Session S36 - General Poster Session III.
POSTER session, Thursday morning, March 23
Exhibit Hall, MCC

[S36.001] Structure and Properties for Two-Photon Absorbing Materials

In our continuing efforts to design nonlinear optical materials, conjugated polyenes that exhibit the two-photon absorption mechanism, are being studied. The accurate prediction of the absorption frequencies and cross-sections for the relatively large molecular systems of interest is difficult due to both the high level of ab initio calculations required for evaluating excited state energies and transition dipole moments, as well as the modeling of the material's behavior in the condensed phase. In this study we present multi-configurational self-consistent field geometry optimization results, in the gas-phase and in solution, for hexatriene, octatetraene, and several of their substituted analogs. Solvent effects are modeled using our newly developed effective fragment potential method. Transition dipole moment calculations are also discussed, from which the two-photon absorption cross-sections are estimated. This approach consists the first stage towards defining a general approach to predict the optical properties of two-photon absorbing materials in solution.

[S36.002] The Design of Reverse Saturable Absorbing Materials Using First Principle Calculations

Electronic structure calculations were carried out to predict properties of reverse saturable absorption (RSA) materials, in particular zinc porphyrins with a combination of halogen and phenyl substituents, that are important for nonlinear optical (NLO) applications. We report results of the electronic spectra for the ground and triplet excited states, as well as ionization potentials, to evaluate the performance of NLO materials for specific applications. Since comprehensive experimental data are not available, we start with the basic porphyrin unit, and thereafter systematically replace the peripheral hydrogens of porphin with halogen and phenyl groups to study substituent effects. The results for halogenated porphyrins and halogenated tetraphenyl porphyrins provide an insightful account of substituent effects in these materials. The computed spectra and IP's are in excellent agreement with available experimental data.

[S36.003] Analysis of Dielectric Spectra of Polymers as a Function of Frequency and Temperature

The broadband dielectric spectra as a function of temperature and frequency of most polymeric materials usually shows several broad and sometimes overlapping peaks, each corresponding to different relaxation processes occuring in the polymer. To perform the analysis of these peaks to obtain their relaxation parameters, several choices must be made. First the data to analyze can be the real part of the dielectric constant, the imaginary part or both simultaneously. Then, the data as a function of frequency at several constant temperatures can be analyzed, and from the results, relaxation plots are constructed which give the dependence of the relaxation time with temperature (Arrhenius or Vogel-Fulcher type). Alternatively, the whole data can be fitted utilizing both, the frequency and temperature as independent variables in a two dimensional approach, in which case the type of temperature dependence must be assumed. Also, a distribution of relaxation times can be assumed a priori (e.g the Havriliak-Negami model) or the distribution can be extracted directly from the experimental data. Finally a choice of fitting procedure to minimize \chi^2, the sum of the square residuals must be made: use standard Marquardt-Levenberg minimization, Monte Carlo Simulated Annealing minimization or integral equation inversion. In this work we compare all these choices for the analysis of \epsilon^\star(ømega,T), using synthetic data and also experimental data of chosen polymers in order to determine the advantages and difficulties of each approach.

[S36.004] Molecular Hyperpolarizabilities of Dithienyl Polyenes from First Principle Calculations

Dithienyl polyenes are known to form stable bipolaron-like cations when oxidatively doped in solution. Previous theoretical studies of polaronic and bipolaronic enhancements in such materials employed semi-empirical molecular orbital theory. In this study we employ density functional theory to optimize the molecular conformations of the neutral, cationic, and dicationic forms of a series of dithienyl polyenes and subsequently calculate the second-order hyperpolarizabilities, gamma, using ab initio theory. We show that gamma increases with the number of ethene repeat units for a given charge. We also report gamma values for dications calculated using open shells as well as results for larger polyenes.

[S36.005] Transport in Polypyrrole in the Presence of Gases

The polymer polypyrrole (Ppy) is of particular interest for applications because it is one of the most mechanically stable of the highly conducting polymers. In addition to the obvious applications for lightweight conductors Ppy has a reversible resistance change under exposure to gases which have led to its use in gas-specific sensors, the "electronic nose". We will present data on bulk Ppy films and gas sensors that have been exposed to water and ammonia. We find that ammonia drives a metal-insulator transition, similar to that as seen in films prepared at temperatures of -40, 0 and 20 C. The evidence for this is most clearly seen in the extrapolation of the conductivity at zero temperature. In the most highly conducting film the conductivity is finite at zero temperature, implying the existence of extended states and metallic behaviour. This shows that the conductivity cannot be described by phonon mediated hopping alone. Instead, we employ a model based on a mixture of highly conducting regions separated by barriers through which the electrons tunnel, a model supported by frequency-dependent conductivity measurements on the same films. The description is also compared with thermo-electric power data.

[S36.006] Effect of Confinement on the Dielectric Perimttivity of Emeraldine Base and Weakly Doped Polyaniline at Radio Frequencies

We report our study on the effects of confinement on the dielectric properties and behavior of emeraldine base and weakly doped polyaniline using radio frequency dielectric spectroscopy. Confining the polymer in random porous matrices of pore sizes ranging from 2.5 nm to 100 nm permits us to study the effect of polarons and bipolarons on the observed dielectric relaxation processes. Measurements on bulk samples show that doping leads to an increase in the number of polarons and bipolarons. These charge carriers are responsible for the observed dielectric relaxation and the relaxation times get faster with increased doping. A progressively non-Debye like relaxation behavior is observed with increased doping due to the presence of multiple paths for the system to relax. Preliminary studies on the dielectric permittivity of emeraldine base polyaniline in bulk and confined to 2.5 nm pores at 300 K show that confinement leads to smaller observed relaxation times. Results on these and other weakly doped samples of polyaniline will be presented. This work was supported in part by Naval Research Office Grant N00014-99-1-0558 and NSF Grant DMR-9872689.

[S36.007] Effect of Source Bandwidth, Focusing and Fluence on the Depth Of Cure in Polymer Dental Composites

Photo-curable polymer dental composites are widely used in restorative dental applications. These composites are typically cured using a conventional curing lamp with broad band visible irradiation between 400-500 nm. Argon ion laser-based sources are now available in dentistry for curing applications. This work reports on the dependence of depth of cure on the wavelength bandwidth, the focusing geometry and the irradiation fluence of the curing light source. The depth of cure resulting from a narrow band irradiation source such as the 488 line of the Argon ion laser is observed to be higher than that resulting from broadband irradiation sources such as the curing lamp or the multiline Argon ion laser with lines between 450-500 nm. For the same total irradiation energy deposited into the polymer a focused beam yields higher depth of cure than a non-focused beam.

[S36.008] Polymer-Containing Droplets in a Matrix, Dielectric Constant, Conductivity, and Electric Fields

When a droplet from one phase of a phase-separated solution of two immiscible homopolymers is injected in the matrix of the other phase and a homogeneous electric field is applied, the droplet can deform either parallel or perpendicular to the electric field direction. The direction of the deformation has been predicted by Taylor (Taylor, G. I. Proc. R. Soc. Lond. A 1966, 291, 159-166) and Torza et al. (Torza, S.; Cox, R. G.; Mason, S. G. Phil. Trans. R. Soc. Lond. 1971, 269, 295-319) in terms of the Leaky Dielectric Model (LDM) in DC and AC fields. In our work, the LDM has been successfully extended to accurately predict large deformations in both DC and AC fields, as commonly observed in polymer-polymer-solvent systems in our laboratory. A droplet shape transition as a function of the electric field has also been investigated and possible explanations are suggested. The transient response of droplets has also been studied. (Supported by NSF, DMR-9521265)

[S36.009] Self-Assembly of ABC Triblock and ABCA Tetrablock Copolymers: Theory and Experiment

By a combination of theory and experiment, we investigate the factors that control self-assembly of ABC triblock and ABCA tetrablock copolymers. The focus of this study is to understand the effect of attaching or inserting a third block (block C) into AB diblock and ABA triblock copolymers. Theoretical predictions of self-assembled mesophases in ABC triblock and ABCA tetrablock copolymers have been obtained by implementing self-consistent mean-field theory in real space. Experimental studies have focused on investigating complex morphologies of nearly monodisperse ABC and ABCA block copolymers, where A, B, and C blocks are PS, PI, and PS-r-PI (or P2VP), respectively. The mesophase morphology was determined by transmission electron microscopy using differential staining of individual blocks to reveal the microstructure.

[S36.010] Modeling Block Copolymer Self-Assembly through a New Approach to Density Functional Theory

Inhomogeneous polymer systems, which range in complexity from homopolymers near surfaces to microphase-separated block copolymers, often show unpredictable behavior in experiments. Gaining insight into this behavior has been a difficult challenge for molecular modeling since standard theories capture very little detail about polymer segments and interactions. Meanwhile, full simulations of self-assembled structures are computationally demanding because of large system sizes. Previous studies of polymers through density functional theory (DFT) have been limited to 1-dimensional and simple 2-D systems due to the need to couple a single molecule simulation to the DFT. Here, we introduce a method to study more complicated systems of polymers, including 3-D structures. The key feature of the technique is the replacement of the single chain simulation by a system of equations following the polymer path integral approach. The effects of segment size disparity, compressibility and solvent effects on block

[S36.011] Designing Polymeric Bicontinuous Microemulsions

Bates and coworkers [Phys. Rev. Lett. 1997, 79, 849] have recently discovered polymeric bicontinuous microemulsion in symmetric ternary blends of AB diblock copolymer with A and B homopolymer. In this phase, the A and B homopolymers separate into two continuous random networks with a diblock monolayer at the A/B interface. This offers a tremendous opportunity to form stable polymeric alloys from immiscible polymer pairs. The challenge now is to determine the optimum conditions for forming these microemulsions. With this goal, we examine the properties of the diblock monolayer. The monolayer has to be sufficiently flexible to form a microemulsion, and we find that this does not occur unless the copolymer is small. However, if it is too small then the monolayers attract each other. This causes them to collapse into a copolymer-rich lamellar phase expelling the homopolymer, thus destroying the microemulsion. Hence, there exists an optimum size of the copolymer. However, we show that the situation can be improved by using a mixture of small and large diblock copolymers.

[S36.012] Phase separation in 3d-binary mixtures with hard mobile particles

We simulate the motion of spherical particles in a phase-separating binary mixture. By combining cell dynamical systems (CDS) equations with Langevin dynamics for particles, we show that the addition of hard particles significantly changes both the speed and the morphology of the phase separation. At the late stage of the spinodal decomposition process, particles significantly slow down the domain growth, in qualitative agreement with earlier experimental data. The results are compared with our earlier study performed for the 2d-systems.~(V. V. Ginzburg, M. Paniconi, F. Qiu, G. Peng, D. Jasnow, and A. C. Balazs, Phys. Rev. Lett. 82), 4026 (1999)

[S36.013] Comicellization in the Block Copolymer/Surfactant Mixtures in Polar Selective Solvent

We have developed the theory for the micelle formation in dilute solution mixtures of A-B diblock copolymers and ionic surfactants S in polar selective solvents. In the case when soluble B-blocks are longer then A-blocks, spherical micelles with B-corona and A-core decorated by surfactant molecules S can be formed. The conditions for the formation of pure micelles and comicelles in this mixture are determined using the synthesis of the calculation of the free energy of the core of the mixed micelles made by us in the previous article [1] and the theory of the micelle formation describing this process as a set of association-dissociation reactions [2]. The equilibrium distribution of the contrions in the surrounding of the isolated comicelle is calculated by numerical solving of the Poisson-Boltzmann equation. It is shown that the block copolymers and amphiphilic molecules mix well in the aggregates (comicelles) in a wide range of concentrations of the components in salt free solution. In the case of high concentration of added salt there is a region on the phase diagram where the distribution function for the aggregation of the components in the micelles is bimodal, corresponding to the equilibrium coexistence of mixed micelles and pure micelles made of surfactants. Adding surfactant to the solution of the block-copolymer micelles leads to the increasing of the micellar weight due to the adsorbtion of the the surfactant molecules in the cores of aggregates. This is in good agreement with recent experimental findings of the article [3] on the mixtures of PS-PEO/CPC in water solutions.

References.

1. A. L. Borovinskii, A. R. Khokhlov, Microphase Separation in a Mixture of Block Copolymers in the Strong Segregation Regime. // Macromolecules, 1998, v. 31, p.1180-1187.

2. A. L. Borovinskii, A. R. Khokhlov, Micelle Formation in the Dilute Solution MIxtures of Block-Copolymers. // Macromolecules, 1998, 31, No. 22, p. 7636.

3. L. M. Bronstein, D. M. Chernyshev, G. I. Timofeeva, L.V. Dubrovina, P. M. Valetsky, A. R. Khokhlov // Langmuir, to be published, 1999.

[S36.014] Microstructural change and phase behavior in hydrogen bonding polymer solutions hydrogen bonding polymer solutions

Water-soluble polymers typically display a lower critical solution temperature (LCST) in aqueous solutions. This behavior is believed to be related to the reason these polymers even dissolve in water, their tendency to strongly hydrogen bond with the solvent. Hydrogen bonding is known to affect the local chain structure of the polymer by altering the trans-gauche equilibrium as some conformations can have significantly stronger interaction with water when compared to others. This could lead to an alteration of the local structure. If the polymer behavior in solution is studied using coarse grained models, this change has to be accounted for by a variation in the persistence length. Small angle neutron scattering was used to study the polymer/D2O solutions at different concentrations and temperatures. The polymer was found to exhibit excluded volume chain behavior as expected, but the persistence length was a strong function of concentration. If the variation of persistence length with concentration is not accounted for in the theoretical description of the thermodynamics of these systems, it could lead to spurious dependencies on the Flory-Huggins interaction parameter. The effect of the variation of the persistence length on the phase behavior will be explored.

[S36.015] Effect of Added Star copolymer on the Phase Behavior of Immscible Polymer Blends

We theoretically investigate the phase behavior of mixtures of immscible A/B linear polymer blends with a A-B star copolymer as a compatibilizer. Phase diagrams are calculated by using Random Phase Approximation, varying the architecture and the comonomer fraction of a star copolymer and compared with that of corresponding mixture with a diblock copolymer. When a (A)m-(B)n type star copolymer with equal numbers of A and B arms (m=n) is added, critical temperature is independent of the number of arms, whereas it strongly depends on the number of arms if the comonomer fraction of the star copolymer differs from 0.5.

[S36.016] Study of Miscibility in Binary Mixtures by Molecular Simulations

The miscibility behavior of binary mixtures of polymeric and small organic molecules was studied using a combination of modified Flory-Huggins theory and molecular simulation techniques. Two different atomistic approaches were used. In the first one, the local interaction between the polymer fragment and the organic molecule was calculated and the energy of mixing was estimated using a coordination number. In the second approach the energy of mixing was calculated directly from bulk simulations. The phase behavior of binary mixtures consisting of poly methyl methacrylate and 4-n-pentyl-4-cyanobiphenyl was investigated using both the approaches.

[S36.017] Phase behavior of a compressible polymer solution.

Recently, Chhajer and Gujrati[1] studied a simple model of compressible polymer solution in a mean-field analysis using a magnetic mapping due to deGennes. The effect of compressibility was of primary concern in their calculations. Their work showed that many conventional concepts of free volume were erroneous. However, their analysis was limited in that no interactions were allowed between the solvent and the polymer. We have considered an extension of their model by allowing this interaction and have solved it on a Bethe lattice of coordination number q. Our results reduce to those by Chhajer and Gujrati in the limit as q goes to infinity. We solve the resulting equations numerically and obtain the phase diagram, where we find both UCST and LCST. The effects of polymer-solvent interactions will be discussed.

[1] M. Chhajer and P. D. Gujrati, J. Chem. Phys., 109, 9022, (1998).

[S36.018] Dynamics of Spherullitic Growth in Blends of Semicrystalline and Non-Crystalline Polymers

The spatio-temporal growth of polymer spherulites has been investigated theoretically based on time dependent Ginzburg-Landau equation (known as TDGL - Model C), by coupling non-conserved crystal order parameter and conserved compositional order parameter. In the description of the total free energy, a double-well local free energy density signifying metastability of crystal ordering is combined with a non-local free energy term representing interface gradient, along with Flory-Huggins free energy of mixing. An additional coupling term is added in the free energy description to account for the interplay between the two non-equilibrium processes of crystallization and phase demixing. Two-dimensional calculations are carried out using material parameters of blends of polypropylene isomers (i-PP and a-PP) and Ethylene-Propylene Diene Terpolymer (EPDM). Competition between two non-equilibrium processes of liquid-liquid demixing and crystallization give rise to a variety of patterns.

[S36.019] Phase Transitions, Relaxation Behaviors, and Structures of Hairy-Rod Polyimides

A series of organo-soluble hairy-rod polyimides, BBPA(n), was synthesized from BPDA dianhydride and CnBBPA diamines with multiple side chains of varying number of methylene units. Side-chain crystallization and melting are observed from differential scanning calorimetry (DSC) measurements. The transition temperatures increase with increasing side-chain length. Wide-angle X-ray diffraction (WAXD) analysis confirms the side-chain melting of BBPA(18). Dynamic mechanical (DM) results of these polyimides reveal two relaxation processes. The low temperature process is a cooperative motion and the high temperature one is a noncooperative motion. WAXD studies on stretched film of BBPA(18) and BBPA(16) suggest an interdigitated packing of the crystalline alkyl side chains. Side chains of other polyimide materials typically fill in the space between backbones in the amorphous state. Several rigid backbones aggregate to form a bundle, and these bundles pack in a hexagonal lattice.

