host’ method. A solvent-induced homogenous mixing (SIHM) technique was developed to embed the polymer into the liquid crystal matrix. Order parameters were obtained from polarized absorption and fluorescence spectra. The dynamics of intra- and interchain energy transfer in the aligned conjugated polymer samples will be reported.

[R1.019] Vibrational relaxation of photoexcited heme in Mb

Photoexcited heme shows a broad, red-shifted absorption spectrum. Relaxation of the "hot" photoproduct spectrum is well described using independent timescales for narrowing (400 fs) and blue shifting (0.4-4 ps) as the system returns to equilibrium. The vibrational relaxation pathway in Mb is explored by using samples with a modified local heme environment (e.g., His93->Gly mutation and protoporphyrin IX->porphine substitution). The His93->Gly mutation experiment demonstrates that the covalent bond between the iron and the proximal histidine has little effect on the overall vibrational relaxation of the "hot" heme. In contrast, the protoporphyrin IX->porphine substitution experiment demonstrates the importance of the van der Waals contacts between the heme and the protein/solvent matrix in cooling the locally hot heme.

[R1.020] Ultrafast Localization Dynamics and Evolution of the Spatial Extent of Solvated Electrons in D_2O/Cu(111)

Time- and angle-resolved two-photon photoelectron spectroscopy (2PPE) is used to investigate femtosecond dynamics of electron localization and solvation in ultrathin ice layers adsorbed on Cu(111). Electrons, optically excited in the metal, transfer into the conduction band of the ice layer giving rise to a feature e_CB in the angle-resolved 2PPE spectra. Electrons localize within the first 50 fs to form a state e_S at 2.9 eV above the Fermi level. The initially positive dispersion of the first moment of e_S and e_CB shows pronounced changes with time, which indicates a transition from a delocalized to a localized state. After 200 fs an apparent negative dispersion is observed for the localized state, which contains information about the momentum distribution of the localized state with width \Deltak. Model calculations, which describe the angle-resolved spectra, lead to the conclusion that \Deltak changes with the binding energy and is related to the spatial extent of the solvated electron, as will be discussed in detail.

[R1.021] Ultrafast Excited State Dynamics and Structures in Self-Assembled Molecular Aggregates of Helicenocyanines

A series of novel phthalocyanines – with and without Zn metallated cores – that have helicene groups substituted on the periphery were investigated. These compounds exhibit a tendency to self-assemble into molecular aggregates (likely dimers) even at very low concentrations (10^-7 M) in common organic solvents. This aggregation dramatically alters their optical properties compared to the monomer. New and/or reshaped absorption bands broaden coverage throughout the UV and visible regions. Emission measurements suggested an intramolecular coupling between the helicene groups and the Pc core. Transient absorption results indicated a fast (several hundred fs) process that may be related to a coherence effect in the excited state. X-ray techniques provided insight on the size distribution of the aggregates and the coordination of the Zn center. Work at ANL was supported by the US Dept of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, under contract W-31-109-Eng-38

[R1.022] Using pulsed X-ray diffraction to study protein dynamics with picosecond time resolution

With the advent of third-generation synchrotron sources, diffraction data useful for X-ray structure analysis can be acquired with exposure times shorter than 100 ps. An application of this time resolution is the study photochemical reactions that can be triggered in a crystalline sample by a laser flash. For this purpose, the ID09 beamline of the ESRF was equipped with a high-speed mechanical chopper to isolate single X-ray bursts from the pulse train of the synchrotron and a mode-locked laser that is phase-locked to the synchrotron, to obtain the necessary timing accuracy. We studied the oxygen storage protein myoglobin, with the aim of finding the escape pathway of the oxygen from its binding site, which is not obvious from the known X-ray structures. By using CO as a substitute for oxygen, we could take advantage of the photosensitivity of the Fe-CO bond to trigger the ligand release by a laser. The L29F mutant of myoglobin proved to be an interesting test case, because it shows ligand dynamics correlated with conformational changes of the protein on a time scale of 100 ps.

[R1.023] Superconductivity

[R1.024] New Robust Statistical Mechanics for High-Temperature Superconductors

The new statistical "complete boson-fermion model" (CBFM) of superconductivity [1] -in which both the BCS and BEC theories are contained as special cases- is used to calculate the superconductor transition temperatures T_C in both 2D and 3D systems. The model includes both two-electron and two-hole pairs in freely variable proportions, along with unpaired alectrons. For perfect electron/hole-pair symmetry and weak coupling one has BCS theory; for no hole-pairs and no unpaired electrons one gets the BEC T_C-formulae. Using the BCS model interaction for electron-phonon coupling and with no adjustable parameters the calculated T_C's compare well with experimental data, provided only that one departs moderately from perfect electron/hole-pair symmetry. These T_C's are also predicted in both 2D and 3D to be higher for hole- than for electron-superconductors, in agreement with general empirical trends.

[1] V.V. Tolmachev, Phys. Lett. A 266, 400 (2000); M. de Llano and V.V. Tolmachev, Physica A 317, 546 (2003)

[R1.025] Magnetotransport in NdNi_2B_2C and PrPt_2B_2C single crystals

Transport and magneto resistance measurements have been performed in NdNi_2B_2C and PrPt_2B_2C single crystals. In this study, we present susceptibility and resistivity measurements in the ab-plane and c direction from 2 K to 300 K. Magneto resistive behavior was studied from 0 to 8 Tesla. In NdNi_2B_2C, the magnetoresistance is negative below of the Neel temperature, T_N \sim 4 K, when H is parallel to the ab plane, and is positive with H\parallel c direction. Above T_N, the magnetoresistance shows positive values. At higher magnetic field and high temperature the contribution is positive, in both directions. PrPt_2B_2C shows no magnetic order at any temperature and is superconducting with T_c = 6 K. In this crystal we also determinate the upper critical fields H_c2.

A. Durán acknowledges a scholarship from IIM-UNAM. R. Escudero acknowledge financial support from CONACyT for grant G-0017; and UNAM-DGAPA, project No.IN-105597.

[R1.026] The Specific Heat of UGe_2: Effects of Pressure and Magnetic Field

At ambient pressure (P) UGe_2 undergoes transitions from paramagnetic, to ferromagnetic (FM), to coupled charge-density wave (CDW) spin-density wave (SDW) phases on cooling. Within a narrow range of P, and at temperatures (T) below \sim0.7 K (but depending on sample quality), superconductivity (SC) coexists with both the CDW-SDW and FM phases. The \gammaT term for the specific heat (C) increases with increasing P, most rapidly between \sim1 and \sim1.4 GPa, the region in which SC develops, then approaches a constant value at 1.8 GPa. The \gammaT term decreases in a magnetic field. In the CDW-SDW region of the phase diagram C includes an exponential term that represents the CDW-SDW and decreases in a magnetic field. The electronic entropy increases rapidly in a narrow interval of P in the vicinity of the CDW-SDW transition, which suggests that it might be broadened first order. A transition to SC is not observed to 0.35 K in the range of P where it is expected to occur, but there are upturns in C/T, absent at other P's, that suggest the onset of SC.

[R1.027] Anomalous Hole-Concentration Dependence of the Hall Coefficient of Oxygen-Intercalated La214 Superconductor

The Hall coefficient of La_2CuO_4+d was measured at room temperature and 77.4 K for d = 0.015 - 0.110. The behavior of the Hall number (p_H) defined as a quantity inversely proportional to the Hall coefficient as a function of the hole concentration p^+ was quite different from that of La_2-xSr_xCuO_4. The p_H at room temperature showed jumps at p^+ = 0.065 and 0.107 which accompanied an integral ratio change in p_H/p^+ such as 1.2 : 2.3 : 3.6, being close to 1:2:3. Above p^+ = 0.107, however, p_H decreased with increasing p^+, which indicates the localization of the holes. The anomalous changes were less distinctive at 77.4 K. The two anomalous points at room temperature are very close to the hole concentrations for the two-dimensional Wigner lattice formation of 4x4 (p^+ = 0.0625) and 3x3 (p^+ = 0.111), which was recently proposed from the optical reflectivity measurements of La_2-xSr_xCuO_4+d by Y.H. Kim and P.H. Hor [Modern Physics Letters B15, 497 (2001)]. Thanks are due to the Hokkaido Tokai University Shigeyoshi Matsumae Memorial Foundation, NSF (DMR9122043), ARPA (MDA972-90-J-1001), the Robert A. Welch Foundation (E1207), and the state of Texas.

[R1.028] Thermoelectric Properties of 123 superconductor with Europium rare earth at low temperatures

In this work we studied the thermoelectric properties in the superconductor 123 compound with rare earth Europium. We have measured a variety of its properties at low temperature, including thermoelectric power, electrical conductivity and thermal conductivity as a function of temperature and oxygen contents. We show the calculated curves of the thermoelectric figure of merit, resulting from those data.

[R1.029] Magnetic Exchange Interactions of Quaternary Intermetallic Superconductors

The magnetic exchange interaction constants J_ex^par for parallel to the c-axis (H//c) in quarternary intermetallic superconductors, RNi_2B_2C (R = rare earth elements), were estimated by substituting the anisotropic Weiss-temperatures (for magnetic field, H , parallel or perpendicular to c-axis), where J_ex^per(for H is perpendicular to c-axis) of R^+3 ions sublattice in RNi_2B_2C is assumed by that of Ho^+3 ions one. The magnetic susceptibility, \chi , showed the Curie-Weiss behavior at the high tempearture regions and two different Weiss-temperatures. From the high-temperature series expansion of \chi(T)¡¯s and the anisotropic Weiss temperatures, the magnetic exchange interaction constant J_ex^par of each R^+3 ions sublattice in RNi_2B_2C (R= Tm, Er, Ho, Dy, and Tb)were obtained. The magnetic ordering in these compounds at high temperature regions will be compared with the previous crystalline electric field effect results.

[R1.030] Temperature Dependence of the Raman Spectral Peaks for a Magnesium Diboride Superconductor

The recently discovered magnesium diboride (MgB2) polycrystalline superconductor has again stimulated the interest in conventional BCS superconductors that, as opposed to most high-temperature superconductors, can possess the high levels of critical current density so essential for magnetic field applications of superconducting materials. However, much remains to be learned about the structural mechanisms behind the magnesium diboride superconducting system. One of the most useful tools for an analysis of the structural features most involved in phonon-mediated superconductivity is the technique of Raman spectroscopy. In this study, we have collected the Raman spectra of a magnesium diboride polycrystalline superconductor over a large range of wavenumber shifts at temperatures ranging from 18K - 300K. We have observed the conventional gradual softening of the major Raman-active phonon mode with an increase in sample temperature. Immediately above and below the critical temperature (measured as around 38 K onset for our sample) we collected the spectra at a finer grid of temperatures in order to more accurately observe the specific phonon mode shifts as the sample passed through the superconducting transition. The Raman spectra were collected over multiple averaged scans with a GaAs PMT and a Triax 550 spectrometer equipped with a holographic Super-Notch Plus filter and were excited with 150 mW of 4880Å argon ion laser radiation at the sample surface. These spectra have provided us with further insight into the specific phonons responsible for the mediation of the Cooper pairing of the electrons on which BCS superconductors are inherently dependent.

[R1.031] Excitation-wavelength and temperature dependence of Raman-active phonons in the spin-ladder system Sr_14-xCaxCu_24O_41, \itx =0, 6, 9.

The so-called spin-ladder compound Sr_14-xCaxCu_24O_41 contains alternating layers of CuO-chains and quasi 1D two-legged CuO ladders. It support superconductivity at high pressure and heavy doping, but also temperature and doping dependent antiferromagnetism and lattice instabilities. The spin-ladder system may thus provide important information on the interplay between spin, electronic and lattice excitations in strongly correlated systems in general, including the conventional high-T_c cuprate superconductors. Here, we report on a Raman scattering study of undoped and moderately doped Sr_14-xCaxCu_24O_41. The chain-polarized Raman spectrum exhibits a complicated pattern of one- and two-phonon modes that renormalize below \sim200 K, signaling a lowering of the lattice symmetry possibly induced by charge ordering effects. Furthermore, the phonon spectrum is strongly dependent on excitation wavelength, demonstrating the resonant character of these modes. The resonance effect is qualitatively similar to what has been observed in the high-T_c superconductor Y123, indicating a common origin in electronic excitations localized to the CuO-chains.

[R1.032] Analysis of electronic structure of selected ruthenocuprates by electron energy loss spectroscopy

Te distortions and defects in the RuO_2 sublattice of RuSr_2RECu_2O_8 (RE=Gd, Eu)(Ru-1212)can affect not only the magnetic properties but also the inter-layer charge transfer in this compound.Microscopic origin of the differences, and relationship between microstructure and electronic structure of the superconducting and non-superconducting Ru-1212[1] are investigated by electron energy loss spectroscopy (EELS) in a transmission electron microscope. The EELS analysis of related SrRuO_3 revealed the correlation between changes of the Curie temperature and the electronic structure manifested as the Ru M- (L-) ionization edges and the O K-edge in EEL spectra, as well as changes in composition achieved by different annealing. In this study, interplay between the Ru M and L-edges and Cu L-edges and O K-edge will be discussed.

[1] P.W. Klamut et al. Physica C, vol 341-348, 455 (2000) This work is supported by NSF-DMR-0105398 and by the State of Illinois under HECA. Work at Argonne is supported by the US Department of Energy, Basic Energy Sciences-Materials Sciences, under contract #W-31-109-ENG-38.

[R1.033] Spectral Properties of Systems Near Metal-Insulator Transition: Coherence-Incoherence and Dimensional Crossover

We have studied single-particle excitations in ARPES in several layered systems (Sr_2RuO_4, NaCo_2O_4 and (Bi,Pb)_2M_3Co_2O_9 where M=Ba, Sr) that display a crossover in the c-axis transport, from insulating-like, at high temperatures, to metallic-like at low temperatures, while being metallic over the whole temperature range in the plane. We have found sharp, quasiparticle-like excitations in the low-temperature 3D-like phase, and their absence in the effectively 2D, high-temperature phase. Similarities with phenomena seen in high temperature superconductors will be discussed.

This work was supported by the DOE under contract number DE-AC02-98CH10886.

[R1.034] Gap anisotropy and defect induced local density of state modulation in YBa_2Cu_3O_7-\delta

Taking into account gap anisotropy and a weak and extended defect, we calculate the modulation of energy-dependent local density of states for cuprate superconductor YBa_2Cu_3O_7-\delta (YBCO). These are observable by scanning tunneling experiment. When there exists a sub-dominant s-wave component in addition to dominant d_x^2-y^2-wave gap, quasi-one-dimensional-like modulations are obtained which are most conspicuous at higher biases and easily understood by the local nesting effect for a Fermi liquid. If the proposed modulations are observed, it would lead to a unified picture among angle-resolved photoemission, inelastic neutron scattering, and scanning tunneling measurements that YBCO is a good ``Fermi liquid'' with a sub-dominant s-wave component in the superconducting gap.

[R1.035] Interaction of Ferromagnetism and Superconductivity in multilayers

We investicate the interaction of Ferromagnetism and Superconducttivity in multilayer system in two limits: First, when Nd \gg L we find spontaneous pancake vortices in each superconducting layer or Josephson vortices according to whether the magnetization m is perpendicular or parallel to the layers. Here N is the total number of layers, d is the interlayer spacing, and L is the linear size of layers. We then argue how this model can be applied to the superconducting weak ferromanet material. Secondly, when Nd \ll \Lambda we find width of the spontaneous stripe domain structure becomes much smaller compared with the bi-layer sysmtem. Here \Lambda=\lambda^2/d is the effective pentration depth and \lambda is the bulk London penetration depth in layers.

[R1.036] Magnetic resonance in RuCu1212: a mixed valency issue

99Ru Moessbauer (MES) experiments on Ru2Sr2GdCu2O8 (RuCu1212) have been performed by DeMarco et al.[1] The MES results are inconsistent with mixed valency, as suggested by NMR studies.[2] The MES hyperfine (hpf) field is ~58 T, indicating a high-spin Ru state (S=3/2) with a 5+ valency. The isomer shift indicates a 4.5+ Ru state. SrRuO3 MES results show a hpf field of ~33T and a 4+ valency with a low spin Ru state (S =1). In RuCu1212 MES, no such 4+ Ru state is observed. For RuCu1212, the linewidths of the sharp MES lines are close to the theoretical lower limit. If electron motion occurs for mixed valent Ru, this must be fast, otherwise line broadening should have been observed. However, fast electron hopping requires also hpf fields of roughly 45 T, which are not observed. NMR [2] shows a sharp 60-T subspectrum and a very broad 30-T subspectrum. The above MES data donot show a 30-T subspectrum. Thus, RuCu1212 powder magnetic resonance studies suggest that RuCu1212 contains another phase, likely to be associated with the 30-T NMR subspectrum. Such studies on aligned powder or single crystal RuCu1212 may prove otherwise. [1] DeMarco, Boekema et al, Am Phys Bull 45 (2000) 729, and DeMarco, Tallon et al, Phys Rev B65 (2002) 212506. [2] Tokunaga et al, Phys Rev Lett 86 (2001) 5767, and Kumagai et al, Phys Rev B63 (2001) 189509. Research is supported by NSF-REU, NHMFL and WiSE@SJSU.

[R1.037] Evolution of spin structure under magnetic field in Nd2CuO4

We use neutron diffraction to study magnetic field effect on magnetic ordering in the parent compound of electron doped cuprates superconductor Nd2CuO4. Previous work has mostly focused on effect of a magnetic field alinged within the CuO2 planes. In view of recent work on superconducting Nd1.85Ce0.15CuO4, we have decided to determine the effect of a c-axis aligned field on Nd2CuO4. In zero field, Nd2CuO4 exhibits noncollinear antiferromagnetic ordering below 270K (TN1) and two spin reorientation transitions at 70 (TN2) and 30 (TN3) K, respectively. When we applied magnetic field perpendicular to Cu plane, we confirmed that there is no change for the magnetic Bragg (1/2,1/2,0) and (1/2,3/2,0) below TN2. However, we observed slight enhancement of (1/2, 3/2, 0) at 100K (phase I) above H=4T. The significance of these results in relationship to the superconducting Nd1.85Ce0.15CuO4 will be discussed.

[R1.038] Entangled electron current through normal-superconductor interfaces

We study the tunneling current emitted from a BCS bulk superconductor into a bulk normal metal through an interface with a finite area, within the limit of vanishing temperature and voltage bias. We derive a local 3D tunneling Hamiltonian which accounts for the different hopping behavior at various electron energies. The equivalence between the pictures of two-electron emission and Andreev reflection is established. We analyze the radial dependence of the density current for different contact radii and barrier heights, and identify several transport regimes. We also calculate the current through two small orifices at a certain distance. Our prediction for the dependence of current on distance differs markedly from those based on calculations which implicitly assume energy independent hopping. Our formalism provides a method to compute transport properties through extended NS interfaces with arbitrary hole shape within the tunneling regime.

[R1.039] Magnesium diboride wire application to high power superconducting dc cables

In 1967, R. L. Garwin and J. Matisoo considered the possibility of constructing a 100 GW, 1000 km, dc superconducting transmission line based on the then newly discovered type II material, Nb_3Sn, refrigerated by liquid helium at 4.2 K.^1 Their paper is viewed by many as the seminal study on the practicality of superconductivity for use in electric transmission cables, and influenced the design of the high temperature superconducting cables now undergoing demonstration worldwide refrigerated by liquid nitrogen, although at much lower power capacities due to the high cost of high temperature superconducting wire. However, the recent discovery of the 39 K MgB_2 superconductor and its promise of cheap, high performance wire may enable the multi-gigawatt capacity transmission line Garwin-Matisoo envisioned. In this presentation, we will rescale their study for MgB_2 cooled by liquid hydrogen at 21 K, which will be used as an additional energy delivery agent as well as a cryogen.

^1R. L. Garwin and J. Matisoo, Proc. IEEE 55, 538 (1967).

[R1.040] Insulators and Dielectrics

[R1.041] Electrical Conductivity in Insulator

ABSTRACT

In insulating solid(Plastic Sheet)of 0.73mm thickness, the conduction process was ohmic at low D.C. electric feilds, but the feild strength increased the conductivity became feild dependent at high feilds and it exhibited some conductivity and the variation in conduction current was none-ohmic.The mechanism of electron transfer between two metallic electrodes separated by insulating material has received considerable attention. The electron transfer current was studied on 0.73mm plastic sheet and(I-V),(log I-log V),(log J-E^1/2)and (log o- 1/T) relations have been studied and the value of slope,electronic dielectric constant and activation energy for nature of conduction mechanism and process have been determined.The electrical conductivity measurements were carried out at room temperature (32.5 celcius)under high D.C. electric feilds of the order of 10^6 volt/meter.The sample of insulator(plastic sheet) was sandwiched between the aluminium electrodes of designed experimental cell,The effect of very high varying feilds at 32.5 celcius temperature,the electrical conduction has been proposed on the data obtained.The non-ohmic behavior in the sample seemed to start at an electric feild 3x10^6 volt/meter.In this case on data obtained it was concluded that "SCHOTTKY EMISSION MECHANISM" has been proposed. The activation energy was calculated by plotting(log o-1/T)characterstics at running temperature and it was found 0.325ev which is less than 1.0,It confirms predominance of Electronic Conduction. I=current in ampere V=volt T=temperature O=conductivity

[R1.042] In situ USAXS studies of nano-particle growth in a premixed flame.

Combustion of organo-metallic or halide vapors and aerosol liquid sprays can be controlled to produce enormous quantities of nano-structured powders. Such flame processes are common in the production of fumed silica, and pyrolytic titania on an industrial scale with primary particle sizes on the order of 10 nm. These nano-particles are typically connected through sintering bridges, ionic bonds or van der Waals forces into ramified, mass-fractal aggregates. The study of this promising technology for nano-particle production has been hindered by the kinetics of particle growth, typically on the order of milliseconds, at high temperature, 2000°C. Using synchrotron radiation and specialized scattering instrumentation capable of simultaneously measuring nano- to colloidal scales (1 nm to 1µm) we demonstrate the feasibility of in situ growth studies in these systems and were able to follow in situ the growth of silica nanoparticles, namely the evolution of primary and agglomerate particle diameter and mass fractal dimension df.

[R1.043] Dosimetric properties of new europium doped KBr phosphors

In this work, dosimetric properties of new sintered europium-doped KBr phosphors subjected to beta irradiation are investigated. The obtained results show that these phosphors exhibit promising thermoluminescence properties that made them a viable alternative to substitute the conventional alkali halides crystals of similar composition for dosimetric purposes, considering as important advantages the simplicity and economy of the fabrication. The thermoluminescence response shows a linear dose dependence up to order of some Grays, which is higher than the linearity presented by the crystals of similar composition. Also, the thermoluminiscence fading is stabilized faster than that of the crystals do.

[R1.044] NEGATIVE POISSON'S RATIO IN CRYSTALS WITH JAHN-TELLER PHASE TRANSITIONS

Microscopic theory of the negative Poisson's ratio in crystals with structural phase transitions is developed. Analysis is presented for the dielectric crystals undergoing structural phase transitions from the tetragonal to the orthorhombic phase. The dysprosium vanadate type crystals are considered as an example. In such a type of crystals the structural phase transition may be induced by the external uniaxial pressure applied along the tetragonal axis. The mechanism of the structural phase transition is based on tuning up the energy gap between the ground and excited Kramers' doublets of the dysprosium cation. As a result of the gap variation the electron correlation caused by the virtual phonon exchange may be reduced or enhanced and the critical temperature is decreased or increased correspondingly. The pressure along the tetragonal axis compresses (expands) the crystal along the z-axis, simultaneously inducing the compression (expansion) in one of the x,y-axes in the crystal plane. The connection between the Poisson's ratio and the microscopic parameters (the electron-strain interaction constants, the phonon exchange strenth) is found. The temperature dependence of the Poisson's ratio is calculated.

[R1.045] Quantized and Limited Distances Between Nuclear Particles in Atoms and Beams, Between Atoms in Molecules, in Nanostructures, in Bodies and Beams, Due to the Electron Positron Lattice (Epola) Structure of Space.

Each epola unit cube (edge 4.4 fm) expands when entered by a moving guest nuclear particle and contracts when left by it. Thus the involved epola particles vibrate with frequency proportional to the velocity (v << c) of the guest particle. These vibrations create electro-magnetic (EM) waves of de Broglie Wavelength (dBW), spreading in the epola with the speed of light. Their interference creates the Accompanying Wave (AW) of the motion, a wave-guide-like channel in the epola, pre-formed for the unresisted motion of the guest particle. Starting a motion requires pumping of "inertial" energy from the guest particle to the epola to create the AW; on stopping, the inertial (kinetic) energy is pumped back. Being a physical entity, the AW has a cross-section width able to resist the penetration into it of AW's of other moving guest particles, causing quantizations and limitations of distances between particles. For stability, an electron orbit length must contain an integral number of dBW's of the electron. Only then are the AW's of each circling of the electron identical to the AW's of all previous circlings. They thus create a rotating standing wave pattern with no centripetal acceleration of the electron, contained in one of the half-wave loops of the pattern. References: M.Simhony, Invitation to the Natural Physics of Matter, Space and Radiation, World Scientific, 1994 (292 pp.). ISBN 981-02-1649-1. Website: www.word1.co.il/physics

[R1.046] Electronic structure and poling characteristics of NaBiTiO3 system

The electron structure of the NaBiTiO3 system with ABO3 type perovskite structure was studied by using self-consult charge-discrete variation-X¦Á(SCC-DV-X¦Á)method, and the effect of A, B position substitute on poling characteristic was also analyzed. The results show that spontaneous polarization is existed in the NBT piezoceramics system, the piezoelectric properties are enhanced because of the displacement of the B position ions, the compound substituent of Ba, Sr and Mn ion can decrease coercive field, and increase the strength of spontaneous polarization, so the dielectric properties are improved.

[R1.047] First-principles analyses of the defect stability in Sn-doped indium oxide

First-principles calculation has been performed to investigate the stability of the defects in Sn-doped indium oxide (ITO). Stability of the defects was calculated as a function of chemical potentials of the constituent atoms of ITO and Fermi energy of electron, where we investigated several different defects beside the ones studied previously by the experimental techniques. As a result chemical potential region of oxygen to crystallize the In_2O_3 was found to be almost the same as that of SnO_2, which explains the physical reason why high efficient doping of Sn atom to In_2O_3 crystal can be realized without causing precipitation of SnO2-like complexes and solid elemental Sn. Moreover, different from a suggestion based on the previous experimental analysis, the strongly bound complex composed of the Sn atom which is surrounded by three nearest oxygen atoms at regular site and an interstitial oxygen atom at quasi-anion site was suggested to be most stable under the condition of high Fermi energy. Then, density of states analysis showed that this stable defect made impurity levels around top of valence band, which suggests that this defect is a origin of the carrier compensation for the ITO films with very low resisitivity.

[R1.048] Dosimetric characterization of chemical-vapor-deposited diamond film irradiated with UV and beta radiation

Diamond is an excellent prospect for clinical radiation dosimetry due to its tissue-equivalence properties and being chemically inert. The use of diamond in radiation dosimetry has been halted by the high market price; although recently the capability of growing high quality polycrystalline has renewed the interest in using diamond films as detectors and dosimeters. In the present work we have characterized the dosimetric properties of diamond films synthesized by using chemical vapor deposition. The thermoluminescence (TL) of UV and beta exposed samples shows a glow curve composed of at least four peaks; one located around 587 K presents excellent TL properties suitable for dosimetric applications with ionizing and non ionizing radiation. The TL excitation spectrum exhibits maximum TL efficiency at 220 nm. The samples show regions of linear as well as supralinear behavior as a function or irradiation dose. The linear dose dependence was found for up to sixteen minutes of UV irradiation and 300 Gy for beta irradiated samples. The activation energy and the frequency factor were determined and found in the range of 0.32 - 0.89 eV and 1.1x10^2 – 2x10^8s_-1, respectively. The observed TL performance is reasonable appropriate to justify further investigation of diamond films as radiation dosimeters.

[R1.049] Defects production in UV alkali halides doped with europium

The defect production in alkali halide with doped divalent impurities and exposed to ionizing radiation can be explained by means of the creation of self-trapped excitons (STE), which are formed by either the excitation of halogen ion or trapped electrons in V_K centers coming from prior halogen ion ionization. Radiative recombination of self-trapped exciton produces a characteristic excitonic luminescence and no radiative recombination causes Frenkel defects (F-H centers). In the present work we provide experimental evidence that indicates that same crystals when exposed to a non ionizing radiation, such as UV near to 230 nm, similar Frenkel defects are generated. This situation is remarkable since 230 nm photons (5.3 eV) cannot directly produce excitons because their energy is below of the creation excitons energy around 7.7 eV. Thermoluminescence and optically stimulated luminescence techniques were used for investigating comparatively the effects of both ionizing and no ionizing radiations. We found that for ionizing and non ionizing radiation the luminescence signal was composed by two bands; the main part of the luminescence response was a broad band centered at 420 nm, which is ascribed to the well known energy transition 4f^65d(t_2g)-4f^7(^8S_7/2) of Eu_2+ ion and other additional band near to 460 nm which it has perhaps an intrinsic origin. In addition, we addressed the participation of the F and F_Z centers in the TL and OSL processes. In this way, we found that TL signal are strongly correlated with F centers (470 K TL peak in KCl:Eu_2+) and F_Z centers (370 K TL peak). Furthermore, through a selective photostimulation process, we also obtained evidence that the F center is involved in the OSL process. The present results point the existence of a common mechanism for the formation of Frenkel defects under ionizing and no ionizing radiation, by means of a possible excitonic mechanism.

[R1.050] Comparative study of elastic constantd of \alpha-, \beta- and Cubic- silicon nitride

Silicon nitride is an important structural ceramic and dielectric insulator. Recently, the new high pressure cubic phase of silicon nitride in spinel structure has attracted a lot of attention.^[1] We have carried out a detailed ab-initio calculation of all independent elastic constants for all three phases of Si_3N_4 by using the Vienna Ab-initio Simulation Package (VASP) in both LDA and GGA approxmations. The results for \beta-Si_3N_4 are in reasonable agreement with a experimental measurement on single crystal samples.^[2] For cubic-Si_3N_4 , The three independent elastic constants are predicted to be C_11 = 504.16 GPa, C_12 = 176.66 GPa, C_44 = 326.65 GPa and a bulk modulus B = 286 GPa. This value is very close to the experimental value of 300 GPa.^[1] All these results will be compared with those obtained by using the OLCAO method based on localized orbital approach.^[3]

[1]. Wai-Yim Ching, Yong-Nian Xu, Jukian D. Gale, and Manfred Ruhle, J. Am. Ceram. Soc. 81, 3189 (1998) [2]. R. Vogelgesang, M. Grimsditch, and J. S. Wallace, Appl. Phys. Lett. 76, 8 (2000) [3]. W.Y.Ching, Lizhi Ouyang, and Julian D. Gale, Phys. Rev. B61, 13, (2000)

[R1.051] Temperature dependence of nuclear quadrupole coupling constant of ^7Li and ^93Nb in LiNbO_3^*

It is well established that the ferroelectricity of LiNbO_3 originates from the displacement of positive ions, lithium and niobium, relative to negative oxygen ions below T_c. The local structural environment of Li and Nb in the crystal is similar at room temperature. However, as the temperature increases they move opposite directions along the c-axis: Li ions move to the center of oxygen triangles while Nb move to the center of oxygen octahedra. The temperature dependence of the nuclear quadrupole coupling constant(QCC) of Li and Nb is quite different: the QCC of ^7Li increases while that of ^93Nb decreases as the temperature increases[1]. In order to understand this opposite tendency we calculated the electric field gradient(EFG) at the two sites due to the first and second nearest neighboring oxygen ions, by considering the effective charges. The calculated results turn out to be in good agreement with the temperature dependence of QCC for both cations in the crystal. \ * Supported by the National Research Laboratory Program(MOST). \ [1] H. J. Kim and S. H. Choh, J. Korean Phys. Soc. \b28, 513 (1995).

[R1.052] Raman study of CaFe_2O_4

The polarized Raman spectra of single crystals of CaFe_2O_4 (space group Pnma, #62, Z=4) were studied between 15 and 523 K. Most of the Raman allowed modes (14 A_g + 7 B_1g + 14 B_2g + 7 B_3g) were identified and assigned to definite atomic vibrations in close comparison with results from lattice dynamics calculations and the Raman spectra of isostructural compounds. The temperature variation of the spectra is discussed.