[S36.020] Characterization of Laterally Attached Main-Chain Liquid Crystals

Most common polymeric liquid crystals (PLCs) have mesogens that are linked at their ends to the polymer backbone by a flexible spacer. Main-chain PLCs with mesogens linked laterally by flexible spacers have been studied much less. This type of PLC has the potential of exhibiting interesting phase properties due to the lateral attachment of the mesogens to the backbone, which could result in additional orientational ordering. In this investigation, polyesters with laterally linked mesogens containing one and four carbon spacers and varying terminal group lengths have been synthesized. Characterization of the monomers as well as the polymers was carried out using standard techniques. Differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), and polarized light microscopy (PLM) have revealed the existence of both crystalline and liquid crystalline phases. DSC experiments show the presence of multiple phase transitions for both monomers and polymers. PLM and WAXD results indicate the existence of liquid crystalline phases of higher order than nematic phases.

[S36.021] How do we extract the three chi's that describe a compressible blend from SANS ?

We demonstrate that a lattice model of a compressible blend is characterized by three bare chi parameters; chi-01 (between void and polymer1), chi-02 (between void and polymer2) and chi-12 (between polymer1 and polymer2). We propose a methodology to extract the corresponding three effective chi's from the scattering intensity and additional information on the compressibility and partial molar volumes. We have also defined and obtained a single effective chi for the blend (without RPA) and compared this single effective chi with the other three effective chi's. In all calculations, the athermal part was removed exactly resulting in an interaction part which remains finite over the entire composition regime. From thermodynamics, we know that this single chi is not the same as chi-12, and we shall determine when the two are close or similar. We have also obtained the values of the three chi's at the critical point for different systems to display their utility. We have carried out the calculations in different ensembles and find that the value of chi's depend on the ensemble chosen.

[S36.022] Use of Parallel Tempering for the Simulation of Polymer Melts

The parallel tempering algorithm(C. J. Geyer, Computing Science and Statistics: Proceedings of the 23rd Symposium of the Interface, 156 (1991).) is based on simulating several systems in parallel, each of which have a slightly different Hamiltonian. The systems are put in equilibrium with each other by stochastic swaps between neighboring Hamiltonians. Previous implementations have mainly focused on the temperature as control variable. In contrast, we vary the excluded-volume interaction in a continuum bead-spring polymer melt, as has been done for lattice polymers already(Y. Iba, G. Chikenji, M. Kikuchi, J. Phys. Soc. Japan v. 67, 3327 (1998).). The "softest" interactions allow for substantial monomer overlap such that pivot moves become feasible. We have benchmarked the algorithm by comparing it to the chain breaking algorithm used on the same system. Possible applications of the algorithm include the simulation of polymer systems with complex topologies and combining the method with the Gibbs ensemble technique for the phase behavior of polymer blends.

[S36.023] Structural properties of self-assembled polymers with H-bonding in solution

The term `polymer' is usually reserved for long chains or networks of monomers connected by covalent bonds. When non-covalent interactions are used, such as hydrogen bonds, the polymerisation process is reversible, and materials with novel properties emerge. We will are presenting a highly organised self-assembled polymer involving H-bonding interactions. In order to introduce a second level of organisation in these structures, we have investigated the possibility of obtaining columnar order by p-stacking. In bulk and in alkane solvents, the polymers form highly organised superstructures with a columnar architecture. Small angle neutron scattering have been used to investigate the structural properties and the conformation of these aggregates. The effects of concentration and temperature on the self-assembled polymers have been studied in different solvents. The length of the columns increases with the concentration and becomes less sensitive above a certain value. While the radius of the cylinders remains constant.

[S36.024] Structural Transitions Induced by a Recombinant Methionine-Trigger in Silk Spidroin

Control of beta sheet formation is an important factor in the understanding and prediction of structural transitions and protein folding. In genetically engineered silk proteins this control has been achieved using oxidative triggers. A genetically engineered variant of a spider silk protein, and a peptide analog, based on the consensus sequence of Nephila clavipes dragline silk, were modified to include methionines flanking the beta sheet forming polyalanine regions. These methionines could be selectively reduced and oxidized, altering the bulkiness and charge of the sulfhydryl group to control beta sheet formation by steric hindrance. Biophysical characterization and monitoring of structural transitions and intermediates were accomplished through attenuated total reflectance infrared spectroscopy (ATR-IR) for solution state structures in both oxidized and reduced forms. For solid state structural characterization, IR microscopy and reflectance IR experiments were performed. Electron diffraction data as well as circular dichroism studies provide structural corroboration for all experiments in which reproducible sample preparation was achieved.

[S36.025] Molecular Dynamics Simulations of Poly(ethylene oxide)/LiI Melts:Molecular Weight Dependence.

Molecular dynamics simulations has been performed on solutions of diglyme/LiI and 11 repeat unit PEO/LiI at 363 K and 450 K for two compositions ether oxygen : Li (EO:Li) = 15:1 and 5:1. As for PEO/LiI solutions, at low salt concentration EO:Li = 15:1 we found static and dynamic behavior consistent with a separation of the solutions into salt-rich and polymer-rich (PEO-like) domains. The composition of Li+ cation coordination in diglyme/LiI solutions deferred significantly from the one in 11 repeat unit PEO/LiI solutions. The numbers of ether oxygen atoms coordinated around Li+ cation as well as the number of "free" ions were found to decrease with decreasing molecular weight. Unlike for PEO/LiI, we found essentially no temperature dependence in the range of 363 K - 450 K of the amount of free ions, ion pair and higher aggregates in diglyme/LiI solutions. In spite of the higher anion and cation self-diffusion coefficients in diglyme/LiI at EO:Li = 15:1 then in PEO/LiI the collective charge diffusion coefficient and the conductivity were found to be comparable or even lower for diglyme/LiI. However, at EO:Li = 5:1 composition both ion self-diffusion coefficients and conductivity were higher for diglyme/LiI. EO-Li bond lifetime was shown to correlate well with torsional correlation time for complexed dihedral angles around -O-C-C-O- bond. While an average EO-Li bond lifetime was estimated to be of the order of 0.1 ns in diglyme/LiI solutions at 450 K, an average diglyme/LiI bond lifetime lasts of the order of 10 ns. Both EO-Li and I-Li bonds showed very similar behavior at EO:Li=5:1 salt concentration for diglyme/LiI and PEO/LiI systems.

[S36.026] Investigation of the Effect of an Alkali Salt on the Conformation of Poly(ethylene oxide) in the Melt by Small Angle Neutron Scattering and Molecular Dynamics Simulations

Recent molecular dynamics simulations of poly(ethylene oxide)(PEO)/LiI melts predict a decrease in the radius of gyration of the PEO chains due to changes in conformation as a result of the complexation of Li+ with the ether oxygens. Small angle neutron scattering(SANS) measurements from PEO and PEO/LiI melts at 361 K with an ether oxygen atom to LiI ratio of 15:1 are shown to verify the prediction. The characteristic ratio for the chains in a pure PEO melt was also found to be in accord with updated calculations and a previous SANS study. This research was supported in part by the Division of Materials, Office of Basic Energy Sciences, U. S. Department of Energy, at ORNL, managed by Lockheed Martin Energy Research Corp. under Contract DE-AC05-96OR22464, by the National Science Foundation- Division of Materials Research through NSF CARRER award DMR 96-24475 and by a gift from the Eveready Battery Company.

[S36.027] Diffusion of hard spherical particles in gel-like systems: Reaching the continuum limit on a lattice

Lattice models are very useful to study the diffusion (random or biased, with or without constraints) of a particle. However, the diffusion coefficient D generally depends on the type of lattice used. In order to obtain experimentally relevant results, one needs to consider the continuum limit, i.e. the limit where the size of the lattice parameter is infinitely small in comparison with the size of the probe and obstacles. A procedure to reach this limit is presented and applied to several systems. The case of a periodic system with isolated spherical obstacles is studied, and a general relation between D, the excluded volume and the dimensionality d of the problem is obtained. The case of straight, infinitely long fibers placed (periodically and randomly, with and without crosslinks) along the three orthogonal axes is also examined. Finally, a representation of a realistic gel formed of irregular, crosslinked fibers placed along random directions is described. Our diffusion data for pseudo-spherical particles in the continuum limit are compared to the predictions of various obstruction models of migration.

[S36.028] Electrophoresis of Composite Molecular Objects: The Relation between Friction, Charge and Ionic Strength in Free-Solution

We studied the mobility of streptavidin end-labeled ssDNA fragments in free solution capillary electrophoresis. Our data show that the contribution of the streptavidin label to the net electrophoretic mobility of the complex depends on the buffer ionic strength because it is actually related to the persistence length of the ssDNA fragment. Therefore, the ionic strength of the buffer affects both the free-solution mobility of naked ssDNA and the effective slowing down due to the streptavidin label. We show that our experimental results can be interpreted using recent polyelectrolyte electrophoresis theories. We thus confirm that the so-called free-draining picture of local ssDNA electrophoretic friction and charge is not valid.

[S36.029] Migration of Long Polyelectrolytes in a Structured Microfluidic Channel

We report on the computer simulation of a polyelectrolyte confined in a sub-micron size channel. An applied electric field drives the charged macromolecule, while deep wells placed periodically along the channel act as entropic traps that hinder displacement. We thus attempt to echo recent experiments investigating the behavior of long DNA strands inside microfluidic structures fabricated on chip. The simulation is performed in two dimensions using a Monte Carlo bond fluctuation algorithm. The DNA molecule is modelled by a self-avoiding chain, and hydrodynamic interactions are neglected. Realistic electric field lines inside the channel are calculated, and both DC and AC fields are considered. The mobility of the DNA is obtained as a function of field, channel geometry and molecular size. Our results uphold the idea that simple microfluidic devices can serve as effective tools for polymer size separation.

[S36.030] Structure Formation in Salt-Free Solutions of Amphiphilic Sulfonated Polyelectrolytes

Self-assembled systems have long attracted attention due to their practical importance in many technical and biological fields. Dodecyl-substituted poly(para-phenylen)sulfonates (abbreviated PPPS) are highly charged polyelectrolytes which in the uncharged state have been investigated extensively and an intrinsic persistence length of 15 nm has been reported. Due to their hydrophobic side chains, PPPS are compatible with water only as micellar aggregates and tend to form supramolecular structures even at concentrations as low as 10^-5mol_mon.units/l. Because of the rodlike conformation of PPPS, this self-assembly leads to aggregates of anisotropic shape. Therefore, depolarized light scattering was employed to yield complementary information about structure and dynamics of these complex fluids. Aqueous solutions of PPPS at room temperature undergo a structural transition at a critical concentration of c_crit.=0.016 g/l. This transition is characterized by a strong increase of scattered intensity in forward direction and dynamic depolarized scattering. Above c_crit. the cylindrical micelles (L=310 nm, d=3.1 nm, N_radial=12) self assembly into large ellipsoidal clusters of size in the \mu m range. Due to the strong increase of depolarized scattered intensity there has to be a preferential orientation of the micelles inside those clusters, which thus represent a lyotropic mesophase. By combining static and dynamic light scattering for the low q-range as well as small angle x-ray scattering for the higher q-range it is possible to determine size and shape of each aggregation step. \par Decreasing the molecular weight of the PPPS has profound influence on the micellar length and hence on c_crit. which is close to the overlap concentration (c \sim 1/L^3) allowing for the observation of the polyelectrolyte effect.

[S36.031] Molecular Dynamics Study of Poly(ethylene oxide) in Aqueous Solutions

We have performed molecular dynamics simulations of Poly(ethylene oxide)(PEO) in aqueous solutions. Polymer conformations and structure of the solution were investigated as a function of polymer molecular weight, solution composition and temperature. Systems of PEO chains with molecular structure H-[-C-O-C]n-H where n=2,3,12 and 54 were investigated for concentration domain of polymer weight fraction 0.1-1.0 and temperature range 298-450 K. Local conformations of polymer and hydration structure were found to be weakly dependent on molecular weight. We found that local conformations can be conveniently analyzed in terms of O-C-C-O sequences (conformational triads), which could be classified as "hydrophobic" and "hydrophilic" based on the composition dependence of their populations. The influence of temperature and composition dependencies of 1) populations of hydrophilic and hydrophobic conformers; 2) the extent of PEO-water hydrogen bonding; and 3) the PEO global conformations and correlations on the phase behavior in these systems is discussed.

[S36.032] Competitive adsorption of polyelectrolytes and model proteins

We study the competitive adsorption between polyelectrolytes and model charged proteins on charged surfaces. We use a molecular mean field theory which includes steric, van der Waals and electrostatic interactions. The equilibrium amounts of adsorbed flexible polyelectrolyte and model protein are obtained as a function of the charge density on the surface, the bulk salt concentration, the degree of ionization of the proteins and the polyelectrolyte charge distribution. The role of the flexibility of the polymer chain on the adsorption behavior will be discussed in detail. Some arguments for the competitive adsorption kinetics will also be presented.

[S36.033] Use of a gradient programmer to study the polyelectrolte effect

We present the results of a viscometric study of the polyelectrolyte effect in xanthan, a stiff polyelectrolyte, dissolved in water with no added salt. These results are obtained by a novel method (Reed, W. F.,private communication) incorporating a gradient programmer to vary the concentration. This greatly speeds the acquisition of data as a function of concentration, and makes possible certain experimental geometries that are difficult or impossible otherwise. Unfortunately, it also complicates the analysis of data. Issues such as dead volume between detectors and high shear in capillary viscometery will be discussed, and their effect on the interpretation of viscometry data will be addressed.

[S36.034] Effective-Medium Gaussian Chain Theory for Nondilute Polymer Solutions Confined to a Slit

Theoretical formulation for the thermodynamics of a nondilute polymer solution in a confined space is presented. The interactions between polymer chains are taken into account as an effective potential field in which a chain of the Gaussian conformation grows. The dependence of the potential on the concentration was determined so that the osmotic pressure of the bulk solution reproduces the one calculated in the renormalization group theory. The chemical potential, the density profile, and the dimension of the polymer chain were calculated as a function of the concentration for different slit widths. Comparison of the results with those obtained in the computer simulations reveal that chains can enter the slit space more easily and the depletion layers at the slit walls are thinnner than expected for the Gaussian chain. To improve the approximation, we took into account the monomer density correlations and a two-dimensional nature in the chain contraction. We found the latter was more effective to bring the theoretical result closer to the simulation results.

[S36.035] Influence of Water Molecules on the Structure and Dynamics of a Polymer Electrolyte

The local structure and segmental dynamics of polymer electrolyte PEO-LiClO4 with addition of water were studied using neutron diffraction and quasi-elastic neutron scattering (QENS). The neutron diffraction data measured from lithium isotope substituted samples show that both the structure around Li cation and environment about counter anion are altered by the introduction of water. This study clearly demonstrates that the water molecules interact with both Li ions which are coordinated by ether oxygens and counter anions which are separated from Li ions by polymer chains. In addition, the QENS data indicates that segmental relaxation of the polymer chains in electrolyte is strongly influenced by the presence of water molecules. These results explain the observation that the variation of conductivity during the absorption of water into polymer electrolytes. This work was supported by Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy, under contract No. W-31-109-ENG-38.

[S36.036] Comparison of STEM and SAXS Data from Ethylene-Based Ionomers

Previous work in our group has determined the size, shape and spatial distribution of the ionic nano-aggregates in poly(ethylene-ran-methacrylic acid) [P(E-r-MAA)] partially neutralized with Zn. The nano-aggregates are spherical in shape and are randomly and uniformly distributed throughout the material. The nano-aggregates are nearly monodisperse in size with a mean diameter of ~ 2.1 nm. Their size is independent of degree of neutralization and thermal treatment. SAXS data from these samples will be quantitatively analyzed using the size and shape information from STEM to determine the electronic density difference between the matrix and the nano-aggregates.

[S36.037] Ionic Nano-Aggregates in Styrenic Ionomers: Influence of Ion-Pairs, Neutralization Method and Thermal History

Ionomers based on styrene have been studied using scanning transmission electron microscopy (STEM) to directly image ionic nano-aggregates. Additional morphological information has been obtained by small-angle x-ray scattering (SAXS). The influences of ion-pairs, neutralization method and thermal history on the shape, size, size distribution and spatial distribution of ionic nano-aggregates have been surveyed. Ion-pairs are formed by the neutralization of carboxylic acid or sulfonic acid groups with Cs and Zn. Neutralization methods include solution neutralization and drop-wise neutralization. Thermal treatments include 0, 18, 72 and 240 hours at 120°C. Preliminary STEM results indicate that the ionic nano-aggregates exhibit a variety of shapes and sizes not previously considered.

[S36.038] Modeling Failure in Filled Polymer Composites

The use of nano sized fillers in polymer composites have been shown to dramtically increase the toughness of the composite. However, the mechanism by which these fillers improve the strength of the polymer matrix is still not clearly understood. Here, we use Molecular Dyanmics simulations to study the effect of filler geometry and interaction between the filler and the polymer matrix on the molecular mechanisms that control failure in these systems. We vary both the temperature and the rate at which the composite is loaded to study any rate dependent effects in these composites.

[S36.039] Organically Modified Aero-Sol Gel Silica for Elastomer Reinforcement

We have developed facilities to produce organically functionalized silicas using a novel, room-temperature, aerosol, chemical reactor (ASG reactor). This reactor can produce exceedingly high surface area nano-structured materials (up to 800 m2/g) with tuned interfacial chemistries.

This poster will present our results on dynamic mechanical properties of elastomer compounds with ASG-organically modified silicas and comparison with conventional carbon black, conventional precipated and fumed silica as well as blends of the conventional materials. The mass-fractal structure as determined by SAXS and SALS, as well as conventional gas and DBP absorption measurements and microscopy will be presented.

Hyeon-Lee, J.; Beaucage, G.; Pratsinis, S. E. (1997) Chem. of Mat. 9, 2400. Hyeon-Lee, J.; Beaucage, G.; Pratsinis, S. E.; Vemury, S. (1998) Langmuir 5751.