[R1.053] Simultaneous EPR and optical spectroscopy of pure and Rh-doped barium calcium titanate crystals

The large electro-optic coefficients and high holographic sensitivity of barium calcium titanate (Ba0.77Ca0.23TiO3, BCT) make it promising candidate for various applications, since it has no drawback related to the phase transition at 280 K like barium titanate. The nominally pure and Rh doped BCT crystals were investigated with the help of optical absorption spectroscopy, light induced absorption change measurements and electron paramagnetic resonance (EPR) in the temperature range between 4.2 and 300 K. Analyzing angular dependencies of the EPR lines we succeed to identify low-symmetry and nearly cubic centers of iron trace impurity. Four other paramagnetic defects were found after illumination: Ti3+, Rh2+, O- and Pt3+. All these defects are participants of charge transfer processes. At the light energy E about 1.3 eV the holes abandon Ti4+ creating the paramagnetic electronic Ti3+. At E > 2.3 eV O2- ions capture the holes and create hole O- centers and simultaneously Ti3+ concentration increases. Nearly located non-controlled or intentionally introduced impurities (Sr, K, Na, Ca...) may serve as pins for the fixation of the electrons and holes. Additional correlated changes were found at E > 3.2 eV (band-band transition), when EPR lines of Rh2+, Pt3+ appear and intensities of Ti3+, O- and Fe3+ essentially increase. Models of defects involved in these light induced processes are proposed.

[R1.054] RAMAN SCATTERING STUDIES OF Pb_1-x Sr_x TiO_3 (x = 0 to 1.0) FILMS GROWN BY METALORGANIC DECOMPOSITION (MOD)

Pb_1-x Sr_x TiO_3 (x = 0 to 1.0) films of thickness \sim 4 \mu m have been prepared on Pt substrates by the metalorganic decomposition (MOD) technique. X-ray diffraction results show that the films are polycrystalline with a perovskite tetragonal phase at room temperature for x < 0.6 and a cubic phase for x > 0.6. Room temperature Raman spectra show a systematic variation of lattice vibrational modes with composition. The most notable changes in the Raman spectra with x are the coalescence of A_1(3TO) and E(3TO) modes into one at approximately x = 0.6, and a considerable softening of A_1 (2TO) mode. Temperature dependent Raman spectra of Pb_0.4Sr_0.6TiO_3 film (ferroelectric to paraelectric phase transition temperature \sim room temperature) show a reduction in the intensity of the characteristic phonon modes with an increase in temperature; however, the intensity of modes persist up to \sim150 ^\circC indicating a diffuse phase transition. For temperatures >150 ^\circC broad features remain that are characteristic of a disordered paraelectric cubic phase. The temperature dependence of dielectric permittivity shows a broad maximum, and the ferroelectric hysteresis loops persist up to \sim150 ^\circC in agreement with the Raman measurements.

[R1.055] NMR measurements in a hydrogen/helium slush at 4.2 K

Matrix isolation of various atoms in solid hydrogen presents both pure and applied research possibilities. When single atom properties are measured with NMR in the background of a quantum solid, insight into electronic interactions and quantum diffusion may be obtained. A cell has been constructed which, when filled with liquid helium, may have various gases injected into it. If this gas is a mixture of hydrogen and, say, boron, NMR may be performed on both the H and the B nuclei. Crystal or amorphous structures and atomic diffusion may be investigated. Design and construction of the apparatus will be presented. The first pulsed NMR data on H will be presented and interpreted.

[R1.056] Theory

[R1.057] A New Physics Simulation Machine

A new physics simulation language, byte code compiler and stack machine interpreter are described. This machine is capable of interactively solving research problems in quantum and classical mechanics, as well as simulating simple scenarios involving electromagnetic field production and the evolution of chaotic systems. On the educational side, the language is powerful enough to model most of the solutions of ‘introductory’ physics textbook problems. A near-term development goal of this project is to provide the capability to model molecular motors such as ATP synthase in the simulation machine.

[R1.058] Application of the Coupled-Cluster Method to the Single-Impurity Anderson Model

The Coupled-Cluster method (CCM) is applied to the well known single-impurity Anderson model wherein hybridization may occur between a localized \textitf-orbital and a filled Fermi-sea of conduction electrons. The basic ansatz of the CCM is to write the true wave function \Psi in terms of a ground-state function \Phi_0 as |\Psi\rangle=e^\tildeS|\Phi_0\rangle where the operators \tildeS describe the excitations of the system. For this particular system we choose |\Phi_0\rangle=f^_\sigma|0\rangle_f|F\rangle where |F\rangle denotes a filled sea of conduction electrons while |0\rangle_f is the unoccupied f-orbital. A simple expression for the ground-state energy is derived and comparisons are made with other methods.

[R1.059] Jahn-Teller Effect: A Plaquette Expansion Approach

A number of years ago Hollenberg et al.\ developed a closed form expression for the ground-state energy density for a general (extensive) many-body problem, written in terms of the Lanczos tridiagonal matrix form of the Hamiltonian. A plaquette expansion may be written for this matrix wherein the physics is encapsulated entirely through the connected moments of the Hamiltonian. We apply this method to study the ground-state energy of the linear E\bigotimes\epsilon Jahn-Teller effect.

[R1.060] Generalized Moments Expansions

For a number of years linked-cluster expansions have been a major part of techiques used for the investigation of quantum Hamiltonian systems. Such methods include the ``\textitt-expansion" of Horn and Weinstein [Phys. Rev. D \textbf30, 1256 (1984)] as well as the derivative expansions such as the Connected Moments Expansion (CMX) and the Alternate Moments Expansion (AMX). In this work we have derived a general expression for any moments expression of which both the CMX and AMX are particular cases.

[R1.061] Berry phase effects in a generalized Kronig-Penney model

Berry's phase (BP) has been shown to play an important role in a variety of seemingly dissimilar areas of physics: optics, molecular physics, nuclear resonance, etc. In solid-state physics, an understanding of the BP was crucial in developing a complete theory of electric polarization in dielectrics, and it has been shown that the semiclassical equations of motion for Bloch electrons should be modified by BP effects.(See e.g. R. Resta, J. Phys.: Condens. Matter 12, R107 (2000).) Why, then, is the BP not more widely known? We suggest that this is because there is a lack of simple model systems that exhibit BP effects. To address this, we have constructed a generalized Kronig-Penney model that produces BP effects. Model parameters are varied to show how the BP itself changes; in particular, in the limit where our non-symmetric model acquires inversion symmetry, the Berry phase is seen to go to expected values. We then illustrate how the existence of a BP can be seen in selected phenomena, for example the motion of electron wave packets.

[R1.062] Computational Themes

[R1.063] Quantum Molecular Dynamics with Non-Gaussian Wavepackets

The time-dependent variational principle provides a rigorous framework for developing approximate quantal equations of motion for many-body systems. To date, most calculations use Gaussian single-particle wavepackets. Such wavepackets represent the exact solution of both the free-particle and harmonic oscillator systems; these exact solutions do not represent the physical conditions of strong Coulomb interactions in warm dense matter. In the development of the equations of motion for self-similar exponential wavepackets, which are the exact solutions of the l=0 bound-state Coulomb system, we emphasize the importance of constraining the time-dependent variational principle with the continuity equation. We have compared Rutherford scattering trajectories with Gaussian and exponential wavepackets.

[R1.064] Three-Dimensional Simulation for Laser-Plasma Interactions; a Quasi-Static Approach

The interaction of a high intensity laser with ionizing gas and plasmas is of current interest for both Laser-Wakefield Acceleration and x-ray generation. Three-dimensional simulations of these interactions have been conducted using fully electromagnetic particle-in-cell calculations; however,long-interaction time simulations have been limited due to the computational cost. We discuss the quasi-static approximation, which allows simulations of longer interaction times, as well as the method used to implement this approximation using an object oriented structure. We also present preliminary results of a 3D simulation in the quasi-static approximation of a laser with a fluid plasma. We compare results of this three-dimensional simulation with WAKE [1], a two-dimensional quasi-static simulation, for effects which are predominantly 2D in nature, e.g., self-focusing. We also present preliminary results for instabilities which have a 3D structure, filamentation.

[1] T. M. Antonsen, Jr. and P. Mora, PRL 69(15), 2204, (1992)

[R1.065] Numerical Computation of Higher Order Derivatives to Arbitrary Precision

The numerical computation of higher order derivatives to arbitrary precision is shown to be related to the inverse of a Vandermonde-like matrix. The well-known formulas for lower derivatives and lower precision are obtained as special cases of the general formulation. This formulation is useful in problems requiring double, quad and infinite precision arithmetic.

[R1.066] Computational Methods incorporating symbolic Steps for solving the Poisson and Inhomogeneous Helmholtz Equations conforming to cylindrical Boundaries

It is a goal to solve the Helmholtz equation and the Poisson equation in a manner retaining insight of the functional structure. The conventional 2 or 3 dimensional integrals involved with solving such problems via Green's function often is prohibitive. It is often easier in terms of computational or analytical labors to use finite element or iterative schemes to solve such differential equations than to boldly integrate the source density with the Green's function in two or more dimensions. The purpose of this presentation is to demonstrate a method for solving such problems within the cylinder (where the source is confined) with a version of the element method and a computational method for solving such problems in the surrounding source-less region in analogy to long-range issues of electrostatics and long-range issues of diffusion. This outer region is modeled and/or calculated by using a finite difference scheme in a small intermediate region and with an appropriate term-by-term basis matching procedure in a suitable spherical boundary. This proximate spherical boundary serves to generate the homogeneous solution to either the Laplace or homogeneous Helmholtz equation with the appropriate spherical harmonic basis.

[R1.067] Development of a Field-Aligned Integrated Conductivity Model Using the SAMI2 Open Source Code

The SAMI2 open source code is a middle and low latitude ionspheric model developed by the Naval Research Lab for the dual purposes of research and education. At the time of this writing the source code has no component for the integrated magnetic field-aligned conductivity. The dependence of human activities on conditions in the space environment, such as communications, has grown and will continue to do so. With this growth comes higher financial stakes, as changes in the space environment have greater economic impact. In order to minimize the adverse effects of these changes, predictive models are being developed. Among the geophysical parameters that affect communications is the conductivity in the ionosphere. As part of the commitment of Texas A amp; M Univeristy-Commerce to build a strong undergraduate research program, a team consisting of two students and a faculty mentor are developing a model of the integrated field-aligned conductivity using the SAMI2 code. The current status of the research and preliminary results are presented as well as a summary of future work.

[R1.068] WebTOP: Interactive 3D Web-based Simulations for Teaching Waves and Optics

WebTOP is 3D interactive computer graphics system designed to help students learn about waves and optics. It has been used to help teach undergraduate introductory physics and optics classes. It has sixteen modules that treat the following topics: waves, geometrical optics, reflection and refraction, polarization, interference, diffraction, lasers and scattering. WebTOP simulations have the following characteristics. First, they are three dimensional, i.e., they have navigation controls that allow the user to rotate the scene, pan it, or zoom into it. Secondly, they are interactive. The user can change the parameters either by typing the values into boxes, or by using the mouse cursor to move the corresponding widget in the scene. Thirdly, the simulations are animated, when animation is appropriate. Furthermore, the simulations include vcr-type controls that allow the user to record a session for later retrieval and viewing. Finally, these modules run inside a web browser. They can be run from our website, http://webtop.msstate.edu or be downloaded from this website and run locally. In addition to the simulations, each WebTOP module includes a short description of the theory used, and sets of recorded examples and suggested exercises. WebTOP is sponsored in part by the National Science Foundation (DUE 9950569).

[R1.069] Simple and unified derivation of conjugate gradient and variable metric minimization

Simple derivations of the most popular methods for finding the minimum of a function of many variables are presented in a unified manner at a level appropriate for an undergraduate computational physics course. In particular simple derivations of the conjugate directions method, the conjugate gradient method, and the variable metric method are described and some generalizations of these methods are also discussed.

[R1.070] Electronic Structure

[R1.071] A finite difference method for the solution of the coupled Schrödinger and Poisson equations for quantum dot systems

With the advent of recent advances in epitaxial crystal growth technology, such as molecular-beam epitaxy, which have enabled the fabrication of atomically sharp heterojunction interfaces, there has been a growing interest in spatially quantized systems. Of these systems, quantum dots (QD) have attracted much attention. Here we present a finite difference method for the solution of the coupled Schrödinger and Poisson equations for a number of QD systems. These systems consist of vertically aligned multiple QDs with varying numbers of electrons. The effect of the inter-dot separation on the energy levels of these QDs is investigated using this finite difference technique. The method has the advantages of being relatively fast and adaptable to any QD geometry. The method is outlined briefly and the results from these calculations are presented here, with the intention of using them as stepping stone to investigating more complex systems.

[R1.072] Variational anisotropic model of Wannier excitons compared with fractional-dimensional space approach

Binding energy of Wannier excitons in a quantum well of thickness L is studied using two models: a two-parameter trial wave function and a continous fractional-dimensional space with dimension alpha between 2 and 3. Since both models provide quantitative measures of the exciton spatial anisotropy as L changes, we give physical arguments for a plausible definition of alpha = alpha (L).

[R1.073] Tight-Binding model for Rubidium

The NRL tight-binding total energy method was applied to Rubidium, a material which is known as an utralsoft metal. We fit LAPW calculations of high symmetry structures onto a non-orthogonal tight-binding Hamiltonian. This Hamiltonian accurately reproduces the LAPW band structures, density of states and total energies as a function of volume. In addition, the tight-binding scheme determines various quantities that were not fitted, such as elastic constants and phonon frequencies in agreement with experimental values. We also explored the applicability of this model in performing molecular dynamics simulations and its extension to other alkali metals.

[R1.074] First-principles study of gradient corrections to the local density functional on the structural properties of ionic solids

We have studied the structural properties of ionic crystallie solids by means of first-principles total-energy calculations using the full-potential Linearized Augmented Plane Waves (LAPW) method. The calculations are based on the Density Functional Theory and we have used the Local Density Approximations (LDA) and the Generalized Gradient Approximation (GGA)for the exchange-correlation potential, in order to analyze the gradient effects. We present results for the lattice parameter and bulk modulus for ionic solids with NaCl structure. From a comparison of our results with experimental values, we find that LDA give errors of 3% and 25% for the lattice parameter and the bulk modulus, respectively. The inclusion of the GGA systematically improve these quantities, for the lattice parameter the errors are menor to 1% and about of 5% for the bulk modulus. The importance of gradient corrections in ab-initio calculation of ionic systems are emphasized.

[R1.075] Polymer Physics II

[R1.076] Infrared and Raman study of phase separation in binary n-alkane mixtures

The phenomenon of micro-phase separation occurs in certain solid solution of binary mixtures of n-alkane in room temperature. The blends of C36H74/C24D50 have been studied at different molar concentration ratios. The reflectance micro-infrared spectra and the Raman spectra have been obtained for the previously mixed, melted and quenched samples. The spectra changing in time show the proceeding process of phase separation. Similar polymers align themselves inside the micro-domains. The alignment results in a periodic intermolecular potential, often called the crystalline field, which induces line splitting, broadening and/or intensity changes. This has been clearly shown in the IR and Raman spectra taken from the various spots of the sample. Different band shapes in the region of 1400 cm-1 to 1500 cm-1 represent different contribution of amorphous and orthorhombic phases. The splitting and different band intensity at about 1090 cm-1 represent different mole fraction of the components in a certain small area of sample. This research is in early stages of development and only preliminary results will be presented.

[R1.077] Mass transport in thin polymer films during AFM-assisted nanolithography

An AFM-assisted nanolithography in thin polymer films is the subject of this presentation. We have developed the model describing three stages of the AFM-based nanolithography process in polymers: non-uniform electric field formation inside thin (20-50 nm) polymer film; polymer softening due to localized Joule heating a fraction of polymer film heated above the glass transition point; and mass transport of dielectric polymer liquid in the direction of electrically biased AFM tip.

The model is based on the numerical solution of three-dimensional non-uniform heat equation together with modified Navier-Stokes equation for uncompressible non-Newtonian polymer liquid in cylindrical coordinate system. The electric field in the polymer film is evaluated through the method of images. The pressure acting on the dielectric liquid is associated with a strong gradient of electric field induced by the AFM tip resulting in very fast (fractions of milliseconds) mass transport of polymer liquid and in formation of raised nanostructures (1-100 nm).

This approach suggests novel experimental lithography technique, which must be conceptually different from all lithography techniques (including thermo-mechanical writing - MILLIPEDE, AFM-assisted chemical modification in polymer resists, e-beam, ion-beam etc.) in polymer materials on nanoscale reported in the literature up to date.

[R1.078] In–situ Rheo-SAXS and Rheo-WAXD studies of Shear Induced Structures in Model Polyethylene Blend

The effect of chain length on shear-induced crystallization in model blend polyethylene melts was studied by in-situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide-angle X-ray diffraction) techniques. Model polyethylene blend was prepared by solution blending of low molecular weight polydisperse PE (Mw = 50,000, MWD = 2) with 10wt161,000, MWD = 1). While shear-induced oriented crystalline structures were not observed in SAXS and WAXD of the single component low molecular weight polydisperse PE (shear rate = 60 1/s, shear duration = 5 s at 115ºC ), the blend showed oriented crystalline structures after shear and these structures were stable at temperature near the nominal melting point. SAXS and WAXD results clearly show that the high molecular weight species (or long chains) in the blend directly affect the formation and stability of the orientation-induced crystalline structures in polymer melts under flow. This study verifies that upon cessation of flow, the longer chain molecules remain oriented and can form stable precursors for nucleation; on the other hand the shorter chains relax and loose their orientation rapidly due to short relaxation time.

[R1.079] Structure of Secondary Crystals in Ethylene-Based Ionomers

A typical DSC thermogram of an ethylene-(meth)acrylic acid ionomer displays two melting endotherms: one near 100^oC reflecting the melting of primary ethylene crystals, and one at 40-60^oC which we have shown via simultaneous SAXS/WAXS/DSC to arise from the melting of interlamellar secondary crystals. Dynamic DSC (DDSC) confirms that the two peaks reflect a bimodal crystal thickness distribution, rather than a superposition of melting and recrystallization events. The melting temperature of these secondary crystals, estimated to be 2.5-3.5 nm thick, is sensitive to annealing history. DDSC also indicates that these secondary crystals melt irreversibly, as expected if each must be individually nucleated. The 2-D SAXS patterns of highly-oriented blown films of such ionomers show intense peaks, arising from the polyethylene lamellar crystallites, along the direction of principal orientation. Comparing the azimuthal variation in SAXS peak intensity at temperatures below and above the low-temperature endotherm reveals that the secondary crystallites are significantly oriented, but less so than the primary lamellae. Thus, the secondary interlamellar crystals also have a lamellar (anisotropic) habit, rather than resembling fringed micelles (isotropic).

[R1.080] Structural Studies of Ethylene-1-Octene and Ethylene-Norbornene Random Copolymers by NMR and WAXD

The properties of two series of melt-quenched, random ethylene copolymers (comonomer content < 15 moldiscussed. Changes in the crystallite properties with increasing comonomer content, including crystallite thickness reduction from ^13C T_1 NMR relaxation times and chain packing from the line widths of crystal NMR spectra, were found to be independent of comonomer type. Analyses of the non-crystalline regions revealed differences. Copolymers with norbornene showed a larger reduction in the peak position of the WAXD amorphous halo relative to copolymers with the same content of 1-octene. The NMR resonance of the amorphous CH_2 backbone units was broader in the copolymers with norbornene. Both observations are due to significant conformational differences in the non-crystalline chains with different comonomer type. Interestingly, the overall decrease in ^13C T_1 times of the amorphous CH_2 backbone units with increasing comonomer content was the same for both copolymer systems. Hence, in the range of comonomer content studied, the rates of fast motions for ethylene segments in the backbone are independent of comonomer type.

[R1.081] Morphology Evolution in Polytetrafluoroethylene as a Function of Melt Time and Temperature: Single- and Multi-molecule Folded Chain Single Crystals and Banded Structures

Examination of the evolution of the crystalline morphology of dispersed PTFE emulsion particles of various molecular weight resins as a function of time in the melt indicates that substantial molecular motion on the substrate occurs; large, angular particles, considerably larger than the original dispersion particles, form first for short melt times, followed by development of both planar folded chain single crystals and single molecule single crystals and banded structures with parallel double striations. The molecules in the single molecule single crystals and bands are parallel to the substrate, an individual double striation appearing to consist of a “double edge”, folded chain lamella, more or less normal to the substrate. In two “nano”-emulsion samples used (TE 5070 (DuPont) and 18749/26 (Ausimont)), comparison of measured molecular weight and particle morphology suggests chain folding in the as-polymerized single crystal particles.

[R1.082] Observation of Double Glass Transition in Cold Crystallized Poly(phenylene sulfide)

The glass transition and crystallization of poly(phenylene sulfide), PPS, were studied using modulated differential scanning calorimetry (MDSC), density, and wide-angle X-ray scattering (WAXS). Amorphous samples were prepared through a rapid liquid Nitrogen quench from the melt and then were cold crystallized at 100C just above the glass-transition temperature of 88.5C. The glass transition was measured from the inflection point of the reversing heatflow curve of the MDSC. During crystallization Tg shifts from 90C to 100C by the time PPS achieves its ultimate crystallinity at a treatment time of 1500 minutes (25 hrs.). Tg also becomes broader as treatment time is increased, suggesting a broader distribution of the relaxation times. The degree of crystallinity measured by both density and WAXS increased with increasing isothermal crystallization time to an ultimate value of 0.185. When PPS is cold crystallized just above the Tg of an amorphous sample, two distinct relaxation processes are observed. Samples with treatment times below 25 hours had a Tg of 90C, while those treated for this time or longer had a Tg of 100C. A double glass transition was observed for sample treatment times between 660 and 1440 minutes. This is evidence of two distinct relaxation processes in the material. For treatment times less than 660 minutes there is only a single glass transition temperature corresponding to the first relaxation. As the isothermal treatment time was increased beyond 660 minutes a second Tg appears, signaling the onset of the second relaxation mechanism. This transition in the mode of relaxation was observed through examination of the reversing heatflow curve and in the Avrami fit of the density data.

[R1.083] Molecular relaxation process of isotactic polystyrene studied by real-time dielectric spectroscopy and small and wide angle X-ray scattering

The molecular relaxation processes of cold-crystallized isotactic polystyrene (iPS) have been investigated using real-time dielectric spectroscopy. Wide and small angle X-ray scattering studies were performed during, and subsequent to crystallization, to determine degree of crystallinity and lamellar thickness. The purpose of our study is to compare the restrictions imposed on molecular mobility by crystals in the vicinity of the alpha (glass transition) relaxation, for bulk films (reported here) and thin films of iPS. In separate experiments, dielectric and X-ray data were collected during isothermal crystallization at temperatures Tc = 140°C or 170°C for various times, followed by reheating. From dielectric loss tangent data we observe that the glass transition temperature, Tg shifts to lower frequency during crystallization, indicating the relaxation time increases as crystals constrain the amorphous phase. The frequency at which the alpha relaxation occurs shifts two orders of magnitude higher for Tc = 170°C compared to Tc = 140°C, indicating reduced relaxation time for higher crystallization temperatures. The dielectric data were well fitted to a Havriliak-Negami model, and the parameters describing the distribution of relaxation times and dielectric relaxation strength were obtained. The dielectric strength and symmetric broadening parameter both decrease, while the central relaxation time and asymmetric broadening parameter increase with crystallization time.

[R1.084] Characterization of Fiber and Bulk of Poly(trimethylene Terephthalate) by Quantitative Thermal Analysis

Quantitative thermal analyses fibers of poly(trimethylene terephthalate) (PTT) are presented based on the measured heat capacity by standard differential scanning calorimetry (DSC) and temperature modulated differential scanning calorimetry (TMDSC) and compared with earlier results for bulk PTT . The heat capacities of the solid and liquid states of semicrystalline PTT are reported from 5 K to 570 K. The low temperature heat capacity of solid PTT is linked to the vibrational spectrum, using the ATHAS method. The experimental heat capacities of liquid PTT can be expressed by: CpL(exp) = 211.6 + 0.434T J/(K mol). The semicrystalline fiber and bulk PTT are analyzed using the extrapolated vibrational heat capacity of the solid and the total heat capacity of the liquid as baselines. The glass transition temperature of amorphous PTT occurs at 310-315 K with a dCp of 94 J/(K mol). For 100is 30±2 kJ/mol. Using quasi-isothermal TMDSC, the apparent reversing and nonreversing heat capacities were determined from 220 to 540 K. Additional time-dependent, reversing contributions are linked to reorganization and recrystallization, while the major melting is irreversible. Truly reversible and time-dependent irreversible heat effects were separated. With these data one can compute crystallinity changes with temperature and the mobile amorphous fractions.

[R1.085] Effects of chain configuration on the crystallization behavior of poly(lactic acid)

Research interest in poly(lactic acid) stems not only from its environmentally appropriate synthesis and potential applications, but also from its complex crystalline forms. The building block, lactic acid, has two stereoisomers, L-type and D-type, which can form polymers of different regio-regularity. In addition, a blend of L- and D-homopolymers can form stereocomplex crystals with melting temperature much higher than homopolymer crystals. Based on the characteristic ratios measured, poly(lactic acid) chains are found to be inherently rigid. The D-L junctions have been characterized. The distribution of D- and L-block lengths has been analyzed. The configuration of the chains has a profound influence on the overall crystallization rate and crystalline perfection. Based on spectroscopic evidence, the molecular parameters governing the magnitude and specificity of interchain interactions in both homopolymer and copolymer stereocomplexes have been elucidated. Thus, the molecular origin of the thermal stability of stereocomplexes can be better understood.

[R1.086] X-ray Characterization of Row Crystallized Polymers

Polymer melt crystallization is strongly influenced by flows that orient some macromolecules that become fibrillar nuclei for subsequent epitaxial growth of folded chain lamellae. For polyethylene (PE) and poly(ethylene terephthalate) (PET), the resulting row structures have the additional feature that the ribbon-like epitaxial crystals twist coherently; the same sort of lamellar twisting gives rise to banded spherulites in quiescent crystallization. In oriented row crystallization, the crystallographic b axis is normal to the flow direction, while the average orientations of a and c axes are functions of the length of the twisted lamellae. A simple quantitative model with cylindrical symmetry accounts for the textures observed by wide-angle X-ray diffraction. The same model accounts for the small-angle X-ray pattern appearing to be well oriented, regardless of the length of the twisted lamellae and associated orientation of the c axis.

[R1.087] Crystallization Kinetics and Morphology of Thin Films of PEO/PMMA Blends

The morphology and crystal growth kinetics in thin films of PEO/PMMA blends were studied optical microscopy in the reflection mode. Samples (ca. 100 nm thick) were prepared by spin-casting a dilute dichloroethane solution onto a cleaned silicon wafer and subsequent drying under vacuum. Crystal growth rates were measured as a function of blend composition, crystallization temperature and film thickness. This presentation will specifically focus on studies of dendritic growth in 30/70 PEO/PMMA blends. Studies of the temperature dependence of the crystal growth rate and the morphology indicate that both are a complex function of the thermal history.

[R1.088] Lamellar Morphology of Metallocene Random Propylene Copolymers studied by Atomic Force Microscopy

Four sets of propylene based random copolymers with co-units of ethylene, 1-butene, 1-hexene and 1-octene, in a wide range of co-monomer contents up to 10 mol percent (including co-unit and other defects), were studied after both rapid and isothermal crystallization from the melt. Etched film surfaces were imaged so as to minimize catalyst and co-catalyst residues. As the concentration of the gamma polymorph increases with increasing comonomer content or increasing crystallization temperature, the thickness and lateral extension of the observed lamellae decreases rapidly. Spherulites are formed in copolymers with non-crystallizable units (1-hexene and 1-octene) up to 3 mol percent total defect content, and were observed right up to 7 mol percent total defect content in those with partially crystallizable co-monomers (ethylene and 1-butene). However, lamellae were observed in the surfaces of all copolymers analyzed, even in the most defective ones, highlighting the importance of the gamma polymorph in propagating lamellar crystallites in polypropylenes with a high concentration of defects. The morphology of equivalent microtomed bulk specimens will be comparatively discussed.

[R1.089] FTIR, DSC, WAXS and density study of cold crystallized isotactic polystyrene

The techniques of Fourier transform infrared (FTIR) spectroscopy, wide-angle X-ray scattering(WAXS), density measurement and differential scanning calorimetry (DSC) were used to study the cold crystallization behavior of isotactic polystyrene (iPS) films cold-crystallized at 140C and 170C for various times. It was observed from FTIR that the intensity of the peak at 981 cm-1 increases with crystallization time and this reflects the manner of crystallization. The degree of crystallinity was obtained from DSC by heat of fusion calculation, from WAXS by the crystal peak area after amorphous phase subtraction and from density measurement by two-phase model density calculation. Crystallinity determined by any method exhibits a correlation with the absorption peak intensity ratio I981cm-1/I1026cm-1 , obtained from FTIR. Such a correlation provides us a way to study the crystallization process for iPS thin films. The FTIR spectra of crystallized thin film was taken for films spin cast from solution on KBr disks. For the first time, crystallinity of iPS thin films was obtained by the reference to the correlation curves between the FTIR peak intensity ratio and crystallinity developed for bulk film.

[R1.090] Broadband Dielectric Investigation of Amorphous and Semi-Crystalline Polylactides

Molecular dynamics of poly (L-lactide) and several L-lactide/meso-lactide random copolymers were investigated in the frequency domain using broadband dielectric spectroscopy. The dielectric relaxation spectra of fully amorphous and crystalline samples reveal the influence of crystalline content and microstructure on chain motion in the amorphous phase. Differences in relaxation strength of the segmental processes were observed in these samples. While the strength of the crystalline samples increases with temperature, that of the amorphous samples changes only very little or in the opposite direction with temperature. This behavior will be discussed in the context of a rigid amorphous phase. As expected, mean segmental relaxation time is longer and its distribution is broader (at lower frequencies) in samples with higher crystallinity. Differences in the details of the relaxation processes as a function of the crystallinity and morphology will be discussed.

[R1.091] Breakup of Spiral and Concentric Ringed Spherulites in Polymer Crystallization

[R1.092] Controlling Crysallization Properties of Poly(ethylene oxide) Thin Film by Geometric Confinement

Semi-crystalline thin films of poly(ethylene oxide) were prepared by spin coating. Geometric confinement was implemented in different ways including: decreasing film thickness, nanopatterning the substrate, adding nano-particles. Morphology was measured by atomic force microscopy(AFM), melting temperature and lateral modulus were tested by shear modulus force microscopy(SMFM). The morphology was found to change from spherulite to shish-kebab, and then to finger patterns as the film thickness decreases from 200003 to 12003. The melting point was found to decrease gradually from 340K to around 320K. Both nanopatterning of the substrate and addition of functionalized nano-particles have the effect of inducing heterogeneous surface nucleation which resulted in a decrease of the spherulite size. The effects of increased nucleation sites on the melting point and hardness as a function of film thickness will be discussed.

[R1.093] Effect of fiber on shear-induced crystallization of i-PP in UHMWPE/i-PP and Aramid/i-PP fiber composites

Shear-induced crystallization of the isotactic polypropylene (i-PP) matrix in fiber composites containing ultra high molecular weight polyethylene (UHMWPE) fibers or Aramid fibers was studied by in-situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) techniques. The maximum fiber content was 10 wt composite and the shearing conditions included the application of step shear with rate = 60 1/s and duration time = 5 s at 145 ^oC. The development of lamellar structures (by SAXS) and crystal orientation (by WAXS) was strongly affected by the type of fiber present. It was found that Kebab structures were more likely to develop in the presence of UHMWPE fiber than Aramid fiber. In addition, the orientation of i-PP lamellae was greater in the UHMWPE composite than that in the Aramid composite, even though the crystallization rate was similar in both systems. As the UHMWPE phase was non-crystalline at the measurement temperature (145 ^oC), our results suggest that a high degree of molecular interactions between the oriented PE chains and the surrounding iPP chains makes the molten UHMWPE phase an effective nucleating agents for iPP crystallization under shear. The epitaxial effect of Aramid fiber on the crystallization of iPP was not a dominating factor under the shear conditions.

[R1.094] Study of the Reversibility in the Crystallization Behavior of Statistical Ethylene/Styrene Copolymers by Classical and Temperature Modulated Differential Scanning Calorimetry

The crystallization and melting behavior of statistical copolymers of ethylene and styrene (0 - 11 molwere studied using classical and temperature-modulated (TM) differential scanning calorimetry. The evolution of the reversible part of the excess heat capacity was measured during the slowest stage of primary crystallization and during secondary crystallization using TM-DSC in the quasi-isothermal mode. Evolution of the degree of crystallinity and the melting temperature(s) during primary and secondary crystallization was recorded with classical DSC. We will discuss the effect of crystallization temperature and copolymer composition on the primary and secondary crystallization behaviors and show correlations between the evolution of the excess heat capacity, the degree of crystallinity and the melting temperature(s). We will also show that TM-DSC experiments carried under quasi-isothermal conditions enable us to obtain information on the establishment of constraints during secondary crystallization, which can be linked to the evolution of the multiple melting behavior of these copolymers as a function of crystallization time. In turn, these studies shed some light on the relevance of the two existing models of reversible crystallization/melting for secondary crystallization processes by lamellar thickening or by secondary crystal formation.