[S36.040] Percolation of Filled Rods in a Phase-separating Blend

We study the phase separation of a 30/70 binary blend with filled rod-like particles. The rods are preferentially wet by the minority component of the binary mixture and thus are forced by the blend to be localized in restricted areas. The rods are aligned due to this restriction and form percolating network. On the other hand, the minority fluid is also stretched by the anisotropic shape of rods and a fluid percolation is also formed even though the system is very off-critical.

[S36.041] Photo-Induced Morphology Development in Free-Radical Initiated Polymer Dispersed Liquid Crystal Films

Agile optical elements consisting of random (PDLCs) or gradated (HPDLC) dispersion of liquid-crystal (LC) droplets in an amorphous polymer matrix are of intense interest for display, shutter and beam steering applications. Morphologies are created through photoinduced polymerization and subsequent phase separation of the liquid crystal from a multi-functional monomer syrup. Recent efforts with highly functional free-radical monomer and non-reactive LC molecules indicate that gelation of the matrix resulting from the rapid increase in molecular weight associated with the free-radical polymerization occurs in concert or before phase separation. Additionally, the size, shape and distribution of the subsequent nanoscale liquid crystal droplets are very sensitive to composition of syrup (LC content and presence of surfactant molecules). Static small-angle X-ray scattering and high resolution scanning electron microscopy (SAXS/HRSEM) were utilized to examine the two-phase structure of these polymer dispersed liquid crystal films (PDLC). The effect of varying the composition of a syrup on the resulting two-phase morphology was explored. Static SAXS experiments indicate that increasing the LC content increases the samples' nanostructural heterogeneity as indicated by increases in intensity at low q and the invariant (Q). These trends are supported by HRSEM analysis, which confirm changes in the two-phase morphology evidenced in the SAXS patterns. Porod analysis also revealed a systematic increase in the internal surface area as the LC concentration was increased. Curing of the system in the beam at a synchrotron during data collection revealed unexpected temporal development of the scattered intensity with time. Initial dynamic results will be presented.

[S36.042] Polymer Adsorption from Supercritical Fluids: Technique Development and Preliminary Results

In this presentation an in-situ, high-pressure, surface plasmon resonance (SPR) spectroscopic technique is described and used to investigate the adsorption/desorption behavior of polymers from supercritical fluid (SCF) solutions. The goal of this work is to develop an understanding of the physico-chemical properties of the polymers and the SCF solvents that determine the properties of the polymeric surfaces formed at varying distances in pressure-temperature (P-T) space from a phase boundary. The utility of the SPR technique is demonstrated in an experimental program with three distinct features. I) Phase behavior experiments are performed to map the P-T space where the polymer dissolves in solution. II) Polymer adsorption/desorption experiments are performed, in-situ, using the newly-developed SPR technique to monitor the kinetics of this process. III) The polymer coating is characterized with techniques such as atomic force microscopy to relate film properties to process conditions. The results from the SCF experiments are compared to those with the same polymer dissolved in a liquid solvent.

[S36.043] Effects of pressure and temperature on the static and dynamic properties of PE via NpT Molecular Dynamics Simulations

Isobaric/Isothermal Molecular Dynamics Simulations of a melt of 100 bead united atom polyethylene chains have been performed at multiple pressures far above Tg. The trajectories are validated via activation volume and enthalpy calculations in addition to transition rates. The pressure dependence of torsional cooperativity is presented for the first time. Local segmental motion is analysed via changes in the distribution of relaxation times for geometric autocorrelation functions as a function of pressure and temperature.

[S36.044] Effect of Solvent Structure and Polymer Architecture on Polymer Conformational Behavior Using High-Pressure Dynamic Light Scattering

A high-pressure dynamic light scattering (DLS) technique is described for characterizing the conformational behavior of a polymer in a high-pressure solvent environment. This DLS technique is used to investigate the changes in conformation and mobility of a polymer as it responds to systematic changes in solvent quality and polymer architecture. The focus of this presentation is the characterization of several polyolefin-alkane solvent systems. These model systems are selected because of their closely matched energetics. The impact of polymer architecture on conformational behavior is studied by using a well-characterized polybutadiene with varying amounts of ethyl branches in the polymer backbone. The impact of solvent structure on polymer conformational behavior is studied using three pentane isomers, n-pentane, isopentane, and cyclopentane. For each pentane isomer, solvent quality is also controlled by varying both pressure and temperature in the single-phase region allowing characterization of the conformational behavior of polymer-solvent mixtures very close to and far away from the phase boundary. This work compliments the Small Angle Neutron Scattering (SANS) data collected by our group on the same polymer-solvent mixtures.

[S36.045] A study of the annealing of poly(ethylene-co-octene) by standard DSC

The annealing behavior of ethylene-octene-copolymers with 12 and 25 mass-% 1-octene was analyzed. The irreversible annealing process consists of, at least, two different events, crystallization and reorganization. These two processes are separated and quantitatively characterized by exponential laws with different time constants. Annealing at constant temperature leads to arrested, global equilibria. At lower temperatures the system continues to crystallize and is again far from this equilibrium which requires continued annealing. Simultaneous to the annealing process there is a considerable amount of reversible crystallization and melting. This process is best described as a temperature-dependent local thermodynamic equilibrium. --- Supported by the Div. of Materials Res., NSF, Polymers Program, Grant # DMR-9703692 and the Div. of Materials Sci., Office of Basic Energy Sciences, DOE at Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corp. for DOE, under contract number DE-AC05-96OR22464.

[S36.046] In-situ X-ray scattering study on melting of polybutylene succinate

Multiple melting endotherms on differential scanning calorimeter (DSC) heating scans have been observed in many semi-crystalline polymers. Extensive studies including the time-resolved X-ray scattering have been conducted to elucidate the origin of the multiple melting endotherms. Among several models proposed to account for the multiple melting endotherms, dual or more lamellar thickness model and melting-recrystallization are the main two models applied to the interpretation of multiple melting endotherms. However, evidences to support the different models are back and forth and the origin of this phenomenon still remains the subject of debate. In the course of studying the melting behaviors of polybutylene succinate (PBS) in our laboratory, we also observed the multiple melting endotherms during the DSC heating scan. In order to elucidate the structural origin of the multiple melting endotherms observed in the PBS, we conducted the time-resolved small and wide angle X-ray scattering experiments utilizing the symchrotron radiation source. In this presentation, we will discuss the melting mechanisms associated with the multiple melting endotherms based on our unique SAXS and WAXS patterns and their changes on heating.

[S36.047] Crystallization of polyethylene blends at high Peclet numbers

Crystal growth velocity, growth arm spacing and growth arm thickness have been measured for binary blends of high density polyethylene and two different highly branched metallocene polyethylenes. These blends crystallize at high Peclet numbers, in contrast to low Peclet numbers for other blends for which such measurements have been made. The ratio of growth velocities of blend and neat polymer is found to be approximately the mole fraction of the slowly crystallizing species, whereas the morphological parameters are nearly independent of concentration. These results are contrasted with those for blends which crystallize at low Peclet numbers, and for which diffusion control of the morphology is important. It is suggested that redistribution of polymer species at the growth front occurs by convection in the present case.

*Work supported under NSF/GOALI grant DMR-9629825.

[S36.048] Flory Model of Polymer Crystallization, Kauzmann Paradox and Gibbs-DiMarzio Theory of Glass Transition

The Flory model of crystallization of polymers is well known and forms the cornerstone of the Gibbs-DiMarzio theory of glass transition. The model has no known exact solution and the original calculation [1] was shown to be incorrect [2]. Still it is interesting to know the order of the phase transition, if it has one. We have studied the thermodynamics of the model in the limit of infinite molecular weight. We have solved it exactly on a recursive lattice with coordination number q=4, relevant for a tetrahedral lattice. Our results show that there is a continuous, i.e. a second-order, transition at which the entropy of the system is continuous. It is finite at all temperatures and approaches 0 as T goes to 0 so that the system is never completely ordered except at T=0. As the temperature is raised above T=0 the system begins to disorder with a degree of disorder that depends on T, which is in accordance with the analysis of Gujrati and Goldstein [2]. Since there is no first order transition there is no Kauzmann paradox. Similarly there is no possible metastable extension in the model which is central to the Gibbs-DiMarzio conjecture for an ideal glass transition. Thus, our solution does not justify their conjecture.

[1] P.J. Flory, Proc. R. Soc. London Ser., A234, 60 (1956) [2] P.D. Gujrati, J. Phys. A: Math. Gen., 13, L437 (1980), P.D. Gujrati, M. Goldstein, J. Chem. Phys., 74(4), 2596 (1981)

[S36.049] Relationship between Crystal Thickness and Isothermal Crystallization Temperature for Determination of Equilibrium Melting Temperature for Syndiotatic Polypropylene

Syndiotatic polypropylene (sPP) was used to investigate the relationship between isothermal crystallization temperature (T_c = 70-115^oC), crystal thickness and subsequent melting using simultaneous synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) in conjunction with DSC. The thickest lamellar crystals melt at the end of the DSC endotherm. At this temperature, the SAXS intensity (corrected formelt scattering) showed a diffuse profile, and the crystalline feature in the WAXD pattern was completely absent. This crystal thickness was estimated using an approach based on the single lamella structure factor, which will also be compared to the value determined by the interface distribution function. The equilibrium melting temperature obtained this way will be contrasted by other methods such as the Hoffman-Weeks approach.

Acknowledgement: This work was supported by by a NSF grant (DMR 9732653).

[S36.050] Dynamical heterogeneity and non-Gaussianity in glassforming liquids and polymers

There is increasing evidence that cooperative particle motion and dynamical hetereogeneity play an important role in the transformation of a supercooled liquid to a glass. It has also been long recognized that the distribution of particle (or monomer) displacements in such liquids is strongly non-Gaussian at intermediate times, exhibiting a long tail to large displacements. In particular, the non-Gaussian parameter introduced by Rahmann, which characterizes the shape of the distribution, looks qualitatively similar to generalized susceptibilities recently introduced to quantify dynamical heterogeneity. As yet, however, the fundamental connection between the two is unclear. In this paper we use molecular dynamics simulations of model supercooled liquids and polymer melts to explore the relationship between the non-Gaussian nature and cooperative nature of particle and monomer motion above the glass transition.

[S36.051] NMR relaxation study of components dynamics of PI/PVE blend system

Although the PI/PVE(polyisoprene/polyvinyl ethylene) blend is well known as a miscible polymer blend system, there is still considerable controversy about the dynamics of the individual components in this blend. In this study, variable field NMR relaxation measurements have been performed on melt states of PI, dPVE and their blends. Five samples having different compositions of PI/dPVE (100/0, 70/30, 50/50, 30/70 and 0/100 in wt.) were made from PI(M_n = 1,400, MWD = 1.11) and dPVE(M_n = 2,250, MWD = 1.05). For these samples, ^13C-T_1, NOE and ^2H-T_1 have been measured through a wide range of temperatures. The effect of blending on components dynamics approximately corresponds to some temperature change for both components. In comparison with pure samples, while the dynamics of PI in blends have been decreased to the extent corresponding to 10 - 25 K temperature decrease, we have found that blending cause dPVE dynamics to be increased to the extent corresponding to temperature elevation of 15 - 30 K. Computer simulations of this blends system are also in progress.

[S36.052] Effect of Linkage Groups on Motional Cooperativity in Secondary Relaxations of Some Glassy Polymers

In our previous research we found that the secondary relaxation of bisphenol-A (BPA) polycarbonate is due to a cooperative motion which includes several repeat units. Clearly the linkage between neighboring BPA segments must play an important role. It is expected that a more flexible linkage would require less motional cooperativity. To demonstrate this idea, polyformals based on the comonomers BPA, tetramethyl-BPA and methylenechloride with controlled sequence structure were synthesized and DMA studies were conducted. The results show that the secondary relaxation behavior of copolyformals is not dependent on the block length of BPA segments, which suggest that the molecular motion does not require in-chain cooperation. The lower in-chain motional cooperativity of BPA-polyformal is attributed to the lower rotational energy barrier of the formal linkage.

[S36.053] Segmental Dynamics in a Model Blend of Alkanes

C_24H_50 and C_6^2H_14 blends have been investigated as a model miscible polymer blend. We have measured the segmental dynamics of C_24H_50 and C_6^2H_14 blends, using ^13C T_1 and NOE measurements in the C_24H_50 component and ^2H T_1 measurements in the C_6^2H_14 component. We have performed the T_1 and NOE measurements as a function of temperature and composition. Use of the low molecular weight alkanes permits differentiation of the chain ends, and provides a monodisperse system in both components. From these measurements, correlation times can be calculated for the C-H bond motions of the alkane chains. At 64°C, the dynamics of the larger molecule change by a factor of 3 across the composition range, while the dynamics of the smaller molecule change by a factor of 4. A comparison with molecular dynamics simulations is planned.

[S36.054] TEST FOR DISPERSION OF HYPERSONIC WAVE VELOCITES IN NITROBENZENE USING BRILLOUIN SCATTERING TECNIQUES

Brillouin Scattering was used to measure the velocity of hypersonic waves in liquids at frequencies from 2.5GHz to 7GHz. The hypersonic wave velocity at this range of frequencies was measured for water and acetone. These values were found to be constant as frequency was changed and the results compared to the standard values for the wave velocity of 1487 meters per second for water and 1177 meters per second for acetone as was expected. The hypersonic wave velocities were then measured for nitrobenzene at a broad range of frequencies. The results showed continuously increasing velocities starting at 1350 m/s at 2.5GHz to 1540 m/s at 7GHz. Consequently this data shows dispersion in nitrobenzene for the frequencies tested.

[S36.055] Solvent-Induced Crystallization in Poly(Ethylene Terephthalate)

The solvent transport in poly(ethylene terephthalate) (PET) and related phase transformation were investigated. Based on Harmon's model for Case I (Fickian), Case II (swelling) and anomalous transport, the data of mass uptake were analyzed. Pure Case I or Case II behavior did not appear in the PET-acetone system. The mass transport in PET is accompanied by a large-scale structural rearrangement, which leads to the induced crystallization of the original amorphous state. Solvent-induced crystallization was studied by wide angle X-ray scattering (WAXS), small angle x-ray scattering (SAXS), Differential Scanning Calorimeter (DSC), and Fourier Transform Infra-Red (FTIR), which is different from the thermal crystallization. During this process, the matrix is under compressive stress which causes different kinetic path of crystallization in comparison with that by thermal annealing. The crystallization process was proposed in terms of the long period L, the crystal thickness lc and the thickness of amorphous layer la, calculated from the linear correlation function and interface distribution function. The variation of trans conformation is used to monitor this process.

[S36.056] Effect of hydrogen termination of Si surface on the growth of silver films on Si(111)

HF-etching of Si substrates is known to passivate Si surfaces against oxidation by hydrogen termination of Si dangling bonds. Contact angle measurements of tiny water drops on Si(111) surfaces with and without etching show 90^o and 65^o, respectively, indicating a lower surface free energy for the former. We have investigated the structure and morphology of Ag films (t = 30, 70 and 120 nm) deposited in an MBE system on Si(111) substrates with and without HF-etching. In situ RHEED, and x-ray diffraction measurements show highly ordered epitaxial Ag<111> growth on HF etched Si(111) and a polycrystalline growth on unetched Si(111). AFM images show a relatively smooth surface on the former, although the RMS roughness increases with the increase of film thickness. Results of cross-sectional high resolution TEM studies will be presented.

[S36.057] Varying the growth of Manganese on Si(111) with temperature and coverage

Mn is known to form islands with a \sqrt3 \times \sqrt3 reconstruction when deposited onto a heated Si(111) surface. The minimum height of these manganese silicide islands is 4 Åand can be varied by changing the rate of Mn deposition and the deposition temperature of the Si(111) substrate. This dependence will be shown through the statistical analysis of scanning tunneling microscopy images. In addition the deposition location dependence for submonolayer coverages of Mn will be discussed. The probability of Mn clusters to occupy the faulted half of the Si 7 \times 7 unit cell changes depending on the deposition temperature of the substrate, and whether the annealing is done during the Mn deposition, or post deposition.

[S36.058] Noble Metal Row Growth on Si(5 5 12)

The growth of one-dimensional, nanometer-scale structures is of current interest. A viable template for such growth is the high-index Si(5 5 12) surface, because it forms a single domain reconstruction composed of row-like features. These rows consist of structural units found on other Si surfaces, such as pi-chains, dimers, and tetramers. We have recently examined the growth of noble metals on this surface (Au, Ag) using scanning tunneling microscopy (STM). At moderate annealing temperatures (450 C) and low coverages (below 0.25 ML), they form an array of overlayer rows. These rows nucleate along the more reactive Si tetramer rows, and have an inter-row spacing equal to the underlying substrate (5.4 nm). At higher temperatures (550 C+), the Au and Ag rows develop slightly different internal structures and remove all underlying Si rows. As the coverage is increased to 0.5 ML, Ag forms the most ordered types of rows with strong 2x and 3x periodicity along the row direction. Au is much more disordered, but continues to demonstrate row-like growth with weak 3x periodicity. At much higher temperatures (800 C), Au causes facets to occur where the rows again become ordered. In summary, all of the coverages and temperatures investigated here result in row-like structures, indicating that Si(5 5 12) is a promising template for the growth of 1D nanometer-scale structures.

[S36.059] Metastable-atom stimulated desorption of H^+ ion from H_2O adsorbed alkalated Ni(110) surfaces

Metastable helium atoms (He*) have attracted much attention not only as a primary beam for metastable deexcitation spectroscopy (MDS), which provides information about electronic states on the outermost surface, but also as a potential tool for lithography. In addition, we have recently discovered the ion desorption stimulated by the impact of He* [1], which may also be used for the characterization of the outermost surface. In the present study, we have clarified that He* stimulates H^+ desorption from the K- or Na-covered Ni(110) surface adsorbed by H_2O molecules. Measurements were carried out using a pulsed-discharge type He* source and a time-of-flight method. The kinetic energy of He* was found to be less than 0.3 eV. It is concluded that H^+ desorption by He* is caused by a hole created on the valence levels via the Auger deexcitation of He*. The H^+ desorption by He* may be understood within the framework of Menzel-Gomer-Readhead (MGR) model.

[1] M. Kurahashi and Y. Yamauchi, submitted to Surf. Sci.

[S36.060] Stability of the trace map and localization in one-dimensional chains

We show that the localization properties of one-dimensional chains can be studied using the stability of the trace map of the transfer matrices. This approach shows that the nature of the fixed points of the map, determine the nature of the spectrum. We also show that the localization properties of the wave-function can be obtained using the Lyapunov exponents of the trace map, used in the sense of chaos theory. As an example of this approach, we investigate localization in the Harper equation.

[S36.061] Electronic structure of extended defects on the (111) surfaces of noble metals.

We describe the use of multiple-scattering methods for determining the electronic structure in and around extended defects on metal surfaces, with particular emphasis on long-range effects due to surface state scattering. Here we are interested in making contact with STM studies near islands and steps on noble-metals, and in quantifying the interactions in the near-surface region.

In our approach we avoid impractical supercell/slab approximations by solving exactly the Dyson equation for the defect introduced as a perturbation to the clean surface; for steps we exploit periodicity along the step edge. We discuss some of the numerical aspects of our solution to this problem.

To illustrate the method we give results obtained for various systems including atomic corrals, terraces formed from ascending and descending steps, dressed step edges, and for supported magnetic atom chains.

[S36.062] Spin-Polarization of Potassium Adsorbed Fe(100) Surfaces

The spin-dependent electronic structure of ferromagnetic surfaces and their interaction with adsorbates have been studied to explore the surface specific magnetic properties. While spin-sensitive electron spectroscopies have been mostly used to investigate several surface layers, spin-polarized metastable deexcitation spectroscopy (SPMDS) has proved its extremely high sensitivity for detecting the spin polarization of adsorbates and the outmost surface of ferromagnetic materials. Most of surfaces studied by SPMDS are high work function surfaces where the Auger neutralization (AN) process takes place after the resonance ionization of metastable helium atoms. The MDS spectrum for the AN process reflects the self convolution of the occupied state density. On the contrary, at a low work function surface the Auger deexcitation process takes place primarily and the MDS spectrum reflects the occupied state density directly. We have conducted SPMDS measurements, using a spin-polarized metastable helium atom beam, for potassium adsorbed surfaces of iron films deposited on MgO(100) substrates. The SPMDS spectra clearly show a negative polarization at the potassium 4s band to the majority spin of the iron film.

[S36.063] Phase change properties of Ge2Sb2Te5 compared to Ge4Sb1Te5 with respect to data storage applications

To be able to adjust material properties to the demands of rewritable optical storage applications (high data density and transfer rates) we have investigated and compared the phase change characteristics of thin sputtered Ge2Sb2Te5- and Ge4Sb1Te5-films. Both crystallize into a rocksalt structure at 157C, and 150C, respectively. The material with the higher content of Ge shows a significantly higher activation energy (EA=3.7eV in comparison to EA=2.24eV), as confirmed by temperature dependent electrical measurements. This results in a larger incubation time for laser modification on the ns-scale. Ge2Sb2Te5 shows a second transition into a rather complex hexagonal structure at 310C (EA=3.64eV). The optical properties of both phases are slightly different. Laser modified areas are always in the first phase as confirmed by TEM and SAD. Hence there is a risk of a coexistence of two phases which would lead to an increase of the noise level in storage applications. This can be avoided by using a crystalline matrix with rocksalt structure. By capping the single phase change films with a thin dielectric layer the transition temperatures and activation energies are increasing for both materials, which might be attributed to changes of the tension state at the interface.

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[S36.064] Time Limiting Factors of Laser Induced Amorphization and Crystallization on the Micron Length and Nanosecond Time Scale for the Optical Data Storage Medium Ge2Sb2Te5

We have studied the reversible amorphization and crystallization as a function of applied laser power and pulse length in order to identify the time limiting processes. Amorphization occurs as soon as the melting temperature is reached. After the pulse ends the melt quenches into the amorphous state. There is no evidence of kinetic superheating, since the melting temperature turns out to be independent of the pulse length. This leads to an elementary scaling law relating the laser power P with the respective minimum time t(P) for amorphization: the square root of t(P) is proportional to one over P. For the crystallization of as deposited amorphous films we have found a threshold time of 100 ns. This time limit is identified with the minimum incubation time needed to reach the steady state nucleation rate. Hence the limiting process is the formation of critical nuclei. In contrast the investigation of the crystallization of laser produced amorphous marks reveals no incubation time. The complete erasure of amorphous marks is possible within 10 ns. This shows the presence of quenched-in nuclei inside the amorphous marks. Thus in that case the limiting process is the growth of the preexisting nuclei.

[S36.065] Static Symmetry and Dynamic Symmetry at Critical Point

Symmetry is the ``same measure" and related to similarity. When temperature (T) is lower than the critical temperature T_c, any system will form a static structure as symmetric as possible, such as the formation of perfect crystal after phase transition. Above the critical temperature T_c the system will assume the highest possible dynamic symmetry (isotropicity and homogeneity). After rejection of the Gibbs paradox statement (discontinuous similarity-entropy relation) and higher symmetry-lower entropy in statistical mechanics textbooks, we have established a new theory of higher similairty-higher symmetry-higher entropy-higher stability relation. Symmetry is not an ``yes-or-no" (symmetric or nonsymmetric) property, it is continuous. Gibbs' distinguishability-nondistinguishability (an ``yes-or-no" property) and its relation to entropy has been changed to a continuous similarity-entropy relation. Gibbs paradox statement (Entropy of mixing to form solid assemblages, liquid and gas mixtures or any other analogous assemblages such as mixed quantum states, decreases with the increase in the property similarity of the subsystems) has been rejected. The new theory (similarity principle, or the higher entropy-higher similarity relation, see: http://www.mdpi.org/lin/similarity/similarity.htm) can be used to solve all the outstanding problems in phase transitions. This new theory conforms very well with and explains perfectly all the symmetry breaking phenomena. Reference: S. -K. Lin, J. Chem. Inf. Comp. Sci. 36 (1996) 367-376. Downloadable from www.mdpi.org/lin.

[S36.066] A Method for the Determination of the Dielectric Tensor of Arbitrarily Oriented Anisotropic Thin Films Employing a Rotatable Polarizer

We introduce a method to measure the dielectric tensor in anisotropic thin films. The method employs a rotatable polarizer, thereby eliminating the need for sample rotation or changes in angle of incidence. Employing a general theory for analyzing light propagation in an anisotropic medium, relationships are derived for use in ellipsometry measurements. The technique is used to determine the indices of refraction and the principal axes for a calcite crystal, demonstrating the validity of the method. In addition, we describe the use of the technique and associated theory for thin films and interfaces.

[S36.067] Using 2nd sound triangulation for the localization of hot spots

Some applications of superconductivity use liquid He below 2 K. Among these are test stands for evaluating superconducting accelerating cavities. Some of these tests stands contain equipment for the detection and localization of hot spots. Brute force approaches using tens and sometimes hundreds of contact thermometers have been used for this purpose. To the present apparently no use have been made of the 2nd sound property of HeII for detecting and locating these hot spots. An effort to do just that is under way in the Ramp;D phase of one of the high energy physics experiments being considered at Fermilab. The status and details of this effort will be described.

[S36.068] Determination of pore-size distribution in thin films using PALS

There are few probes capable of characterizing nano/meso-scale porosity in thin (sub-\mum) films, especially when the pores are closed and inaccessible to gas absorption methods. Positronium Annihilation Lifetime Spectroscopy (PALS) is used to determine the pore size distribution in low-K meso-porous methylsilsesquioxane thin film which appears to have a nominally closed pore structure. The standard calibration for converting Ps lifetimes to pore sizes fails for pores larger than a few nm. An extended calibration has been developed and is compared with data over a wide range of pore sizes. A model of Ps trapping and annihilation in isolated pores is presented and is used to convert continuum lifetime distributions (determined using the lifetime fitting program CONTIN) into pore size distributions. The results are compared with TEM analysis. Work supported by NSF grant ECS-9732804 and SEMATECH.

[S36.069] Reducing Gas Sensing Mechanisms in Thick Porous SnO_2 Layers

\begin center Abstract \end center

\vspace1cm

\parindent0.5cm A quantitative model of the gas-sensing processes occurring at the surface of thick porous tin-dioxide layers is presented. The model is based on the assumption that the conduction mechanism is governed by the Schottky potential barriers at the junctions between adjoining grains. The potential barrier heights are modulated by the surface coverage of the SnO_2 grains with negatively charged chemisorbed oxygen species.

To describe the interaction of reducing gas molecules with the chemisorbed oxygen, a method of ``conditioned adsorption'' has been developed. The central idea is to assume that the reducing gas molecules are ``adsorbed'' (i.e. react) only on pre-adsorbed oxygen.

Experiments were performed to characterize the conductance variation with changes in concentration of ethanol C_2H_5OH vapors in air for a SnO_2 -based gas sensor. The theoretical response curve, which was calculated by using this model, was compared with our experimental results. The predictions made my the theoretical model are in good agreement with the experimental data.

[S36.070] Magnetic Screening Of Wires & Other Resistive Sheath Structure In The Sanford Effect Range With New Conductivities

Because of array stability interest, individual Al wires in the range[1] r=6 to 12mum amp; Imax=8e4 to 1.6e5 Amp are studied by simulation with ALEGRA and analysis, using I(t) forms of the Sandia Saturn generator w,w/o pre-pulses amp; qualitatively improved conductivities(sig)[2]. Radial motion of expanded wires is found dominated by resistive quasi-static(RQS) magnetic field behavior(skin depths exceeding dimensions) because sig is greatly reduced by the Mott transition, giving approximately Ez(r)=const. Then J(r) amp; Joule heating \simsig(r), amp; wire coronas are first cooled by expansion amp; then heated relative to cores, first because sig/Cv*rho is larger and then unstably thru sig\simT^3/2 at low rho. Jmax moves from core to corona, largely SCREENING Bth from cores, integral sig*dAz increases, amp; voltage falls. A thin surface tamper of insulator reduces screening amp; voltage fall. Nearly hydrostatic balance with RQS JxB forces is seen except when rapidly rising I(t) recompresses wires. Collective resistivity[3] also occurs in the low rho corona tips. [1]Sanford et.al. PRL 16Dec96 [2]Desjarlais BulAPS Nov99 [3]Forslund et.al. PF Jul72

[S36.071] Topological invariants from stable homotopy groups

Non-trivial global topology of principal gauge bundles over space-time is known to describe many physically important properties of nature such as the existence of spin, the quantization of electric charge, and the existence of topological objects like monopoles and instantons. We use Bott periodicity to explore the non-trivial homotopy groups \pi_q(G) for a range of values of q, where G is one of U(N), Sp(N) or O(N) at large N and we discuss the physical objects which are described by \pi_q(G) at each q.

[S36.072] On the non-equilibrium at intermediate energies HIC

The local equilibrium process has been studied for the ^93Nb+ ^93Nb at different incident energies and the equilibrium has been observed at different time steps. With the evolution of time step, central region reaches equilibrium more quickly. Accompanying the emissions of energetic particles, the equilibrium can be reached gradually from center to outside. The equilibrium temperature is 5 to 7MeV, and the chemical potential is 30MeV. The thermal- equilibrium is faster than chemical one. And for the peripheral collision, the equilibrium process is also observed.

[S36.073] Nonradiating sources and the electromagnetic potentials

We report a new description of monochromatic nonradiating current distributions that is based on the electromagnetic potentials. This enables us to address the question whether the potentials associated with a spatially localized nonradiating source can possess (or not) a measurable physical significance outside the source region. In particular, it is well known that, under electrodynamic conditions, the electromagnetic fields produced by a localized nonradiating source vanish everywhere outside the source's support. We address here the question of physical observability of the associated external potentials in connection with the Aharonov-Bohm effect. We consider, in particular, the Coulomb gauge, although a physically equivalent description holds in other gauges. It is shown that, under electrodynamic conditions, the potentials in the exterior of a localized nonradiating source necessarily vanish if one makes a particular gauge choice, i.e., they are there as unobservable as the corresponding nonradiating fields. This does not necessarily hold under static conditions in which one can have non-trivial potentials with physically observable effects in regions where the fields vanish, e.g., in the Aharonov-Bohm effect. We also report a previously unknown hierarchy of nonradiating current distributions, both spatially localized and non-localized ones.

[S36.074] Purely radiating and nonradiating scalar, electromagnetic and weak gravitational sources

It has been known for some time that localized sources to the scalar wave equation and Maxwell's equations exist which do not radiate. Such sources, referred to as non-radiating (NR) sources, generate vanishing fields outside their spatial support which prevents them from interacting with nearby objects by means of their fields. Work on NR sources dates back to Sommerfeld, Herglotz, Hertz, Ehrenfest and Schott who studied these objects in connection with electron and atom models. NR sources have also appeared extensively in inverse source/scattering theories as members of the null space of the source-to-field mapping.

In this presentation, we provide a new description of scalar, vector or tensor NR sources and of a complementary class of sources, namely, sources that lack a NR part, i.e., `purely radiating' sources. We show that the class of square-integrable localized purely radiating scalar, electromagnetic or weak gravitational sources is exactly the class of solutions - within the source's support - of the homogeneous form of the associated partial differential equation relating the sources to their fields, i.e., purely radiating sources are themselves fields. As a consequence of this result, NR sources are shown to be inseparable components of a broad class of physically relevant sources, thereby having a physical significance that transcends their use in wave-theoretic inversion models. Localized NR sources are characterized in connection with the concept of reciprocity as non-interactors. The role of NR sources in absorption of radiation and energy storage is addressed. The general theoretical results are illustrated with the aid of a one-dimensional (1D) electromagnetic example corresponding to a transmission line system (equivalently, a 1D plane wave system) with uniformly distributed sources/loads.

[S36.075] Rigorous solution of transient propagation of electromagnetic waves through a medium: causality plus diffraction in time

We have found the rigorous solution of transient propagation of electronmagnetic waves through a medium. The rogorousness enables the solution to exhibit its apparent consistency with the Einstein causality. Thus, we confirm that faster-than-light or superluminal propagation of electromagnetic waves is not possible. Evanescent transmission gives rise to the diffraction in time, which is the actual reason for deformation of group propagation. Based on the principle of diffraction in time, superluminal group propagation can be understood. The findings are also instructive for understanding the time problem for particle tunneling.

[S36.076] Pressler's Quantumization of the Atom and The Left-handed Electron's Internal Structure

A new theory of superconductivity will be presented. In addition, I will reveal the answer to one of the most intriguing questions in science since the elusive electron particle was discovered by J. J. Thomson: Exactly what is the electron particle and why does it exhibit both particle and wave characteristics. Specifically, how does the electron exhibit an intrinsic magnetic field and, inside the atom, how does it exhibit angular momentum. I will describe, in detail, particle pair production. The fundamental physical theoretical parameters, the physical mass and charge, of all elementary particles is introduced. The fundamental neutron particle’s internal structure is also illustrated. The electron radius is estimated to eight significant figures. I will present a novel theory concerning atomic structure, the position and nature of the electron inside the atom, and the nature of bonding, i.e., the covalent bond is described in terms of the interactions of atomic magnetic fields. Precise bond angles and distances of the molecule are considered. This new concept is consistent with experimental evidence and adheres strictly to the valence-shell electron-pair repulsion (VSEPR) model presently used in chemistry. I will explain the atomic model concept as being a true harmonic oscillator; periodic motion of the electron at resonant frequency produces radiation at discrete frequencies or line spectra because the electron is under the action of two restoring forces.

[S36.077] Applications and New Representation for Classical Relativity

We have experimentally tried to prove the time dilation and length contraction so that we might make new equipment. During the experiment, we have measured time and length, but we've encountered unclear concepts of time for both Galilean and Special relativity. We recognized that representation of time has to be changed, also the concept of length contraction could have been explained by using representation of time. A system moving uniformly spends more time traveling than an inertial system for Galilean time relation t=t^\prime(Arthur Beiser, Perspectives of Modern Physics, McGRAW-HILL Book Company, 1969). Since time dilates, the length contraction must exist in order to be proved mathematically for special relativity. When a new concept of time has been applied to the Doppler effect, we obtain same results.

[S36.078] Zero Point Energy and the Quantum Hypothesis

Planck radiation law introduced zero point energy. Einstein-Stern speculated that zero point energy alone may imply the radiation law. Einstein dropped it in 1916, but Boyer revived it in 69. Indeed, \emphZero point energy and the quantum hypothesis are separate assumptions that do not replace each other. The Quantum hypothesis has no substitute. We show that(1)\emphBoth Einstein and Boyer solutions are different from the radiation law, and inconsistent with it. (2)Planck "obtained" zero point energy because he \emphassumed it in his derivation. (3)Einstein-Stern assumed, \emphunknowingly, the correct \hbar ømega /2 zero point energy, and not the concerning \hbar ømega . (4)\emphZero point energy and the quantum hypothesis, combined together, are equivalent to the radiation law. (5) The quantum hypothesis for the energy with \hbar ømega quanta, is equivalent to a quantum hypothesis for the entropy with \hbar ømega /T quanta. The energy radiation law is equivalent to an entropy radiation law s=\hbar ømega øver T \left[1øver e^\hbar ømega øver kT-1 +1\right]+k\ln \left[1øver e^\hbar ømega øver kT-1 \right] But \emphzero point entropy is zero even if zero point energy is not.