[R1.095] Effect of complex flow kinematics on the molecular orientation distribution in injection molding of liquid crystalline copolyesters

Properties of liquid crystalline polymers (LCPs) depend critically on the molecular orientation distribution, which in turn can be dramatically influenced by flow fields during processing. Our group has previously applied in situ x-ray scattering to measure orientation distributions in steady, isothermal complex channel flows of LCPs. It was found that the complex orientation states arise from the competition of inhomogeneous shear and extension. Here we consider the extent to which these concepts translate to the more complex transient amp; nonisothermal case of injection molding, through ex situ studies of molecular orientation distributions in injection molded plaques. These studies employ a new, low-cost aromatic copolyester based on the mesogen dihydroxy-a-methylstilbene. We find strong similarities in the type of orientation states observed in both cases. Further, systematic changes in the relative importance of shear and extension through changes in the plaque thickness lead to changes in orientation distribution that would be anticipated from our evolving understanding of the effect of mixed shear amp; extensional flows on orientation. These results verify that idealized isothermal studies not only elucidate fundamental flow/orientation relations, but also serve as a useful intermediate step towards understanding true processing conditions.

[R1.096] The Phase Behavior of Liquid Crystalline Polymers Containing Sulfone Group in Side Chain

The phase behavior of side chain liquid crystalline polymers, (tetradecylsulfonyl) methyl-substituted polyoxyethylene (14SEO) and (octylsulfonylhexylthio) methyl-substituted polyoxyethylene (8S6EO), was studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM), FT-IR, small and wide angle x-ray scattering (SAXS and WAXS) and rheology. POM showed that both 14SEO and 8S6EO had ordered smectic phases at room temperature. From SAXS measurements, it was found that the layer thickness of the smectic phase corresponds to twice the extended side chain length. Above the isotropization temperature (Ti), no scattering peaks were observed for 8S6EO, while broad scattering peaks were still persistent for 14SEO. It is believed that the broad peak after the Ti is related to the characteristic block copolymer-like correlation hole peak in the homogeneous state. On the basis of the SAXS results, rheological measurements were also performed in order to identify whether Ti of 14SEO was truly related to the order-disorder transition (ODT).

[R1.097] Geometric “Chirality” from “Umbrella” Molecules

A new series of liquid crystals with the unique combination of a discotic triphenylene core surrounded by six tails containing rod-like cyanobiphenyl as end groups were synthesized. The triphenylene core and cyanobiphenyl end groups are connected via different number of methylene units (n = 6 – 12). The phase behaviors and structural identifications of most of these molecules include a liquid crystal phase (N) and two crystalline phases. Under a direct current electric field, the cyanobiphenyl groups align along the electric field and there is a drastic change in the optical birefringence. Each individual molecule becomes “umbrella”-shaped, and the molecules stacked together to form columns. This anisotropic geometry generates a geometric “chirality” although the atomic structures of these molecules are achiral.

[R1.098] Supramolecular and Molecular Structures in Aromatic Polyimides Containing Cyanobiphenyl Side-Chains

A series of newly designed polyimides, which are composed of aromatic polyimide backbones and side chains containing 4-cyanobiphenyl mesogens, has been synthesized (abbreviated as BPDA-nCBBP, n represents the number of methylene units). Most of the crystal forms have been identified as triclinic lattices with large unit cells. The number of chains in those unit cells ranged from six to eight. The fact that large unit cells in these polymers include various numbers of chains leads to an important issue; how are the chains packed into these large unit cells. It has been found that a waved layer packing model with microphase separations between the main chains and the side chains can best fit our X-ray diffraction experimental results. This new concept of molecular packing may also provide explanations for some other unsolved experimental observations in polymer ordered structures.

[R1.099] Morphological Chirality and Crystal Twinning in Different Length Scales of a Chiral Liquid Crystalline Polyester

The chiral liquid crystalline polyester is synthesized from (R)-(-)-4'-ƒç-[2-(p-hydroxy-o-nitrophenyloxy)-1-propyloxy]-1-undecyloxy-4-biphenyl carboxylic acid via an A-B type condensation polymerization. The resulting polymer exhibits Chiral Smectic A, Chiral Smectic C and Twist Grain Boundary Smectic A phases, as indicated by DSC, WAXD and PLM. Helical morphologies exist in the latter two phases. By melt crystallization and solvent evaporation, flat single lamellar crystals have been obtained with a basic monoclinic unit cell with a=1.03nm, b=0.47nm, c=6.43nm and ƒ×=83„a by SAED and WAXD fiber pattern. SAED results also showed that two kinds of crystal twinnings existed in this polymer: micro-twinning within one single lamellar crystal and rotation-twinning between two lamellae. Morphological helical structures can also be obtained in the crystal form in this polymer. The helical lamellar crystals possess the same crystal structure as their flat counterparts and all helical crystals show a right-handed twist with pitch lengths on a micrometer size by TEM and AFM.

[R1.100] Physical aging in a polymer glass subjected to carbon dioxide pressure jumps

We report results from tensile creep tests performed on an epoxy resin in the presence of carbon dioxide at different pressures (Pco2) and at a constant temperature below the glass transition temperature Tg. Time – Pco2 superposition was applied to the data to account for the plasticization effect due to the interaction between the carbon dioxide molecules and the polymer. In addition, physical aging of the epoxy films was investigated using sequential creep tests after carbon dioxide pressure down-jumps at constant temperature and also after temperature down-jumps at constant carbon dioxide pressure. The isothermal pressure down-jump experiments showed physical aging responses similar to the isobaric temperature down-jump experiments. However, the aging rate for the CO2–jump was slightly lower than that for the T-jump and the retardation time for the Pco2-jump experiments was up to 6.3 times longer than for the T-jump conditions. The results are discussed in terms of classical physical aging and structural recovery frameworks and a speculation about the differences in the energy landscape resulting from Pco2-jump and T-jump experiments is also made.

[R1.101] Studies of Poly(vinyl chloride) Based Endotracheal Tubes From the Microscopic to Macroscopic Scale

The endotracheal tube (ET) is a polymeric conduit that forms a closed system of pulmonary ventilation that is most often used to allow delivery of air to critically ill patients via intubation. Currently used ETs cause a wide variety of clinical problems including laryngeal edema (inflammation), severe morbidity, and occasionally death. To investigate the origins of this behavior, mechanical, chemical, morphological, and biocompatibility characterization of injection-molded (Endotrol) tubes of poly(vinyl chloride) (PVC) containing ~35 wtplasticizer was conducted. Experiments included fourier-transform infrared spectroscopy, gel permeation chromatography, differential scanning calorimetry, accelerated solvent extraction, uniaxial tensile testing, high-resolution force spectroscopy, atomic force microscopy, and plasticizer leaching. We intend for these studies to form the basis for future ET materials selection and design.

[R1.102] Optical Spectroscopy and Modeling Studies of Poly(di-n-alkylsilanes)

The influence of thermal treatments on the photophysics in \sigma-conjugated poly(di-n-alkylsilanes) (i.e., butyl, hexyl, octyl, decyl and dodecyl) have been investigated. In situ measurements of both thin film absorption and photoluminescence reveal numerous distinct structural conformers. The exact distribution of these conformations is sensitive to both thermal quenching and subsequent heating. Molecular modeling studies of large oligomers and ab initio calculations of representative tetramers and hexamers are used to identify energetically favorable chain conformations. Zero-energy calculations identify deep local minima corresponding to simple 7/3, 9/4 and 15/7 helices. Models of repeating dyads identify a 14/6 helix and other candidate low energy structures. These results are compared to the experimental data.

[R1.103] Excitation Energies of Fluorene-Based Polymers and Oligomers- Ab Initio Approach

Organic light emitting polymers and oligomers are conjugated materials that are being considered for use in numerous optical devices e.g. light emitting diodes, flat panel displays etc. The key to commercialization of these product lies in producing stable blue light emitting polymers and oligomers. Fluorene-based polymers and their derivatives are possible candidates for blue light emitting materials.In this work, we investigate the geometrical, electronic and optical properties of fluorene-based \pi-conjugated monomers that include phenylene, thiophene and other units using computational quantum mechanical approach. The relaxed potential energy surface was generated for some systems using the Hartree-Fock method (HF/3-21G*). Fully optimized ground state structures were obtained using HF and various density functional theory (DFT) approaches (B3LYP/3-21G*, B3LYP/6-31G*, B3P86/6-31G*, B3PW91/6-31G*, MPW1PW91/6-31G*). The energies of the first 20 singlet-singlet transitions were then obtained by applying the corresponding time-dependent (TD) density functional theory (TD-B3LYP/6-31G*, TD-B3P86/6-31G*, TD-B3PW91/6-31G*, TD-MPW1PW91/6-31G*) to the previously optimized geometries. The results of calculations were compared with experimental values when possible. The main trends of these calculations will be presented and discussed.

[R1.104] Nanoimprinting of Photonic-Bandgap Devices in Ionically Self-Assembled Monolayers

Photonic Crystals (PCs) are a new class of materials providing new opportunities for the enhancement control of the propagation of light in waveguides and laser action in 2D distributed feedback structures. However, in order to possess nonzero even-order nonlinear optical susceptibilities, a material must lack a center of inversion at the macroscopic level. As a result several novel methods for creating noncentrosymmetric materials incorporating organic molecules with large molecular susceptibilities have been developed over the past decade. Using commercial ionic polymer dyes, ionically self-assembled monolayers (ISAMs) provide a new platform to produce such noncentrosymmetric arrangement of nonlinear optical chromophores Originally developed by Chou, nanoimprinting techniques provide a powerful alternative to e-beam lithography for definition of photonic structures in ISAM films. Here we report the nanoimprinting of photonic structures in such films.

[R1.105] A versatile fiber-coupled system design for Photon Correlation Spectroscopy and Fabry-Perot Interferometery

A new and useful experimental setup has been built utilizing optical fiber coupling of scattered light into existing photon correlation spectroscopy (PCS) and Fabry-Perot (F-P) interferometery systems. The setup is very versatile and allows measurements over a broad range of scattering angles and sample temperatures along with considerably faster and easier alignment of system optical components. A major advantage of the set up is the feature allowing simultaneous PCS and F-P measurements. We compare characteristics such as spectral resolution, contrast, signal-to-noise, light collection efficiency, etc. to conventional systems where fiber coupling has not been employed. The effects of using multimode fibers were investigated in detail and a variety of practical issues and limitations will be discussed. The system was tested using simple organic solvents. Currently, measurements are initiated on neat melts of poly(ethylene oxide), (PEO) and PEO-melt/LiClO_4 solutions.

[R1.106] Low Voltage Electron Microscopy of Polymer and Organic Molecular Thin Films

We have been investigating the capabilities of a low voltage electron microscope (LVEM) operating in transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), scanning electron microscopy (SEM), and electron diffraction (ED) modes. The microscope operates at a nominal accelerating voltage of 5 kV and fits on a tabletop. The mean free path for total electron scattering was calculated to be 14 nm for organic samples at ~5 kV. The total end point dose for the destruction of crystallinity at 5 kV was measured from the fading of Bragg reflections in ED patterns to be 5 x 10-4 C/cm2 and 3.5 x 10-2 C/cm2 for polyethylene and pentacene, respectively. Images taken of several organic materials have shown high contrast for low atomic number elements and a resolution of better than 2 nm. The materials studied include thin films of the organic semiconductor pentacene, diblock copolymers, electrospun polymer fibers, dendrimers and gold nanoparticles.

[R1.107] Ultrasonic degradation of polysaccharides studied by multi-angle laser light scattering

Ultrasonic degradation of polymers is a well-studied field. It is understood that the degradation process results from shock waves generated by cavitation and collapse. That is, cavities formed when the pressure wave is at a minimum violently collapse as the pressure increases. The rapid collapse sends out shock waves into the material and the main-chain rupture is believed to stem from this rapid motion of solvents. We explore this process using the experimental technique of multi-angle light scattering (MALLS). MALLS provides the molecular weight, RMS radius and virial coefficient of polymers in dilute solution. We monitor the change in these parameters as a function of time exposed to ultrasound for three polymers: Xanthan, Hyaluronic acid (HA), and Lambda Carrageenan (LC). These polysaccharides are chosen because they have similar initial molecular weights (~10 ^6 g/mol) but different architectures. Xanthan is a wormlike chain, HA is an expanded random coil (large persistence length) and LC is a contracted random coil (small persistence length).

[R1.108] Quantification of Monolayer Surface Coverage by Forward Recoil Spectrometry

Typical experimental means of characterizing the coverage of a self assembled monolayer (SAM) involve measuring the thickness of the SAM layer or characterizing the surface energy. Here, we present a new experimental method that can determine the surface coverage of a deuterated SAM. Forward recoil spectroscopy (FRES) also know as elastic recoil detection (ERD) is used here not to probe the thickness of the SAM layer, but instead to determine the areal density of deuterium, ([D] / cm2). By comparing against a known standard, the areal density of deuterium in the unknown SAM can be determined directly. A series of standards with varying thickness were used to improve precision. In order to improve the accuracy, the effect of beam damage was eliminated by extrapolating to zero beam dose. By knowing the chemistry of the SAM and the experimental areal density of deuterium, the molecular areal density, the footprint, and the packing density of the SAM can be directly calculated.

[R1.109] Fabrication and Characterization of Chemically and Topographically Patterned Substrates

Our interest in chemically functionalized nano-structured surfaces arises from their potential technological importance in nanobiotechnology. Recent papers report the fabrication of topographically patterned substrates at biologically relevant length scales, and surface functionalization progressed with the use of self-assembled monolayers (SAM) to modify surface properties. We have used a combination of these techniques to fabricate a chemically and topographically patterned biomimetic surfaces for the study of cell-substrate interactions. Using PDMS stamps and a combination of protein adsorbent and protein repellant SAMs, we have been able to direct protein adsorption to the tops of line patterns between 150 and 1000 nm wide.

A it is difficult to characterize the chemistry of such small patterned samples, which is a fundamental issue in the understanding of surface phenomena. No study has been done so far to characterize these types of surfaces. We have found a method to probe the chemistry and orientation of these SAMs using Near Edge X-ray Adsorption Fine Structure (NEXAFS), which is sensitive enough to discriminate between differing chemistries on the tops and sides/grooves of lines 150 nm wide.

[R1.110] Conformation-Assisted Fluctuation of Density and Nucleation in Polymer Melts

The phase separation kinetics of density and conformation in polymer melts is studied by the linearized time-dependent Ginzburg-Landau equations. In the present model of the free energy density, there are two order parameters which are the density and the conformational change. A new nucleation mechanism is proposed in which the fluctuation in density and the fluctuation in conformational change are coupled by the mixed derivative term and the cross gradient term of these two parameters. These terms are important factors for the phase separation kinetics, since the density and the conformation further each other and induce the phase separation both in density and conformation. The dynamic structure factors for both density and conformational change are calculated. The relevant small x-ray scattering and depolarized light scattering experimental results are compared to test this model.

[R1.111] Theory of Reversibly Associating Copolymer-like Clusters in Melt

We theoretically consider phase behavior of various mixtures consisting of two monodisperse functionalized homopolymers (denoted by A and B) capable of forming clusters between functional groups (stickers) using the Landau-Ginzburg formalism. In these model systems, resulting molecular architectures via clustering resemble block copolymers, and when the associations between stickers are strong enough, various types of microphases are possible. Minimizing the free energy with respect to the cluster distribution and the parameters describing the microphase, the stability of phase structures is investigated as a function of thermodynamic and material variables. Using the second harmonic approximation the phase behaviors of various types of copolymer-like clusters in melt are extensively investigated and their possible phase structures are discussed.

[R1.112] Effect of chain topology of block copolymer on micellization: ring vs linear block copolymer

The aggregation of amphiphilic block copolymers in solution to form micelles has attracted great interest in recent years because of its importance in industrial applications. Many studies on these systems have mainly focused on a di- or triblock copolymer and much less attention was given to other architectures such as ring block copolymer. Recent experimental work has extended those works to include ring block copolymer, made by end-linking the triblock copolymer. Although the micellization of the ring block copolymer seemed to be favored over that of the linear triblock copolymer, two block copolymers showed similar values of cmc in experiments. In the present work, micellization of ring block copolymer (ring-B9A8) was simulated by Brownian dyanmics and micellar behavior is compared with triblock copolymer (A4B9A4) to investigate more systematically the effect of molecular architecture. Critical micelle concentration (cmc), average aggregation number and micellar distribution are compared with corresponding quantities measured for linear triblock copolymers having the same chain length and composition. Simulation results show that the cmc of ring-B9A8 is smaller than that of A4B9A4. The difference is explained by simple mean-field type theory.

[R1.113] Molecular Simulation of Polymer Crystallization: Nucleation from Pre-oriented Melt

Molecular dynamics simulations are performed using a united atom force field for polyethylene to characterize homogeneous nucleation and its enhancement in the presence of high levels of orientation. Nucleation can be accelerated by orienting the material on the molecular scale, in accord with experimental observations. Using this approach, we have prepared pre-oriented melts of 20 C400 chains by applying a uniaxial stress to the melt at 425 K and allowing the system to evolve for 1.6 ns. We then remove the stress and quench the system to several different temperatures. The formation and growth of nuclei are investigated by analysis of director orientation and local order parameter. The results are indicative of competing rates of orientation relaxation and nucleation, the balance of which changes with quench temperature. The structures of the resulting crystallites are also characterized.

[R1.114] Monte Carlo simulations to investigate dynamics of concentrated polymer solutions

Processes on different length scales affect dynamic properties of polymer solutions. The dynamics of chains in concentrated solutions are strongly affected by the local environments of their chain segments. While it is well known that the local density plays an important role it is still difficult to quantify the effect. In this work, we investigate dynamics of concentrated solutions with Monte Carlo simulations of a generalized bond-fluctuation model. The simulations employ local, non bond-crossing moves and account for attractive interactions between occupied nearest neighbor sites. In order to sample configuration space efficiently, we combine standard and expanded ensemble Monte Carlo methods. We will discuss parallel implementation of the algorithm and present first results for self-diffusion coefficients and mobilities for polymer solutions at different concentrations and temperatures.

[R1.115] Monte Carlo Studies of Polymer Chains in Aqueous Solutions

Solubilities of chain in water as measured by Henry's Law constant is controlled by the availability of sufficiently large cavities to accommodate the solute (entropy term) and the interaction of solute with the solvent molecules (energy term). In this study, different configurational factors such as bond length, bond angle, torsional energy have been applied to a Lennard-Jones chain model, and their effects on the solubilities are assessed.

[R1.116] Coarse-Grained NPT Molecular Dynamics of Polymer-Layered Silicate Nanocomposites

Exfoliated polymer-layered silicate nanocomposites exhibit many desirable properties. In situ polymerization is one method used to prepare such nanocomposites. Success of this approach depends significantly on the interactions of the monomer (and curing agent) with the silicate sheets and the subsequent impact on the polymerization reaction between the silicate layers. Coarse-grained MD simulations of a stack of silicate sheets immersed in binary fluid mixtures were conducted to examine the influence of interlayer proximity and sheet-fluid interaction parameters on the infusion behavior. Conclusions include: 1) NPT simulations, using Berendsen's technique to control system pressure, differed appreciably from NVT simulations; 2) substantial partitioning occurred, altering interlayer composition from bulk composition; and 3) a range of solvent-sheet interaction parameters exhibited formation of multiple layers of fluid between silicate sheets and steadily increasing intercalation rates, suggesting that exfoliated structures could be formed at sufficiently long simulation times.

[R1.117] SELF DIFFUSION IN NANO FILLED POLYMER MELTS: A MOLECULAR DYNAMICS SIMULATION STUDY

SELF DIFFUSION IN NANO FILLED POLYMER MELTS: A MOLECULAR DYNAMICS SIMULATION STUDY* T. G. Desai,P. Keblinski, Material Science and Engineering Department, Rensselaer Polytechnic Institute, Troy, NY.

Using molecular dynamics simulations, we studied the dynamics of the polymeric systems containing immobile and analytically smooth spherical nanoparticles. Each chain consisted of N monomers connected by an anharmonic springs described by the finite extendible nonlinear elastic, FENE potential. The system comprises of 3nanoparticles and the rest by freely rotating but not overlapping chains. The longest chain studied has a Radius of gyration equal to particle size radius and comparable to inter-particle distance. There is no effect on the structural characteristics such as Radius of gyration or end to end distance due to the nanoparticles. Diffusion of polymeric chains is not affected by the presence of either attractive or repulsive nanoparticles. In all cases Rouse dynamics is observed for short chains with a crossover to reptation dynamics for longer chains.

[R1.118] Anchoring of a nematic liquid crystal at a polymer surface

A commonly used technique to orient liquid crystal molecules in contact with a polymer is to rub the polymer surface with a soft cloth. This has the effect of both digging grooves and aligning polymeric chains in the surface. The relative importance of these two effects has long been debated. To study this question we have performed atomistic molecular dynamic simulations of molecules of 5CB in contact with polyvinyl alcohol. In this way it is possible to investigate the effects of interchain spacing, tacticity, chain orientation, and surface topography on the preferred orientation. The resulting preferred direction of planar anchoring appears to depend on all of these factors.

[R1.119] Film Formation and Roughness in Aqueous Solution of Hydrophobic and Polar Groups on an Adsorbing Substrate: Computer Simulation Study

A computer simulation model is used to investigate film formation on a discrete lattice. The substrate is designed by placing a layer of absorbing constituents (S) at the bottom of the lattice. Water solvent (A), polar (B), and hydrophobic (C) constituents are then mixed in the lattice. Nearest neighbor interaction among these constituents are considered along with their molecular weight. Metropolis algorithm is used to move the constituents particles stochastically. Water constituents are allowed to evaporate. As the simulation proceeds, concentration of water reduces and the mobility of B and C decreases accordingly. Phase separation between B and C components seem to emerge with water constituents trapped in between. Roughness increases with the solvent concentration. Snaps and figures will be presented to monitor the surface growth as the data will become available.

[R1.120] Phase separation kinetics in thin polymer films: dynamic self consistent field theory

We study various aspects of phase separation in binary polymer mixtures confined between two hard walls. To this end we employ two dynamic extensions of self consistent field theory: In `dynamic SCFT' the densities are evolved in time and the use of an Onsager coefficient proportional to the monomer densities represents the dynamics of point-like particles. The method of the `external potential dynamics' makes use of the temporal propagation of the effective external fields. A constant Onsager coefficient in this method implies non-local dynamics of polymers. In polymer blends that are confined between two walls that both attract the same kind of polymers the differences between the underlying dynamics are studied by analysing the formation of enrichment layers. In films with selectively attractive walls we examine the dynamics of capillary waves of an interface formed parallel to the walls. We also address the question how a phase transition from a completely homogeneous mixture between the walls to the localized phase takes place.

[R1.121] Fluorophore Conformation in Green Fluorescent Protein: A Molecular Dynamics Study

Green Fluorescent Protein (GFP) is a widely used fluorescent marker exhibiting two excitation peaks, a strong peak at 398 nm and a second one at 475 nm. The fluorescence is from the chromophore p-hydroxybenzylideneimidazolinone, which is attached to a central helix surrounded by a b barrel made of 11 b strands. Two different forms of the fluorophore - a protonated/neutral fluorophore and a de-protonated/anionic fluorophore - are considered to be responsible for the two distinct spectroscopic states. Although GFP is highly fluorescent, denatured-GFP is non-fluorescent, indicating that the environment of the protein plays an important role in its fluorescence behavior. Molecular dynamics simulations were carried out for the isolated fluorophore and for the whole GFP molecule. By comparing the conformational changes of the fluorophore in both cases, we find that the flexibility of the central fluorophore in GFP is significantly restricted due to the rigid nature of the protein, thus affecting its electronic structure. Both the protonated and the de-protonated fluorophores were studied.

[R1.122] Effect of Endfunctionality and Molecular Weight of Reactive Polymers on Reaction Kinetics and Interfacial Properties at Immiscible Polymer Interface: A Monte Carlo Simulation Approach

In present study, a binary A/B immiscible polymer system containing endfunctional polymers (i.e. reactive polymer A or B) was investigated to study both dynamic and static interfacial properties such as interfacial fluctuation, interfacial tension and thickness. When mono-endfunctional or di-endfunctional reactive polymers were added into the immiscible blend system, the copolymer conformations formed by the reactions at the interface were observed and the reaction kinetics was also calculated by using the Monte Carlo method. The copolymer coverage is defined as the number of reacted polymers per the interfacial area. At a low concentration of reactive polymers, the copolymer coverage increases twice for di-endfunctional polymer system than for mono-endfunctional polymer system. However, in the case of a relatively high concentration of reactive polymers, the saturated value of interfacial coverage is not proportional to the number of endfunctional sites. Through the reactions between two di-endfunctional polymers, the copolymers have several conformations such as loop and tail and the distribution of conformations is found to be related the reaction rate between reactive polymers at the equilibrium state of the reactive system. We also present the effect of molecular weight of the reactive polymers on the interfacial properties, which is significantly dependent on the reaction rate.

[R1.123] Solid Phase DNA Amplification: A Simple Monte Carlo Lattice Model

Recently, a new type of PCR called solid phase DNA amplification, has been introduced where surface-bound instead of freely-diffusing primers are used to amplify DNA. This type of amplification is limited to two-dimensional surfaces and therefore allows the easy parallelization of the PCR process in a single system. Furthermore, solid phase DNA amplification could provide an alternate route to DNA target implantation on DNA chips for genomic studies. We propose a simple Lattice Monte Carlo model of solid phase DNA amplification. We study the growth, stability and morphology of isolated PCR colonies under various conditions. Our results indicate that, in most cases, solid phase DNA amplification is characterized by a geometric growth and a rather sharp size distribution. These results are qualitatively different those obtained for liquid PCR processes which are usually characterized (at least initially) by an exponential growth and a broad population distribution. Various non-ideal effects are studied, and we demonstrate that such effects do not generally change the nature of the process, except in extreme cases.

[R1.124] Molecular Simulations of Protein-Polyelectrolyte Complexes

We present the results of Monte Carlo and Molecular Dynamics simulations of complexation between protein and polyelectrolyte chains in dilute and semidilute solutions. We have established that the binding between polyelectrolytes and proteins occurs in such a way that the oppositely charged groups on the protein are close to the polyelectrolyte leading to effective electrostatic attraction between the two. In dilute solutions the complex is usually formed at the end of the polyelectrolyte with polyampholyte chain elongated and aligned along the polyelectrolyte backbone. We also observed that initially collapsed polyampholytes undergo coil-globule transition by forming a complex. The structure of polyampholyte-polyelectrolyte complex is analyzed by tail and loop distribution functions. In semidilute solutions the complexation between protein and polyelectrolytes leads to formation of the reversible network, in which proteins play role of the junctions connecting polyelectrolyte chains.

[R1.125] Thin films of asymmetric triblock copolymers: Monte Carlo simulations and Self-Consistent Field Theory.

We use computer simulations and self-consistent field theory to study the morphology of asymmetric A_.125B_.75A_.125 triblock copolymer thin films confined between two homogeneous surfaces (walls). Morphology is investigated as a function of the film thickness and the strength of the wall-polymer interaction. For very thin films we observe cylinders perpendicular to the walls for a wide range of wall-polymer interaction. With increasing film thickness other morphologies (e.g. parallel cylinders) are becoming more stable. Phase diagram obtained from computer simulations agrees qualitatively with that predicted by the theory.

[R1.126] Computer Simulation of Associating Ideal Chains

Monte Carlo computer simulation of associating ideal chains is used to examine properties near the gelation transition. The chains are modeled as two beads connected with a spring. We analyze the results for functionalities (f) 2 and 3. For binary association (f=2), distributions of rings and linear chains are numerically obtained and compared to analytic results. For both functionalities (f=2 and 3) we numerically determine correlation functions and thermodynamic properties, including specific heat as a function of temperature and concentration. We compare our simulation results with theoretical calculations.

[R1.127] Long Range In-Plane Order of Oriented Diblock Copolymer Thin Films by Graphoepitaxy

Previous work by Russell and coworkers has demonstrated that controlling the interfacial energies and wetting behavior of an asymmetric diblock copolymer enables the control of the orientation of its microphases. In particular the cylindrical phase can be readily aligned perpendicular to a substrate when it is placed on a surface that is neutral to both components of the copolymer. The minor phase, PMMA may then be removed using UV radiation leaving a nanoporous template. In this work, we will report long range, in-plane ordering of the hexagonally packed nanopores that is achieved using graphoepitaxy. The long range ordered and vertically aligned diblock copolymer film can be used to produce arrays of catalytic nickel dots, which grow vertically aligned carbon nano-fibers (VACNF), resulting in a well ordered array of VACNFs.

[R1.128] Structural evolution and phase characterization of polyelectrolyte/surfactant complexes in aqueous solution

The self-assembly behavior of polyelectrolyte/surfactant complexes was examined in dilute aqueous solution. We have chosen to study the interaction of a model polyelectrolyte [sodium poly(styrene sulfonate)] with cetyltrimethylammonium chloride (CTAC), a cationic surfactant, at varying ionic strength. At a certain charge ratio, the polyelectrolyte/surfactant assembly will collapse into a highly-ordered, insoluble complex. It is the evolution of structure in these systems as a function of surfactant composition and ionic strength that is of particular interest. Soluble structural characterization was performed via small angle neutron scattering in deuterated water. Small angle X-ray scattering was used to follow the phase behavior of the solid complexes above the insolubility point. The transition from soluble to insoluble complex and the mechanism of collapse is of particular interest in this study.

[R1.129] Rate of Reaction and Crosslink Density of Bifunctional Monomers (Application to Olefinic and Acrylate Functionality): Computer Simulation

Rate of reaction and crosslink density of bifunctional monomers (olefinic (A) and acrylate (B)) in a solvent are studied via a computer simulation model. A fraction p of lattice sites are randomly occupied by A and B at a given concentration pA and pB. All other remaining sites represent an effective solvent medium. Monomer and solvent are mixed for a given period of time before polymerization is initiated. If A or B attempts to move into a site occupied by another functionality (A or B), a bond is formed with a reaction probability when both units have at least one unsaturated bond: the probability of reaction is KAA, KAB, and KBB for a radical initiated photo-polymerization. Once reacted, those monomers become immobile. Three systems are considered:(i) pA/pB = 1, (ii) pA/pB = 5, (iii) pA/pB = 2. Crosslink density and rate of reaction are studied as a function of polymer concentration. Preliminary data appears to support experimental observations.

[R1.130] A Lattice Model for the Simulation of Diffusion in Heterogeneous Polymer Systems

The diffusion of molecules through heterogeneous media is not simple Fickian diffusion on the length scale of the heterogeneities. Pulse field gradient NMR measurements of diffusion reflect this by yielding apparent diffusion constants which depend on the time over which diffusion is observed. Equations for simple cases of tortuous diffusion and restricted diffusion are available where some fraction of the media is assumed to be permeable and some fraction is impermeable. In many polymer systems, the media has domains which are more permeable and less permeable. In the limit of long times and above the percolation threshold, diffusion in such systems can be characterized with effective medium theory. To characterize diffusion as a function of time in heterogeneous media and at all compositions, a lattice model is presented based on an approach developed by Ediger. The lattice model gives results which match effective medium theory at long times and the produces tortuous and restricted diffusion above and below the percolation limit when one domain is made impenetrable. It shows a wide range of behavior of the apparent diffusion constant on observation time which is intermediate between tortuous diffusion and restricted diffusion for various ratios of the diffusion constant in the more permeable and less permeable domains and for various ratios of solubility between the more permeable and less permeable domains. The model demonstrates that apparent diffusion data from PFG NMR provides a new structural view of heterogeneities in systems with more and less permeable domains.

[R1.131] Coarse-Grained MD Simulations of Layered Silicate Stacks Within Blends of End-Functionalized and Homopolymers

Experimentally, many successful polymer-layered silicate nanocomposites share a design motif of chains end-tethered (or grafted) to a silicate surface. However, economic and processing concerns drive continued exploration of melt-processed systems, whose motif differs in that chains are generally physisorbed to the silicate sheet. Drawing from the former to address challenges in the latter, small additions of end-functionalized chains to a polymer melt may provide enhanced coupling between external layers of silicate tactoids and molten polymer matrix, enhancing the exfoliation via a plate-peeling mechanism. To provide guidance, the influence of different sheet-polymer-functional end group interaction parameters on the infusion behavior has been investigated using coarse-grained MD simulations of layers of silicate sheets immersed in a sea of non-functionalized polymer. The results indicate that an almost completely intercalated structure could be obtained by adjusting the functional group-sheet interaction parameters. Furthermore, multiple layers of polymer formed inside the galleries with intercalation rates increasing steadily with time. This suggests that exfoliated structures are formed at sufficiently long simulation times.

[R1.132] Self-assembled DNA-mediated assembly of the metallci nanopraticles for single electron transistor

DNA molecules have attracted many researchers as building blocks for interconnecting nano-scale materials because of their complementary properties. Here, we present an efficient method of nanoparticle fixation between electrodes using thiol-modified oligonucleotides. Using this method, we have fabricated nanodevices and investigated their electrical properties. Devices have been made with different lengths of DNA molecules. The devices fabricated with 30 or 37 mer DNA strands are found to exhibit p-type semiconducting behaviors, while the ones with 10 mer DNA molecules show the single-electron effects even at 77 K. These results suggest that the 10mer DNA strands act as tunneling barriers in electron transfer and metallic nanoparticles play a role of quantum dots. However, the 30-37 mer DNA strands are too long to act as tunneling barriers. Therefore, the observed semiconducting behaviors are considered to be resulted from the semiconducting properties of DNA molecules themselves.

[R1.133] CHARGE INJECTION IN DOPED ORGANIC SEMICONDUCTORS

A study is done to understand the dependence of injection on the degree of doping of an organic semiconductor. A model organic semiconductor, tetra-methyl triphenyl diamine doped polycarbonate (PC:TMTPD) was used for these experiments. By substituting TMTPD molecules with a TMTPD+SbF6- salt, the degree of doping in the organic semiconductor was systematically varied. Changes in the electrical characteristics of devices with various electrodes were analyzed to yield the dependence of injection on the degree of doping. Along with the doping concentration, the temperature and distance between electrodes was also varied. This provided a better understanding of how doping, temperature and electrode spacing affect device performance.

[R1.134] Theoretical Study of Donor - Spacer - Acceptor Structure Molecule for Molecular Rectifier

Recently, the molecular electronics has attracted strong attention as a ``post-silicone technology'' to establish a future nanoscale electronic devices. To realize this molecular device, unimolecular rectifiering function is one of the most important constituents in nanotechnology [C. Majumder, H. Mizuseki, and Y. Kawazoe, Molecular Scale Rectifier: Theoretical Study, J. Phys. Chem. A, 105 (2001) 9454-9459.]. In the present study, the geometric and electronic structure of alkyl derivative C37H50N4O4 (PNX) molecule, (donor - spacer - acceptor), a leading candidate of molecular rectifying device, has been investigated theoretically using ab initio quantum mechanical calculation. The results suggest that in such donor-acceptor molecular complexes, while the lowest unoccupied orbital concentrates on the acceptor subunit, the highest occupied molecular orbital is localized on the donor subunit. The approximate potential differences for optimized PNX molecule have been estimated at the B3PW91/6-311g++(d,p) level of theory, which achieves quite good agreement with experimentally reported results. This study was performed through Special Coordination Funds for Promoting Science and Technology of the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government.

[R1.135] Electro-optical photonic crystals formed in H-PDLCs by thiol-ene photopolymerization

Electrically switchable 2-D and 3-D photonic crystals are rapidly formed by holographic polymerization-induced phase separation of liquid crystal from a monomer-liquid crystal mixture. We report the fabrication and electro-optical properties of liquid-crystal-filled polymer photonic crystals of orthorhombic F symmetry using thiol-ene photopolymerization. The 2-D and 3-D structures are formed by two beam prism technique using UV 364.8 nm line of Ar ion laser. A 4-beam technique was also employed. The crystals exhibit electrically switchable Bragg diffraction with crystal structure in good agreement with theoretical expectations. These photonic crystals compare favorably with liquid-crystal-imbibed colloidal crystal arrays. The polymerization mechanism is different for this monomer class as compared to conventional multi-functional acrylates. This results in a substantial delay in the gel point. Differences in grating performance between the two material classes will be explored.

[R1.136] Photonic Films Prepared by Plasma Polymerization/Copolymerization

Different polymeric photonic films, including ¼ wavelength high/low refractive index layer stacks and anti-reflective coatings have been prepared by means of plasma enhanced chemical vapor deposition. All these films show a dense bulk structure and a smooth, pin-hole free surface. By controlling the processing parameters, the films can be prepared in accordance with any optical design allowing for selection of notch (bandgap) locations and depths. The ability to deposit polymer films with any refractive index between 1.35-1.65 by copolymerization techniques and to deposit on a variety of substrates yields an effective tool at fabricating a wide variety of non-conventional polymer photonic elements. Due to the highly crosslinked structure of the polymer, these films possess excellent chemical resistance, environment survivability, and good adhesion with substrates. IR and UV-Vis spectroscopies, ellipsometry, SEM, XPS, and AFM have been applied to explore the relationships between structure and the optical properties of the resultant thin films.

[R1.137] Multi-Dimensional Holographic PhotoPolymerization: Fabrication of Organic-Inorganic Photonic Band Gap Gain Medium

Holography offers a versatile, rapid and volume scalable approach for making large area, multi-dimensional, organic PBGs; however, the small refractive index contrast of organics prevents formation of a complete band-gap. The introduction of inorganic nanoparticles to the structure provides a possible solution. In contrast to the multiple steps (exposure, development and infiltration) necessitated by lithographic-based holography (e.g. photoresists), holographic photopolymerization of monomer-nanoparticle suspensions enables one-step fabrication of multidimensional organic-inorganic photonic band gap (PBG) structures with high refractive index contrast. The PBGs are formed by segregation of semiconductor nanocrystals during polymerization of the polymer network. A model describing the migration of the nanoparticles into three-dimensional patterns, encompassing elements of Kogelnik’s coupled wave theory for volume holograms, mass transport and polymerization kinetics, was utilized to select writing conditions and polymerization rates to obtain optimal morphologies for optical gain.

[R1.138] Hyper-Rayleigh scattering excitation profile of nonlinear optical dendrimer

Nonlinear optical (NLO) chromophores with pi-conjugated organic molecules attached with electron donor and electron acceptor groups usually exhibit large molecular first hyperpolarizability (£]). Large £] NLO chromophores when processed with amorphous polymer yield second order nonlinear optical polymers with potentials for various photonic applications. Recent studies have shown that NLO polymers of the guest/host type having a substantial NLO chromophore loading tend to form aggregates. The aggregate formation is believed to be due to strong intermolecular dipole-dipole interactions between NLO chromophores themselves and also between chromophores and polar polymer chains. The aggregate formation diminishes the macroscopic optical nonlinearity in electric field poled guest/host polymers, and render them to become less useful for applications. Large void-containing structures such as dendrimer may effectively decrease the intermolecular interaction, and prevents the aggregation formation. We report here recent results of hyper-Rayleigh scattering (HRS) intensity measurements of dendrimer in solution with different dendrimer concentrations and at several laser excitation wavelengths. The concentration dependence measurement provides information about the loading level above which aggregation is to occur. The wavelength dependence of £] gives information about the effect of regular, well defined three-dimensional architecture on the molecular hyperpolarizability as well as the validity of the two-state model commonly used to describe the wavelength dependence of £]. Dynamic light scattering data are also provided for comparison.

[R1.139] Electrochemical Deposition of Nanostructured Conducting Polymer Coatings on Neural Prosthetic Devices

Micromachined neural prosthetic devices facilitate the functional stimulation of and recording from the central nervous system (CNS). These devices have been fabricated to consist of silicon shanks that have gold or iridium sites along their surface. Our goal is to improve the biocompatibility and long-term performance of the neural prosthetic probes when they are implanted chronically in the brain. In our most recent efforts we have established that electrochemical polymerization can be used to deposit fuzzy coatings of conducting polymers specifically on the electrode sites. For neural prosthetic devices that are intended for long term implantation, we need to develop surfaces that provide intimate contact and promote efficient signal transport at the interface of the microelectrode array and brain tissue.

We have developed methods to rapidly and reliably fabricate nanostructured conducting polymer coatings on the electrode probes using templated and surfactant-mediated techniques. Conducting polymer nanomushrooms and nanohairs of polypyrrole (PPy) were electrochemically polymerized onto the functional sites of neural probes by using either nanoporous block copolymers thin films, "track-etched" polycarbonate films or anodic aluminium oxide membranes as templates. Nanofibers of conducting polymers have also been successfully obtained by polymerizations in the presence of surfactants. The influence of current density, monomer concentration, surfactant concentration, and deposition charge on the thickness and morphology of the nanostructured conducting polymer coatings has been studied by optical, scanned probe, scanning electron and transmission electron microscopy. As compared with the normal nodular morphology of polypyrrole, the nanostructured morphologies grown from the neural electrode result in fuzzy coatings with extremely high surface area. The electrical properties of the polymer coatings were studied by Impedance Spectroscopy (IS) and Cyclic Voltammetry (CV). The significant drop in impedance in magnitude and phase angle is consistent with an increase of the surface area due to the roughened surface morphology.

[R1.140] Electrical transport through covalently-linked multi Zn(II) porphyrin arrays

We have investigated electrical transport properties through covalently-linked multi Zn(II) porphyrin arrays trapped between nano-electrodes. At room temperature, two features are noted. One is the hysteresis of I-V curves and the other is the diode-like behavior. Particularly, the positive and negative voltage pulses applied before sweeping the bias voltage to measure the I-V curves lead to different I-V curves suggesting the possibility for applications of molecular random access memory devices. In addition, we have also measured the photoconductivity. By illuminating LEDs, the conductance is enhanced and the threshold voltages are lowered. Possible transport mechanisms are discussed.

[R1.141] Pattern-Photopolymerization-Induced Phase Separation of Self Assembled Carbon-Nanotube Composites

We employed multi-wave interference mixing technique for fabrication of patterned carbon nanotubes in a polymeric matrix. This technique is a one-shot technique and fast as compared to the existing technologies such as cumbersome precise drilling, photo-masking, etc. The emergence of morphology is characterized in-situ using time-resolved light scattering analysis, optical microscopy and atomic force microscopy(AFM). In the triacrylate/E7/multi-wall carbon nanotube systems, we observed stratified layers in which nanotubes were found to allign perpendicular to the substrate in the low intensity region. Assimilar observation was made in the case of tetraacrylate/E7/multi-wall carbon nanotube systems, also.

Supported by Collaborative Center for Polymer Photonics(The University of Akron) and WPAF/ Air Force Office of Scientific Research

[R1.142] Organic Semiconductor Devices for Chemical and Biological Sensing

Molecular organic semiconductors have attracted a great deal of attention recently in the field of optoelectronics. These materials have potential for a broad range of applications because of their mechanical flexibility, processibity, and tunability of the electronic properties. In this poster, we report the design and development of several organic semiconductor devices for chemical and biological sensing. A fluorescence-based biosensor is demonstrated using an organic light emitting diode (OLED) as an excitation source. In addition, the possibility of biological, gas and humidity sensors based on organic thin film transistor (OTFT) is explored. The preliminary results of these prototypical systems based on OLEDs and OTFTs look very promising. The use of organic semiconductors may lead to new biosensors that are small, easily portable, inexpensive, fast, and capable of detecting low concentrations of specific analytes with high sensitivity and high selectivity.

[R1.143] Structure and Ion Transport Studies of PEO-Based Solid Polymer Electrolytes

X-ray and conductivity experiments have been performed to understand cation transport in polyethylene (PEO)-based organic-inorganic nanocomposite (OIC) solid polymer electrolytes [1], (PEO)14LiTf + OIC. X-ray diffraction patterns show that the addition of OIC enhances amorphicity, which is thought to aid ionic conduction [2]. The resulting disorder is reflected in changes in intensity and width of the prominent PEO peaks, d120 and d014, which are correlated with the observed maximum in conductivity near 40Further support for the correlation between structure and dynamics is provided by Raman, IR, and DSC measurements. The results are also likely to bear upon the existing theories to understand ionic conduction in crystalline [3] and amorphous [2] phases of materials. This work was funded by a NASA grant (NAG3-2588).

[1] L.M. Bronstein, C. Joo, R.L. Karlinsey, A. Ryder, J.W.Zwanziger, Chem. Mat. 13 (2001) 3678. [2] C. Berthier, W. Gorecki, M. Minier, M.B. Armand, J.M. Chabagno, P. Rigaud, Solid State Ionics 11 (1983) 91. [3] J. Gadjourva, Y.G. Andreev, D.P. Tunstall, P.G. Bruce, Nature 412 (2001) 520.

[R1.144] SWNT Orientation in Polymer/SWNT Composites

Orientation of both SWNTs and polymer chains is a major factor in determining the physical and mechanical properties of polymer/single wall carbon nanotube (SWNT) composites. In the present study, polarized Raman spectroscopy was used to quantitatively study the orientation of SWNTs in melt spun polypropylene/SWNT and dry-jet solution spun polyacrylonitrile/SWNT composite fibers. The results indicated that, even at moderate draw ratio, the Herman’s orientation factor of SWNT can reach very high values (~0.9). As compared to the orientation of SWNT, the polymer orientation in PP/SWNT and PAN/SWNT composite fiber is also investigated with IR dichroism and X-ray diffraction techniques. In both cases SWNTs show higher orientation than the polymers.

[R1.145] Hydrothermally Stable Mesoporous Silica and Organosilica Prepared with PEO-PLGA-PEO Triblock Copolymer Templates

Nonionic poly(ethylene oxide) surfactants or triblock poly-(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) copolymers as structure directing templates have been widely used for preparing mesoporous silicas such as SBA-15, SBA-16, and MSU-X. However, the organic modifications of the mesoporous silica framework and the efforts to improve weak hydrothermal stability of mesoporous materials have not been not so successful. In present study, we describe the synthesis of mesoporous organosilicas from the co-condensation of TEOS and 1,2-bis(trimethoxysilyl)ethane (BTMSE) by using PEO-containing triblock copolymer templates such as F127 (EO106PO70EO106; BASF) and poly-(ethylene oxide)-poly(DL-lactic acid-co-glycolic acid)-poly(ethylene oxide) (LGE76, EO43(L23G6)EO43, PEO-PLGA-PEO). More hydrophobic PLGA block, compared with the PPO block in the PEO-PPO-PEO block templates, was chosen to provide more contrast between hydrophilicity and hydrophobicity of the block template such that organosilicate precursor is confined in the aqueous matrix phase. We also report the hydrothermal stability of cylindrical mesoporous organosilicas (MOL-1) from 1,2-bis(triethoxysilyl)ethane (BTESE) using a LGE54 (EO16(L23G6)EO16, PEO-PLGA-PEO) template.

[R1.146] Modification of Polymer Rheological Properties Through the Incorporation of Functionlized Nanoparticles

Polymer rheology can best be controlled when addressed at the segmental level of the polymer molecule. We show that a critical dimension, Ro, exists between the radius of added nanoparticles and the polymer chain which separates a regime whereby the particles increase the internal free volume or reinforce the chain. We blended Au, Pd, and POSS nanoparticles (3-12nm) with polymers (Mw=100K-4M) and measured the glass transition and viscosity as a function of Ro. Transmission Electron Microscopy (TEM) was used to study filler size and aggregation. The effect of fillers in Tg of PS was studied using Shear Modulation Force Microscopy (SMFM). The dynamics of diffusion of fillers into the PS matrix were studied using Second Ion Mass Spectrometer (SIMS) and Neutron Reflectivity (NR) techniques will be reported. The effect of substrate interactions will be examined by comparing the results in supported and free standing films. References: 1. S.Ge, M.H. Rafailovich, J. Sokolov.Physical Review Letter, Vol. 85, 2340-2343, 9/19/2000.

Support from the NSF MRSEC is gratefully acknowledged.

[R1.147] Static and Dynamic Properties of Polyethylene Chains in the Presence of Nanoparticles

The static and dynamic properties of polyethylene chains were studied in the presence of nanoparticles. The simulations were performed on a coarse-grained high coordination lattice using the Metropolis Monte Carlo algorithm. Polyethylene chains of C_60H_122, C_100H_202, and C_316H_634 were simulated at 473 K with one spherical particle that was varied in size. The interactions between the polymer chains and the nanoparticle was also varied. Initial results indicate that the dynamics of the polymer chains were strongly influenced by the interaction parameters and the cut-off radius.

[R1.148] Self-Organization of Nanoparticles in Ultrathin Polymer Films

We are investigating the two-dimensional (2D) organization of nanoparticles in thin, liquid (above glass transition) polymer films with thickness comparable to particle size, i.e. from a few nm to about 20 nm. It is found that tethering the polymer chains to the underlying surface can stabilize the nanoparticles against macroscopic phase separation. Examined as a function of nanoparticle surface coverage, the range of observed morphologies bears intriguing similarities to 2D striped phases familiar from systems such as Langmuir monolayers and ferroelectric thin films. Other nanostructures that have been observed include particle nanorings, ~20 to 80 nm in diameter. These rings are nonequilibrium structures and may reflect drying dynamics of the film during particle deposition. The various nanostructures provide insights into how simple physical processes can create order at sub-100 nm length scales, and may be useful for modification of surfaces for applications such as information storage.

[R1.149] X-Ray Reflectivity of Free-Standing Bilayer Films of Immiscible Polymers

We have performed X-ray specula and diffuse scattering on free-standing bilayer films of immiscible polymers and analyzed the spectra as a function of film thickness. Interfacial width between polymer layers was successfully obtained with relatively high accuracy from new fitting protocol using a Fourier transform method. We show that measuring the integral capillary spectra at the interface between immiscible polymer bilayers may be an alternate way to determine the bending modulus of a thin film.

[R1.150] Supercritical Fluid Introduction of Low-Density Polymer Thin Film Formation

A series of polystyrene (PS) thin films varying the film thickness from 100 to 1000 Åwere exposed to supercritical CO_2 (scCO_2) at the density fluctuation (T=36 ^oC, P=8.2MPa), where an anomalous linear dilation of the film was observed, and were then frozen by flash evaporation of CO_2 without forming voids in the films. In order to characterize the film quality, we used x-ray reflectivity (XR), where XR is sensitive to concentration profiles over small distances. Here we report that uniform low-density ultrathin films can be easily manipulated as a function of magnitude of the swelling in scCO_2. In addition, we shall also discuss the effect on the refractive index and the glass transition temperature of the films.

[R1.151] Off-specular X-ray scattering study of polymer brush interfaces

X-ray off-specular scattering from a series of polystyrene brush films was measured on the BW4 USAX beamline at HASYLAB/DESY to investigate roughness correlation between the two interfaces. Either molecular weight or grafting density was varied to get a desired thickness (57~1070Åof a polymer brush film. For the samples with thicknesses between 98 and 411Åadditional modulation between the Yoneda peak and the specular peak was observed in detector scans. Both resonant diffuse scattering (RDS) for the correlated roughness and dynamical scattering such as a wave guide effect can give rise to this feature. The frequency of the RDS is half of that of dynamic scattering. In our case, the frequencies of this additional modulation are twice of those expected in the scattering from the roughness-correlated interfaces. So, this scattering feature is considered to have dynamical origin. The dynamical fringes observed in the detector scan from a dried film were no longer observed in a swollen state.

[R1.152] Effect of Freeze Drying from Dilute Solution on the Glass Transition Temperature of Polystyrene

It is well known that the structural and dynamic properties of thin polymer films are often different from that in the bulk state. Of particular interest is the cause of the depression in glass transition temperature (Tg) in ultra-thin films. In previous work, the effects of freeze-drying from dilute solution on Tg were investigated by measuring the glass transition temperature of polystyrenes with various molecular weights. A reduction of Tg was found. In this work, using linear and cyclic polystyrenes, the mechanism of this depression is investigated by following the time-dependent recovery of the Tg depression as a function of temperature and confirming that entanglement concentration is not related to this effect.

[R1.153] Activated Dynamics, Fragility and the Glass Transition in Polymer Melts

A microscopic theory of slow segmental dynamics and the glass transition in polymer melts has been formulated using mode-coupling, density functional and activated process ideas. The amplitude of thermal density fluctuations, the packing length, and the backbone characteristic ratio are the critical parameters which quantify the caging forces which favor segmental localization. Free energy barriers, a dynamic crossover temperature, kinetic glass transition temperature, dynamic fragility, and the temperature dependence of the segmental relaxation time have been studied. Model calculations and quantitative applications to specific polymer melts will be presented.

[R1.154] Transition from liquid to brush-like behaviors in ultrathin polymer films

We have performed X-ray specula and diffuse scattering on liquid polymer films and analyzed the spectra as a function of film thickness and molecular weight. The results show that films whose molecular weight is close to the entanglement length behaves as a simple liquid except that the shortest wavelength is determined by radius of gyration (Rg) rather than the monomer distance. When the molecular weight was higher than the entanglement length, the effects of the substrates propagate to the surface scaled as a Rg rather than film thickness. Furthermore films exactly 3Rg thick exhibit brush-like behavior where long wavelength cutoff vector, ql,c scaled as d-0.79.

[R1.155] Rotational Holstein Polarons

For electronic transport in complex molecules, such as liquid crystals, DNA and other organic materials, the interaction between electrons and phonons corresponding to rotation of molecules is important. In a one-dimensional model, we study in detail the properties of rotational polarons appearing as a result of electrons (described by a tight-binding model) interacting with rotational phonons. We consider different coupling and frequency regimes, including weak coupling, adiabatic and non-adiabatic regimes. The anharmonicity of the molecular rotation and the nonlinearity of the electron-phonon (EP) interaction leads to novel physical consequences. In the weak coupling regime, although electron effective mass increases with increasing EP interaction, as in the usual polaron model, the polaronic energy shift and correction to the mass appear in different orders of perturbation. In the strong coupling regime, the nonlinearity leads to weaker renormalization of the electron mass. Moreover, the polaronic bandwidth decreases with increasing EP interaction according to a power law, in contrast to the exponential law in usual (non-rotational) Holstein models. With decreasing force constant, the combination of decreasing phonon frequency and increasing soft anharmonicity leads to increasing self-trapping of electrons.

Supported by NSF-NIRT, US-DOE, and OU-CMSS.

[R1.156] Polymer blend morphology evolution under shear flows

We have extended the dynamic self-consistent field (DSCF) theory, originally formulated in 1D to 2 dimensions. The DSCF theory couples the time evolution of chain conformations, volume fractions and momenta, based on local conservation laws. A modification of the lattice random walk formalism of Scheutjens and Fleer is used to generate anisotropic chain conformations under flow. Here we present a DSCF study of the evolution of the morphology of a sheared polymer blend. We examine the role of interfacial width, copolymer addition and the effect of chain length on the resulting morphology.

[R1.157] Nanoparticles at the liquid-liquid interfaces: assembly, displacement, transport and crosslinking

The self-assembly of particles at fluid interfaces, driven by the reduction in interfacial energy, is well established. However, to nanoscopic particles, thermal fluctuations compete with the interfacial energy giving rise to a particle-size-dependent self-assembly. Here, ligand-stabilized nanoparticles are shown to assemble into three-dimensional constructs at fluid-fluid interfaces where the properties unique to the nanoparticles are preserved. The small size of the nanoparticles leads to a weak confinement of the nanoparticles at the fluid interface that opens new avenues to size-selective particle assembly, two dimensional phase behavior and functionalization. Fluid interfaces afford a rapid approach to equilibrium and easy access to the nanoparticles for subsequent modification. A photo-induced transformation is shown where nanoparticles, initially soluble in toluene, are transported across an interface into water and are dispersed in the water phase. A sheet of nanoparticles' membrane was generated from inter-particle chemical crosslinking through the functional ligands at the fluid-fluid interfaces. The characteristic fluorescence emission of the nanoparticles provides a direct probe of their spatial distribution.

[R1.158] Cell motility on nanotopography

Cell motility is strongly influenced by the structure of the substratum. Understanding cells motility on a surface has significant applications both in vivo and in vitro applications, such as biological sensors and hip replacement. A gradient surface is used to study the effect of the lateral nanotopography on cell motility. A gradient surface is generated by block copolymer and homopolymer blends, where the concentration of the components varies uniformly across the surface. The two homopolymers phase separate on the micron scale and this length scale gradually decrease to the nanoscopic, i.e. microphase separation of the diblock, as the copolymer concentration increases. Quantitative analysis of the speed of cell migration is correlated to the lateral length scale of the surface.

[R1.159] Non-linear dependence of the Flory interaction parameter on the inverse of absolute temperature in polystyrene-block-poly(n-pentyl methacrylate) copolymer

Diblock copolymers comprised of polystyrene covalently linked to poly(n-pentyl methacrylate), PS-block-PnPMA, with weak segmental interaction, are shown to exhibit a closed-loop phase behavior over a narrow range of molecular weight. This block copolymer has a closed-loop type of phase behavior. The endothermic transitions from disordered to ordered state and back into the disordered state, as a function of increasing temperature, are driven by delicate balances between weak segmental interactions and the entropy resulting from the mismatch in compressibility in addition to translational entropy. Flory interaction parameter (¥ö) of PS and PnPMA segments was calculated by fitting the mean-field theory to small angle neutron scattering (SANS) results. A well-known expression of ¥ö = a + b/T, in which a and b are temperature-independent constants and T is the absolute temperature, could not fit the SANS results in the homogeneous regime. The non-linear dependence of ¥ö on 1/T results in the closed-loop phase behavior in the diblock copolymer and the UCST and LCST in the homopolymer blend system.

[R1.160] Pressure Effects on the Closed-Loop Phase Behavior of Poly(styrene-block-n-pentyl methacrylate)

The unusual closed-loop phase behavior of poly(styrene-block-n-pentyl methacrylate) was investigated using a series of high-pressure experiments with the aim of understanding the molecular origins of the newly discovered upper order-to-disorder transition in this non-interacting polymer system. Small angle neutron scattering (SANS) studies under hydrostatic pressure were performed, from which pressure coefficients for the microphase transitions were extracted. This revealed dTUODT/dP and dTLDOT/dP for the upper and lower transitions of approximately –600 and 200°C/kbar. In addition, SANS and birefringence studies were carried out under carbon dioxide pressure. An appreciable expansion of the closed-loop was observed after the addition of as little as 0.01 g/cc carbon dioxide. These results suggest an entropic origin to the upper order-to-disorder transition seen in these materials.

[R1.161] Phase Behavior of Mixures of Block Copolymer and Homopolymers in thin film

The phase behavior of mixtures of block copolymer and homopolymers in thin film was investigated by using atomic force microscopy, field emission scanning electron microscopy and small angle X-ray scattering method. The block copolymer employed in this study was polystyrene-block-poly(methyl methacrylate) (PS-PMMA) with PMMA cylindrical microdomains. Two homopolymers of PMMA and poly(ethylene oxide) (PEO) were used to study the effect of the interaction parameter between homopolymers and PS block on domain spacing of mixtures. The microdomains in all thin films are oriented perpendicularly to the neutral surface made of PS-ran-PMMA copolymer. We found that the miscibility between PMMA homopolymer and PMMA block in thin film was enhanced compared with that in bulk. Also, at a given molecular weight of PMMA homopolymers, the PMMA chains in thin film are more localized at the center of PMMA microdomains than they are in bulk, which results in a larger increase of the lattice spacing (D) in the former. The value of D increased at first and reached a steady value. However, for mixtures consisting of PEO, the saturation of D was achieved at a smaller weight fraction of PEO in the mixtures compared with PMMA. This is attributed to the strong repulsive interaction between PEO and PS, which gives more localization of the PEO at the central region of the PMMA domains.

[R1.162] Solvent-Induced Ordering of Diblock Copolymer Thin Film

Full utilization of nanoscopic patterns by block copolymer ordering requires the control of the orientation and order of the microphase-separated domains over very large areas. Previous work has showed that the asymmetric diblock copolymer of polystyrene and poly(ethylene oxide) P(S- b-EO) can produce an array of cylindrical domains oriented normal to the substrate surface simply by spin-coating. Here the ordering behavior of this cylinder-forming P(S-b-EO) in the solvent vapor was investigated. Copolymer films were prepared by spin-coating, followed by annealed in the solvent vapor, and by solvent-casting in the solvent vapor. Benzene, a good solvent for the two highly immiscible blocks, was used. Long-range order of the PEO cylindrical domains was found. This behavior can be attributed to the strong immiscibility coupled with the mobility of copolymer chains. Interesting results were also found when the polymer solution was cast in the benzene/water mixture vapor. Water condensation and its selective adsorption by PEO leads to an increase of the PEO domain size, in addition to the long-range order. Thus, domain size and lateral order can be controlled easily by the introduction of the water.

[R1.163] Dynamics of Electric Field-Induced Instabilities in Thin Liquid Films

Fluctuations arising from electrohydrodynamic instabilities in thin liquid films can be tuned, by manipulating the balance of electrostatic pressure and Laplace pressure at the liquid-air interface, to produce patterned arrays. By laser scanning confocal microscopy, real-time studies were performed to determine the factors governing the selection of the dominant wavelength and the rate at which this fluctuation grows. Reflection interference fringes from the film surface were used to measure the growth rates of fluctuations and their dependence on electrostatic pressure. The growth rate was found to be exponential and to depend strongly on the applied field and the initial distance between the liquid surface and the opposing electrode. Changing the viscosity by introducing a crosslinking agent was found to retard the fluctuation's growth or could be used to lock-in a fluctuation at an intermediate growth stage. Dynamic control over fluctuating films may have important applications in lithography and microfluidic devices.

[R1.164] Temporal Evolution of Single Layer Film under Confinement in Electric Field

Structure formation at the surface of a single layer film in a heterogeneous external electric field is reported by imposing well-defined topographical stripes patterns. Growth of electrohydrodynamic instabilities under the area defined by those stripes as the upper electrodes is accessible by optical microscopy. Undulations grow beneath stripes leading to the formation of columns of polymer that span between the electrodes. The finite size of the stripes is seen to markedly alter the characteristic wavelength of the fluctuations. The center-to-center distance, \lambda_C-C, between the columnar structures along the length of the upper electrode is found to be independent of electrodes width. However, the \lambda_C-C across the width of the upper electrode is significantly altered. Efforts have focused on understanding the finite size effect, optimization of pattern replication, and the applicability to more complicated pattern design.

[R1.165] Moving Entangled liquid Fronts: Fingering Instabilities

A thin polymer film placed on a non-wettable substrate will rupture resulting in nucleation and subsequent growth of holes. As the hole grows, the fluid inside the hole is collected in a rim. In the absence of an external driving force, the rim of the moving front can become susceptible to fluctuations, becoming amplified, resulting in the formation of fingers. This instability is similar to fingering that occurs in simple liquids subject to an external driving force such as gravitational forces, Marangoni forces (surface tension gradients), and centrifugal forces. We show that the wavelength of the instability, L, like that observed in simple liquids, obeys the same scaling, L\simhCa^-1/3, where Ca is a capillary number. The mechanism of this instability and its relation to slip is discussed.

[R1.166] CO2 Induced Retrograde Vitrification in Thin Polymer Films

Thermally induced and CO2 induced glass transitions of poly(methyl methacrylate) (PMMA) thin films supported by SiOx/Si substrates were investigated using spectroscopic ellipsometry. The glass transition temperature, Tg, of the PMMA/SiOx/Si system increases with decreasing film thickness. We show that PMMA films experience a CO2 induced glass transition, Pg, and that Pg decreases with decreasing film thickness. In addition, the magnitude of the Pg depressions increases with increasing temperature. Moreover, we show that this system exhibits the phenomenon of retrograde vitrification, wherein upon increasing temperature isobarically, the polymer exhibits a rubbery to glass transition.

[R1.167] Stability of Polystyrene Thin Films in CO2

Atomic force microscopy was used to examine the structural stability of polystyrene thin films supported by SiOx/Si substrates in supercritical CO2 environments. The films were annealed at various conditions of temperature and pressure. It is well established that in vacuum, or air, polystyrene becomes structurally ubstable and dewets SiOx/Si substrates when annealed above the glass transition temperature. In contrast, the PS films in CO2 remained stable for thicknesses between 10 and 100 nm. These results are rationalized in terms of the effective interface potential computed from the optical properties of the system, considering the effect of the surrounding media, as well as the absorption of CO2 into the polystyrene layer.