[S36.079] Zero Point Energy may not yield Gravitation

Sakharov proposed that gravity is a zero point energy fluctuation force. Puthoff examined U_1=- \frac \hbar c^3 \pi \Gamma ^2Re \int \limits _0^u_ce^-2uR \frac 1R^2du, the first term of the Casimir-Polder zero point energy potential, and derived Newton's gravitational force. Carlip showed that the force is non-newtonian. Puthoff responded obtaining U_1 \approx \left( - G\fracm^2 R \right)\left( \frac\fracømega _i^2 ømega _c^2 \fracømega _0^2 /ømega _c^2 ømega _i^2 /ømega _c^2 - 1 \right), and claiming that the second multiplier approaches 1.

But we note that by L'hospital rule, the second term approaches - 2ømega _i^2 /ømega _c^2, and leads to U_1 \approx 2\fracømega _i^2 ømega _c^2 G\fracm^2 R, and to a negligible repulsive force. Thus, \textitthe U_1 zero point energy potential does not imply newtonian gravitation

This is further amplified as we show that \textit Newtonian gravitation results from the potential U_N = \frac\hbar c^3 \pi \Gamma ^2 Re \left\ \int\limits_0^u_c e^ - 2uR 2u^2 du \right\, where \Gamma = G\fracm c^3 = \fracm\pi c^2 \hbar ømega _c^2 , u_c = - i\fracømega _c c, ømega _c = \sqrt \frac\pi c^5 \hbar G is Sakharov's cutoff frequency, G is the gravitational constant, m is the mass of each of the particles, c is the light speed, and R is the distance between the particles. U_N \textit is not a component of the Casimir-Polder zero point energy potential, and we don't know if it could be interpreted as some kind of a zero point energy potential.

[S36.080] A Study of Momentum and Spin Relaxation of Hot Electrons in GaAs Under Biasing Electric Field and the Stokes Parameters of Luminescence

Hot electron luminescence spectroscopy is an all optical technique that allows the study of photoexcited hot electrons in direct bandgap semiconductors under highly nonequilibrium and steady-state conditions. The energy of the photoexcited hot electrons is very large compared to their thermal energy and the electrons relax via emission of the longitudinal optical (LO) phonons. The population of photoexcited electrons cascades down from their point of photoinjection to the band minima at point reflecting their steady-state energy distribution. Throughout this process a small fraction of photoexcited electrons recombine with holes localised at the Be acceptor sites to produce a hot electron luminescence (HEL) spectrum. The polarisation properties of hot electron luminescence are investigated under linearly and circularly polarised excitations. It is found that the electric field applied along different directions of the GaAs crystal broadens the lineshape profiles of the LO-phonon peaks and consequently the degree of polarisation changes across them. The electric field induced spectral changes are explained by the field broadening model.

[S36.081] Phonons in InP/InGaAs superlattices

Infrared and far infrared reflectivity as well as Raman scattering measurements have been performed in an [(InP)_5 (In_0.49 Ga_0.51As) _8] _30 superlattice grown by molecular beam epitaxy. The far-infrared reflectivity spectra consists of the superlattice confined and interface modes plus the modes originated in the buffer layer (In_0.49Ga_0.51As) and the substrate (InP). In the infrared spectral region above 1000 cm^-1, only the interferences from the buffer layer (In_0.49Ga_0.51As) and the substrates (InP) have been observed. The geometrical parameters obtained from the analysis of the infrared and far-infrared reflectivity spectra are in good agreement with the X-ray diffraction data. In the Raman spectra, the folded longitudinal acoustic phonon doublet appears around 39 cm^-1. The frequencies agree well with a continuum model calculation. In the optical phonon spectral region, the confined modes corresponding to both constituents are observed. The modes representing vibrations of atoms at both interfaces: InP/InGaAs (230 cm^-1 mode) and InGaAs/InP (240 and 260 cm^-1 modes) are also found.

[S36.082] Effects of Edge-Diffusion and Downward Motion on the Base Percolation Threshold

The effect of different hopping rates for monomer motion, edge diffusion and downward motion on island growth is studied by means of a Kinetic Monte Carlo model. The size distribution, average island size and the change in the threshold for 2-d percolation in the base are followed, for i=1 and for several values of D/F and of the hopping rates.

[S36.083] Investigation on TiN grain growth competition with Level Set Methods

With the increasing importance of density in thin films technology, mainly for microelectronic applications, the study of grain growth competition is now one of the most important issues. We use a 2D Level Set method to investigate the influence of temperature and rougthness on the TiN grain growth. Our model includes the physical known effects of surface deposition, direct deposition from a line source, reemission, redeposition, annealing, masking and shadowing in the deposition. Our results show, as a validation of the model, the existence of bifacets. This has been observed experimentally. We observe, for a given region, the influence of the neighborhood. The effects of the anisotropy are present everywhere.

[S36.084] The effect of an in-plane magnetic field on the universal conductance fluctuations of an open quantum dot

We report on the influence of a parallel component of the magnetic field (B_\parallel) on the low temperature magnetoresistance of an open circular cavity. The structure is patterned by electron beam lithography on a two-dimensional electron gas (2DEG) confined in a GaAs/AlGaAs heterostructure. An electrostatic gate deposited on the whole cavity is used to tune the electron density and to change the state of the sample \emphin situ. Furthermore, the gate voltage has an effect on the shape of both the billiard and the openings. The experiments were performed down to 50 mK by tilting the sample \emphin situ by an angle (\theta) between the normal to the 2DEG and the magnetic field. The shape of the weak localization-like peak and the magnetoresistance oscillations are analysed as a function of gate voltage and \theta. The results show modifications of electron dynamics inside the cavity.

[S36.085] Electronic Properties of a Square Lattice in a Periodically Modulated Magnetic Field

Electronic properties of quantum-dot systems are of great current interest. For a quantum dot array forming a simple square lattice of lattice constant a and with one electron on each dot, the electron motion can be regarded as strongly localized on the lattice sites. The energy spectrum of these two-dimensional lattice electrons under a constant magnetic field exhibit complex self-similarity and Cantor-set-like structure as a function of magnetic field, with rather interesting topological properties. The system is known as a quantum fractal system. In this work we consider such a quantum dot system under a periodic magnetic field with a period b. We have solved the eigenvalue problem of the electrons in this system. It is found that the energy spectrum is strongly dependent on the ratio \beta=a/b. For odd fraction of \beta, there is an additional energy branch with narrow bandwidth. The calculated eigenvalues and eigenvectors are then employed to calculate the static magnetoconductivity \sigma_xx and \sigma_yy. The conductivity exhibits fast quasiperiodic quantum magnetic oscillations, reflecting the complex nature of the energy band structure and population of these bands.

[S36.086] Nonanalyticity of the Electron Distribution Function and Nonlinear Electromagnetic Waves in Semiconductors

Nonanalyticity of the Electron Distribution Function and Nonlinear Electromagnetic Waves in Semiconductors S.S.Rozhkov Institute of Physics, Ukrainian Academy of Sciences, Kyiv, Ukraine

It is shown that propagation of electromagnetic waves in a plasma is nonlinear if the charge carrier distribution function has fractures or, more exactly, nonanalyticies. This result is valid for any plasma. In the case under consideration the nature of the nonanalyticities is connected with spontaneous optical-phonon emission by the nonequilibrium electrons of a semiconductor. The standard wave equation for the electromagnetic field E in a plasma implies that the formula dJ/dt = (\epsilon_o ømega_p^2/4\pi)E for the current density J takes place (eo is the static dielectric constant and wp is the plasma frequency). If the distribution function f(p) is nonanalytical on some set of points of momentum space, then the ordinary formula connecting J and E is violated, and the wave equation becomes nonlinear. In the high-frequency limit we find the function f(p) and derive the wave equation for E in a semiconductor placed in parallel constant electric and quantizing magnetic fields. In such a situation the stationary electron distribution function acquires sharply expressed fractures as a result of runaway of the electrons and spontaneous optical-phonon emission hindering the runaway. Creation and dynamics of dark envelope solitons for the field E are discussed footnote S.S.Rozhkov, JETP 71, 1135 (1990).

[S36.087] CdHgTe:V CRYSTALS - NEW PROMISING PHOTOREFRACTIVE MATERIALS (OPTICAL AND PHOTOELECTRIC PROPERTIES)

The present paper is devoted to a detailed investigation of the optical and photoelectric properties of Cd_1-xHg_xTe:V (x=0.012) crystals. Such crystals were grown for the first time by the Bridgman method. Transport measurements (T=293 K) showed that the samples were highly compensated with a resistivity greater than 10^6 Ømegacm. The electron mobility in the as-grown samples was 300 cm^2/Vs. Vanadium-doped CdHgTe crystals are characterised by the absorption spec-tra with two broad bands 1.02 and 1.48 \muk at 77^o K, which could be attributed to the intracenter and photoionization transitions. The position of exciton reflection band at T=4.5^o K correspond to 791.5 nm. The photoluminescence spectra involves a broad band near 794.0 nm which could be attributed to excitons bound at neutral acceptors (A^oX-line). The photogalvanic-current spectra (PGS) of these crystals consist of the positive bands which are caused by photoionizing transitions of electrons from impu-rity or defect levels to the conduction band. The PGS shows anisotropy: in the impu-rity absorption region the broad bands were observed near 1.23 and 1.29 \muk when light propagates in the <111> and <110> directions accordingly. On the base of ob-tained results it was proposed a model of the photorefractive effect in CdHgTe:V crystals.

[S36.088] The ordered-vacancy compound CdIn_2Te_4: Its structural, electronic and vibrational properties

Five phases of the CdIn_2Te_4 compound have been studied by means of an ab-initio technique. These are the defect tetrahedral I\bar4, I\bar42m and P\bar42m phases, the spinel-like phase Fd\bar3m and the defect rocksalt-like P4/mmm. We find that (i) the defect tetrahedral phases have virtually identical energies; (ii) the rocksalt-like phase is a high pressure phase that appears theoretically around 13GPa, which is considerably higher than experiment; and (iii) the spinel-like Fd\bar3m is a possible metastable phase obtainable from decompression of the rocksalt-like phase. We have calculated the electronic properties of each phase and find semiconducting behavior with a direct band gap at the \Gamma point for the defect tetrahedral phases, but metallic behavior for the spinel-like and the defect rocksalt-like phases. The Raman and IR spectra at zero pressure for the phases I\bar4 and I\bar42m have been computed. We have also calculated the effect of the pressure in the vibrational spectra of the I\bar42m phase. A softening of a E mode appears.

[S36.089] Numerical C-V calculations of CdTe/CdS thin films solar cell

We have calculated the band discontinuites and interface charge density of the heterojunction CdTe/CdS, the calculations are based in the C-V matching method. Our solar cells are builted with thin films done by close spaced sublimation. The significance of the simultaneous determinations of the band discontinuites and interface charges of these solar cells is discussed.

Work partially supported by CONACYT, Mexico, proyect G27713A

[S36.090] Lattice Constant Difference of Silicon Isotopes Determined with X-ray Standing Waves

The lattice constant of crystals depends on their isotopic composition. This is caused by the effect of zero point motion and anharmonicity. The effect is largest at T = 0 and vanishes above the Debye temperature. In the same way as successfully demonstrated recently [1], we used the x-ray standing wave technique to determine the lattice constant difference between a 0.9 \mu ^30Si enriched epitaxial film and a Si substrate crystal with natural isotopic composition in the temperature range from 54K to 300K. The measurements were performed at the RÖMO station at HASYLAB, Hamburg with Synchrotron radiation from the DORIS storage ring, employing a (333) reflection from the substrate at high Bragg angle and detecting electron total yield. The result of the measurements and the data analysis can be compared with two calcutaions [2,3] and are in better agreement with the recent path-integral Monte Carlo simulations [3].

[1] A.Kazimirov, J.Zegenhagen, M.Cardona, Science 282(98)930 [2] S.Biernacki, M.Scheffler, J.P.C.M.(94)4879 [3] C.P.Herrero, Solid State Comm. 110(99)243

[S36.091] Reduced Acceptor Concentration in Undoped GaAs Grown on Low Temperature GaAs

Low temperature photoluminescence (PL) measurements on pseudomorphic AlGaAs/InGaAs/GaAs modulation-doped transistors with a low temperature (LT) GaAs layer in the undoped GaAs buffer layer clearly show a decrease in the PL transition energies of the InGaAs quantum well compared to a structure with no LT GaAs. Self-consistent calculations of the electron and hole bandstructure and the conduction band potential profile suggest that the observed redshift in PL energies can be attributed to band-bending in the undoped GaAs induced by the Fermi level pinning at the undoped GaAs / LT GaAs interface and a decrease in the background acceptor concentration of the undoped GaAs grown by molecular beam epitaxy on a LT GaAs layer.

[S36.092] Optical properties of AlGaP alloys grown by gas source molecular beam epitaxy

We present the optical properties of a series of Al_xGa_1-xP (0 \leq x \leq 0.78) alloy films grown on S-doped GaP (001) substrates by gas source molecular beam epitaxy with elemental Ga and Al, and thermally cracked PH_3 as the source materials. The quality and compositions of the samples were verified by double crystal x-ray diffraction, Rutherford backscattering spectrometry, and ion channeling. Room-temperature pseudodielectric function spectra from 1.5 to 6.0 eV were obtained by spectroscopic ellipsometry. Using the parabolic-band critical point model, room-temperature critical point energies of the E_1, E'_0, E_2, and E'_2 interband transitions and their dependences on composition x were determined from numerically calculated second energy derivatives of the ellipsometric spectra. The vibrational properties were also investigated by Raman scattering. The Raman spectra exhibit GaP-LO, GaP-TO, and "AlP-like" LO phonon peaks. The "AlP-like" LO phonon peak shows a strong compositional dependence.

[S36.093] Exciton-Light Interaction in Surface-Disordered Quasi-Two Dimensional Quantum Well

We derive and analyze the light absorption in a quasi-two-dimensional semiconductor quantum well, which is determined by exciton-surface scattering. In analyzing exciton scattering from randomly-rough well boundaries, we treat the electron- and hole-surface scattering separately since the Coulomb interaction is suppressed in the direction perpendicular to the well plane. We find the essential dependence of the exciton-surface relaxation frequency \nu on microscopic parameters of the boundary defects (r.m.s. roughness height \zeta and correlation length R_c), the average well width d and on exciton characteristics. Specifically, in the case of the sharp exciton ground resonance \nu\propto \zeta^2R_c^2d^-6, whereas for the broad resonance \nu\propto \zeta d^-3. It is unusual and surprising that the latter dependence of \nu on \zeta is linear. Our estimations show that the surface scattering mechanism can well compete with the mechanisms of homogeneous excitonic broadening. At the same time, the surface scattering frequency \nu is substantially determined by the homogeneous one.

[S36.094] Shear Orientation of Viscoelastic Polymer-Clay Solutions

The shear orientation of a viscoelastic clay-polymer solution was investigated by means of flow birefringence and small-angle neutron scattering (SANS). With increasing shear rate a pronounced minimum in birefringence was observed corresponding to the stretching of polymer chains adsorbed between clay particles as well as the orientation of platelets in the flow field. SANS measured the shear-induced orientation of polymer and platelets as a function of shear rate. At rest and at low shear rates a diffuse isotropic ring of scattering intensity was observed. With increasing shear rate an anisotropic scattering pattern developed. At higher shear rates, the scattering anisotropy increases due to the increased orientation of the clay platelets in the shear field. Cessation of shear leads to fast recovery demonstrating the system to be highly elastic.

[S36.095] Formation of Magnesium-Oxygen Complex Impurities in Silicon at Room Temperature

Magnesium, when thermally diffused into silicon, is well- known to behave like an interstitial donor.(1) Our recent study indicates that, for silicon containing proper amount of oxygen, magnesium can pair with oxygen to form magnesium- oxygen complex impurity, which is also an interstitial donor in silicon.(2) Our spectroscopic study further reveals that such magnesium-oxygen complex impurities can even be formed by interstitial magnesium and dispersed oxygen in silicon at room temperature. (1) L.T. Ho and A.K. Ramdas, Phys. Rev. B 5, 462(1972). (2) L.T. Ho, Phys. Stat. Sol. (b) 210, 313(1998).

Acknowledgement: This work was partially supported by the National Science Council of the Republic of China under contract number NSC 88-2112-M-001-002.

[S36.096] Competing Attractors and State Probability

We investigate the Fokker-Planck equation which results from adding Gaussian white noise to a d-dimensional dynamical system. We are particularly interested in higher dimensional systems in the weak-noise limit. In this latter limit, the Fokker-Planck equation reduces to the classical mechanics of a point particle moving in a d-dimensional configuration space. We can now look at stable fixed points of the original system. These points are the local maxima of the probability distribution for the system. In a classical mechanical context, these points are also local minima of action. Comparing the probabilities of various attractors is reduced to comparing the action calculated along paths between attractors. This poster session will discuss a numerical scheme by which a minimum-action path connecting fixed points is constructed from the shallowest paths between them.

[S36.097] The Structure of Eigenmodes and Phonon Scattering by Discrete Breathers in the Discrete Nonlinear Schrödinger Chain

We present linear theory for one-dimensional phonon scattering by discrete breathers in the discrete nonlinear Schrödinger equation using transfer matrix formulation. We focus on eigenmodes in the linearized equation, which plays an important role in the scattering problem. Considering a special class of boundary conditions for both physical and unphysical eigenmodes in the non-traveling region and their continuation into the traveling region, we obtain an intuitive picture of the relation between the occurrence of perfect transmission and the localized eigenmode threshold. The perturbation approach with a transfer matrix formulation in the weak coupling limit predicts both the existence of two localized eigenmode thresholds at finite coupling strength and the structure of perfect transmission and perfect reflection. These results are shown to be applicable to a wide class of nonlinear chains including the phonon scattering problem by the discrete breather in the nearest neighbor Hamiltonian chain with cubic on-site potential.