[R1.168] Solvent Effects on Ordering in Block Copolymer Films

Block copolymers have emerged as an efficient route to create surface patterns on the scale of a few tens of nanometers. Some applications demand defect-free patterning over large length scales. Systems of cylindrical polystyrene-b-polybutadiene and poly(ethylene-alt-propylene)-b-polylactide were subject to solvent processes and then examined by optical and atomic force microscopy. Additionally, in situ grazing incidence small angle X-ray scattering data show the evolution of the in-plane structure as the solvent evaporates. With the use of a solvent atmosphere by solvent casting or solvent annealing, order over several tens of microns can be obtained of cylindrical microdomains oriented parallel to the substrate, a length scale much greater than that achieved by thermal annealing on the same system. Furthermore, by using droplet pinning, the in-plane orientation of the cylindrical microdomains can be directed by the strength and direction of flow within the droplet caused by evaporative flow.

[R1.169] Simulating Filled Diblock Copolymer Morphologies and their Optical Properties.

Photonic crystals are structures in which the dielectric constant is a spatially periodic function and forbidden frequency bands may exist for incident electromagnetic waves. Block copolymers have the potential to be promising photonic crystals due to the regular periodicity in which they self-assemble. Due to limitations on the molecular weight of the polymer species (and therefore the domain sizes of the resulting structures) and the dielectric contrast between different polymeric species in block copolymers the range of forbidden frequencies and the size of the photonic bandgaps are severly limited.

Recent computational investigations into the effects of selectively adding particles to diblock copolymers have revealed a wealth of insight into the potential morphologies of such structures. Here we extend these endeavors and consider the consequences of highly localising the particles at the interface between the A and B domains of a lamellar formation. Furthermore, we simulate the complex interactions between propagating light and the resulting heterogeneous solid material in order to determine the optical properties. We find that even for such simple 1D photonic bandgap materials a rich variety of phenomena ensues.

[R1.170] A simple route to nanostructure- selective solvent swelling

A simple route to generate nano-scale porous films based on P(S-b-MMA) has been demonstrated. Simply by swelling the copolymer film with a selective solvent for the minor, PMMA component, a nanoporous film is produced without further treatment or the removal of either block. Re-annealing the nanoporous film regenerates the original microphase-separated polymer nanostructures. This process is fully reversible over many cycles. The mechanism of formation of the pores is mainly due to the partial reorientation of cylindrical microdomains PMMA to the surface. Thus the surface can be modified from hydrophobic to hydrophilic without any additional surface chemistry.

[R1.171] Ordering of Hard Rods and Spheres in the Diblock Copolymer

We present a hybrid numerical approach to investigate the multi-scale ordering of hard rods and spheres in a symmetric diblock copolymer AB. The repulsive interaction between the A-coated rods and B phase can align the rods parallel to the interface and thus form a lamellar network of rods. The lamellar network of rods provides a locally-nematic host for hard spheres. A wealth of morphologies is observed by tailoring the short-range repulsion between the hard particles and B phase, and also thorough long-range depletion interaction between the rod-like inclusions. To separate the effects of diblock lamellar structure from those of depletion interaction, we also provide a hard wall model for comparison.

[R1.172] Post-Deadline Posters

[R1.173] First principles study of dilute magnetic semiconductors : ZnOCo and GaNCo

With the full potential linearized augumented plane wave (FLAPW) method, we have explored electronic and magnetic properties of dilute magnetic semiconductors such as ZnOCo and GaNCo. The calculations were performed with 72 atoms in a wurtize unit cell in which two Zn and Ga atoms are replaced by two Co atoms respectively. In our calculations, we first searched the total energy minimun varying the positions of Co atoms in ecah system. The structure optimization was also done in our numerical calculatioins. With these results, electronic and magnetic properties of dilute magnetic semiconductors have been investigated.

[R1.174] Intrinsic Asymmetries in Spin-Valves: A Tight Binding Model Study

We have explored the I-V curves, dynamical conductance, and tunneling magnetoresistance (TMR) of 1D magnetic tunneling junction through single band tight binding model calculations. The results indicate that the difference in density of state of two ferromagnetic leads play a major role as one of sources of intrinsic asymmetries in TMR and dynamical conductance at finite bias. Besides, we have displayed that large TMR can be obtained even at high bias due to intrinsic DOS character of half metallic leads

[R1.175] An adjustable Brownian heat engine

A microscopic heat engine is modeled as a Brownian particle in a sawtooth potential (with load) moving through a highly viscous medium driven by the thermal kick it gets from alternately placed hot and cold heat reservoirs. We found closed form expression for the current as a function of the parameters characterizing the model. Depending on the values these model parameters take, the engine is also found to function as a refrigerator. Expressions for the efficiency as well as for the refrigerator performance are also reported. Study of how these quantities depend on the model parameters enabled us in identifying the points in the parameter space where the engine performs either with maximum power or with optimized efficiency. The corresponding efficiencies of the engine are then compared with those of the endoreversible and Carnot engines.

[R1.176] Voltage gain in nanoelectronic GaAs/AlGaAs Y-Branch Switches

Using high resolution electron-beam lithography and wet chemical etching we realized Y-branch switches (YBS) based on modulation doped GaAs/AlGaAs heterostructures. The Y-branch consists of a one-dimensional source, which is split along the branching section into two one-dimensional drains. In addition to source-drain voltages, external electric fields can be applied via side-gates along the branches.

A YBS realized in this way can be used as a compact nanoelectronic differential amplifier. This is demonstrated by applying voltage differences to the side-gates and detecting the resulting voltage difference at the branches. In this configuration differential voltage gain of up to 30 has been observed. Further investigations revealed a bias voltage dependent switching efficiency which increases superlinearly with the applied bias voltage [1].

The bias voltage dependent switching efficiency is interpreted in terms of a capacitive coupling of the branches. This intrinsic coupling was found to enhance the effect of the side-gates. Due to the capacitive coupling of the branches a doubling of the switching efficiency was observed for a bias voltage of 1.75 V.

[1] S. Reitzenstein, L. Worschech, P. Hartmann, M. Kamp, A. Forchel, Phys. Rev. Lett. 89, 226804 (2002)

[R1.177] Rarefied gas dynamics using stochastic rotation dynamics

In the past two decades, Direct Simulation Monte Carlo (DSMC) has been the dominant predictive tool for rarefied gas dynamics. In the non-hydrodynamic regime, where continuum models fail, particle based methods have been used to model systems ranging from shuttle re-entry problems to mesoscopic flow in MEMS devices. A new method, namely stochastic rotation dynamics (SRD), which utilizes effective multiparticle collisions, will be described. It will be shown that it is possible to get the correct transport coefficients for Argon gas by tuning the collision parameters, namely the collision angle and collision probability. Simulation results comparing DSMC and SRD will be shown for equilibrium relaxation rates and Poiseuille flow. One important feature of SRD is that it coarse-grains the time scale, so that simulations in the transition regime are typically five to twenty times faster than for DSMC. Benchmarks as a function of Knudsen number will be given, and directions for further research will be discussed.

[R1.178] Photoexcitation and Control of Coherent Phonons and Electrons in GaAs

The interaction of ultra-short laser pulses with GaAs is investigated via simulations of the coupled electron-ion dynamics using a non-adiabatic quantum molecular dynamics model. The scheme, tight-binding electron-ion dynamics (TED), integrates the electron motion forward in time by numerically solving a unitary form of the time-dependent Schrödinger equation. The ion motion is simultaneously evolved in time through the solution of the Hellmann-Feynman equation. Results will be presented on the photoexcitation of coherent phonons in bulk GaAs showing the polarization dependence of the external laser field and the differences in phase dependence of resonant and non-resonant coherent phonons. i.e., the differences between displacive excitation of coherent phonons and impulsive Raman excitations. Simulations results of the coherent control of both the phonons and the electrons indicating both constructive and destructive interference depending on the pump-pulse control-pulse delay time will be shown. Initial results on the changes in polarization dependence due to the external laser field interacting with the (110) surface of GaAs will also be presented. This work was performed under the auspices of the U. S. Department of Energy by the University of California Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48. Partial support from NATO under Contract No. CRG 941028 is gratefully acknowledged.

[R1.179] Is quantum confinement possible in SiC quantum dots?

Using ab-initio methods, we investigate theoretically the possibility to fabricate silicon-carbide quantum dots. We find that the optical properties of this type of dots would be weakly dependent on size but strongly influenced by the structure of the surface, which in turn depends on the growth conditions. We discuss the conditions where quantum confinement could be observed in SiC quantum dots.

[R1.180] Multi-Frequency Synthesis Using Symmetry-Based Methods in Arrays of Coupled Nonlinear Oscillators

Multi-frequency oscillations in coupled array networks of nonlinear oscillators are demonstrated, theoretically and experimentally, using symmetry-based methods. In particular, two networks of coupled nonlinear oscillators are considered where each network contains N identical, mutually coupled systems. Under certain conditions, the arrays illustrate periodic behaviors where one array oscillates at N times the frequency of the other array. This type of multi-frequency behavior is different from the one observed in periodically forced systems because it is dictated, exclusively, by the symmetry of the network. The experimental demonstration consists of a circuit that is based on coupled over-damped Duffing oscillator equations.

[R1.181] Computational Issues in the Control of Quantum Dynamics

Computational Issues in the Control of Quantum Dynamics Phenomena

Herschel Rabitz Department of Chemistry Princeton University

The control of quantum phenomena embraces a variety of applications, with the most common implementation involving tailored laser pulses to steer the dynamics of a quantum system towards some specified observable outcome. The theoretical and computational aspects of this subject are intimately tied to the growing experimental capabilities, especially the ability to perform massive numbers of high throughput experiments. Computational studies in this context have special roles. Especially important is the use of computational techniques to develop new control algorithms, which ultimately would be implemented in the laboratory to guide the control of complex quantum systems. Beyond control alone, many of the same concepts can be exploited for the performance of experiments optimally tuned for inversion, to extract Hamiltonian information. The latter scenario poses very high demands on the efficiency of solving the quantum dynamics equations to extract the information content from the experimental data. The concept of exploiting a computational quantum control tool kit will be introduced as a means for addressing many of these challenges.

[R1.183] Lattice dynamical investigation of force constants, Raman, and infrared wavenumbers in SrCeO_3 orthorhombic perovskite.

At room temperature SrCeO_3 has a orthorhombic perovskite structure with space group ( Pbnm) . Further it exhibits a first order phase transition at different pressures. Such phase transition has been reported recently by S. Lorident et al. [J. Phys. Chem. Solids,63(2002) 1983] using Raman spectroscopy. Hence in this work, a short-range force constant model (SRFCM) and Wilson G.F. matrix method have been applied for the first time to investigate the phonons in SrCeO_3 perovskite in the orthorhombic phase. Here, we consider two types of short-range forces. They are stretching forces and bending forces. Ten stretching and ten bending force constants are obtained by having a best fitting to the measured Raman frequencies. The stretching force constants Ce - O are dominated over Sr O. It was also observed that the bending force constants O-Ce-O are larger than O-Sr-O. The calculations with the nine stretching and five bending force constants (using iteration method) provide a good agreement for the observed Raman frequencies. All 24 Raman modes (7A_g, 5B_1g, 7B:2g, 5B_3g) and 25 infrared (9B_1u, 7B_2u, 9B_3u) frequencies have been calculated and assigned. The infrared frequencies have been calculated for the first time and assign in their specific mode of vibrations.

[R1.184] Development of a Third-Generation Material Handling System for Electron Beam Irradiation

In order to facilitate the electron beam irradiation of samples at the 150 kW NEO Beam accelerator, a linear motion system (LMSIII) was developed. This third generation system is computer-controlled and designed to handle a total sample cross-sectional area of <2 m^2, with a maximum payload of about 400 kg. This unit is able to take full advantage of the spatial homogeneity of the electron beam for irradiation of samples up to 1.2 m wide. The LMSIII is based on a lead-screw design that can control both the speed and position of the sample very precisely. The sample speed is adjustable from less than 1.0 cm/min up to a maximum of about 30.0 m/min, which allows control of the total absorbed dose for a given set of beam conditions. The modular design of the sample stage allows the implementation of a cooling or heating cell to control the sample temperature during electron beam processing. The sample temperature can be maintained and monitored between 5°C and 350°C using the cooling and heating cells. Experimental data have shown that the cooling cell can be used to lower the sample temperature by almost 50conditions. An atmosphere control module is also being developed which will allow a vacuum or various gases to be introduced to the sample during electron beam processing.

This work was partially supported through NASA grant NCC3-721 and KSU's Research Council.

[R1.185] Design and Construction of a Faraday Cup for Electron Fluence Measurements in the Energy Range from 1 to 5 MeV

Electron beams are used in research and industry in order to develop new materials or change the physical properties of materials of technological interest (e.g. polymeric materials or semiconductors). Usually the physical or chemical effect produced by the electron beam is a function of the dose absorbed by the material and ultimately of the fluence of the incoming radiation. In many of these applications, such as the study of radiation damage in solar cells, the electron fluence is one of the parameters used to characterize the effect of the radiation on the semiconductor material. In this paper a description is presented of the design and construction of a Faraday Cup to measure electron beam fluence values in a 1 to 5 MeV, 150 kW electron accelerator used for radiation processing. At such high powers, the electron beam is continuously scanned back and forth in one direction in order to prevent the burning of the sample, so the design of the Faraday Cup took into consideration the fact that the electron beam is absorbed both in the fluence sensing element as well as in the outer jacket. A virtual instrument was developed using National Instruments development software to control the data acquisition process with the Faraday Cup and associated NIM electronics. Fluence measurements have been carried out in the energy range from 1 to 5 MeV. Examples of fluence measurements on semiconductor devices will be presented.

Work partially supported through NASA grant NCC3-721 and KSU's Research Council.

[R1.186] Influence of Mobile Ions on Nanotube Based FET Devices

Carbon nanotube field-effect transistors often exhibit a hysteresis effect in the gate-voltage dependence. We demonstrate a method to introduce this effect controllably by coating nanotube devices with charged polymers, and present a basic model for explaining the source of the hysteresis. The effect has previously been proposed to be used as a memory element, and we show that for our devices it may also make a suitable humidity sensor.

[R1.187] Geometry-dependent scattering through quantum billiards: Experiment and theory

We present theoretical and experimental studies of geometry-specific quantum scattering in microwave billiards of a given shape. We perform full quantum-mechanical scattering calculations and find excellent agreement with experimental results. We also carry out semiclassical calculations where the conductance is given as a sum over all classical trajectories between the leads, each of the trajectories carrying a quantum-mechanical phase. We unambiguously demonstrate that the characteristic frequencies of the oscillations in the transmission and reflection \emphamplitudes t and r are related to the \emphlength distribution of the classical trajectories between the leads, whereas the frequencies of the \emphprobabilities T=|t|^2 and R=|r|^2 can be understood in terms of the \emphlength difference distribution in the pairs of classical trajectories. We also discuss the effect of nonclassical ``ghost'' trajectories, ie. trajectories that include classically forbidden reflection off the lead mouths.

[R1.188] Inter-Basin Motion Approach to Dynamics of Conformationally Constrained Peptides

In this work the inter-basin motion (IBM) approach [1] is applied to studying dynamics of conformationally constrained peptides, particularly to a non-ideal contact of the system with a thermal bath. The coupling of the system with the thermal bath is expressed in terms of a memory function. The aim of the present study is twofold. First, we present a dynamical diagnosis of three hexapeptide variants, alanine hexapeptide with neutral terminals, alanine hexapeptide with charged terminals and a backbone cyclized alanine hexapeptide,[2], focusing on the transitions between basins rather than between individual locally-stable states. Second, the present study is intended to pinpoint a way for extracting useful information about the strength of the system-solvent coupling and how this interaction affects the propensity of relaxation towards the native state. We show that a slight variation of the value of the memory friction parameter may induce a sizeable change of the relaxation time. In addition, the change of the memory friction parameter produces alterations on short time-scales among the population distributions. Especially, high energy basins seem affected the most. In the Markovian limit, the basin populations computed within the IBM model are compared to those obtained by using state-to-state transition rates in the full master equation approach. The two methods yield similar results when the time scales between intra- and inter-basin dynamics are well separated.

[1] F. Despa and R.S. Berry, J. Chem. Phys. 115, 8274 (2001) [2] -Y. Levy and O.M. Becker, J. Chem. Phys. 114, 993 (2001); Y. Levy, J. Jortner, and O. M. Becker, Proc. Natl. Acad. Sci. USA 98, 2188 (2001); J. Chem. Phys. 115, 10533 (2001)

[R1.189] Continuum electrostatics in (bio)molecular modeling

Like other macroscopic theories, classical electrostatics translates surprisingly well to the molecular size scale. A protein in a physiologic medium can be modeled as a low dielectric body with an embedded distribution of charges, immersed in a high dielectric solvent with a continuous counterion distribution. Such a system is described by the linearized Poisson-Boltzmann (LPB) equation, a 3-dimensional partial differential equation that can be solved by numerical methods to yield the electrostatic potential field in and around the protein. This approach has been used in models of protein folding, intermolecular binding, protonation equilibria, etc. Several such applications will be discussed and the strengths and weaknesses of continuum electrostatic theory at the molecular level will be considered.

[R1.190] Atomic Mutipoles in Macromolecular Simulations

Empirical force fields for macromolecular simulation are made up of a number of simply calculated terms whose sum approximates the Born-Oppenheimer potential energy. To improve this approximation, current efforts are focussed on the non-bond interaction terms, specifically the electrostatic and induction terms. According to most analyses (decomposition or intermolecular perturbation based) these terms dominate. Until recently, these were modelled by fixed "effective" point charges at atomic positions. There is increasing interest in more sophisticated models, including fixed and induced multipolar terms centered at atoms as well as at off-atom positions such as bond centers and "lone-pairs". However the computational cost of these alternatives seems at first glance to be prohibitive. In this talk we generalize the particle-mesh approach to handle these types of electrostatic models, demonstrating that these far more accurate models can be used in simulations at a low to moderate cost overhead.

[R1.191] Observation of Critical Casimir Effect in a Binary Wetting Film: An X-ray Reflectivity Study

We report recent observations of the fluctuation-induced "Casimir force" in a confined fluid near a critical point. The system studied consists of a thin wetting film of binary mixture methylcyclohexane (MC) and perfluoromethylcyclohexane (PFMC) on Si(100), which is in equilibrium with the binary vapor and bulk liquid mixture at critical concentration. The film satisfies an anti-symmetric boundary condition (+,-) such that MC-rich liquid wets the liquid/Si interface while PFMC is favored at the liquid/vapor interface. The total film thickness was controlled by varying the temperature difference \Delta T between the substrate and the reservoir. The x-ray reflectivity results show that for a given \Delta T, the thickness is enhanced for temperatures just below T_c = 46 ^oC, in qualitative agreement with the theoretical expectation. Moreover, the Casimir amplitude \Delta _+,- (at T_c) deduced from the measurements is consistent with the theoretical values obtained previously by Krech [Phys. Rev. E 56, 1642 (1997)].

[R1.192] Fabrication of Ca_2RuO_4 Films Deposited on Various Substrates

We have deposited the Mott insulator Ca_2RuO_4(CRO) films on Si(100), SrTiO_3(110), and NdGaO_3(100) substrates by laser ablation for both observing the first order metal-insulator transition (Mott-transition) and the mechanism of high-T_c superconductivity. A lattice constant of Si, SrTiO_3 and NdGaO_3 substrates is similar to that of Ca_2RuO_4. Substrate temperatures under deposition are between 700 and 780^\circC. The oxygen pressure in chamber is 200 mTorr. The used target is the CRO ceramic. Films deposited between 750 and 780^\circC are crystallized and are very thin. Films grown at 700^\circC are thicker than those at 750^\circC and have a good crystalline. A deficiency of Ca-element with larger kinetic energy than Ru-element is analyzed by electron probe microanalysis. A sharp interface between the CRO film and the SiO_2/Si substrate is observed by scanning electron microscopy. Results performed by Ca-rich CRO targets for Ca-enhancement will be presented then.

[R1.193] Metal Nanowire Resonators

Metallic nanowires are a simple, easily fabricated material with applications in nanoelectronic, nano-plasmonic and nanomechanical systems. We have developed fabrication techniques to create doubly clamped, suspended beams of metallic nanowires. Through a magnetomotive technique, we have excited and detected the fundamental resonant mode of a platinum nanowire beam. We will present our latest results studying the non-linear behavior of this system, as well as our efforts in frequency tuning and parametric amplification.

[R1.194] Polarization-induced three-dimensional electron slabs in III-V Nitride semiconductors

We demonstrate bulk-doping by tailoring polarization charges in the III-V Nitride semiconductor system. We are able to create mobile three-dimensional electron slabs (3DES) without the introduction of donor-impurities in Al_xGa_1-xN alloys by by employing only compositional grading. The electron-slabs exhibit higher mobilities than comparable donor-doped samples, making it an attractive tool for achieving high-conductivity layers. At low temperatures, the three-dimensional carriers show no signs of freeze-out, and coupled with the large reduction of impurity scattering, the mobility remains high (\approx 3000 cm^2/V \cdot s). This enables us to perform low-temperature magnetotransport measurements on the 3DES, making it possible to demonstrate strong Shubnikov de-Haas oscillations in a three-dimensional carrier system in the III-V Nitride semiconductors for the first time. From the oscillations, the effective mass is determined to be m^\star=0.19m_0, the dominant scattering mechanism is identified as alloy scattering, and the alloy-scattering potential is determined to be V_0=1.8eV. This technique of polarization-doping is used to create three-dimensional carrier densities in the range of 10^17 - 10^18/cm^3, and can be extended. Doping is controlled by two factors - the thickness of graded layers, and the alloy composition. The wide electron slabs offer another playground for study of electron transport in three-dimensions in the presence of high magnetic fields.

[R1.195] Secondary emission induced by low-energy ion bombardment of a surface: Role of an adsorbate

The interaction of energetic ions with surfaces is of interest in many applications such as materials processing via discharge etching, where ion-surface interactions affect, e.g., equilibrium plasma characteristics. Depending on characteristics of the incident ions and of the surface, secondary emission of electrons and ions may be effected in various ways. We investigate low energy ion-induced emission in the context of precluding certain well-known mechanisms of impact-induced secondary emission, and present experimental results in which absolute probabilities for ion-induced secondary emission of anions and electrons have been measured as a function of adsorbate coverage and impact energy for low-energy (< 500 eV) positive and negative ions incident on various surfaces. Secondary electron and anion kinetic distributions are presented as well, and the results are compared to those of previous experiments in which the presence of an adsorbate dramatically enhanced secondary emission. It is suggested that the underlying mechanism of this secondary emission involves collisional excitation of a surface state, which subsequently decays to give rise to both anion and electron emission into the vacuum. Supported in part by the US Department of Energy, Office of Science, Division of Chemical Sciences.

[R1.196] The Electronic Structure of Bismuth Bilayers

Using angular resolved photoemission spectroscopy we identified four two-dimensional (2D) bands within a 1 eV binding energy region below the Fermi level. The top two bands that are part of the complex Fermi surface of Bi (111) are located in the projected bulk band gap and exhibit the six-fold rotational symmetry of the top Bi bilayer. The two lower lying bands reside inside the projected bulk band structure. The three-fold rotational symmetry of these bands indicates a weak interaction with the underlying bulk bands. Our observations (crystal properties, symmetry observations, spin degeneracy, band filling, insensitivity to surface contamination, as well as band structure calculations) can not be associated with bona fide surface states but strongly support the existence of a 2D electronic structure originating in the topmost bilayer of Bi(111). The data can be explained with a tight binding (TB) calculation of a bilayer by eliminating the second nearest neighbor interactions that determine the coupling between bilayers in bulk Bi.

Funding by NSF award DMR-0084402 is acknowledged.

[R1.197] Subthreshold Optics of In_xAl_1-xN Films: Electron Localization and Temperature Dependence of Absorption Peaks

The InN, InAlN, and AlN films with thickness ranging from 100 to 8000 Å have been grown on (0001) sapphire substrates, utilizing plasma source molecular beam epitaxy (PSMBE), we have grown films. All the obtained films are epitaxial with no phase segregation, as shown by high resolution X-ray diffraction scans. Characteristic surface pattern of large number of 10-nanometer scale hillocks is revealed by the atomic force microscopy; the size distribution of these hillocks depends of the film composition and thickness, as well as on the buffer layer thickness in the case of In-containing films. Associated with the hillocks, additional absorption peaks are detected far below the fundamental absorption threshold. Counterintuitive temperature dependences of these peaks can be explained in the framework of the model of electron localization near the hillock tips due to built-in electric field

[R1.198] Effect of impurities in the breaking of Gold nanowires

Metallic nanowires, and in particular Au nanowires, have been the focus of many recent studies. Surprisingly large interatomic distances, in the range of 3.6-4.0 Åhave been reported for one atom thick gold nanowires by various experimental groups [1-3]. From a theoretical viewpoint, no one has been able to reproduce the above mentioned large distances for clean nanowires, the maximum obtained values being of the order of 3.0-3.1 ÅIn an attempt to explain these results we have decided to systematically study the effect of impurities in Au nanowires under stress, using an as realistic as possible model. We study the influence of C, H and other impurities in the breaking of Au nanowires [4]. For C atoms, we have concluded that its presence in Au nanowires would result in distances significantly larger than 3.6 Åwhereas the obtained Au-Au distances for the H case are in excellent agreement with the experimental results, suggesting that H impurities are most likely the source of the yet unexplained large Au-Au distances in the range of 3.6 ÅOur results are based on ab initio total energy DFT calculations. In order to have an as realistic as possible wire, we have used a 70 Au atoms structure that we have recently obtained [5] through a TB-MD simulation. This work is supported by FAPESP, Capes and CNPq. CENAPAD-SP is acknowledged for computer time. [1] H. Ohnishi, Y. Kondo and K. Takayanagi, Nature 395, 780 (1998). [2] V. Rodrigues and D. Ugarte, Phys. Rev. B 63, 073405, (2001). [3] S. B. Legoas et al., Phys. Rev. Lett. 88, 076105 (2002) [4] F. D. Novaes, A. J. R. da Silva, E. Z. da Silva, and A. Fazzio, Phys. Rev. Lett., accepted (2002). [5] E. Z. da Silva, A. J. R. da Silva, and A. Fazzio, Phys. Rev. Lett. 87, 256102 (2001).

[R1.199] Coarsening dynamics of faceted crystal surfaces

The Crystalline-Aviles-Giga (\mathcalCAG) equation governs the annealing of a faceted crystal surface, where attachment kinetics is the dominant mass transfer mechanism. The Hamilton-Jacobi-Aviles-Giga (\mathcalHJAG) equation is the associated growth model: \beginequation h_t - \varepsilon \hat\mathcalF (\nabla h) = div \left[D\hatW \left( \nabla h \right) \right] - \Delta^2 h. \endequation In both cases the characteristic length scale of the faceted surface grows in time ( coarsens) through the merging and annihilation of facets. We identify the sharp-interface theories for both \mathcalCAG (\varepsilon=0) and \mathcalHJAG (\varepsilon>0) through a matched asymptotic analysis. The results are novel edge-network dynamical systems (\mathcalENDS), From these, scaling laws for the coarsening rate follow, as well as predictions on surface morphology. We thereby explain the numerically observed accelerated coarsening of \mathcalHJAG relative to \mathcalCAG, as well as understanding the associated transition in surface morphologies. Our approach naturally extends to a broad class of faceted crystal growth problems.

[R1.200] Quantum Monte Carlo study of solid sodium

Quantum Monte Carlo (QMC) calculations, using the variational (VMC) and diffusion (DMC) methods is performed on the body-centered cubic solid sodium. We report calculations of the cohesive energy, the occupied bandwidth and the pair correlation functions (PCFs). DMC calculations including the core polarization potential (CPP) give an excellent value for the cohesive energy of the solid. The calculated bandwidth 3.7 eV is significantly larger than the experimental value of 2.6 eV, but is in good agreement with a recent many body caluculation with a sophisticated approximation.

[R1.201] Zn and Ni impurity ions in d-wave superconducting cuprates

To describe the scattering of superconducting quasiparticles from non-magnetic (Zn) or magnetic (Ni) impurities in optimally doped high T_c cuprates, we propose an effective Anderson model Hamiltonian of a localized electron hybridizing with d_x^2-y^2-wave BCS type superconducting quasiparticles with an attractive scalar potential at the impurity site. Due to the strong local antiferromagnetic couplings between the original Cu ions and their nearest neighbors, the localized electron in the Ni-doped materials is assumed to be on the impurity sites, while in the Zn-doped materials the localized electron is distributed over the four nearest neighbor sites of the impurities with a dominant d_x^2-y^2 symmetric form of the wave function. With Ni impurities, two resonant states are formed above the Fermi level in the local density of states at the impurity site, while for Zn impurities a sharp resonant peak below the Fermi level dominates in the local density of states at the Zn site, accompanied by a small and broad resonant state above the Fermi level mainly induced by the potential scattering. This is exactly what has been observed in the scanning tunneling microscopy experiments. From the calculated spin relaxation functions, we find that the 3d localized electron in both Ni and Zn doped materials displays a weak magnetic oscillation. This result is consistent with the signal of a spin-1/2 magnetic moment exhibited by nuclear magnetic resonance measurements in YBa_2Cu_3O_6+\delta doped with Zn or Ni impurities. The local density of states and their spatial distribution at the dominant resonant energy around the substituted impurities are calculated for both cases, and they are in good agreement with the experimental results of scanning tunneling microscopy in Bi_2Sr_2CaCu_2O_8+\delta with Zn or Ni impurities, respectively.

[R1.202] Shot noise in a chaotic cavity: The role of contacts in Quantum to Classical crossover

Shot noise in a chaotic cavity (Lyapunov exponent \lambda, level spacing \delta, linear dimension L), coupled by two N-mode point contacts to electron reservoirs, is considered as a measure of the crossover between stochastic quantum and deterministic classical transport. The transition proceeds through formation of a number of fully\/ transmitted(reflected) states occupying a compact parts of the cavity phase space. These transmission channels contribute to the mean current \barI, but not to the shot-noise power P. The transmission channels do not exist for N\alt\sqrtk_FL, where the universal Random Matrix theory limit gives P/2e\barI=1/4. For N\agt\sqrtk_FL we expect a suppression of the noise like (k_FL/N^2)^N\delta/\lambda h. These signatures could help to distinguish ballistic chaotic scattering from random impurity scattering in quantum transport.

[R1.203] Dielectrophoresis of graded microparticles in suspensions

Dielectrophoresis of graded microparticles in suspensions \vskip 0.3cm

L. Dong, J. P. Huang, K. W. Yu and G. Q. Gu \vskip 0.3cm

Department of Physics, The Chinese University of Hong Kong Shatin, NT, HK.

Dielectrophoresis is an AC electrokinetic phenomenon that employs the difference in the electric polarizability of microparticles and the suspending media. Under the action of an external electric field, these particles polarize, and experience a force in a nonuniform field. The degree of polarizability can depend on the frequency of the applied AC field. In this work, we consider graded spherical particles in which the material properties can vary continuously in space. These inhomogeneous particles can be more useful and interesting than the homogeneous inclusions. A new theory has been established to study the effective properties of graded composite materials under externally applied field, namely, the differential effective dipole approximation (DEDA). The theory has been applied to two model dielectric profiles, namely, the power-law and linear profiles. Moreover, we have shown that these profiles actually admit exact solutions for the local electric field. We have compared the DEDA results with the exact results for the two model profiles and the agreement is excellent. Based on the DEDA, we investigate the DEP spectrum of a colloidal suspension of graded spherical particles, and compare the results with the DEP spectrum derived from the homogeneous particles.

[R1.204] STM-Induced Light Emission from the Surface of Perinone Derivative Molecular Monolayer Covered Au (100) Substrate

The light emission property on the surface of perinone derivative (PD) molecular monolayer covered Au(100) substrate (PD/Au(100)) was investigated by using an ultrahigh-vacuum tunneling microscopy. The optical spectral and emission intensity measured from the bare Au(100) surface and the PD/Au(100) surface indicated that the observed light emission from the PD/Au(100) surface is dominated by the surface plasmon. The PD molecules showed weak contribution to the observed light emission. The reason is attributed to the strong coupling of PD molecular electronic states in the PD molecular monolayers with the Au(100) surface and the resultant quenching of molecular fluorescence to the Au(100) surface through non-radiative energy dissipation. The results suggest that light emission from molecules on metal substrate maybe enhanced by improving the decoupling of the PD molecular electronic states to the metal substrate.