[S36.098] Nonequilibrium Electronic Fluctuations in Semiconductors in Quantizing Magnetic Fields and Quantum Wires

Nonequilibrium Electronic Fluctuations in Semiconductors in Quantizing Magnetic Fields and Quantum Wires S.S.Rozhkov Institute of Physics, Ukrainian Academy of Sciences, Kyiv, Ukraine

For the steady state of a nondegenerate one-dimensional electron gas which may be very far from equilibrium (have essentially non-Maxwellian energy distribution) we solve the current fluctuation problem and find the linear response of the system to the alternating electric field. This allows us to obtain relations similar to the fluctuation-dissipation theorem or, in other words, to calculate the noise temperature \Theta. At equilibrium we define \Theta = T, where T is a temperature of a thermal bath (phonons, here). It is shown that the nonequilibrium fluctuations may be much more intense than equilibrium ones. We consider electrons of a semiconductor in a quantizing magnetic field and a quantum wire. In both cases the constant electric field E is applied along the direction of free electron motion, and electrons interact with acoustic and optical phonons. We distinguish a few stationary regimes by the E dependences of the current density or average energy \epsilon. It makes sense to single out the regime of saturating the current. In this case the noise temperature Theta significantly exceeds the phonon temperature T and the one-dimensional electronic-subsystem ``temperature" T_e = 2\epsilon = 2ømega/3, where ømega is the optical phonon energy.

[S36.099] Analysis of the Morphology of Fractal Aggregates

We have studied the morphology of fractal aggregates resulting from cluster-cluster simulation dynamics via statistical analysis techniques. For our comparison, we have studied several models including the diffusion limited cluster aggregation model (DLCA), the hierarchial model, and the restricted hierarchial model. Sorensen and Oh (C.M.Sorensen and C.Oh, Phys. Rev. E58, 7545-8 (1998)) have shown that the fractal dimension is analytically related to the aspect ratio for the restricted model. Here, we add an anisotropy to the models to study the effect of such a restriction on the morphology of the resulting clusters and the relation between aspect ratio and fractal dimension.

[S36.100] GROWTH KINETICS OF FRACTAL AGGREGATES IN AEROSOLS

Diffusion limited cluster aggregation (DLCA) kinetics of titanium dioxide aerosols was investigated by dynamic light scattering (DLS). The Knudsen number Kn, which is equal to the mean free path of the medium molecules in the aerosol divided by the radius of aggregates, was changed by varying the pressure of the aerosol confining chamber. The measured mobility radius R shows power law growth with an exponent varied as the system evolved from the transition regime (Knudsen number Kn is larger than or equals 1) to the continuum regime (Kn < 1). This shows that the homogeneity of the aggregation kernel is dependent on the kinetic regime. The values of the homogeneity derived from the experiments are 0.01 with an uncertainty of 0.005 in the continuum regime and -0.54 with an uncertainty of 0.05 in the transition regime. These findings agree well with the kinetic theory regarding fractal aggregation.

[S36.101] Response Analysis of Pricing in a Trading Model with Market Pressure and Price Resistance

A computer simulation model is used to study the variation of stock price with time caused by an abrupt change in price in a market influenced by trading momentum while resisting price fluctuations. In the spirit of the Cont-Bouchaud model, the trading is performed in groups, i.e., number of traders s each with n_s trading group follows a power-law n_s \propto s^-\tau as the number of clusters in percolation at the threshold. Activity a is used to describe the probability trading; the decision to buy or sell depends on a trade probability (W(t+1) = - [K \cdot P(t)]^n + M \cdot [P(t) - P(t-1)], where, K is a force constant (pricing pressure), M is inertia (momentum trading), P is a measure of the market price, and the exponent n is an odd integer. Attempts are made to search for conditions under which one can obtain a log-periodic oscillation in response of price after an abrupt change. For suitable parameters values, an approximate oscillation is observed. Other models are being developed to study the pricing response and some of the results will be presented as the data become available.

[S36.102] Scarring in vibrational modes of thin metal plates

We report the first direct experimental observation of scarring phenomenon associated with transverse vibrational waves in a thin metal plate. The plate has the shape of a full stadium and clamped boundary conditions. Normal modes are imaged using time-averaged holographic interferometry, and modes corresponding to ``bouncing ball'' and higher order periodic trajectories are found. An eikonal approximation of the solution along classical trajectories of the stadium including nontrivial phase shifts at clamped boundaries yields a useful quantization condition for the observed modes.

[S36.103] Transitions in the two-dimensional, underdamped Frenkel-Kontorova model

We study the transitions that occur for a two-dimensional layer of atoms, commensurate with a periodic substrate potential, subject to damping, a thermal bath and driven by an external d.c. force. The mechanism for the locked-to-running transition, which occurs on increasing the driving force beyond a critical value, is investigated in detail. The various kinds of stable states which can be accessed by decreasing the driving force are also discussed.

[S36.104] Nonlinearity vs. Disorder: Stability of nonlinear localised impurity modes

We develop a systematic theory of the nonlinear localised impurity modes and their stability. In the linear limit, such modes are the well-known defect modes which are localised at the impurity sites. We show that nonlinearity may change dramatically the properties of the impurity modes. As an example, we consider a generalised nonlinear Schroedinger equation with a point-like impurity and present an extensive classification and the analytical stability analysis of its localised solutions. The most interesting properties are obtained in the case when the localisation mechanism due to an impurity competes with that due to nonlinearity.

This work is supported by a collaborative DIST grant.

[S36.105] Monte Carlo simulations of a vibrational probe attached on a flexible chain polymer

We conduct Monte Carlo simulations to study the behavior of a vibrational probe labeled on one end of a hard-core tangent sphere chain. We model the vibrational probe as an elastic ball, and the ball is not allowed to overlap with the monomers of the attached chain. The elastic energy of the ball obeys the Hook's Law, and the ball size remains positive. The effects of the polymer chain on the elastic ball are sensitive to the nature of the ball, such as elasticity. As the force constant of the ball becomes larger, the average ball size first decreases and then increases again. In general, the average ball size decreases as the chain length is increased. Further, we find that the ball tends to expand the chain, but it does not affect the scaling law of the average polymer size with chain length within our simulation precision. Our simulations are useful to test the feasibility of using a vibrational probe to determine the chain conformation.

[S36.106] Self-Similarity, Scaling and Renormalization Group Calculation of Thermodynamic Properties of a Non-Ideal Randomly Jointed Chain

A new formulation of the renormalization group method is proposed and used to calculate swelling factor and partition function of a 3-D randomly jointed chain with hard-sphere excluded volume interactions. The results of these calculations are compared with computer simulation data generated for the same microscopic model of a non-ideal polymer chain.Partial results are published in Physica A 272 (1999) 22-47.

[S36.107] ClO_4^- Rotation in a model of Amorphous Polyethylene Oxide

We have studied the rotation of a ClO_4^- molecule in amorphous polyethylene oxide in a molecular dynamics model. We calculate the time Fourier transform of the self correlation function of the rotational kinetic energy. It contains several characteristic vibrational and relaxational modes whose significance will be discussed.

[S36.108] Self-Adapting Fixed-Endpoint Configurational-Bias Monte Carlo

An extension to the configurational-bias Monte Carlo method is presented which allows for the efficient conformational sampling of the interior segments of chain molecules whose interactions include strong bonded terms. The ability to regrow interior segments overcomes the limitations of conventional configurational-bias methods (where the regrowth is always directed to a free chain end) and now allows for the simulations of chain systems with low concentrations of chain ends, that is higher molecular weights, networks, or cyclic structures. An additional biasing (closing) probability is used that guides the bead-by-bead configurational-bias regrowth of interior segments towards its desired fixed target. Previous methods have been developed utilizing a similar scheme, but are limited to chain molecules using force fields of arbitrary complexity for which an accurate closing probability is not known a priori. To obtain an accurate closing probability, it is taken from presimulations, and when unavailable, a uniform probability is used which self-adapts periodically during the simulation to increase efficiency.

[S36.109] Oxides Without Water: The Thermal Stability of Oxide Nanocrystals

It is possible to adapt the high temperature, liquid phase reactions used to make CdSe nanocrystals to the production of highly crystalline nanosized oxides, in particular TiO2, ZrO2 and SnO2. Structural characterization of the materials using HRTEM and XRD indicate that the interior of the nanocrystals is quite perfect, and the samples consist of isolated particles with size distributions of 10-20% on the diameter. Each of these oxides adopts what is thought to be the metastable crystal structure for the bulk phase; at high temperatures, these phases spontaneously convert to the more stable phase. We study this transformation process using both in-situ transmission electron microscopy in addition to macroscopic characterization of powders. In the case of titania, the temperature of this metastable-stable transformation is highly sensitive to the surface chemistry of the nanocrystals. Fully hydrated nanocrystals, for example, undergo both the transformation and grain growth at temperatures as low as 600 C; anhydrous oxide particles made using non-hydrolytic chemistry are stable to much higher temperatures.

[S36.110] Probing radiative recombination in semiconductor nanocrystals with cavity QED

Time-resolved photoluminescence (PL) from semiconductor nanocrystals is characterized by multiple exponential decays ranging from nanoseconds to microseconds, reflecting complex radiative and non-radiative processes and making it difficult to extract information on radiative recombination in these nanocrystals. In this paper, we report studies of radiative recombination in core/shell nanocrystals using an experimental approach based on cavity QED and the fact that only radiative processes are affected by modifications in vacuum fluctuations. We single out radiative processes from the complex decay dynamics in time-resolved PL by embedding nanocrystals in an optical microresonator and by comparing time-resolved PL obtained at energies resonant and off-resonant with relevant resonator modes. A radiative lifetime of order 10 ns is obtained. The relatively short radiative lifetime further indicates that a significant fraction of the PL arises from intrinsic dipole-allowed radiative recombination in nanocrystals.

[S36.111] Atom Scattering From Atomic Surfactants

Our research is aimed at exploring collisions and reactions between gas atoms and liquid surfaces. The role that surface atoms play in

gas-liquid energy transfer can be investigated by monitoring collisions between rare gases and surface active monolayers. Dilute alloys of bismuth (Bi) in gallium (Ga) form solutions in which the surface concentration of solute Bi atoms can be changed from nearly zero to a complete monolayer while the bulk concentration remains constant. Our goals are 1) to explore how energy transfer between argon or xenon atoms and a liquid Bi/Ga alloy solution varies with changes in the surface composition of the Bi/Ga mixture and 2) to determine how the surfactant Bi monolayer changes the microscopic structure of the gas-liquid interface. We find that gas-liquid energy transfer can be controlled by changing only the composition of the top monolayer of the liquid. In the case of high energy Ar and Xe scattering from the alloy, we are able to span the total difference in energy transfer to pure Bi and pure Ga simply by changing the amount of Bi surface monolayer of the Bi/Ga solution. We also find from gas scattering intensity measurements that the Bi/Ga liquid surface is much rougher than pure Ga. At least for high collision energies, it would appear that the approach to thermal equilibrium between atomic gases and atomic liquids can be significantly influenced by the presence of a single monolayer of surfactant atoms.

[S36.112] 2-IR 1-Visible Sum Frequency Generation Study on Co-adsorption of CO and Ethylene

Surface Vibrational Sum Frequency Generation (SFG) is applied to investigate the co-adsorption of CO and ethylene on Pt(111) from ultra high vacuum (UHV) to 1 atmosphere. The co-adsorption of CO and ethylene is a special system for study because CO is a poison for catalytic hydrogenation of ethylene as well as many other olefins. We report the first use of 2-IR 1-Visible SFG to monitor both CO and ethylene simultaneously during the co-adsorption process as well as under reaction conditions. In UHV, co-adsorption was observed when a clean Pt(111) surface was exposed to 5L of both CO and ethylene near room temperature. Pressure dependence up to 1 atmosphere and temperature dependence from 25C to 550C were investigated. For reaction studies, 2-IR 1-Visible SFG was used in conjunction with a packed column gas chromatograph to correlate surface species with reaction kinetics.

[S36.113] Investigation of Vibrational and Rotational Surface Modes of N_2 Molecules Absorbed on in situ Cleaved MgO(001) by Helium Atom Scattering

Helium atom scattering has been used to probe the surface dynamical character of N_2 grown by isothermal adsorption at partial pressures of about 2.5\cdot 10^-8 mbar at temperatures held in the range of 11 to 12 K onto an in situ cleaved MgO(100) substrate. The scattering chamber base pressure during the measurements was in the mid 10^-11 mbar range. N_2 coverages of a full monolayer and submonolayer were studied. With the full monolayer and with an incident beam of 28.8 meV, two Einstein type modes were observed at 13 meV and 6.7 meV. The latter was a parallel vibration as judged by the intensity increase toward the zone boundary. The 13 meV mode favors parallel polarization, however, the intensity was weaker and the argument is not so clear; it could be an overtone of the 6.7 meV mode. With an incident beam energy of 18.8 meV, a third mode at 3.5 meV was seen, with uncertain character. For submonolayer coverage, three modes were observed: one at 2.8 meV (a rotational mode), a weak 5.6 meV mode and a mode at 8 meV. The latter mode decreased in energy with increased coverage becoming the 6.7 meV mode of the full monolayer. The modes were identified by observing the frequency change with different isotopes of N_2.

[S36.114] Frequency Domain Multi-Dimensional Ultrafast Spectroscopy with Doubly Vibrationally Resonant Non-Linear Processes

Because the signals generated in non-linear spectroscopy are directly dependent on the intensity of the input beams, it is sensible to use ultrafast (< 10^-12 s) non-linear spectroscopy, where the peak power of the lasers is higher, and the intensity damage threshold of the sample and solvent is higher than that of slower pulses. This paper presents a comparison of picosecond and nanosecond four-wave mixing systems using simple organic solvents as model systems. In particular, it compares the doubly resonant and non-resonant four-wave mixing intensities and evaluates the discrimination that is possible with suitable time delays. This work establishes detection limits and explores the ramifications for biomolecular spectroscopy.

[S36.115] Line-Narrowing in Doubly Vibrationally Enhanced Infrared Four Wave Mixing (DOVE-IRFWM)

DOVE-IRFWM uses two tunable infrared lasers and one fixed visible laser to probe a sample via two-dimensional four wave mixing. A cross-peak can occur when the two infrared lasers are resonant with separate infrared transitions. In this work, the cross-peak between the acetonitrile (CH_3CN) \nu _2 (CN stretch) mode and \nu _2 + \nu _4 (CN stretch + C-C stretch) combination band in liquid acetonitrile-water mixtures was examined using DOVE-IRFWM. Conventional infrared spectroscopy shows that these transitions are inhomogeneously broadened due to hydrogen bonding interactions with the water. However, the DOVE-IRFWM cross-peak is significantly narrower than the infrared features, and the cross-peak position shifts as the two laser frequencies probe different parts of the inhomogeneous envelopes. The shifts show a positive correlation in the hydrogen bonding-induced changes of the vibrational frequencies.

[S36.116] Doubly Resonant Frequency Domain Vibrational Spectroscopy

Doubly resonant vibrational spectroscopy is a new optical technique, analogous to 2D-NMR, that measures the coupling between molecular vibrations. Vibrations can be coupled by either the strong bonds within a molecule or by the weak bonds between molecules in a condensed state. Two tunable infrared lasers probe vibrational double resonances using non-linear optical four-wave mixing processes. Systematically scanning the frequencies of these lasers produces a two-dimensional spectrum of the vibrational resonances in a sample that is both mode selective and site selective. These capabilities have been demonstrated using acetonitrile (CH3-CN) as a model system. Two-dimensional vibrational spectra will be presented that show selectivity between the \nu _2 and \nu _3 vibrational modes of acetonitrile and selectivity between acetonitrile and deuterated acetonitrile molecules in a mixed sample. Research currently in progress uses ultrafast lasers to improve the sensitivity of the technique and will develop its capabilities to determine molecular structures.

[S36.117] Dynamics of Bond Breaking Studied by Chemical Force Microscopy

Intermolecular forces underline a variety of phenomena in chemical and in biological systems, such as cell adhesion, protein folding and molecular recognition in ligand-receptor pairs. Understanding the dynamics of these interactions is critical for modeling and controlling these processes. The advent of ultra-sensitive force measurement techniques has enabled direct measurement of the bond strength on the relevant length scales. Recent measurements have pointed out the importance of kinetic factors in bond strength and pointed out the necessity to explore the whole energy landscape of a chemical bond. Still, little is known about the response of the bond strength to the environmental variables such as temperature. The analysis of this response provides a way to determine thermodynamic characteristics of the binding interaction. We used chemical force microscopy to measure the temperature dependence of the interaction forces in a well-defined system presenting a finite number of identical bonds. The tip of the scanning probe microscope was modified with distinct chemical functionalities to give rise to the well-defined and uniform interactions with the sample surface. We will discuss the theoretical framework for interpretation of such measurements, as well as the relative importance of thermodynamic and kinetic factors affecting the bond strength in the presence of solvent medium. We also point out the differences in kinetics of bond breaking in single bond systems vs. multiple bond systems.