[R1.205] Compositional analysis of diamond like carbon and carbon nitride films deposited by magnetron sputtering

The growing influence of the amorphous carbon not only as mechanical protective coating , but also of its possible use as electronic semiconducting material have made this material an important one. Incorporation of Nitrogen in a-C:H is believed to improve the semiconducting properties[1]. Moreover Carbon-Nitrogen films are a possible candidate for dielectric, insulating and passivating layers in a variety gallium nitride based device applications. Thin films amorphous carbon, non-hydrogenated, hydrogenated and nitrogenated were deposited on glassy carbon, silicon and quartz using magnetron sputtering of graphite target. Argon and Nitrogen were used as a sputtering gases. For Elemental concentration, films deposited on glassy carbon were used. 2.2 Mev of He++ beam is extracted from accelerator and in directed to the target films. Back and Forward scattered He++ particles were detected by solid-state detectors. The number and the energy of the particles striking the detector is stored electronically. The areal density in atoms per cm2, on the substrate surface was obtained from the shift in the substrate edge and area of carbon and other elements signals in Rutherford Backscattering Spectrum (RBS). Total Hydrogen content of the films were measured with Elastic Recoil Spectroscopy (ERS). Spectrum were simulated using Rutherford Universal Manipulation Program (RUMP).

[R1.206] Mapping Silicon Bonding at Heterostructure Interfaces with Atomic-Resolution TEM and Ab-Initio Modeling

The bonding of silicon at interfaces has always been of general interest for the semiconductor industry even before the time of nanostrucutred devices, but in the new age of nanostructured devices further miniaturization will depend on our ability to design silicon interface properties. To achieve this goal, these interfaces must be characterized directly and at the atomic scale. We use a combination of Z-contrast imaging, electron energy loss spectroscopy, and ab initio density functional theory to reveal the bonding of silicon at various interfaces. In this study, we investigate the interfaces between silicon and gallium arsenid, silicon and thermally grown silicon dioxide and ultra thin strained silicon and silicon dioxide. Our unique characterization tools allow us to compare the bonding of silicon at these interfaces and to separate strain and mixing (doping) effects at these interfaces.

[R1.207] Experimental Realization of Background charge insensitive Single-electron memories

The problem of background charge randomness needs to be tackled for the emergence of practical ultra-high dense single electron devices. Background charge effects in memory structures are relatively easier to overcome than for logic. One possible way is to have a destructive readout of the stored memory bit leading to oscillations in the electrometer, yielding a background charge insensitive single electron memory device. We report on experiments with metallic floating gate single electron memory cells with different barriers separating the floating gate (FG) from the control gate (CG). Discrete single electron charging of the FG across thin (< 10nm) barriers with small hysteresis loops indicating memory effect is seen. A background charge insensitive mode of operation is demonstrated in the case where charging of the FG is done through the island of the SET as opposed to by CG. Methods to improve fabrication and the performance of these devices are discussed.

[R1.208] Universal but not Rouse-like: short chain melt dynamics

The Rouse model is generally used as the standard model for interpretation and analysis of experiments on short chain melt dynamics. Computer simulations on the other hand have provided growing evidence over the last 10 years that there are several quantitative as well as qualitative short-comings of the Rouse model. We will discuss some new simulation results on the scaling behavior or monomer mean square displacements with time and the non-diffusive center of mass motion for times smaller than the Rouse time. We will also show how this behavior crosses over to reptation dynamics for long chains.

[R1.209] First-Principles Study of the Self-Assembled Pentacene Molecules on Metal Surfaces

Oriented thin films of organic semiconducting small molecules have received considerable attention as active semiconductors for device applications such as Schottky diodes and thin-film transistors (TFTs). Among these organic materials, pentacene has been found to have the highest mobilities for hole transport. Understanding the formation of self-organized ad-layers of pentacene would contribute to the fabrication of nanostructures and possibly highly oriented pentacene layers by epitaxy for use in electronic devices. To understand the ordering patterns of pentacene ad-layers on metal surfaces, we investigated the energetics between pentacene molecules with and without metal substrates and analyzed its electronic structure. We used a self-consistent first-principles calculation method based on the density functional theory (DFT) within local density approximation (LDA). The localized pseudo-atomic orbitals (PAO) are employed for a real-space numerical basis set, which was suggested by Sankey and Niklewski, and the Troullier-Martins-type pseudo-potential is used. As results, we found that the ordering patterns can be explained by the energetics between pentacene molecules, and the metal substrates appears not to influence too much on the interaction between pentacenes. To investigate the nature of the self-assembled structure, we calculated the total energies of various configurations for the molecule pattern, e.g., side-by-side and head-to-head ordering or on-top stacking. Depending on its direction, extremely different interaction character between two pentacenes is found and explained by its electronic structure analysis.

[R1.210] Doping and temperature dependence of superfluid weight for high Tc cuprates

Using the improved slave-boson approach of the t-J Hamiltonian [Phys. Rev. B 64, 052501 (2001)] that we developed recently, we report the hole doping and temperature dependence of the superfluid weight. It is shown that at low hole doping concentration x and at low temperatures T there exists a propensity of a linear decrease of the superfluid weight n_s/m with temperature, and a tendency of doping independence in the slope of \fracn_sm^*(x,T) vs. T in accordance of the relation \fracn_sm^*(x,T) = \fracn_sm^*(x,0) - \alpha T with \alpha, a constant. It is also demonstrated that T_c increases with hole doping concentration x, reaches a saturation(maximum) at optimal doping and decreases with increasing x and n_s/m^* in the overdoped region. Such a reflex (decreasing) behavior of T_c is attributed to the weakening of coupling between the spin(spinon pair order) and charge(holon pair order) degrees of freedom in the overdoped region. All of these findings are in agreement with \muSR measurements.

[R1.211] Unification of Classical and Quantum Mechanics amp; Theory of Relative Motion

A systematic survey of relevant pivotal experiments leads us to arrive at (I) vacuum comprises substantial entities called aethers and (II) the velocities of light as measured in vacuum c and by a moving observer c', and the observer's velocity v obey the law of vector addition. (I)-(II) facilitate a General Scheme, which leads to (A) from Newton Mechanics solution for vacuum the fundamental formation of basic material particles having a mass, size, charge, etc. and being a de Broglie wave obeying Quantum Mechanics (B) augmentation in the mass, de Broglie wavevector, etc of a moving particle by a factor \gamma = 1/[1-(v/c)^2]^1/2 (C) length and time contractions of a moving body as measured in the frame in which the body resides (D) coordinate transformation between an inertial frame at rest and one relatively moving, called Galileo-Lorentz transformation (GLT) (E) using the GLT the prediction of null-fringe shift of the Michelson-Morley experiment and the Doppler effect of electromagnetic waves etc (F) inference of various contemporary empirical rules, incl Uncertainty Relation; etc.

[R1.212] Scanning Electron Microscopy of Fully Hydrated Cells

A new capability for scanning electron microscopy of wet biological specimens is presented. A membrane that is transparent to electrons protects the fully hydrated sample from the vacuum. The resulting images are a striking combination of morphological aspects of the whole cell with a wealth of internal details. The technique employs immuno-gold for specific labeling and stains for observing intracellular structures. Tissue slices are directly inspected (with or without stains) at varying magnifications, imaging only the external layer of cells. Simultaneous imaging with photons excited by the electrons incorporates data on material distribution, with potential for multi-labeling and specific scintillating markers.

[R1.213] Structure, Photophysics and the Order-Disorder Transition to the Beta Phase in Poly(9,9-(di n,n-octyl)fluorene)

X-ray diffraction, UV-vis absorption and photoluminescence (PL) spectroscopy have been used to study the well-known order-disorder transition (ODT) to the beta phase in poly(9,9-(di n,n-octyl)fluorene) (PF8) thin film samples through combination of time-dependent and temperature-dependent measurements. The ODT is well described by a simple Avrami picture of one-dimensional nucleation and growth but crystallization, on cooling, proceeds only after molecular-level conformational relaxation to the so called beta phase. Low temperature PL studies reveal sharp Franck-Condon type emission bands and, in the beta phase, two distinguishable vibronic sub-bands with energies of approximately 199 and 158 meV at 25 K. This improved molecular level structural order leads to a more complete analysis of the higher-order vibronic bands. A net Huang-Rhys coupling parameter of just under 0.7 is typically observed but the relative contributions by the two distinguishable vibronic sub-bands exhibit an anomalous temperature dependence. The PL studies also identify strongly correlated behavior between the relative beta phase 0-0 PL peak position and peak width. This relationship is modeled under the assumption that emission represents excitons in thermodynamic equilibrium from states at the bottom of a quasi-one-dimensional exciton band.

[R1.214] STM Studies of the Growth of Ni and Cu Islands on TiO_2(110)-(1x1): Controlling Island Size Distributions

We have investigated the growth of Cu and Ni islands on rutile TiO_2(110)-(1x1) surfaces using scanning tunneling microscopy (STM). Both Cu and Ni islands grow 3-dimensionally at room temperature even at low coverages. The narrowness of the size distributions for both Ni and Cu islands are controlled by the diffusion (D) to deposition flux (F) ratios, with the smallest values for D/F corresponding to most uniform size distributions. Increasing the deposition flux resulted in smaller islands with higher island densities, while increasing the diffusion rate at elevated deposition temperatures produced larger islands with lower island densities. As the metal coverage is increased, the island density increases even up to coverages as high as 2-3 ML, whereas the island diameters do not change much throughout entire coverage range. Islands do grow with increasing coverage, but this growth is mainly due to an increase in island height. Based on the stronger admetal-oxide interaction for Ni compared to Cu, it was expected that the Ni islands might have smaller aspect ratios compared to the Cu islands. Furthermore, the stronger admetal-oxide interactions could result in decreased diffusion rates for Ni compared to Cu. However, we found that Ni islands do not grow flatter on the surface compared to Cu, and there is no evidence for slower Ni adatom diffusion at room temperature. A major difference between Ni and Cu island growth is that the sintering of Ni islands occurs at much higher temperatures. We have shown that the rate limiting step in the sintering of Ni and Cu islands is adatom detachment, not adatom diffusion. Therefore, the slower sintering of Ni islands compared to Cu can be explained by the stronger metal-metal bond strength for Ni.

[R1.215] Giant Increase of Superconducting Transition Temperature of Gallium Infiltrated into Nanoscaled Opals.

Gallium-filled opals with a 100% filling factor have been fabricated via infiltration of liquid gallium into opals (300-nm silica spheres) using a novel high pressure-high temperature technique. AC magnetic susceptibility has been measured as function of temperature at different external magnetic fields. A profound Meissner effect was observed with an onset at about 30 K, suggesting a presence of the superconducting phase, which occupies up to 30% of the infiltrated gallium volume. This temperature is about 30 times higher than the superconducting transition temperature of bulk gallium. An external magnetic field of 1 T completely suppresses the superconductivity. Possible physical models of the effect will be discussed.

[R1.216] Effect of Light Soaking on roughness of a-Si:H film

Device quality undoped hydrogenated amorphous silicon (a-Si:H) films prepared by the conventional glow discharge of silane gas onto Corning 7059 substrates are exposed to heat filtered white light from a tungsten halogen lamp (\approx 35 mW/cm^2). For an exposure of three and a half hours, the dark conductivity decreases by a factor of 1.7 and the photoconductivity by a factor of 2.5. Atomic Force Microscope (AFM) in non-contact acoustical a.c. mode is used to measure the surface roughness of a-Si:H. The light soaking is done from the back of the a-Si:H, through a hole in the AFM stage, so that the roughness can be measured in the annealed and the light soaked state, without the necessity of removing the sample. We find that the r.m.s. roughness estimated from the 2000nm x 2000nm image, increases from \approx 2.5 nm in the annealed state to \approx 3.2 nm after three and a half hours of light exposure. However, for lower image areas (400nm x 400nm) and smaller the value of the roughness as well as its increase after light exposure is much smaller. The results are explained in terms of hydrogen movement at the surface of a-Si:H.

[R1.217] POLYMER COATING IMPROVES STABILITY OF POROUS SILICON AGAINST LIGHT SOAKING

Porous silicon (PSL) prepared by the electrochemical etching of p-type Si wafer shows red photoluminescence (PL) and an asymmetric ESR signal. Exposure to heat filtered white light (LS) decreases PL when PSL is in air. In \approx10^-1 torr He, however, PL intensity first increases for short exposures (exposure time t_x \le 2700s) and then decreases to almost zero for long exposures(t_x \ge 5400s). The increase (decrease) in PL intensity is always accompanied by a decrease (increase) in dangling bonds, measured by ESR. A layer of polystyrene (PS) on PSL decreases ESR by about 50% and PL by about 30%. Further, the PL peak shifts towards blue. Interestingly, PS coated samples do not show any significant change in PL and in ESR even after 16 hrs of LS. This suggests that PS not only protects PSL from ambient air but also changes the electronic structure of PSL surface. These electronic changes are such that LS is unable to increase ESR. This may be responsible for the improved stability of the PSL layers against LS, after coating with PS.

[R1.218] Continuous-Wave Terahertz Spectroscopy of Biomolecules

Terahertz (THz) radiation, lying between 0-300 cm-1, is ideally suited to study the large-amplitude, low-frequency vibrations of biomolecules that are often critical to the function of the biomolecule. In an effort to investigate such motions, we have recorded THz spectra of biomolecules in polyethylene matrices using continuous-wave (cw) THz radiation. The cw THz radiation is generated by pumping a solid-state photomixer with two cw, near-infrared laser sources. THz spectra of biotin and the tripeptides alanine-alanine-alanine, alanine-glycine-alanine and glycine-alanine-glycine have been obtained at 4.2 K and room temperature. At 4.2 K, all of the spectra display a small number of discrete absorptions. At room temperature, the tripeptide samples show considerable broadening, whereas, biotin exhibits relatively little broadening. Vibrational anharmonicity is used to model the line shapes that result from changes in vibrational state populations. In addition, molecular modeling is used to interpret and understand the spectra.

[R1.219] Comprehensive characterization of DC performance of n-p-n GaN-based bipolar transistors

Poor ohmic contacts and high leakage current paths between terminals demand extra caution in characterizing GaN-based bipolar transistors properly and extracting useful information on intrinsic device performance. In the literature, interpretation of device performance varies widely. In some reports only common base operation was achieved [1]. Frequently common emitter characteristics and Gummel plots were reported at low current levels [2] and occasionally they were not consistent [3]. We find that in our devices common emitter I-Vs generally agree with Gummel plot and diode/leakage characteristics, and that device performance can be categorized to three rather distinct regions: 1) low current region dominated by base-collector leakage currents, 2) intermediate current region, manifesting base-emitter recombination current/leakage current and current gain flattening, 3) high current region, degraded by self-heating, possibly the Webster and Kirk effect. Devices analyzed in this work were fabricated using emitter regrowth technique with an emitter size of 20 \times 50 \mu m^2. [4] We find that owing to high leakage path coupled with poor terminal contacts, the extrinsic current gain bEXT measured at a low current level can be erroneously attributed to the gain of the intrinsic transistor \beta_INT, which is generally much lower. As the current is increased, the effect of leakage currents is diminished, and \beta_EXT \rightarrow \beta_INT. This model has been satisfactorily applied to explain both n-p-n and p-n-p bipolar transistor performance. In the intermediate current region we employed a 1-D temperature-dependent short emitter and long base model. The simulation shows that the electron minority carrier diffusion length L_nB or lifetime is the bottleneck in limiting \beta. A value of L_nB = 0.32 \mu m is deduced from our devices. The temperature influence on current gain is also found to be substantial for Al_xGa_1-xN/GaN HBTs with x_Al < 5. In the high current region our devices generally show a rather sharp drop in \beta. Calculation of emitter current density distribution along the emitter width shows that the devices are indeed limited by the Kirk effect, accompanied by other high-injection effects. In short, we have carried out extensive study on current gain of GaN-based bipolar transistors, which provides insight in proper interpretation of device performance and device design.

[1] Cao, X.A. et al., Solid-State Electronics, 44 (2000) 649-54 [2] Kumakura, K. et al., Applied Physics Letters, 80 (2002) 3841-3 [3] Shelton B. S. et al., IEEE Transactions on Electron Devices, 48 (2001) 490-4 [4] Limb, J. et al., Electronic Letters, 35 (1999) 1671-2

[R1.220] Increasing the Size of a Piece of Popcorn

Popcorn is an extremely popular snack food in the world today. Thermodynamics can be used to analyze how popcorn is produced. By treating the popping mechanism of the corn as a thermodynamic expansion, a method of increasing the volume or size of a kernel of popcorn can be studied. By lowering the pressure surrounding the unpopped kernel, one can use a thermodynamic argument to show that the expanded volume of the kernel when it pops must increase. In this project, a variety of experiments are run to test the validity of this theory. The results show that there is a significant increase in the average kernel size when the pressure of the surroundings is reduced.

[R1.221] Growth behavior of carbon nanotubes on multilayered metal catalyst film (Al/Fe/Mo) in chemical vapor deposition

The temperature- and time- dependences of carbon nanotube (CNT) growth by chemical vapor deposition are studied using a multilayered Al/Fe/Mo catalyst on silicon substrates. Within the 600 - 1100 ^oC temperature range in these studies, narrower temperature ranges were determined for the growth of aligned multi-walled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs). Aligned MWCNT growth is favored at lower temperatures (\sim700 ^oC). At 900 ^oC, in contrast to earlier work, double-walled carbon nanotubes (DWCNTs) are found more abundant than SWCNTs. At further elevated temperature, highly defective carbon structures are produced. Defects also are found to accumulate faster than the ordered graphitic structure if the growth of CNTs is extended to long growth durations. Atomic force microscopy, field emission scanning electron microscopy, high resolution transmission electron microscopy, and Raman spectroscopy are used to characterize the catalyst and various types of CNTs.

[R1.222] Formation of in-situ CNT junction by direct lateral growth

We present an approach to form a reliable integration of carbon nanotubes via direct parallel growth method. The method involves in-situ growth of carbon naotubes to bridge predefined junction electrodes of Nb/Co(or Ni), and furthermore, a high degree of ordering parallel suspended nanotubes can be obtained by applying DC bias during the growth. The arrays with robust contacts are unique system for explorations of collective behavior in coupled systems, and are useful for applications in nanoelectronics and NEMS.

[R1.223] Growth and characterization of ZnO nanowires and wire on wire

ZnO nanowires were grown successfully on silicon (001), GaAs (001), and c-plane sapphire substrates by using vapor-liquid-solid epitaxy (VLSE). The substrate was first coated with thin gold film, then together with ZnO and graphite powders, were placed in a Al2O3 boat and was put in a furnace. Constant Ar gas flow through the furnace and the furnace was heated to 920 oC .ZnO nanowires with diameter as small as 30nm could be obtained. The samples all had very good photoluminescence properties, with a sharp bandedge-related emission at around 3.35eV and broad defect-related peak at around 2.5 eV at T= 20 K. The SEM image indicated that the ZnO nanowires grown on GaAs is the thinnest and the nanowires grown on Si are widest of the three. The PL results showed the relative intensity between bandedge-related PL intensity and the defect-related PL intensity from ZnO nanowire are highest for nanowires grown on Si, followed by sapphire and GaAs. By using the sample with nanowires on it as substrate, after depositing gold and using VLSE growth again, we were able to grow ZnO wires on ZnO wires. From SEM measurement, it could be seen the new wires were grown outwardly along six faces of the original hexagonal nanowire. The PL measurement reveals that the wire on wire structure has a defect emission band that is broader than the original nanowire.

[R1.224] Elastic Properties of Lithographically Prepared Polymeric Nanostructures

In the semiconductor industry, it is critical to understand if nanostructuring changes the physical properties of a material appreciably from their bulk values. For example, deep UV lithography is used to print the latent image of a structure into a thin photoresist film. An aqueous solvent then dissolves the exposed region of the film, leaving a nanostructured polymer behind. Later this nanostructure is transferred into the silicon through a reactive ion etch. While the nanostructured polymer is temporary and sacrificial, its mechanical properties are of paramount importance to the quality of the final structure. There are capillary forces from the aqueous developer that can deform or collapse neighboring structures if the dimensions are small enough. To avoid this buckling or collapse, chip designers need materials properties such as the elastic modulus and it crucial if reduced dimensions affect the modulus of the nanostructured polymer. In this work we use Brillouin scattering to quantify the acoustic modes in polymer thin films and nanostructured parallel lines. In the films we see the traditional surface (Rayleigh) and longitudinal guided modes, and these can be used to estimate the bulk and the shear modulus of the polymer film. In the nanostructured parallel lines, we observe the same modes and an additional broad mode that is ascribed to a breathing mode of the line. We study this breathing mode as a function of line thickness in both the lithographically prepared polymeric structures as well as corresponding structures once they transferred into the silicon oxide substrate via a reactive ion etch. The possibility of using these modes to extract reliable mechanical properties is discussed in detail. Temperature variations of the Rayleigh modes in the films and in the nanostructured are also analyzed.

[R1.225] Doping dependence of bose condensation energy and correlations with spectral peak intensity and superfluid weight in high T_c cuprates

Based on our recent holon-pair boson theory of the t-J Hamiltonian (Phys. Rev. B 64, 052501 (2001)) we report the doping dependence of the bose condensation energy, superfluid weight and spectral peak intensity. We find a universality of doping dependence in these physical quantities, by equally showing an arch shape in the variations of their magnitudes with the hole doping concentration. We find that all of these physical quantities scale well with the positive charge carrier (hole) density x, but not with the electron density 1-x for the entire range of hole doping. It is shown that the doping dependence of the condensation energy U at T = 0 K is given by the relation, U(0) \approx \alpha x^2 |\Delta_0|^2 with \Delta_0, the pairing gap at 0 K and \alpha, a constant.

[R1.226] Surface morphology and nanostructure growth characterization of GaAs(331), (311), and (711)

The stability of surfaces, atomic reconstruction, and energy of formation of high index surfaces of GaAs has been emphasized during the last few years due to its potential for basic research and technological needs. In this investigation, several orientations of GaAs double side polished wafer have been used. The experiments have been performed in a molecular beam epitaxy (MBE) combined with an in situ scanning tunneling microscopy (STM) in order to illustrate the atomic structure of the surface of the wafer in the real space. During the growth, reflection high-energy electron diffraction (RHEED) was used to indicate the quality of the surface of the wafer. GaAs (331), which is between the (110) and (111) orientations, is the first surface observed in this study. GaAs (331) A (B) is faceted into (110) and (111) A (B) surfaces along the [-110] direction. The growth interruption and the annealing the surface can affect the uniformity of the facets on the both A and B surfaces. It has been observed that the (111) surface of each facet is preferred for the nucleation of “ribbons”. The second surface, which has been studied is GaAs (311). In addition to observing an (8x1) reconstruction on GaAs (311), two new reconstructions have been observed on GaAs (311). A (4x1) reconstruction was observed on GaAs (311) A and a (2x1) reconstruction on GaAs (311) B. The third surface reported in this study is GaAs (711). GaAs (711) is not a stable surface and we observed it to transfer to a stepped (100) and stepped (511) surface depending on if the growth condition was Ga or As rich with the step flow reversed in direction between the two cases. These results can help to better understand the GaAs surface and to design its use for the growth of quantum wells, wires and dots.

[R1.227] The Use of Interactive Technologies to Improve Student Learning of Physics from Middle School to College

The Physics Department at Randolph-Macon Woman's College, a liberal arts women's college of 720, has traditionally turned out approximately 0.6 majors/year. We have invigorated the program by adding community (e.g. SPS, physical space, organized activities), adding a significant technical component (e.g. web-assisted and computer interfaced labs and more technology in the classes [1]), and incorporating new learning techniques (JITT, Physlets, Peer Instruction [2], Interactive DVD’s, and using the Personal Response System [3]). Students have responded well as evidenced by significant increases in enrollments as well as strong scores on the FCI. As an offshoot of this original project supported by the NSF, we have applied some of these teaching methods to teach younger children and teachers of younger children. In this presentation, we will discuss the implementation of the new curricular developments and the specific changes we have seen and hope to see in student learning.

[1] This work is supported in part by the NSF CCLI Program under grant DUE-9980890. [2] See, for example, the project Galileo website http://galileo.harvard.edu for a description of all of these techniques. [3] The Personal Response System is a wireless response system made by Educue, www.educue.com.

[R1.228] Architecture and Development of DCMP Web Site

The multi-tier implementation of DCMP Web site is discussed. It is based upon newly developed PHP technology. The technology allows for creating dynamic content and scalable solutions for Web site capabilities. There are several aspects as to what type of information is to be on the site. First, it should serve the immediate needs of the researchers in the field, namely, conferences, journals, news, funds, etc. This is currently available on the site, but can be extended and improved if needed. Second, the site will reflect the connection between Condensed matter physics and the technological breakthroughs that drive the economy. Third, the site will carry an educational mission helping educate the general public, and on the other hand, help young people to start their careers in the field. The content of the DCMP Web site is under active development. It depends upon wide involvement of DCMP members.

[R1.229] Effect of Boundary Curvature and Local Slip in Nanofluidic Shear Flow

Continuum and molecular dynamic simulations have shown that molecularly rough surfaces give rise to a reduction in the effective slip length. In particular, implementation of a local no-slip condition at the liquid-solid interface has been shown to produce a negative slip length, leading to a shift of the apparent boundary into the flow domain. Using molecular dynamics simulations, we have investigated the behavior of a Lennard-Jones fluid near a sinusoidal boundary allowing for the possibility of local slip. For long wavelengths of the substrate modulation, we recover the predictions for the effective slip length obtained by hydrodynamic calculations [1]. As the wavelength is reduced, the effect of the boundary curvature becomes more pronounced leading to a sharp decrease in the effective slip. These simulations systematically probe the transition from the hydrodynamic regime to flow behavior in the presence of molecular scale roughness. Funding from the NSF CTS00-88774 is gratefully acknowledged.

[1] D. Einzel, P. Panzer and M. Liu, PRL, Vol. 64, 2269 (1990).

[R1.230] Correlating Structure and Crystallization Conditions for Ethylene Copolymers by Real-Time SALS

The melting behavior of a homogeneous ethylene-butene copolymer (M_w = 70,000 g/mol; \rho = 0.90 g/cm^3; 6.4 mol % comonomer) is studied by the simultaneous measurement of transmitted light and small angle light scattering (SALS) under cross polarized (H_V) and parallel polarized (V_V) optical alignments. Measurements were performed on samples crystallized at several temperatures ranging from room temperature to 110 ^oC (16 ^oC above the peak melting temperature (94 ^oC) of quenched samples) for various crystallization times (1 hr to 72 hrs). We show that the final melting temperature (T_m^f) can be determined from our SALS measurements. T_m^f values we obtain from H_V and V_V SALS measurements are in good agreement with values from differential scanning calorimetry (DSC) and small angle x-ray scattering (SAXS) for samples with the same thermal history. From SALS we are able to evaluate how the crystallization conditions determine the segregation between the first forming and latter forming crystals and, the number of crystal populations observed by DSC. We are also able to determine the time and temperature dependence of the fraction of the first forming crystals. We discuss the implications of our results for understanding how chain microstructure and crystallization conditions affect the structure of copolymers of ethylene and \alpha-olefins.

[R1.231] Molecular Dynamics Simulation of the SH3 Domain Aggregation Suggests a Generic Amyloidogenesis Mechanism

We use molecular dynamics simulation to study the aggregation of Src SH3 domain proteins. For the case of two proteins, we observe two possible aggregation conformations: the closed form dimer and the open aggregation state. The closed dimer is formed by ``domain swapping'' --- the two proteins exchange their RT-loops. All the hydrophobic residues are buried inside the dimer so they cannot further aggregate into elongated amyloid fibrils. We find that the open structure --- stabilized by backbone hydrogen bond interactions --- packs the RT-loops together by swapping the two strands of the RT-loop. The packed RT-loops form a \beta-sheet structure and expose the backbone to promote further aggregation. We also simulate more than two proteins, and find that the aggregate adopts a fibrillar double \beta-sheet structure, which is formed by packing the RT-loops from different proteins. Our simulations are consistent with a possible generic amyloidogenesis scenario.

[R1.232] Ab-initio description of domain walls in permalloy

In terms of the fully relativistic Screened KKR-method domain walls corresponding to 180 Deg. und 90 Deg. domains are described theortically on an ab-initio level. In particular eventual splitting of 180 Deg. domains into 90 Deg. domains are discussed. These calculations are quite some importance for a discussion of domain wall resistivities, see also session G29.

[R1.233] Field-Theoretic Simulations of Polymer Solutions Under Confinement

We used field-theoretic simulations to study the equilibrium behavior of a polymer solution under good solvent conditions confined to a slit of width L. In particular, we obtained the density profiles across the slit for different values of the monomer excluded volume over a wide range of concentrations C. We also obtained mean field results for the density profiles. The effective correlation length \xi_eff was extracted from the density profiles and compared to the mean field result (valid in the limit of high concentrations). For small excluded volume parameters we found that \xi_eff is well described by the mean field result for all concentrations. For larger excluded volume parameters the correlation length exhibits a C^-3/4 scaling behavior for intermediate concentrations, which is compatible with the behavior expected for this system in the semi-dilute regime.

[R1.234] Application of the renormalization group method in wireless market intelligence

We use a renormalization group method, similar to that developed for random spin chains, to infer information about the layouts of cellular wireless networks.

[R1.235] Drying and Cracking of Nanoparticle Suspensions

Directional drying drives a sol-gel transition in aqueous suspensions of silica nanoparticles. Under a variety of conditions, a striking array of evenly-spaced parallel cracks invades from the drying edge. We measure the dynamics of gelation and cracking by analyzing digital images obtained with bright field microscopy. We image the distribution of water in a drying film with CARS microscopy. Our findings suggest that capillary forces drive the flow of water from the bulk to the drying edge, calling into question existing models of drying and cracking in these systems.

[R1.236] Bandstructure and Charge Density Waves in NbSe_3

I report on new bandstructure measurements of the linear-chain inorganic conductor NbSe_3. Experimental bandstructures are obtained from high-purity undoped samples with angle-resolved photoemission spectroscopy. NbSe_3 is known to carry two independent charge density waves (CDW's) and it is thought that fluctuation effects are enhanced due to the one-dimensional nature of the material. I find no evidence of such fluctuation effects, as the measured valence bands remain metallic to within ten Kelvin of the higher transition temperature. From the band crossings, Fermi contours have been extracted. These are compared to the Fermi surface predicted by a density functional theory calculation. Signatures of the CDW transition - the Peierls structural distortion, modification of the valence bands, and the destruction of some of the Fermi surface - are also discussed.

[R1.237] CHEMICAL BONDING AND TOTAL ENERGY CONFORMATIONAL ANALYSIS OF THE ORGANIC SUPERCONDUCTOR k-(BEDT-TTF)2Cu[N(CN)2]Br.

The two-dimensional charge-transfer organic salt k-(BEDT-TTF)2Cu[N(CN)2]Br reveals a complex and intricate crystalline structure. This is a common feature of a large family of organic superconductors. The magnetic characterization of k-(BEDT-TTF)2Cu[N(CN)2]Br shows changes in the superconducting order parameters (Tc and l) as a function of the cooling rate. This behavior was attributed to an impediment in the normal dynamics of the structure. It seems that the changes in the order parameters are due to a crystal blocking effect in the arrangement of the “staggered” ethylene groups at the end of the BEDT-TTF molecule. Preliminary quantum molecular simulations suggest an energy difference between the electronic energy of the “staggered” and the “eclipsed” molecular conformations. However, the order of magnitude of such energy difference requires further investigations. In this poster we discuss our recent results in this problem. Particularly, we focus into the chemical interaction between the BEDT-TTF molecule and the Cu[N(CN)2]Br chain as well as a comparative analysis between the predicted vibration spectra for the conformers.

[R1.238] The Growth of Microcrystalline Silicon and Silicon-Germanium Thin Films using ECR-PECVD

A high rate growth method of hydrogenated microcrystalline silicon and silicon-germanium has been developed with very low hydrogen dilution ratio, using a remote ECR-PECVD process. The key variable was the hydrogen dilution, the ratio of hydrogen to silane, ranging from 3.3 to 10, adding helium systematically. Phase transition from amorphous to microcrystalline states was observed as the amount of added helium varied. Hydrogenated microcrystalline silicon films with more than 70 % of crystalline volume fraction were formed at growth rates of 3.2 Åsec at low substrate temperature below 300 ^oC with a hydrogen dilution ratio as low as 3.3. The structural, electrical and optical properties, by Raman shift, x-ray diffraction, dark and photo conductivity, activation energy, and photosensitivity measurements, were investigated to grow good quality microcrystalline films at the low hydrogen dilution ratio with high growth rates.

[R1.239] XANES Data on Trace Quantities of Iron in Hydroxyapatite Structures

Trace elements such as iron are of interest in both biologically and geologically formed apatites. They are thought to occupy substitutional sites at the concentration of about 200 ppm. Most likely metal atoms replace the calcium atom in one of \textbftwo non-equivalent calcium sites. The inorganic mineral structure hydroxyapatite (which comprises 30% of human and animal bone) consists of Ca_5(OH)(PO_4)_3 in a hexagonal crystal structure designated in Herman-Maugin crystallography notation as P63/m or as Number 176 in the International Tables of Crystallography (ITC). Hydroxyapatite formed under geological conditions has the same crystal structure. Hydroxyapatite can also be fabricated synthetically, but has limitations in terms of crystal growth size. The experimental technique of X-ray Absorption Near Edge Structure (XANES) and X-ray Absorption Fine-structure Spectroscopy (XAFS) were used to evaluate the oxidation state of iron. Data was taken at the X-9B line at the National Synchrotron Light Source at Brookhaven National Laboratory.