[S36.118] Enhanced transferability of embedded atom method potentials of ruthenium using density functional theory

Most metal surfaces undergo relatively large re-constructions which prohibit ab-initio quantum mechanical energy calculations due to the large amount of atoms in the system. Many-body semi-empirical methods are used to calculate ground-state properties of real materials. The Embedded Atom Method (EAM) is computationally fast and widely used for semi-empirical quantum mechanical calculations on large and complex realistic metallic systems. Typical implementations of EAM are done by fitting the EAM functions to experimental data such as lattice parameter, lattice structure, elastic constants, and sublimation energy. These properties sample a narrow region of configuration space around bulk equilibrium. However, there are limitations in the applicability of EAM potentials fitted to bulk equilibrium properties. Problems including clusters and surfaces, that are in a completely different region of configuration space than that of bulk are beyond the capabilities of such equilibrium bulk-fitted potentials. We present Ru EAM potentials with increased transferability by obtaining a parametrization of them using physical properties obtained by ab-initio methods that sample a wider region of configuration space. In this work, we use the Density Functional Theory Pseudopotential Method (DFTPM) to increase the properties database. Our EAM potentials with enhanced transferability are used to calculate the surface relaxation and reconstruction that occur on different surface orientation of ruthenium. Supported by: NSF, DOE

[S36.119] Comparison of the solvation free energies calculated by the extended scaled particle theory and the Monte Calro simulation of the hard body system

One of the quantitative methods for fast calculation of the hydrophobic effect on the hydration free energy of the large molecule, ex. protein, is the extended scaled particle theory (XSPT). In the statistical mechanics, chemical potential of the molecule is calculated by integrating phase space while the target molecule is hypothetically scaled up. The XSPT uses the assumption for the phase space integration accompanying the scaling up of the solute molecule from the material point (scaling parameter ( \lambda =0 ) to the real size ( \lambda =1 ). Monte Calro simulation is applied to the hard body system in order to evaluate the validity of the XSPT. Hypothetical scaling up the solute molecule with the solvent molecules of fixed size in the XSPT, which corresponds to the hypothetical scaling down of the all solvent molecules with the solute molecule of fixed size in the alpha-shape, is performed in the Monte Calro simulation. In the case of the solute is the fused spheres, H.S.Ashbaugh proposed the approximation of the solvation free energy of the solute molecule by using the Voronoi polyhedron. We also compared the results of the XSPT with both this Voronoi polyhedron approximation and the Monte Calro results.

[S36.120] Representation of electronic excited states by conditional wavefunction

Hartree-Fock scheme is an ordinary method to calculate the zeroth order approximation for non-relativistic electronic excited states of atoms and molecules. The accuracy of zeroth order hamiltonian affects the efficiency of higher order estimation of the Hamiltonian and the Green's function. To improve the preciseness of zeroth order Hamiltonian, we try to include the relaxation of electronic excited states into zeroth order approximation by using conditional wavefunction representation instead of Hartree-Fock method. Our method is illustrated by the calculation of electronic double-excited states of Helium and single-excited states of Neon. Further extention of our formulation for multiple-exfcited states are also discussed.

[S36.121] The role of the external heavy atom effect on the fine structure splitting in ^2\Pi MgNe^+

In open-shell diatomic van der Waals complexes ARg, (where A is an open-shell atom or ion and Rg is a rare gas) some molecular terms exhibit strong R-dependence of the fine-structure splitting (fss). Molecular orbital analysis has shown that for the BAr, LiAr, LiNe and CAr systems, [Sohlberg and Yarkony, J. Phys. Chem. 101, 3166 (1997); J. Chem. Phys. 107, 7690 (1997); 111, 3070 (1999)] this strong R-dependence arises from the mixing of Rg character into what are nominally partially-occupied A orbitals. Rg monocations typically have large spin-orbit coupling constants (\xi(Ne^+) = 521 cm^-1, \xi(Ar^+) = 955 cm^-1) and therefore even a small degree of mixing can have an appreciable effect on the fss in the molecular region. Remarkably, in sharp contrast to the above systems, the fss in ^2\Pi MgNe^+ shows very weak R-dependence [Reddic and Duncan, J. Chem. Phys. 110, 9948 (1999)]. Multi-reference CI calculations and molecular orbital analysis will be presented.

[S36.122] Photodissociation Spectroscopy of Ca^+(C_2H_4)

We have studied the photodissociation of the Ca^+(C_2H_4) complex using a reflectron time-of-flight mass spectrometer. As the wavelength of the dissociation laser was varied between 440 - 750 nm four absorption bands were found. Ca^+ was the only photofragment observed. Isotope substitution experiments and ab initio electronic structure calculations were used to support the assignments of these bands to metal-centered transitions correlating with the 3d and 4p levels of Ca^+. Spectral analysis of the bands gives vibrational constants for the Ca^+--C_2H_4 intermolecular a_1-stretch in the 1^2A_1, 2^2B_1, and 2^2B_2 states, and for the CH_2-CH_2 a_1-wag and the HCH a_1-bend in 2^2B_2. Calculations predict a ground state equilibrium structure with a C_2V \pi-bonding geometry and a dissociation energy of De'' = 0.551 eV. Both theoretical and experimental results show that the 4p\pi(2^2B_1 and 2^2B_2) excited states are relatively weakly bound at long range by electrostatic interactions. Finally, these results are contrasted with previous studies of the more strongly interacting Mg^+(C_2H_4) and Al^+(C_2H_4) complexes.

[S36.123] Microwave Investigation of Sulfuric Acid Monohydrate and its Deuterated Isotopomers

We report the first microwave spectroscopic investigation of the 1:1 complex H_2O-H_2SO_4 and several of its deuterated isotopomers. The complex is prepared in situ via reaction of water and SO_3 using a co-injection source in which H_2O (D_2O/H_2O) vapor is introduced into the early stages of an Ar + SO_3 expansion. The J=1<-0, 2<-1, and 3<-2 a- and c- type spectra for eight isotopomers have been measured and are consistent in all cases with that of a near-prolate rotor with appreciable dipole moment components along the a- and c- inertial axes. The spectra are complicated by internal motion presumably of the water unit which splits the a-type K_-1= 0 transitions and strongly affects the c-type Q-branch transitions. The possible internal motions giving rise to the observed spectral splittings as well as the ground state structural parameters of the H_2O-H_2SO_4 complex will be discussed and compared to recent DFT structure calculations [1,2].

1. Re, S.; Osamura, Y.; Morokuma, K. J. Phys. Chem. A 103, 3535 (1999). 2. Arstila, H.; Laasonen, K.; Laaksonen, A. J. Chem. Phys. 108, 1031(1998).

[S36.124] Microwave Investigations Of C_5H_5N-SO_3 And HCCCN-SO_3: The Principle Of Hard And Soft Acids And Bases Applied To Partially Bonded Systems

Microwave spectroscopy has been used to study the Lewis acid-base adducts C_5H_5N-SO_3 and HCCCN-SO_3. The spectrum of C_5H_5N-SO_3 indicates a short N-S bond length of 1.9155(5) Åand free rotation of the SO_3 unit. In contrast, HCCCN-SO_3 is more weakly bound with an N-S distance of 2.568(8) Åslightly less than the expected van der Waals distance of 2.9 ÅThe NSO angles are 98.922(4)^o and 91.8(4)^o for C_5H_5N-SO_3 and HCCCN-SO_3 respectively. A Townes and Dailey analysis of the ^14N quadrupole coupling constant of C_5H_5N-SO_3 indicates a transfer of 0.54 electrons upon formation of the dative bond. This is a physical measurement of the “soft” portion of the chemical interaction and comparisons are made with other adducts of SO_3. Bonding is considered in light of Pearson’s concept of Hard and Soft Acids and Bases,^a noting the correlation of adduct properties, such as electron transfer and bond length, with the energy gap between the donor and acceptor orbitals.

a. Pearson, R. G. J. Am. Chem. Soc., 1963, 85, 3533.

[S36.125] Superhalogen Behavior of FeO_4 MnO_4

Superhalogens are a class of cluster compounds that possess electron affinities in excess of 3.6 eV which is the electron affinity of chlorine, the most electronegative element in the periodic table. For clusters containing oxygen, a superhalogen has to satisfy the composition MO_n, where 2n=k+1. Here k is the maximal valence of the central atom M and n is the number of oxygen atoms. This requirement is fulfilled for MnO_4 as the maximal valence of Mn is 7. However, FeO_4 should be a closed shell system as the maximal valence of Fe is 8. Consequently, it should possess a relatively low electron affinity. We have performed self-consistent calculations of the total energy and geometry of FeO_4 and MnO_4 clusters in the neutral and anionic configurations using gradient corrected density functional theory and molecular orbital approach. The electron affinity of MnO_4 was found to be 5 eV and as expected, is a superhalogen. Contrary to expectation, FeO_4 too was found to have a large electron affinity, namely 3.8 eV and thus, belongs to the superhalogen class. The origin of this large electron affinity of a closed shell system is shown to be a manifestation of the special bonding characterstic of d-electron systems.

[S36.126] An integrated symbolic/numeric approach for the calculation of atomic transition matrix elements and energies using many-body perturbation theory.

A program for the calculation of atomic properties using an integrated symbolic and numerical approach is described for arbitrary excitations (any number of particles and holes including the multiconfiguration states) from closed-shell atoms. Transition matrix elements and energy formulas up to third-order perturbation are generated via the symbolic programming language Mathematica. This is accomplished by symbolically manipulating the second-quantized form of the energy operators within the framework of Rayleigh-Schrödinger perturbation theory to obtain the correct wavefunction for a particular excitation. Once the MBPT formulas are generated with symbolic program, angular reduction is performed symbolically. The radially dependent MBPT formulas obtained through this symbolic process are numerically evaluated employing a relativistic basis set for an accurate calculation of atomic energies and transition amplitudes, providing a completely integrated, seamless, and robust computational method capitalizing on the systematic precision of MBPT and the disparate attributes of both computer algebra systems and languages specializing in numerical efficiency and precision. Numerical results which illustrate the entire process are given.

[S36.127] Binding of Cd ions to the cell wall of B. Subtilis-- an EXAFS study

Understanding metal complexation in aqueous solutions and biological systems is essential to studying metal contaminant distributions in the environment, as well as for developing remediation techniques. Cell walls have been shown to facilitate the precipitation of metals, but the exact mechanism and binding sites for the metal ions is largely unknown. The ability of the Extended X-ray Absorption Fine Structure (EXAFS) technique to probe the local environment of the absorbing atom has been used to study the complexation of Cd on the cell walls of B.Subitilis. Results show a gradual change from carboxyl to phosphoryl environment with increasing ph. The proportion of atoms binding to the corresponding functional groups have been quantified and a model for adsorption proposed.

[S36.128] Similarity principle and rejection of Gibbs paradox

Gibbs Paradox says that entropy of mixing or assembling decreases discotinuously with the increase in the property similarity. After the rejection of the Gibbs paradox statement (see papers cited at website http://www.mdpi.org/lin/), the similarity principle has been developed: If all the other conditions remain constant, the higher the similarity among the components is, the higher value of entropy of the mixing, the assembling or the chemical bond formation process will be, the more spontaneous the mixing, the assembling or the chemical bond formation process will be, and the more stable the mixture, the assemblage or the chemical bond will be. The similarity principle is very useful. If one wants to mix substances, increase the similarity (of relevant properties); if one plans to separate the substances as phases, reduce their similarity! Then, the desirable processes of mixing or separation will happen spontaneously. Normally by changing temperature ( similarity is related to Boltzmann factor) and pressure, one can control the similarity and in turn, direct the process towards the desired direction. Higher temperature and pressure leads to higher similarity. This theory is important in understanding molecular recognition, self-organization, molecular assembling and molecular replication.

[S36.129] NMR chemical shift of a benzene molecule in liquid water

We perform first-principles calculations to determine the nuclear magnetic resonance (NMR) chemical shifts of a benzene molecule in liquid water. A question of importance is how the hydrophobic hydration will affect the magnetic properties of the solute. To address this question we employ a recently developed ab initio formalism which is appropriate to compute NMR chemical shifts in extended systems [PRL 77, 5300 (1996)]. We consider a system composed of a benzene molecule surrounded by 32 water molecules with periodic boundary conditions. The snapshots used to compute the averaged chemical shifts are obtained from ab initio molecular dynamics calculations which make use of a LCAO localized basis set [PRB 53, R10441 (1996)]. This method succesfully describes the structure of water clusters, as well as reproduces the radial distribution functions of liquid water. Since NMR chemical shifts are sensitive to hydrogen bonded environments, our calculations would be useful in describing the nature of the interaction between the liquid and the benzene molecule.

[S36.130] Classical Theory of Molecule-Surface Scattering: Application to C_2H_2 Scattering from LiF

A classical mechanical theory for the scattering of small, rigid molecules is developed which includes both translational and rotational energy and momentum transfers between the surface and projectile. The results of calculations with this model are compared with recent high-precision measurements of the scattering of C_2H_2 from a clean, ordered LiF surface. The comparisons between calculations and measurements indicate that the observed angular distributions are dominated by single collisions with the surface and their shapes are due largely to translational energy (multiphonon) transfers. The calculated results for rotational transitions of the scattered molecules are also in reasonable agreement with measurements.

[S36.131] Theory of Inelastic Electron Tunneling through adsorbates, and relation to their vibrational lifetimes

We study the relation between the controlled excitation of vibrations in adsorbates by Scanning Tunneling Microscope (STM), and the lifetimes of these modes when de-exciting by electron-hole pair mechanism. To this end we present a Green Function formalism that accounts for the two processes in a unified way. Implementation of the formalism in a Linear Combination of Atomic Orbitals (LCAO) approach [1] has shown a good agreement with experiments for the case of Acetylene on Cu [2]. We show the connection between our Green Function approach and the perturbation cluster approach of [3]. The way the lifetimes of different vibrating modes are sorted, and the relative intensities of their inelastic peak in the spectroscopy with STM, are discussed and their relation is clarified. The concrete case of the CO molecule is analyzed. [1] N.Mingo and K.Makoshi, submitted to Phys. Rev. Lett.; Surface Sci. 438 (1999) 261-270. [2] B.C.Stipe, M.A.Rezaei and W.Ho, Phys. Rev. Lett. (1999)1724, and references therein. [3] M. Head-Gordon and J. C. Tully, J.Chem.Phys. 96(5), p.3939, 1992.

[S36.132] Investigation of Chemisorption of Oxygen on Metal Surfaces by Embedded Cluster Model

We have calculated the chemisorption energy of oxygen on metal surfaces by the embedded cluster model ^2, 3 which constructs an embedding potential using a set of molecular orbitals (MO) obtained for a large cluster. The method to construct an appropriate embedding potential is investigated from calculations of oxygen adsorption on Al(100) and Au(100) surfaces by varying several conditions, for instance, the choice of cluster, the localization procedure and use of the fractional MO occupancies. To examine the reliability of this model, we calculate the cluster size dependence of the binding energy which should be converged with increasing cluster size. The electron density distribution in the environment region is compared with that obtained from band calculations.

[S36.133] High Speed Cinematography of Cracks Spreading under Failure of Amorphous Metallic Alloys

Amorphous metallic alloys are unique high strength materials that under low temperature straining (300 - 77 K) are absolutely thermomechanically unstable against the catastrophic plastic shear. Its velocity is close to the transverse sound velocity ct. That is why experimental studying of shear crack propagation in amorphous alloy ribbons at low temperatures needs high-speed methods of observations. Samples of the NI78Si8B14 and Fe70Ni10B20 amorphous alloys ribbons were tensile tested in a pulse testing mashine. The motion of the main crack front during ductile shear failure was studied by means of a high-speed film camera (SFR-2M) with a frame frequency of 2x106 s-1. Loading of the sample was synchronized with both the pulse light source and the high-speed camera. Results of observations: a) the velocity of of shear crack propagation is close to the maximum theoretical limit 0.9 ct; b) a pulsating motion of of the crack is observed with a retardation of crack motion at the moment of branching or changing the orientation of the crack surface that became faceted; the process of shear crack propagation is step-like.

[S36.134] Pulsed laser processing of crystalline and amorphous Si

The paper introduces a 3D computer simulation model of the melting and crystallization process of amorphous and crystalline Si induced by pulsed laser irradiation. The model takes into account the dependence of thermal and optical properties of Si on temperature using the adiabatic approximation. Some results of the model are compared in order to evidence how this dependence affects the thermal field during heating and cooling. In the presence of melting two source terms are introduced in the heat equations of liquid and solid phases to account for the energy deposited by the pulse and for the latent heat due to the state change. By solving these equations, the local temperature and intensity of the latent heat source term are calculated as a function of the laser pulse time profile. The beginning of the melting process in a given volume element of surface heat affected zone is defined by the time at which the local melting temperature is reached; the end of the melting process is identified by the time where the latent heat source term becomes zero. A criterion based on the volume and surface free energies enables also to calculate the characteristic time at which starts the local nucleation process in liquid phase. The comparison of these times for amorphous and crystalline Si is critical to establish which phase is effectively allowed to form. Moreover, the delay with which the various points of the molten zone nucleate the crystalline phase enables also the grain size to be estimated; the criterion is based on the possibility for a given point of the molten phase to nucleate a new grain instead of being involved in the growth of another grain nucleated earlier.

[S36.135] After quench effects in beta-phase CuZnAl SMA

Critical applications of SMA need a guaranteed behaviour. In SMA metallurgy, it is usual to quench the alloys after some homogeneization and betatization thermal treatments. The quenched alloys are in metastable state, and thus some time evolution is expected. It is well established that when transforming to the martensite phase, the martensite can experience a stabilization, which, in some Cu based alloys may be up to 100K if the transformation is done immediately after the quench. Appart from the classical stabilization of martensite, time evolutions can be detected in beta phase after the quench. A correct description and a methodology on the detection and evaluation of these effects is needed in order to obtain high reliability of SMA devices. These evolutions affect the macroscopic parameters during at least hundreds of hours, and affect also the martensitic transformation temperatures. This work is aimed to describe the time evolution behaviour in terms of macroscopic parameters as electrical resistance, density, and calorimetric signal (energy dissipation), observed after the quench in CuZnAl alloys. These parameters can be considered as representative of the internal state of the sample, and can be connected to the Ms behaviour for long time evolutions in beta phase near room temperature. The observed dependences on the sample characteristics are also discussed.