[R1.240] Shot Noise Suppression in Photodiodes Associated with Multiplication Gain

We identify a new shot noise suppression mechanism in avalanche photodiodes (APDs) associated with multiplication gain. The shot noise is suppressed due to the history dependence of impact ionizations in thin \sim0.1\mum or heterostructure APDs. The shot noise of an APD is S(f)=2eIMF(M)\gamma\Deltaf, where e is the electron charge, I is the amplified photo current, M the average multiplication gain, F(M) the so called excess noise factor associated with the variance of the gain of individual carrier, \Deltaf the bandwidth, and \gamma is the Fano factor which has so far been assumed to be 1 in the literature. While the ``traditional'' local-field APD theory, which assumes that the impact ionization process is history-independent (i.e., Markovian), is a good model for uniform, thick APDs, it is not appropriate for thin or heterostructure APDs where impact ionizations are strongly history dependent and new generations of carriers are temporally correlated. Using a Monte Carlo simulation we demonstrate that for a thin Al_0.6Ga_0.4As/Al_0.2Ga_0.8As heterostructure APD, \gamma at low gains (M=2-4) is suppressed to \sim0.7 and restores to a value of 1 at higher gains. We thus reconcile the apparent conflict between the measured F(M)<1 in some experiments and the requirement (by definition) that F(M) be \geq1. This shot noise suppression mechanism is independent of known mechanisms such as electron-electron interaction, Pauli exclusion principle, or tunneling current across potential barriers.

[R1.241] Crossover from Anomalous to Conventional Antiferromagnetism in Pd-Doped UPt_3 Studied via Cantilever Magnetometry

UPt_3 exhibits an anomalous form of antiferromagnetism, which has been speculated to be fluctuating in time rather than static. Substitution of Pd for Pt, however, is known to induce conventional AFM behavior. In an attempt to confirm the existence of an AFM quantum critical point in U(Pt_1-xPd_x)_3 at x_C = 0.006, as recently determined by \muSR measurements, [1] we have made high-sensitivity measurements of the magnetization on single and polycrystals of that system using cantilever magnetometry over the temperature range 0.4 \le T \le 8.0 K and in magnetic fields up to 6 T. While the AFM transition is readily observable in samples with 0.014 \le x \le 0.050, we find no evidence for the transition at x \le 0.010, in contrast to \muSR measurements which clearly signal entry into the conventional AFM state for 0.007 \le x \le 0.050. Based on these results we are unable to rule out a scenario whereby anomalous, possibly time fluctuating, AFM at low Pd concentrations evolves gradually into the conventional AFM state at higher concentrations. We are now studying the temperature-dependence of the magnetization and susceptibility at low temperatures to look for alternate signatures of the quantum critical point.

[1] A. de Visser et al. Phys. Rev. Lett. 85 (2000) 3005.

Supported by the Petroleum Research Fund of the American Chemical Society, Sigma Xi-Scientific Research Society GIAR Award

[R1.242] Annealing behavior of the Hf/Si(001) system

Previously refractory metal silicides were investigated for potential use as ohmic contacts and interconnects in the manufacture of integrated circuits. More recently formation of a silicon- rich hafnium silicide, and its subsequent oxidation, appeared as a promising route to production of an amorphous, high-k dielectric. We have investigated the formation and morphology of hafnium silicide films on Si(001) using XPS and AFM. Hf has been evaporated on clean Si(001) to yield a Hf/Si(001) system. To form HfSi_2 by solid state diffusion, the Hf/Si(001) system has been annealed at 1000^oC . Upon annealing, XPS indicates the formation of HfSi_2 and AFM shows coarsening of island features on the surface. The average grain size HfSi_2 has been found to be 0.3\mum.

[R1.243] Analysis of Hard and Soft Spheres in one Dimension

We examine the phase transition that occurs in various one dimensional granular systems. We observe a phase transition in which the bottom layers begin to form a solid-like region while a fluid-like region remains on top. The solid regime begins to form once a certain critical temperature is reached. We used two types of computer simulations to test our theoretical model. We determine the first order nature of the phase transition and verify the functional form of the density of the system.

[R1.244] Structural Recovery in a Model Epoxy: II Physical Aging Responses after Temperature and Relative Humidity Jumps

One cause of the long-term dimensional changes in glassy polymers is the gradual evolution of the viscoelastic behavior through aging processes in the glassy state. Many applications involve changes in relative humidity (RH), under which the materials exhibit aging processes that may differ from those in constant RH conditions. Work done by Zheng and McKenna has shown that the dilatation response after relative humidity jumps has a similar phenomenology to, but different kinetics from, that obtained in temperature jumps. In the present work creep tests are performed during structural recovery of the sample after RH and T-jumps using Struik’s sequential loading protocol. The tests include intrinsic isopiestics, memory effect, and asymmetry of approach, which are indirectly represented by the shift factors of the retardation times. The creep compliances as a function of time were analyzed by the KWW function to estimate the retardation time \tau. We will also describe similarities and differences in creep responses in RH and T-jump experiments.

[R1.245] Growth and field-effect-transistor fabrication of ZnO grown by laser ablation

We report on the growth of ZnO layers on various kinds of substrates using laser ablation and on the fabrication of field-effect transistors (FETs) utilizing either silicon dioxide or ZnMgO as the gate insulators. ZnO layers were deposited on glass, silicon, or sapphire substrates. Depending on the substrates, the growth temperature was varied between 200 and 550^oC. The oxygen partial pressure during growth was varied between 50 and 800 mTorr. We also introduced a post-growth thermal-annealing (TA) process in oxygen ambient in order to improve both the the optical and the electrical properties of the ZnO layers. While as-grown layers on sapphire show electron concentrations in mid-10^18 cm^-3 range, the annealed samples show one order of magnitude lower electron concentrations with higher mobility values indicating the effectiveness of the TA process for improving the quality of the ZnO layers. We fabricated long-channel FET devices on ZnO layers grown on glass and sapphire substrates. Devices on both substrates showed FET operations. We will present structure and substrate dependence of the device characteristics.

[R1.246] Analysis of dynamic hydrogen (H2) generation

The focus of this research is on-demand hydrogen generation for applications such as electric vehicles and electric appliances. Hydrogen can be generated by steam reformation of alcohols, hydrocarbons and other hydrogen containing complexes. Steam reformation can be represented as a simple chemical reaction between an alcohol, commonly methanol, and water vapor to produce hydrogen and carbon dioxide. A fuel cell can then be employed to produce electrical power from hydrogen and air. Numerical and experimental techniques are employed to analyze the most appropriate reforming fuel to maximize H2 yield and minimize by-products of which carbon monoxide is the most harmful

[R1.247] Statistics of conductance of one-dimensional disordered samples

We use the tight-binding Anderson model to study the statistical transport properties of one-dimensional disordered potentials. For completely random (white-noise like) potentials the distribution function P(T) of the transmission coefficient T_L of a sample of length L has been thoroughly studied. Two qualitatively different regimes of transport, ballistic (L << l) and localized (l << L), leads to two different distribution functions for conductance - Gaussian and log-normal correspondently. Here l is the localization length. Disordered potentials with long-range correlations give rise to existence of a mobility edge at some energy E_c that separates extended and localized states.^1 We show that in the vicinity of the mobility edge the statistical distribution of conductance exhibits a sharp transition and calculate analytically and numerically the parameters of the distribution function on both sides of E_c. The method we use is based on the exact mapping of the discrete Schrodinger equation to a two-dimensional classical dynamics of a kicked linear oscillator. We established a direct relation between the statistics of the canonical trajectories of this 2D map and the mean value of T_L.

[1] F.M. Izrailev and A.A. Krokhin, Phys. Rev. Lett. 82, 4062 (1999).

[R1.248] Wave Propagation in Semi-Infinite Photonic Crystal Structures

The problem of electromagnetic (EM) wave propagation through a semi-infinite photonic crystal lies in many photonic crystal (PC) integrated optical structures. For instance, coupling of EM wave into and out of a PC waveguide needs to consider the scattering by an air to semi-infinite PC interface. In a PC waveguide bends and branches, every end can be assumed as a semi-infinite structure. By introducing a semi-infinite PC, one does not need to consider multi-reflection in a finite structure, greatly releasing the numerical difficulty typical to the usual finite-difference time-domain method. To solve the general propagation problem for a semi-infinite PC, we employ the transfer-matrix method on the basis of plane wave expansion. It turns out that the problem is closely related to the eigen-modes of the transfer-matrix for a unit cell of the PC. As examples, we have used this approach to investigate the propagation of EM wave from air into a semi-infinite PC, from a semi-infinite PC into air, and from one semi-infinite PC to another semi-infinite PC. In addition, we also study the dependence of coupling efficiency of a PC waveguide on field profile of the incident wave.

[R1.249] Scanning Tunneling Optical Resonance Spectroscopy

We have modified a scanning tunneling microscope (STM) to interrogate the optical bandgap as a function of position within a semiconductor microstructure. A tunable solid-state Ti sapphire laser with fiber optic coupling was used to illuminate a semiconductor surface beneath a STM tip. The photoenhanced portion of the tunneling current was spectroscopically measured using a lock-in technique that utilized a spatial light modulator on the laser illumination. A comparison of the in-situ STORM measurements on chemical vapor deposited InP epilayers to the bulk values of the optical bandgap as determined by transmission spectroscopy, photoluminescence, and theoretical phenomenological values based on x-ray diffraction will be presented. The application of this technique to nanostructured materials such as nanotubes and quantum dots as well as future microsystems will be discussed.

[R1.250] Molecular nanomagnets as contrast agents for Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is a non-invasive technique used in medicine to produce high quality images of human body slices. In order to enhance the contrast between different organs or to reveal altered portions of them such necrosis or tumors, the administration of a contrast agent is highly convenient. Currently Gd-DTPA, a paramagnetic complex, is the most widely administered compound. In this context, we have assayed molecular nanomagnets as MRI contrast agents. The complex [(tacn)_6Fe_8(\mu_3-O)_2(\mu_2-OH)_12]Br_8·9H_2O^1(Fe_8 in brief) has been evaluated and shorter relaxation times, T_1 and T_2, have been obtained for Fe_8 than those obtained for the commercial Gd-DTPA. No toxic effects have been observed at concentrations up to 1 mM of Fe_8 in cultured cells. Phantom studies with T_1-weighted MRI at 9.4 Tesla suggest that Fe_8 can have potentiality as T_1-contrast agent.

^1Wieghardt K Angew Chem Intl Ed Engl 23 1 (1984) 77

[R1.251] Electrostatic properties of colloidal particles in non-polar media

Charge effects in non-polar colloids are important in printing, electronic displays, and petroleum processing. However, little is understood about the fundamental nature of charge origin, counterion distribution, and interparticle interactions. We present measurements of single particle electrokinetics and interparticle interactions and discuss their impact on emerging pictures of electrostatics in these systems.

[R1.252] The Growth of Microcrystalline Silicon and Silicon-Germanium Thin Films using ECR-PECVD

A high rate growth method of hydrogenated microcrystalline silicon and silicon-germanium has been developed with very low hydrogen dilution ratio, using a remote ECR-PECVD process. The key variable was the hydrogen dilution, the ratio of hydrogen to silane, ranging from 3.3 to 10, adding helium systematically. Phase transition from amorphous to microcrystalline states was observed as the amount of added helium varied. Hydrogenated microcrystalline silicon films with more than 70 % of crystalline volume fraction were formed at growth rates of 3.2 Åsec at low substrate temperature below 300^oC with a hydrogen dilution ratio as low as 3.3. The structural, electrical and optical properties, by Raman shift, x-ray diffraction, dark and photo conductivity, activation energy, and photosensitivity measurements, were investigated to grow good quality microcrystalline films at the low hydrogen dilution ratio with high growth rates.

[R1.253] X-RAY EMISSION AND ISOTOPIC SHIFT DURING ELECTROCHEMICAL LOADING OF HYDROGEN AND DEUTERIUM IN PALLADIUM AND NICKEL

Results from a multi-year, research effort, are summarized. The effort was developed within the framework of a cooperation between ENEA and SRI, and was mainly oriented towards: 1. Performing Electrochemical experiments to investigate the emission of X-rays during electrochemical hydrogen loading of thin metallic films of Pd and Ni, 2. Monitoring the inventory of marker elements in the electrochemical cells, and 3. Replicating experiments involving double structure cathodes, that have been performed previously at SRI and Osaka University, in which heat ^3He and tritium were observed.

[R1.254] Hall Probe micromagnetometry of single grain crystalites from bulk superconducting matrices

Experimental characterisation of the transport properties of grains and grain boundaries in bulk superconductors has long been a key aim in development of superconducting wires and tapes for power applications. While global measurements rely heavily on modelling and interpretation, local characterisation such as magnetic imaging offers greater scope in distinguishing the roles of intragrain and intergrain properties. However, the most direct way to gain information about the grains themselves is to first disconnect the grains from one other by grinding a sample down, then measuring the properties of the resultant powder. Most measuring techniques do not have sufficient sensitivity to characterise a single grain, while measuring the whole powder leads to uncertainties due to grain inhomogeneity and misalignment. Here we demonstrate a relatively cheap and simple micro Hall probe technique that can measure the magnetisation of very small samples (currently down to 10 micron in size) with sensitivities which surpass that of macro techniques such as a squid magnetometer by at least an order of magnitude. A key advantage of our technique over other Hall probe approaches is that the background field is rejected to 1 part in a million, so that high field measurements are easily achieved. Furthermore, the magnetisation may be measured during a continuously swept field so that the dynamics of vortices in single crystalites may also be investigated.

[R1.255] Computer Simulations of Two- and Three-Dimensional Colloid-Polymer Systems

Histogram reweighting Monte Carlo and finite-size scaling methods are used to determine the phase diagrams and colloidal pair correlation functions of confined (two-dimensional and quasi two-dimensional) and three-dimensional neutral colloid-polymer systems. The colloidal particles are modelled as hard spheres and the polymer molecules as hard chains. The polymer-to-colloid size ratio is varied from the colloid limit (large colloidal particles and short polymer) to the protein limit (small colloidal particles and long polymer). Phase diagrams are obtained and the critical points of liquid-liquid coexitence of colloid-rich (polymer-poor) and colloid-poor (polymer-rich) are located for various values of the polymer-to-colloid size ratio.

[R1.256] Coulomb drag effect between two dimension electron gas systems in nanometer separation

The Coulomb drag effect between two 2D-electron-gas (2DEG) systems is studied. The distance d between the two 2DEG systems is considered to be in the order of nanometer. Contrast to the systems with distance d larger than10nm, in which the effective interaction decays exponentially with momentum transferq and becomes negligible for large q near 2k_F , the effective potential between the two systems with smaller distance (in the order of nanometer) becomes prominent and cannot be negligible as the momentum transfer near 2k_F. Besides, it is found that the effective potential between the two systems depends also on the Fermi energy of the two 2DEG systems. In the situation of small distance and/or low Fermi energy, it is found that the interaction between two 2DEG systems does not decay to zero, instead, becomes obviously non-negligible. In fact, the effective Coulomb interaction is found to be prominently important for momentum transfer from q=0 to q \geq 2k_F. Therefore, the contribution of Coulomb drag effect which origins from the Coulomb interaction happens not only at the small momentum transfer near zero momentum transfer but also the large momentum transfer near 2k_f. The Coulomb drag effect depends not only on the strength of the effective Coulomb interaction but also on the intensity of interaction absorption of electron gas which can be described by \left| funcIm\chi \right| ^2 \frac1\sinh ^2(\fracømega 2k_bT). The intensity of interaction absorption near q=2k_F is much stronger than the one near q=0. These phenomena causes the contribution from large momentum transfer becomes much more important as the distance between two 2DEG systems and/or Fermi level of electron gas are small enough.

[R1.257] Magneto-Optical Imaging of High Temperature Superconductors Under Tensile Strain

Magneto-optical imaging utilizes the active Faraday rotation of polarized light in the presence of a magnetic field. The affected light can then be captured using digital imaging technology and provide information on the strength and pattern of field penetration within a sample. This technique, applied to YBCO thin film coated conductors, provides for a visual investigation of sub-millimeter aberrations and fracture defects. The further application of post processing techniques have provided a more quantitative analysis of varying patterns used in the YBCO coated conductor investigations. We apply a theory provided for geometries where the conduction path is perpendicular to the applied field and develop a means for determining the critical current of a micro-bridge sample. We also perform a Maxwell curl calculation on the intensity distribution of various images, which reveal boundaries for current paths via relative screening current determination, and striations that we believe to be precursors for fracture from cooling related strain.

[R1.258] Application of Semi-Definite Programming for Many-Fermion Systems

The ground state energy and other important observables of a many-fermion system with one- and two-body interactions only can all be obtained from the first order and second order Reduced Density Matrices (RDM's) of the system. Using these density matrices and a family of associated representability conditions one may obtain an approximation method for electronic structure theory that is in the mathematical form of Semi-Definite Programming (SDP): minimize a linear matrix functional over a space of positive semidefinite matrices subject to linear constraints. The representability conditions are some known necessary conditions, starting with the well-known P, Q, and G conditions [Claude Garrod and Jerome K. Percus, Reducation of the N-Particle Variational Problem, J. Math. Phys. 5 (1964) 1756-1776]. The RDM method with SDP has great potential advantages over the wave function method when the particle number N is large. The dimension of the full configuration space increases exponentially with N, but in RDM method with SDP the dimension of the objective matrix (which includes RDM's) increases only polynomially with N. We will report on the effect of adding the generalized three-index conditions proposed in [R. M. Erdahl, Representability, Int. J. Quantum Chem. 13 (1978) 697-718].

[R1.259] Adsorption Isotherm Studies of Methyl Bromide on MgO

This research involves the adsorption of methyl bromine and methane onto highly-uniform magnesium oxide powder. Methyl bromide was condensed onto the MgO substrate at temperatures between 175 K and 179 K. The layering behavior of the gas molecules was studied by a series of vapor pressure isotherms, using a high-accuracy, computer-controlled system. The isotherms clearly show first layer formation at all temperatures, followed by a continuous layer growth to saturation. Isotherms will be presented and future work discussed. TJH and TEB research sponsored by the Department of Energy EPSCOR Grant No. DE-FG02-01ER45895. JZL research sponsored by start-up funds from the University of Tennessee - Knoxville and by the Division of Materials Sciences, Office of Basic Energy Sciences, U.S. Department of Energy, under contract No. DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC.

[R1.260] Short-Range Order in Linear Systems

For classical linear systems in the continuum in which the particles interact with nearest-neighbor forces, the pair correlation function at short distances can be expressed exactly in a simple form. Results are given for various hard-core interaction potentials, in which the attractive potential is either constant (square-well potential), linear, or V-shaped. The first form is related to the depletion interaction.

[R1.261] Infrared Active Phonons in the Negative Thermal Expanding Compound, ZrW2O8

ZrW_2O_8 is unusual in that it contracts as it is heated, a phenomenon known as negative thermal expansion. We will present studies of infrared reflectivity and conductivity vs. frequency for a pressed pellet sample of ZrW_2O_8, a compound with cubic lattice structure. Our measurements range from 20 K up to room temperature and from 12 cm^-1 to 5000 cm^-1 encompassing the entire range of the infrared active phonons in ZrW_2O_8. The phonon spectra exhibit considerable temperature dependence, particularly at low frequency. It is of interest to note that our infrared measurements show two regions of strong absorption in frequency ranges where the phonon density-of-states inferred from time-of-flight neutron scattering is very low. We will compare IR results with other measurements and discuss implications for understanding the nature of this unusual compound.

This material is based upon work supported by the National Science Foundation under Grant No. DMR-0071949.

[R1.262] STRUCTURAL PROPERTIES OF LIQUID SODIUM SURFACES FROM AB INITIO MOLECULAR DYNAMICS

We have performed extensive first-principles simulations of the liquid-vapour interface of sodium using ensemble density functional theory molecular dynamics. Our runs were equilibrated at two temperatures (400 K and 500 K) above the melting temperature of sodium (\mathrmT_M=391 K), using unit cells containing \sim 160 atoms, periodic boundary conditions, and starting from two slab geometries that have different cross-sections for the simulation cell. Density profiles at the surface have been found to display oscillations similar to those observed experimentally in metals [eg.~H.~Tostmann, \textitet al., Phys.~Rev.~B, \textbf59 783, (1999)], indicating the formation of ionic layers parallel to the surface. The x-ray reflectivities derived from the simulations show qualitative similarity to those found in experiments. The wavelengths of the ionic oscillations are consistent with the Friedel oscillation wavelength, suggesting that this could be the driving force for the formation of layers near the surface. We have investigated the finite size effect comparing our simulations with classical molecular dynamics runs performed with different slab thicknesses. Other properties examined are diffusion coefficients and in-plane coordination and ordering.

[R1.263] \muSR Study of Magnetism and Magnetic Inhomogeneity in (U,Th)Pt_3

We investigated the onset of static antiferromagnetism in U_1-xTh_xPt_3 for 0 \le x \le 0.05 via zero magnetic field \muSR. For 0.009 \le x \le 0.05 we find the low-temperature AFM ordered state to be described by the same multicomponent depolarization function as used for U(Pt,Pd)_3. However, the transition is quite broad compared to U(Pt,Pd)_3. For x = 0.005 and 0.006, no direct evidence for static ordering is found, but we observe strong depolarization over part of the sample volume. Both results point to magnetic inhomogeneity in the sample. We are currently carrying out materials analysis to determine whether this inhomogeneity results from chemical or structural disorder, or from intrinsic phase-separation.

[R1.264] Transfer of Spectral Weight in MnSi.

Recently we have performed detailed studies of the optical properties of the weakly helimagnetic metal MnSi (T_C=30 K) which have shown some surprising results. In this poster we concentrate in the transfer of spectral weight. As MnSi enters the helimagnetic phase, we observe a red shift of the plasma frequency, which indicates that spectral weight is transferred to high frequencies. This is in contrast with what occurs in other magnetic materials such as manganites, hexaborides and Mn-doped GaAs where the opposite is found. The described phenomenon is related to the helically ordered phase since it disappears in the the presence of a magnetic field larger than 0.6 T where MnSi is a ferromagnet.

[R1.265] Residual Stress and Its Effects on Electronic Properties in Quantum Dots

Quantum dots have been proposed as one of the promising nano devices that have many applications, such as high efficiency solar cells, information storage etc.. They are usually fabricated by growing nano-meter sized materials on various substrates. The differences of materials properties between the quantum dots and the substrate materials create residual stresses inside the quantum dots. The stress also causes defects and micro-structural changes in the nano devices. The electrical, optical, photovoltaic properties and device lifetime can be detrimentally affected.

Numerical modeling methods have been utilized to study the residual stresses and their effects on the electronic properties in quantum dots. A finite element method has been used in the residual stress simulations. Both thermal expansion differences and lattice mismatch between quantum dots and substrate materials are included in the residual stress modeling. The time-independent Schrö dinger equation, which includes the strain-induced potential and the potential caused by the semiconductor heterostructures, has been solved with a finite difference method. The wave functions and the energies of the confined states have been obtained. These allow us to compute the electronic and transport properties of the nano-devices.

[R1.266] The Coulomb drag effect between two dimension electron gas systems in nanometer separation

The Coulomb drag effect between two 2D-electron-gas (2DEG) systems is studied. The distance d between the two 2DEG systems is considered to be in the order of nanometer. Contrast to the systems with distance d larger than10nm, in which the effective interaction decays exponentially with momentum transferq and becomes negligible for large q near 2k_F , the effective potential between the two systems with smaller distance (in the order of nanometer) becomes prominent and cannot be negligible as the momentum transfer near 2k_F. Besides, it is found that the effective potential between the two systems depends also on the Fermi energy of the two 2DEG systems. In the situation of small distance and/or low Fermi energy, it is found that the interaction between two 2DEG systems does not decay to zero, instead, becomes obviously non-negligible. In fact, the effective Coulomb interaction is found to be prominently important for momentum transfer from q=0 to q\geq 2k_F. Therefore, the contribution of Coulomb drag effect which origins from the Coulomb interaction happens not only at the small momentum transfer near zero momentum transfer but also the large momentum transfer near 2k_f. The Coulomb drag effect depends not only on the strength of the effective Coulomb interaction but also on the intensity of interaction absorption of electron gas which can be described by \left| funcIm \chi \right| ^2 \frac1\sinh ^2(\fracømega 2k_bT). The intensity of interaction absorption near q=2k_F is much stronger than the one near q=0. These phenomena causes the contribution from large momentum transfer becomes much more important as the distance between two 2DEG systems and/or Fermi level of electron gas are small enough.

[R1.267] Resistive transitions in non-uniform quench-condensed superconducting Bi films

We report measurements of the resistive transitions of quench-condensed superconducting Bi films. Samples were directly deposited onto both clean glass substrates and over well-oxidized Al ground planes. The transition temperatures (Tc's), as derived from fits to the Aslamazov-Larkin fluctuation theory were found to have significant enhancement for high-resistance (R/square over 300 ohms) films over the ground planes. However a corresponding presence of resistive tails with exponential temperature dependence below Tc indicates that this enhancement is due to morphological differences. For films below 300 ohms, fits were in good agreement with the universal theoretical prediction. Although superconducting Bi is known to be amorphous, these results suggest that in very thin layers the film thickness is not uniform.

[R1.268] Universal theory for the determination of both screw and edge dislocation densities for GaN and related materials using high resolution x-ray diffraction

Abstract : Universal Theory for the determination of both Screw and Edge dislocation Densities for GaN and related materials using high resolution x-ray diffraction.

Simon Bates:: Bede Scientific Inc.

High resolution x-ray diffraction is a powerful tool for the measurement of dislocation density within bulk substrates and epi-layer materials. The broadening of the x-ray diffraction peak along specific directions can be directly related to specific types of dislocations once the instrumental function has been removed. However, until now there has not been a single theoretical approach for modeling all accessible reflections. A self consistent ‘mosaic block’ model of both screw and edge threading dislocations will be presented. The model predicts peak broadening along the omega axis for the symmetric as well as the in-plane asymmetric Bragg reflections driven by screw dislocations and finite correlation length effects. The change in the measured omega peak width as a function of the 2Theta and Omega offset angle allows the direct extraction of the screw dislocation density. Edge dislocations give broadening along the omega axis for out of plane asymmetric Bragg reflections only. By analyzing the change in the omega peak width as a function of the Chi offset allows the extraction of the edge dislocation density. The full model will be presented along with its application to a series of GaN samples with known electrical properties.

[R1.269] Measurements of Protein-Protein Interactions in Solutions by Chromatography

The protein-protein interaction in solutions have generated a great deal of interest among structural biologists since [1] showed a correlation between protein crystallisability and a virial coefficient B2 describing such interaction. The work [1] demonstrated that many proteins crystallize in conditions where the B2 becomes slightly negative, indicating net attractive interactions between protein molecules. We present a new efficient method for extracting second virial coefficients B2 of protein solutions from retention time measurements in size exclusion chromatography (SEC). We measure B2 by analyzing the concentration dependance of the chromatographic partition coefficient. We show the ability of this method to track the evolution of B2 from positive to negative values in lysozyme and bovine serum albumin solutions. Our SEC results agree quantitatively with data obtained by light scattering.

1. A.George and W.W.Wilson, Predicting protein crystallization from a dilute solution property. Acta. Cryst., D50:361--365, 1994.

[R1.271] Advances in x-ray powder pattern data collection and its impact on unit cell indexing

Abstract : Advances in x-ray powder pattern data collection and its impact on unit cell indexing.

Simon Bates:: Bede Scientific Inc.

Indexing of powder patterns is one of the major bottle necks for achievement of full structural solutions from powder data. The largest experimental uncertainty that limits the effectiveness of powder pattern indexing is a precise determination of the measured peak positions. Parallel beam x-ray diffraction optics allows the measurement of peak positions without the influence of any defocusing. When combined with a transmission geometry or +/- reflection geometry, the parallel beam optics allow the measurement of a powder sample using Bonds technique for absolute lattice parameter determination. In Bonds technique adapted from single crystal measurements, the powder pattern is collected from 0 to positive max_2Theta and then from 0 to negative max_2Theta. The folding of the two patterns removes any remaining peak positional errors. For the application of Bonds technique to be most effective, a goniometer with absolute linear positional encoding is required. Data will be presented on a wide range of powder samples covering metals and alloys, minerals and organic materials. It will be shown using Bond’s method that for all material types the absolute peak positions can be determined over the complete measurement range with a positional uncertainty of the order of 0.005 degrees 2Theta. With measured peak positions of this positional accuracy, in most cases no special effort is required to index the full pattern.

[R1.272] Tunneling Studies of Metallic Nanoparticles using a Soft Landing Technique

Nanometer size clusters are produced by a "soft-landing" technique ^1 designed to miminize chemical interactions with the substrate. An ultrathin metal film is deposited on an adsorbed xenon layer held below 50 K in an ultrahigh vacuum environment. The subsequent desorption of the xenon results in the clusters "soft-landing" on the substrate. Prior to the fabrication of these clusters, preformed electrodes are deposited on the substrate using standard electron beam lithographic techniques. These electrodes serve as tunneling electrodes and gates while performing spectroscopic measurements using a cryogenic, ultahigh vacuum scanning tunneling microscope. A complete discussion fo the experimental issues and the latest results will be presented. Supported by Army Research Office Grant, DAAD 19-00-1-0147.

^1 Huang, L., Chey, S., and J. H. Weaver PRL 80 4095

[R1.273] Spin and lattice effects in the Kondo lattice model

The magnetic properties of a system of coexisting localized spins and conduction electrons are investigated via bosonization within an extended version of the one dimensional Kondo lattice model in which electron-lattice and on-site Coulomb interactions are explicitly included. The results show that intrinsic inhomogeneities with the statistical scaling properties of a Griffiths phase appear, and determine the spin structure of the localized impurities. The appearance of the inhomogeneities is enhanced by appropriate phonons and acts destructively on the spin ordering. The inhomogeneities appear on well defined length scales, can be compared to the formation of intrinsic mesoscopic metastable patterns which are familiar in two-fluid systems.

[R1.274] Magnetism in the dilute Kondo lattice model

The one dimensional dilute Kondo lattice model is investigated by means of bosonization for different dilution patterns of the array of impurity spins. The physical picture is very different if a commensurate or incommensurate doping of the impurity spins is considered. For the commensurate case, the obtained phase diagram is verified using a non-Abelian density-matrix renormalization-group algorithm. The paramagnetic phase widens at the expense of the ferromagnetic phase as the f-spins are diluted. For the incommensurate case, antiferromagnetism is found at low doping, which distinguishes the dilute Kondo lattice model from the standard Kondo lattice model.

[R1.275] Transport in three-dimensional metal quantum dot superlattices in self-organized mesoporous silica thin films.

Three-dimensional quantum dot superlattices were synthesized by Ship-In-A-Bottle method with silica mesoporous thin films. The diameter ( 3.8 nm ) and position of Platinum metal quantum dots can be designed by the crystal morphology of silica mesoporous film which have Pm3n cubic symmetry. The diameter and arrangement of dots were strictly controlled by host materials. With the use of conventional semiconductor fabrication process, host mesoporous film can be cut under 500nm range. The I-V characteristics and sidegate dependence were measured with sub-micron contacts to metal superlattices. The observed tunneling current oscillation will be discussed.

[R1.276] Exposing High School Students to High Technology *

SWT is attempting to address two critical areas of concern for Texas: 1) the increasing shortage of home-grown scientists and engineers being educated in American universities, and 2) the continuing underenrollment of women and minorities in these programs. Part of the solution requires engaging high school students in interesting activities relating to materials science and engineering. This provides them concrete, hands-on experiences to explore their own technical interests, which could influence their approaching college/career choices. To accomplish this, SWT has joined with San Marcos High School in just such a program. In the initial efforts, advanced-placement students were brought to SWT for a full day of thin film materials fabrication and electronic device testing. Working in teams, the students fabricated thin film resistors in the SWT Microfabrication Lab. In the Electronics Lab, students characterized their devices. The results of this preliminary effort will be discussed. * Supported in part by a US Dept. of Education Smaller Learning Communities grant.

[R1.277] Orbital Contribution in 5f itinerant antiferromagnet UNiGa_5 and UPtGa_5

UNiGa_5 and UPtGa_5 which are isostructural to the heavy fermion superconductor PuCoGa_5 exhibit itinerant antiferromagnetism with T_N=86 K and 26 K, respectively. Note that the nearest neighbor coupling of magnetic moments within the (0 0 1) plane is different between these iso-electronic compounds. In this work, the itinerant antiferromagnetism of UNiGa_5 and UPtGa_5 were studied in terms of magneto-striction and magnetic form factors.

The neutron diffraction reveals the existence of remarkable magneto-strictions around T_N, indicating the large spin-orbit coupling in this system. The magnetic form factors of both compounds show that the orbital moments are quenched systematically corresponding to their behavior in the magnetic susceptibility. The result indicates the important role of orbital moment on the itinerancy of 5f electron in this system.

[R1.278] Measurement of the Contribution of CO* High Energy Triplet States to the Total Yield of CO(a) in the Dissociative Recombination of CO2+ and HCO+/HOC+ Ions with Electrons

The flowing afterglow technique, in conjunction with absolute spectroscopy, has been applied to the dissociative recombination of CO2+, HCO+, and HCO+. The aim was to determine the radiative cascading contribution from high energy triplet states, mainly CO(a´) and CO(d), to the total yield of the long-lived CO(a) state. In the case of CO2+, the absolute yield of the triplet states was found to be 0.17±0.03. When combined with our previously reported value* of 0.29±0.10 for the total spectroscopic yield of CO(a), this implies that about 59from radiative cascading. A similar, but more complex analysis has been performed on HCO+/HOC+ recombination. The results show that CO(a) appears to be populated mainly by direct formation from recombination of the lower energy isomer, HCO+. The total yield of CO(a´) and CO(d) from HOC+ recombination was found to be 0.76±0.12, indicating that there is little direct formation of CO(a) from HOC+ + e-.

* M. P. Skrzypkowski et al., J. Chem. Phys. 108 8400 (1998)

[R1.279] Mimicking Biological Tissues and Probing Soft Surfaces

Hyaluronic acid (HA) is a linear anionic polysaccharide and is the major component of the extra cellular matrix. It plays an important role as structural constituent of tissues, is attached through receptors to migrating cells and has been found recently to play an important role inside cells. We have developed a bio-mimetic system by anchoring HA films to solid supported membranes through an intracellular HA-binding protein p32. This protein was modified by genetic-engineering so that it could be specifically anchored to fluid supported lipid bilayers. The local HA-film thickness and the surface viscoelastic moduli were measured by analyzing the Brownian motion of colloidal probes hovering over the film. A novel dual-wave reflection interference contrast microscopic technique was developed that enables the measurement of the absolute film thickness with 4 nm resolution and thus allows the establishment of correlations between surface viscoelastic parameters and the film thickness. This technique was applied to study the influence of excess salt and cross-linkers on the film thickness and viscoelasticity of the HA layer. The dual-wave method was also applied to the study of adhesion of vesicles on the ultra-thin HA-layers.

[R1.280] Heat capacity, magnetic susceptibility, and resistivity of delta-Plutonium below 300K

We have measured the heat capacity, magnetic susceptibility and resistivity of alpha Plutonium at temperatures between 10K and 300K and at magnetic fields as high as 14 Tesla. The specific heat which is almost field independent indicates a gamma value of 22 mj/molK and a Debeye temperature of 157K. There are features in the specific heat at 22K and 29K suggesting transitions of an as yet unknown nature. The magnetic susecptibility has been measured between 75K and 300K and is almost temperature independent.

[R1.281] Interface strain effects on superconductivity of YBCO ultra thin film

Ultra thin cuprate superconducting films have many physical properties which are different from those of the thicker counterparts because of the lattice mismatch induced strain that persists till the thickness is above a critical value. While many previous reports on strain effects do exist, the exact cause of the thickness dependence of superconductivity remains inconclusive without having strained free samples as a control. This work compares two sets of samples with one containing interface strain and the other strain relieved. The strained samples were grown and measured as is while the equally thick strained relieved samples were obtained through step by step thinning, using ion etching. The strain effect was confirmed by X-ray diffractometry while the change in Tc was utilized as a judge of oxygen content or disorder. We also used angle resolved XPS to study the depth profile of strain through chemical shift. We will discuss the role interfacial strain plays in relation to oxygen order-disorder transition and its consequence to superconductivity.

[R1.282] Texas Intense Positron Source (TIPS)

The Texas Intense Positron Source (TIPS) is a state of the art variable energy positron beam under construction at the Nuclear Engineering Teaching Laboratory (NETL). Projected intensities on the order of the order of 10^7 e+/second using ^64Cu as the positron source are expected. Owing to is short half-life (t1/2 ~ 12.8 hrs), plans are to produce the ^64Cu isotope on-site using beam port 1 of NETL TRIGA Mark II reactor. Following tungsten moderation, the positrons will be electrostatically focused and accelerated from few 10’s of eV up to 30 keV. This intensity and energy range should allow routine performance of several analytical techniques of interest to surface scientists (PALS, PADB and perhaps PAES and LEPD.) The TIPS project is being developed in parallel phases. Phase I of the project entails construction of the vacuum system, source chamber, main beam line, electrostatic/magnetic focusing and transport system as well as moderator design. Initial construction, testing and characterization of moderator and beam transport elements are underway and will use a commercially available 10 mCi ^22Na radioisotope as a source of positrons. Phase II of the project is concerned primarily with the Cu source geometry and thermal properties as well as production and physical handling of the radioisotope. Additional instrument optimizing based upon experience gained during Phase I will be incorporated in the final design. Current progress of both phases will be presented along with motivations and future directions.

[R1.283] Field Transformation in Irregular Waveguides

Periodic photonic crystals have been used to guide light by introducing defect states. Here, we introduce another class of waveguide structures having irregular elements that we discovered from optimization solutions. We have considered conducting wall waveguides with step-wise variations in width. A multi-resolution optimization strategy was used to solve the inverse problem,given a desired wavelength-dependent scattered field. We have found structures that exhibit virtually perfect field transformation with remarkable transformation functionality. Among these, we achieved a structure that results in a frequency-dependent transformation that allows an incident mode at two closely spaced frequencies to undergo different and almost perfect transformations in an element that is just several wavelengths in dimension. This result, which suggests a wavelength division multiplexing application, has now been verified in a microwave experiment. In another structure with a dimension of two wavelengths, at the same frequency, one mode was totally reflected while the other totally transmitted, suggesting applications in a laser cavity mirror for mode control. We believe that these functionalities were possible because of the degrees of freedom in the irregular waveguide structures and the evanescent field content generated. Realization at optical wavelengths requires nanometer features. We have developed a fabrication approach using electron beam lithography and lift-off that should allow a 1.55 micron device to be achieved.

[R1.284] Multi-layer Parallel Beta-Sheet Structure of Amyloid Beta peptide (1-40) aggregate observed by discrete molecular dynamics simulations

New evidence shows that oligomeric forms of Amyloid-Beta are potent neurotoxins that play a major role in neurodegeneration of Alzheimer's disease. Detailed knowledge of the structure and assembly dynamics of Amyloid-Beta is important for the development of new therapeutic strategies. Here we apply a two-atom model with Go interactions to model aggregation of Amyloid-Beta (1-40) peptides using the discrete molecular dynamics simulation. At temperatures above the transition temperature from an alpha-helical to random coil, we obtain two types of parallel beta-sheet structures, (a) a helical beta-sheet structure at a lower temperature and (b) a parallel beta-sheet structure at a higher temperature, both with inter-sheet distance of 10 A and with free edges which possibly enable further fibrillar elongation.

[R1.285] Magneto-transport of Antimonide-Based Compound Semiconductor Structures

We have studied the magneto-transport properties of antimonide-based compound semiconductor quantum wells. This 6.1-A family of semiconductors includes InAs and AlSb, whose significant conduction band offset (approximately 2 eV) affords great flexibility in their bandgap engineering. The samples under investigation consist of a twenty-period InAs/AlSb superlattice on a GaAs substrate, with an additional single InAs quantum well of varying width. For investigations involving further-reduced dimensionality, we are gating the structures, using silicon oxide as a gate insulator on the surface of the GaSb capping layer in order to minimize gate leakage. These extended studies use electron-beam lithography to define the split gates, thereby electrostatically creating one-dimensional constrictions.

[R1.286] Magneto-Optical Observations of YBCO (110) thin films

Magneto-optical imaging (MOI) method uses Faraday rotation of an incident light beam in Bi doped iron garnet to form images of a spatially varying magnetic field. While MOI has been widely used to study YBCO superconducting thin films, most were conducted on c-axis oriented samples on which vortex current circulates along the ab-planes, as is generally true for Abrikosov vortices with external magnetic field applied along the sample normal. A natural question to ask then is ¡°what if these vortex currents are to flow across the ab-planes¡±, as is the case for a-axis or (110)-oriented samples which has c-axis aligned on the film surface while the external field is still along the normal to the surface. We have observed unique vortex patterns, in contrast to those of the c-axis films, which resemble what¡¯s observed as the Josephson vortices on Bi-2212 samples under a tilted external field. We will present the experimental details and our efforts to interpret them.

[R1.287] Mode Analysis for Ultracold Plasmas

We model ultracold plasmas created by laser photoionization. Numerical simulations indicate that the plasmas become stable after the initial expansion. Using initial conditions from experiments and simulations, we use fluid transport equations to analyze modes of the electron plasma.

[R1.288] Infrared Absorption Spectroscopy of Ferromagnetic (In,Ga,Mn)As

(In,Ga,Mn)As is a new type of magnetic semiconductor which relatively little is known about*. It is half way between two better known magnetic semiconductors, (Ga,Mn)As and (In,Mn)As. Compared to (In,Mn)As, (In,Ga,Mn)As has a much higher Curie temperature (above 100K, and increases with increase in Mn content). The (In,Ga,Mn)As sample used in our experimentation was grown on and lattice matched to InP. The substrate temperature during growth was Ts=200-260 ^\circC. Our experiments consists of comparing FTIR data over a range of 15000-100 cm^-1 at several different temperatures (from 4K to 300K) in order to gain some insight into the band structure of (In,Ga,Mn)As.

[R1.289] Elliot-Yafet spin relaxation in n-doped semiconductors

We calculate the spin relaxation time of conduction electrons in n-doped bulk gallium arsenide. We consider the Elliot-Yafet spin-relaxation mechanism, driven by Coulombic-impurity and electron-electron scattering. We find that these two scattering mechanisms result in relaxation times of equal order of magnitude, but with disimilar dependences on doping density and temperature. Our theoretical results are compared with experimentally measured spin relaxation times in gallium arsenide.

[R1.290] Plasma Deposition of Nanometer-Thick Polymer Films on Carbon Nanotube Surfaces

Ultrathin films of pyrrole were deposited on the surfaces of carbon nanotubes using a plasma polymerization treatment. High-resolution electron transmission microscopy images revealed that an extremely thin film of the polymer layer (2-7 nm) was uniformly deposited on the outer and inner surfaces of the nanotubes. Nanotubes of all sizes exhibited equally uniform ultrathin films, indicating well-dispersed nanotubes in the fluidized bed reactor during the plasma treatment. In particular, the inner wall of the nanotube was also coated with a uniform ultrathin film of only 1-3 nm. Time-of-flight secondary ion mass spectroscopy experiments confirmed the highly branched and cross-linked polymer thin films on the carbon nanotubes.

[R1.291] Photomodulation study of the influence of CdCl2 in large area CdS thin films grown by Sputtering

We present the results obtained in semiconductor CdS thin films grown in a large area (450 cm2) by photomodulation (PM). The thin films were deposited on conducting glass (SnO2:F-7W/ð) by Radio Frequency-Planar Magnetron Sputtering with a substrate temperature (Ts) of 250 °C, an Argon pressure of 20 mTorr, a radio frequency power of 300 Watt and with a deposition time (td) of 60 min. A thermal treatment with CdCl2 in these thin films at different times of annealing was carried out. The PM spectra show a better crystal quality in the CdS thin films annealed with CdCl2, which is corroborated with atomic force microscopy (AFM) and observed in the efficiency of the solar cells elaborated with these films.

[R1.292] Growth and coverage measurements of BaO/Sr/Si(001)

The introduction of a submonolayer of an alkaline earth element at the surface of a Si(001) substrate has been shown to allow the subsequent epitaxial growth of crystalline alkaline earth and/or perovskite oxides (i.e., BaO, SrTiO_3, etc.). This remarkable achievement allows the use of crystalline, high-dielectric constant oxides as the gate insulator in a field-effect transistor. However, the atomic structure, and, surprisingly, even the atomic coverage are not agreed upon. We used x-ray fluorescence to probe the interfacial Sr coverage in high-quality crystalline BaO/Sr/Si(001) heterostructures. Varying amounts of BaO (3-13 ML) were deposited onto a 1/2 ML Sr layer grown on Si(001) and capped with thick layers (2000 Å) of Al. Comparison to an implanted Sr/Si standard sample reveals an average resulting Sr coverage of .36 +/- 0.06 ML.

[R1.293] Probing DPPC Bilayer Spreading on Chemically Modified Substrates

The diffusion dynamics of dipalmitoylphosphatidylcholine (DPPC) lipid bilayers on glass substrates were investigated in real time using fluorescence microscopy. DPPC doped with 0.1 molslip and mounted in a temperature controlled fluid chamber. Initially, micron-size, non-fluorescent voids were observed in the lipid bilayer. Upon raising the sample temperature above the fluid transition temperature of DPPC (Tm = 41°C), the micron-size dark voids were replaced by a fluid lipid bilayer phase, followed by spreading of the lipid bilayer across the glass substrate. Interestingly, as the solution returns to room temperature, the non-fluorescent patterns reappear at the original positions. We hypothesize that the non-flourescent voids are due to defects in the glass surface or dewetting caused by localized surface contamination. Future experiments include micropatterning the glass substrate with hydrophobic and/or hydrophilic polymers to tune the lipid bilayer diffusion properties.

[R1.294] Peculiarities of rotation of polarization angle of light reflected from semiconductor surface layers

Reflection spectra and energetic spectra of polarization angle of light reflected from semiconductor surface layers give information about zone structure of materials. The abstract deals with the study of peculiarities of reflectance from size quantized inverse layer of semiconductors and the rotation of polarization angle of light reflected from the same layer. The method of forming size quantized inverse layer CdTe was informed in [1]. Rotation of polarization angle of light in dependence of light wavelength was investigated in the spectral range of reflectance minima. It was shown that both the shape of reflectance spectra and energetic spectra of light are strongly dependent on the ellipticity of polarized light and the incident angle of light. Terms of the ellipticity of incident angle were suggested, according to which the rotation of polarized light were observed.

1. G.Sh. Shmavonyan, Physica solidi status (b), Vol. 229, No.1, p. 89-92, 2002.

[R1.295] Spin-charge separation and Kosterlitz-Thouless confinement-deconfinement transition in 2+1 dimensions

We consider the compact 2+1-dimensional U(1) lattice Abelian Higgs model with matter fields in the fundamental representation. The dual theories in the compact and non-compact cases have the same form, differing merely in the constraints in the functional integral for the partition function. We derive a continuum limit for the compact case showing that in 2+1 dimensions, it is a sine-Gordon theory with an anomalous gradient term arising out of anomalous scaling of the gauge field due to matter-field fluctuations. This is a field-theoretical description of a three-dimensional gas of point charges with logarithmic interactions. We use the Callan-Symanzik equations to demonstrate that this anomalous sine-Gordon theory has a massless and a massive phase in 2+1 dimensions. The renormalization group flow of the coupling constants of the theory are found, showing that it exhibits a Kosterlitz-Thouless phase transition. The stiffness parameter of the theory has a universal jump at the transition, given by the dimensionality. We relate the confinement-deconfinement transition in the model to electron fractionalization in strongly correlated electron systems.

[R1.296] A point collocation method for electron beam column calculation

A computational methodology based on the coupling of point collocation method and mesh-free calculations has been developed to solve the numerical solutions of partial differential equations for an electron beam system. Typically, electron beam column design has been simulated by finite difference method with grid and finite element method based on mesh generation. However, the new method can readily calculate a high-aspect-ratio structure employing only nodes instead of grid or mesh generation. The accuracy of this method is proved through careful analysis on the error between numerical and analytic solutions of a simple modeling. The miniaturized electron beam columns consisting of electron emitter, source lens, deflector, and Einzel lens are calculated by the method. The results show good agreements with previous calculations and experimental data by others. We will discuss the basic concept of the new method and its applications in this paper.

[R1.297] APPLICATION of NANOMETER-MULTILAYER OPTICS for X-RAY ANALYSIS

With the growing number of applications in X-ray analysis, more and more dedicated X-ray optics are required, optimized for the spectral range they are intended to be used for. Three different types of curvature of laterally graded multilayer mirrors are used for X-ray analysis experiments: parabolic, elliptic and planar, which result in parallel, focusing and divergent beam conditions, respectively. Ni/C-multilayer-optics have been well established in X-ray diffraction using Cu Ka-radiation. Intensities of more then 109 cps together with a low beam divergence DF < 0.02° and a superior suppression of Cu Kb-radiation I(Cu Ka1) : I(Cu Kb) ³ 106 are realized with the Twin-Mirror-Arrangement (TMA). At present, parabolic, elliptic and planar nanometer-multilayer X-ray optics are available for different geometries and spectral lines. The typical X-ray beam characteristics, like intensity, monochromasy, divergence, beam width and brilliance can be conditioned by combining one multilayer optics with either a different optic and/or with a crystal monochromator. High brilliance monochromatic sub-mm X-ray spots can be generated by combining two nanometer-multilayer optics. Applications in the fields of X-ray reflectometry, µ-diffraction, X-ray µ-lithography and µ-tomography are the potentials of these high brilliance collimating monochromator systems.

[R1.298] Light Scattering Study of the Nematic - Smectic A Phase Transition in Binary Mixtures of a Calamatic and a Bent-Core Liquid Crystal

Light scattering measurements were performed above the nematic - smectic A transition temperature on the calamatic liquid crystal octyloxycyanobiphenyl (8OCB) doped with the bent-core molecule P7PIMB. The intensity of the scattered light due solelyto bend fluctiations, as well as the relaxation time of the fluctuations, were measured as functions of temperature and dopant concentration. The bend elastic constant \mathbfK_33 was found to decrease as the dopant concentration was increased. Close to the nematic - smectic A transition temperature, but still outside the non-hydrodynamic region, \mathbfK_33 can be expressed as a constant plus a diverging term that is proprotional to the correlation length. Data indicate that the bare correlation length associated with smectic fluctuations decreases with increasing dopant concentration.

[R1.299] Analytical calculation of the spin-resolved pair distribution functions in a partially spin-polarized electron gas

We pose the problem of calculating the spin-resolved electron-pair distribution functions g_\sigma \sigma'(r) in a partially spin-polarized electron gas (EG) through the solution of a variationally derived differential equation for the probability amplitudes \sqrtg_\sigma\sigma'(r). The induced spin-dependent potential is approximated by an educated two-component generalization of the one recently proposed by Kallio and Piilo [Phys. Rev. Lett. 77, 4237 (1996)] for a one component electron gas which is shown to satisfy an important list of known sum-rules on g_\sigma \sigma'(r). We present numerical results for a paramagnetic and a fully spin-polarized three-dimensional EG over an extensive range of density. This approach allows unprecedented accuracy for the spin-resolved pair functions as gauged by comparisons with the state-of-the-art Quantum Monte Carlo data.

[R1.300] Mimicking Biological Tissues and Probing Soft Surfaces

Hyaluronic acid (HA) is a linear anionic polysaccharide and is the major component of the extra cellular matrix. It plays an important role as structural constituent of tissues, is attached through receptors to migrating cells and has been found recently to play an important role inside cells. We have developed a bio-mimetic system by anchoring HA films to solid supported membranes through an intracellular HA-binding protein p32. This protein was modified by genetic-engineering so that it could be specifically anchored to fluid supported lipid bilayers. The local HA-film thickness and the surface viscoelastic moduli were measured by analyzing the Brownian motion of colloidal probes hovering over the film. A novel dual-wave reflection interference contrast microscopic technique was developed that enables the measurement of the absolute film thickness with 4 nm resolution and thus allows the establishment of correlations between surface viscoelastic parameters and the film thickness. This technique was applied to study the influence of excess salt and cross-linkers on the film thickness and viscoelasticity of the HA layer. The dual-wave method was also applied to the study of adhesion of vesicles on the ultra-thin HA-layers.

[R1.301] Dynamic-Mechanical Analysis of Monodomain Nematic Liquid Crystalline Elastomers

Dynamic-mechanical analysis was performed in the glassy, nematic and isotropic states of several monodomain nematic liquid crystalline elastomers (LCE) which differ in their degrees of anisotropy and internal microstructure. It was found that the type of network crosslinker makes a significant difference in the equilibrium properties of these elastomers, in particular, in their effective anisotropy. In spite of these differences, the observed dynamic-mechanical behaviour was very similar. The fact that there is a consistently high and wide loss over the whole nematic region, where storage modulus G’ behaves non-monotonically, is most likely an indicator of the fact that the dynamic-mechanical response is not linear. Master curves have been built between the glassy state and the nematic-isotropic phase transition, where the modulus reaches a low-level soft plateau. Above the nematic-isotropic transition temperature Tni, the modulus rises substantially, since internal relaxation is no longer able to reduce the elastic response – and further time-temperature superposition fails. The dynamics of these elastomers are dominated by power laws, which was confirmed by the successful procedure of the master curve inversion (time-frequency inversion) to describe the static stress relaxation. Interestingly, it was found that mechanical properties characterized by power laws (in time) of stress relaxation match very well with the dynamic properties, where power laws (in frequency) were also observed in the dynamic modulus in the appropriate range of temperatures. The work demonstrates the potential for the use of nematic liquid crystalline elastomers in many acoustic and vibration damping applications.

[R1.302] Simulating Phase Separation Dynamics with Unconditionally Stable Time Steps Equation

Eyre has recently derived unconditionally stable time steps for the Cahn-Hilliard equation, a model for conserved scalar phase separation dynamics. As a result, the stability criterion no longer determines the size of the time step, allowing accuracy criteria to be considered instead. We show that accuracy provides the theoretical upper limit \Delta t \sim t^2/3 for a time step at asymptotically late time t. In this case, after taking n steps, one reaches a time t \sim n^3! Next, we classify all possible stable first order steps, expanding on the parameter range given by Eyre. Finally, we study the time scaling of the truncation error to all orders in \Delta t. From this analysis we conclude that the theoretical maximum time step is unattainable as a stable algorithm and that Eyre's proposed algorithms give \Delta t \sim t^1/3. We find an efficient algorithm capable of \Delta t \sim t^4/9.

[R1.303] ``In-plane'' magnetoresistance in [(Co + 10%Cu)/Cu]N multilayers

We measured the magnetoresistance (MR) in the temperature range 5K<T<300K of some [(Co + 10%Cu)(t(Co + 10%Cu))/Cu(t(Cu))]N thin film multilayers that were prepared by DC magnetron sputtering. t(Co + 10%Cu) was fixed at 35.2 Å, 4.8Åamp;#8804;t(Cu)amp;#8804;41.6 Å, and 8amp;#8804;Namp;#8804;15, for a total multilayer thickness of approximately 600 Å. The objective was to examine AMR and normal MR behaviors as a function of temperature and copper layer thickness, and to look for oscillatory behavior in the MR as a function of t(Cu). Some of the samples showed AMR while others showed normal MR. No well defined oscillatory behavior is seen over the temperature range.

[R1.304] Electron Diffraction Studies of Polar Surfaces of Rock Salt Oxides

Surfaces of compounds that involve at least two types of atoms, bearing charges of opposite signs, whose repeat unit (perpendicular to the surface) bears a non-zero dipole moment are known as polar surfaces. While the bulk termination of such a surface is unstable due to diverging electrostatic potential, they can be stabilized through modification of the surface electronic structure viz-a-viz surface reconstructions. We have carried out high-energy electron diffraction studies on the polar MgO(111) surface and have determined the atomic structure of the r3xr3 reconstruction on this surface. The structure that we obtain is magnesium terminated and is different from one previously proposed for this surface [R Plass, Phys. Rev. Lett. 81, 4891 (1998)]. Since electron diffraction (at low angles) is very sensitive to bonding electron distribution, it is possible to further quantify the electronic state of the various surface atoms. Employing chi-2 refinements we were able to determine that the surface Mg atom and the O atoms in the second layer have partial charges that are different from those of bulk.

[R1.305] Microstructure Evolution in Al-Cu-Fe Quasicrystalline Thin Films

Transmission Electron Microscopy (TEM) was performed to study the microstructure evolution in Al-Cu-Fe quasicrystalline thin films. Thin films were grown by magnetron sputtering on sodium chloride crystals which were subsequently dissolved in water to acquire free-standing films. Nanocrystalline films were found in the as-deposited sample. When annealed at 400oC the films changed to metastable crystalline phases that transformed into icosahedral phases upon further annealing at 500oC. TEM imaging combined with electron diffraction revealed various features associated with the phase evolution in the crystalline-quasicrystalline phase transformation. Some grains in the film functioned as sacrificial grains allowing others to grow into icosahedral phases. Elements near the boundary of the sacrificial grains diffused to form the icosahedral phases, resulting in fragments in the center of the grain. The oxide layer of the film was amorphous aluminum oxide that exhibited poor adhesion to the quasicrystalline films.

[R1.306] Effect of exchange interaction and temperature on the parametric correlations of the conductance peak heights in quantum dots

In closed dots the evolution of a Coulomb-blockade conductance peak height as a function of an external parameter (such as a magnetic field or the shape of the dot) can be studied by, e.g., calculating the correlation between the peak-height for different values of the parameter. Parametric correlations of the peak height in a chaotic or weakly disordered quantum dot are well described in the framework of Gaussian processes (GP), which generalize the Wigner-Dyson random matrix ensembles to systems that depend on an external parameter [1]. Recent studies have explored the effect of temperature and exchange interaction on the conductance using a universal Hamiltonian for chaotic or diffusive quantum dots. Here we explore the parametric correlations of the conductance in the framework of this universal Hamiltonian, using a closed expression for the conductance peak in the presence of spin and a constant exchange interaction [2]. In particular, we study how the peak-height correlator is modified by temperature and the exchange interaction.

[1] Y. Alhassid and H. Attias, Phys. Rev. Lett. 76, 1711 (1996); Phys. Rev. B 54, 2696 (1996).

[2] Y. Alhassid and T. Rupp, cond-mat/0212126 (2002).

[R1.307] UPtSn - a Kondo insulator?

UPtSn can exist in two different crystal structures, depending on preparation/annealing conditions. The electronic properties of these two forms, hexagonal and cubic, are very different and controversial. Electrical resistivity and specific heat of a cubic form of UPtSn are reminiscent of Kondo insulators. We have studied intrinsic properties of both forms of UPtSn by alloying on the U-site and in magnetic fields. Crystallographic, electrical resistivity, specific heat, and magnetization data will be presented and discussed. Supported by DOE grant no. DE-FG02-99ER45748 and National High Magnetic Field Laboratory.

[R1.308] Suppression of Kondo effect by 3d-host ferromagnetism in Sm1-xCexMn2Ge2

Lattice, Ce L3-edge, magnetic susceptibility, and electrical resistivity measurements on the polycrystalline Sm1-xCexMn2Ge2 compounds are presented. The lattice parameter a increases linearly with the increase of Ce concentration x. The Ce-L3 x-ray absorption spectra indicate that Ce in this series is nearly trivalent, and Ce valence decreases slightly with the increase of x. Magnetic susceptibility results show that SmMn2Ge2 exhibits reentrant ferromagnetism. It is found that the antiferromagnetic (AF) phase in SmMn2Ge2 is destroyed by 1.1% Ce substitution for Sm. Thus, for x= 0.011, only ferromagnetic (FM) phases exist below room temperature. The temperature-dependent resistivity results support that in this compound series, the crystalline-field-modified Kondo scattering due to the Ce ions is effectively suppressed by the strong 3d-host FM fields in these materials.

[R1.309] Wavepacket motion of BEC atoms in optical lattices

We study the motion of wavepackets of Bloch waves for BEC atoms in optical lattices. We calculate the Berry phase and angular momentum terms that appear in the semiclassical equations of motion and study the feasibility of their observation in these systems. We also study the effects of the interparticle interaction on these phenomena.

[R1.310] High Field Phase Diagram of PrOs_4Sb_12

Magnetic phase diagram of a new heavy fermion superconductor, and the first Pr-based, PrOs_4Sb_12 was studied in magnetic fields to 32 T. Our results suggest that the field-induced long range magnetic order disappears near 14 T. In fields higher than 14 T, we observe a new anomaly implying an existence of an additional high field phase. Implications of these results on a controversial issue of the crystal field configuration of Pr in this system will be discussed. Supported by NSF, grant no. DMR-0104240 and National High Magnetic Field Laboratory.

[R1.311] Shape Descriptors for Scanning Probe Recognition Microscopy

Direct investigation of, and interaction with, biological objects at the macromolecular level will provide insight into multiple physical regulatory processes. Scanning probe microscopy (SPM) techniques have the potential to provide a direct interaction with living specimens at the macromolecular scale. A key enabling capability is to replace the current x-y raster scan with site-specific direct investigation. In the present research we will discuss the site-specific recognition techniques that are appropriate for tubular and globular biological features. The SPM image will be input to an image segmentation and boundary detection algorithm to extract closed boundaries of features in the image. The boundary information will be parameterized using Fourier descriptors, which are rotation invariant descriptors to be used for recognizing the segmented shape.

[R1.312] Atomic-Scale Surface Modification and In-Situ Tip Preparation Using STM Manipulation

Single atom manipulation with a scanning tunneling microscope (STM) tip on crystal surfaces requires an extremely fine control over the tip-atom-surface junction. The shape of the STM-tip and the chemical elements that constitute the tip-apex are vital for a successful atom manipulation with atomic scale precisions. Here we report an in-situ tip preparation technique useful to fabricate stable STM-tips with a known chemical element at the tip-apex. The experiments are conducted at an ultra-high-vacuum conditions on a Ag(111) surface at 4.8 and 75 K sample temperatures. During the experiment, the STM-tip, made of polycrystalline tungsten wire, is gently dipped into the substrate and the tunneling voltage is increased to 3 V. The penetration depth is precisely controlled. The shape of the holes created by the tip dipping indicates that the tip becomes sharper by repeating the procedure. This is due to the local heating during the tip-sample mechanical contact that re-shapes the structure of the tip-apex. In addition, variation of the tip-height and tunneling voltage during the procedure results in different impact force. By applying suitable impact force with the STM-tip, atomic scale surface steps and Ag islands can be created locally. This entire procedure will be useful to conduct new nanoscale experiments or to test the strength of the material at an atomic level.

[R1.313] Cubic GaN Formation in Mn/GaN Multilayer Films on 6H-SiC(0001)

Group III nitrides have been extensively studied for their optical properties that are suitable for blue lasers. More recently the magnetic properties of transition metal doped GaN have attracted growing attentions because they may exhibit ferromagnetic order at room temperature. Using molecular beam epitaxy, we have grown GaN films delta-doped with Mn in a single layer, as well as in multiple layers. High-resolution transmission electron microscopy images of the cross section of these films were recorded and analyzed to measure the lattice structures, using digital diffractograms calculated by fast Fourier transform. We found that structure in the pre- and post-doping layers of both types of films match the hexagonal 2H-wurtzite GaN. Interestingly, the cubic zinc blende GaN phase was present in the films with multilayers of Mn delta doping. Both films have edge, mixed, and screw dislocations, as well as stacking faults and inversion domain boundaries, and their densities are drastically reduced within and beyond the doping regions. The implications of this study towards further research on Mn doping GaN will be discussed at the meeting.

[R1.314] X-RAY ASORPTION AND EMISSION STUDIES OF HIGH-TEMPERATURE SUPERCONDUCTING SUPERLATTICES

We use resonant inelastic x-ray scattering (RIXS) to study high-Tc, 65K, superconducting supperlattices (SL). (Ba0.9Nd0.1)CuO2+x /CaCuO2 SL were grown by pulsed-laser deposition [1]. Using tunable synchrotron radiation we made site-selective excitations of the SL. We determine the effect of varying the oxygen concentration during growth on in-plane and apical oxygen sites of the SL unit cell. We use x-ray absorption/emission spectroscopy to compare the electronic structure of the oxides layers with that of the SL. We observe insulating and metallic behavior of the constituent layers and the SL, respectivley. The soft-x-ray fluorescence reflects bandlike O 2p states. RIXS gives excitonic-like ''optical'' final states previously only observed for metal-ion core edges.

[1] G. Balestrino et al. Appl. Phys. Lett. 79 99 (2001)

Part R of program listing