[S36.136] Pressure-Dependent Phase Stability in PS-PBMA Diblock Copolymers

Diblock copolymers of poly(styrene-b-n-butyl methacrylate) are used as model systems to study ``lower-critical ordering" type phase behavior, in which microphase separation is observed upon increasing temperature. This phase behavior contrasts with the classical enthalpy-driven microphase separation in block copolymer melts, where microphase separation is observed upon decreasing temperature. These systems have been extensively characterized using small-angle neutron scattering with hydrostatic pressure as the dependent variable; large positive pressure coefficients of the transition temperature of order 150 K/kbar are observed. From these data we obtain an empirical relationship for the Flory-Huggins interaction parameter as a function of temperature and pressure and estimate the effects of pressure on order-order transition boundaries in the phase diagram.

[S36.137] First principles Investigations of the Lattice Dynamics of the Ru(0001) and O(1\times1)/Ru(0001) Surfaces

The recently developed density functional perturbation theory in the framework of a mixed-basis pseudopotential method [1] allows a parameter free characterization of lattice dynamical properties of transition metals and its surfaces, which are of technological importance because of their unusual catalytic properties. Here we present ab initio calculations of phonon dispersion relations of the Ru(0001) and O(1\times1)/Ru(0001) surfaces. For Ru bulk we find pronounced phonon anomalies, which have been confirmed experimentally. They are reflected in unusual dispersion relations of the clean Ru(0001) surface. For the O(1\times1)/Ru(0001) surface we observe a strong dispersion of the vertically polarized oxygen mode along \bar\Gamma-\barK-\barM in good agreement with experiment [2]. In addition, our calculations provide interpretations of other experimentally observed modes. [1] R. Heid and K.-P. Bohnen, Phys. Rev. 60, R3709 (1999) [2] T. Moritz et al., Surf. Sci. 427-428, 64 (1999)

[S36.138] Growth of carbon nanotubes by gas source molecular beam epitaxy

There has been increasing interest in carbon nanotubes because of their potential applications in electronic devices and high-performing nanoscale materials. Carbon nanotubes have been synthesized by arc discharge, laser ablation, and chemical vapor deposition, etc. It was also reported that carbon nanotubes could be grown from carbon vapor under ultrahigh vacuum conditions without the presence of catalyst. In this paper, the growth of carbon nanotubes by using gas source molecular beam epitaxy with the presence of catalyst was first reported. The Si substrates were thermally oxidized with the silicon dioxide layer thickness of 100 nm. Ni layers with different thickness were deposited via e-beam evaporation. The substrate was then introduced into a gas source molecular beam epitaxy chamber. The carbon nanotubes growth was performed by introducing pure acetylene with the temperature of the substrate ranging from 600 to 750 degree. The carbon nanotubes were studied by using Raman spectra and scanning electron microscopy. The formation of carbon nanotubes was strongly influenced by the thickness of Ni layer and the growth temperature. It is found that the end of carbon nanotube is Ni droplet while the base is in a hole. A growth mechanism was proposed.

[S36.139] Low Temperature Buffer Layer for High Quality SiGe Growth

Relaxed SiGe attracted a lot of interest due to the strain Si/SiGe HEMT and integration of the III-V devices on Si. Low temperature buffer method has been used to obtain high quality SiGe with Ge content up to 0.9. However, few experimental results have been reported to explain the mechanism of this method. In this work, low temperature Si or SiGe buffer layers were grown at 200~450 ^oC and characterized by atomic force microscope, photoluminescence and Raman spectroscopy. High quality SiGe layers with different Ge contents have been obtained with this method. It was found that the surface morphology and density of the point defects of the low temperature buffer layer influenced the quality of the SiGe layer grown on them. The buffers grown at 200~250 ^oC had such rough surface that a lot of threading dislocation formed in the followed SiGe layers. The buffers grown at 380~450 ^oC had such low density of point defects that they did not work well to nucleate the dislocations and annihilate dislocations. A mechanism was proposed to explain the role of the low temperature layer on improving the quality of SiGe layer.

[S36.140] Flow experiments in superfluid ^4He near T_\lambda

We report here, data from an experiment studying flow of superfluid helium through an array of sub-micron wide slit-apertures. The data is obtained by applying an AC drive voltage to the diaphragm in a Helmholtz resonator cell. We have observed a staircase pattern in the measurements of the amplitude of diaphragm motion as a function of applied drive. At a temperature approximately 60 microKelvin below the transition we observe critical phase difference less than 2*Pi. This indicates that the array must be close to Josephson regime, unexpectedly for this temperature range. Dependence of the resonant frequency on the amplitude indicates nonlinear current-phase characteristic as well. The work is being carried out at JPL, California Institute of Technology under contract to NASA.

[S36.141] Studies of Bi-modal Distribution and Spatial Distribution of Self-assembled Ge Dots on Si (001)

The uniformity and positioning of self-assembled dots are critical for the pragmatic applications, such as signal processing and quantum-dot based devices. However, the bi-modal size distribution of Ge dots, which is a typical result in the spontaneous system of Ge on Si (001), has become an obstacle to the formation of uniform self-assembled dots. In this work, we study the bi-modal morphology and size distribution of self-assembled Ge dots on Si (001) substrates. It is found that the number ratio of pyramids to domes decreases with growth temperature and then increases. The turning point takes place at about 600^oC, resulting in the formation of mono-modal morphology distribution of Ge dots. Being grown at this temperature, highly uniform Ge dots have been accomplished with the height non-uniformity of about +/-3%, and all the Ge dots are dome-shaped with the average base size of 70 nm. It is believed that enhanced diffusion is the dominant mechanism at the temperature below 600^oC, and the intermixing between Si and Ge becomes dominant at the temperature above 650^oC. Further studies are still under way in order to gain insight into the mechanisms and better control the bi-modal distribution. The other part of the work discusses our recent studies of spatial distribution of Ge dots. We will demonstrate the ability to control freely the positioning of self-assembled Ge dots on selectively grown Si mesas. The spontaneous growth of Ge dots on pre-grown Si mesas is a promising way to control the positioning of self-assembled dots. This result can be extended to other heterogeneous systems.

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[S36.142] Resistive Detection of Single Electron Motion and Spin Resonance

For any of the quantum computing proposals involving spin states in solids, the spin orientations of individual qubit has to be measured at the end of the operations. In the design of the spin-resonance-transistors, a conventional field-effect-transistor (FET) channel is used to accomplish this seemingly almost impossible task. Toward this end, we have performed a sequence of exploratory experiments in Si and GaAs based FET devices. Current noise spectrum in a short channel Si FET, known as the random telegraph signal, is measured for different temperatures and gate voltages. Using a GaAs/AlGaAs heterostructure, we have detected electron spin resonance of a spin-split Landau level by a transport measurement in microwave fields. The extension of our results to the actual implementation of the spin-resonance-transistors will be discussed.

[S36.143] Phase Diagram of Doped Heisenberg Chains

A family of exactly solvable models describing a spin S Heisenberg chain doped with mobile spin-(S-\frac12) carriers is studied by means of the thermodynamic Bethe Ansatz. The models generalize the supersymmetric t--J model which is obtained for S=\frac12. In absence of a magnetic field the models show a new type of spin-charge separation: in the spin sector of the theory one has two types of massless excitations, which can be identified as the spinons in a level-2S SU(2) WZW model and the critical mode of the minimal unitary model \cal M_2S+1, repsectively. In a magnetic field a spin gap phase leading to doping-induced plateaus in the magnetization curve is found.

[S36.144] Effects of Resonant Tunneling on Quantum Transport Through Molecular Structures at Finite Temperatures

We consider quantum transport in molecular two-terminal structures based on organic molecules. Some of those systems have a switching behavior essential for application(C.P. Collier et al., Science 285, 391 (1999))^,(J. Chen et al., Science 286, 1550 (1999)). The molecular analogs of resonant tunnel diodes (RTD) are realized on groups of two, or more, molecular (aromatic) groups separated by the tunnel barrier. The overall performance of the device strongly depends on the level structure and geometry of the molecule and potential (and charge) distribution across the device. The gradual deterioration of the performance of the molecular RTD with elevated temperature is analyzed with a realistic model.

[S36.145] One-Dimensional Migration of Ga atoms as Local Probe of Adsoption Energy Variation

Recently we have found that scanning tunneling microscope (STM) images of bar structures are obtained when Ga atoms on an H-terminated Si(100) surface are observed at 100 K. We reveal that the STM images correspond to a one-dimensional thermal diffusion of single Ga atom which is terminated by dihydrides. In order to verify this, we performed first-principles calculations for the electronic states of this system. As a result, we found that the calculated stable position of a Ga atom and the direction of the diffusion agree with experimental results. The energy barrier for the diffusion of a Ga atom is found to be 0.2 eV, which is a reasonable value for explaining the experimental results. Moreover, the calculated temperature range, where the one-dimensional diffusion takes place, agrees with experimental results. From these results, we conclude that the bar is an image of one Ga atom, directly reflecting the spatial probability distribution of Ga atomic motion.

[S36.146] Long-Range Anticlinic Coupling of Tilt Orientation of Surface Layers in Smectic A Liquid Crystal Freely Suspended Films

Freely suspended films in the smectic~A phase often have tilted, smectic C-like surfaces. The relative orientation of these tilted surface layers can be either synclinic (tilting in the same direction) or anticlinic (tilting in opposite directions). In the case of chiral materials with anticlinic surfaces an external electric field can induce a transition into the synclinic state. Here we report both the film thickness and temperature dependence of the critical field required to induce this transition in one material.

[S36.147] Photoemission Characterization of Al/Alq_3 and Al/LiF/Alq_3 Interfaces

The interfaces formation of Al on tris-(8-hydroxyquinoline) aluminium Alq_3 and Al on LiF/Alq_3 is investigated by using X-ray and ultraviolet photoemission spectrascopy (XPS and UPS). We observed significant modification of O1s, N1s and Al2p core level spectra as Al was directly deposited on the Alq_3 surface. The Alq_3 molecular orbital features in the UPS spectra were also quikly destroyed. In contract, a dramatically different behavior was observed for Al on the LiF/Alq_3 interface. With only about 5Åor less of LiF deposited on the Alq_3 as a buffer layer, the reaction between Al and Alq_3 is significantly suppressed. A well-defined gap state is formed in UPS spectra. The Alq_3 molecular orbital features in UPS shift to higher binding energies but remain easily recognizable. Both the core level spectra and the gap state suggest that the Alq_3 anion is formed in the presence of Al and LiF.

[S36.148] A study of mobile dislocation densities and velocities through mechanical testing

The use of transient mechanical tests such as repeated load relaxations and repeated creep tests provide some insight on the respective contribution of mobile dislocation densities and velocities to the plastic strain-rate. Improvements of the second technique are exposed as well as results on Ni3Al polycrystals. Correlations are found between dislocation exhaustion rates, work-hardening coefficients and the amplitude of yield point at reloading after the relaxations.

[S36.149] Multilayer water condensation and desorption on hydrophobic and hydrophilic surfaces

We are studying water condensation and desorption on hydrophilic (e.g. Pt (111)) and hydrophilic (e.g. octane films on Pt(111) and graphite (0001)), at temperatures around 100-150K. On hydrophilic surfaces the behavior is well known from a manifold of earlier studies. On hydrophobic surfaces there are two qualitatively new behaviors; (i) The effective sticking coefficient can be much less than one because of the much weaker monomer H2O interaction with the surface, compared to hydrophilic surfaces. As a consequence the initial nucleation and growth rate of the film varies strongly with temperature and water vapor pressure (on hydrophobic surfaces) for the studied temperature range. (ii) The required mass of water to reach a coherent and completely covering film is much larger on the hydrophobic surface, i.e. the film morphology differs on the two types of surfaces. These differences are also reflected in differences in the phase transition from amorphous to crystallineof the ice film upon heating through the transition temperature. The measurements were primarily done by temperature programmed desorption. Theoretical modeling can semiquantitatively reproduce the nucleation and growth behavior on the hydrophobic substrate.

[S36.150] Brillouin light scattering in (NH_4)_3H(SO_4)_2 = crystal

The Brillouin scattering from a single crystal from the superprotonic conductor family with common formula Me_3H(AO_4)_2 where Me =3D NH_4, Rb, Cs; A =3D S, Se has been studied in a wide temperature range around the ferroelastic-superprotonic phase transition. Step-like anomalies in hypersonic velocity and damping at the phase transition are determined. Usually, the behavior of LA phonons changes in the framework of Landau's theory at the improper ferroelastic phase transition. In our case the step-like anomalies of the sound velocity of the hypersonic phonons do not agree with Landau theory. An important similarity (step-like anomalies of opposite sign) has been found in the acoustic behavior of other crystals in this family(S.G. Lushnikov and I.G. Siny Ferroelectrics 106 (1990) 237). The Temperature dependence of sound velocity may be explained by taking into account the acousto-ionic interaction.

[S36.151] Temperature variation of the density of vibrational states of = Na_1/2Bi_1/2TiO_3

The majority of relaxor ferroelectrics are perovskite-type materials with disorder in the B-sublattice of the ideal perovskite structure and common formulae AB'_xB"_1-xO_3. In contrast, Na_1/2Bi_1/2TiO_3 (NBT) crystals have disorder in the A-sublattice. The present work consists of results of lattice dynamics studies of NBT in the temperature range 290 K to 50 K by inelastic neutron scattering. The generalized density of vibrational states G(E) of NBT, obtained from INS spectra in the incoherent approximation, varies in the segments 4 to 14 meV and 20 to 55 meV in the 290 K to 120 K temperature range. However, the changes of G(E) upon lowering temperature from 120 K to 50 K are small and lie within experimental error. These changes are discussed within the framework of a modern theoretical model.

[S36.152] The effect of construction technique on 2D triangular piles

In order to test competing theories(J. Wittmer et. al, \textitJ. Physique I) 7, 39 (1997). (S. B. Savage, \textitProceeding of the Third International Conference on Powders amp; Grains) A.A. Balkema, Rotterdam, Netherlands (1997). of granular statics, we perform experiments on 2D triangular piles of cohesionless granular material to determine the effect of construction technique on the stress characteristics. Photoelastic disks allow us to examine the internal stress directly. The triangular piles are formed either by release from a localized source above the center of the pile causing avalanches during formation, or by release from a device that allows a more uniform rain of particles to form the pile. Examining nearest neighbor angle distributions, we find that the localized technique creates a structure with preferred directions that are much more pronounced than those found when using the raining technique. The subsequent stress chains in the bulk of the localized source pile lie along a preferred direction close to the angle of repose. This preferred direction is missing in the raining case. Work in progress examines stress propagation in the same material due to point contacts.

[S36.153] Lateral Diffusion of a Phospholipid on Oil/Water Interface:

It is generally assumed that monolayers constitute an appropriate model for hemi-leaflets of bilayer membranes. In addition, monolayers are better controllable than bilayers relative to lateral pressure, composition and adsorbates. Here, we report a study of direct comparison of the translation diffusion coefficients of a probe lipid in the multibilayers and monolayers of dilauroylphosphatidylcholine (DLPC). Our findings are that the diffusion in the bilayers are slower by an order of magnitude relative to that in the monolayers. We then attempt to replicate the difference by examining the DLPC diffusion in the monolayers at the Oil/Water interface with dodecane (C _12), tetradecane (C_14) and hexadecane (C_16) as the oil phase.

[S36.154] Relativistic effects on electronic structure and phase stabilities of some Pt-based alloys

We examine the phase stability and the ordering tendencies of some Pt-based fcc random alloys using the generalized perturbation method (GPM) implemented in the linear muffin-tin orbitals (LMTO) basis. The reference medium for the GPM is chosen as the completely disordered state of the alloy and its electronic structure is described in the coherent potential approximation(CPA). Ordering tendencies and phase stability are examined via effective pair interactions and their lattice Fourier transforms. Relativistic effects on the ground state cohesive properties and the ordering tendencies are determined by carrying out nonrelativistic and fully relativistic (in some cases scalar relativistic) LMTO-CPA calculations. In all cases considered, namely X-Pt with X= Ti, V, Ni, Cu, Pd and Au, the correct ordering tendency is obtained, although in most cases the ordering tendency is found to be overestimated in the GPM. As expected, relatvistic effects are most prominent in AuPt, where the nonrelativistic description shows a tendency towards L1_1 ordering and the correct result, i.e. phase segregation, is obtained only in the fully-relativistic description. The sensitivity of the ordering tendency to factors such as lattice relaxation and volume per atom is examined briefly. Finally, the effect on the phase stability, of adding a third component, such as V in CuPt or Ni in AuPt, is studied by extending the formalism to the case of a ternary alloy. The central issues in this work are an examination of the effectiveness of the LMTO-CPA-GPM method in being able to predict the ordering tendencies in a large class of Pt-based alloys and the importance of the relativistic effects on their ordering or segregation.

[S36.155] Absolute Coverge of the Saturated Cs/Si(111)7\times7 Interface

We have performed Rutherford Backscattering Spectrometry (RBS) measurements of the absolute coverage of films of Cesium on Si(111)-7\times7. The room temperature saturation coverage was found to be 0.51 \pm 0.02 Monolayers (ML = 7.83\times10^14 Atoms/cm^2). Determination of the absolute coverage is a key step in identifying the structure of the interface. Samples are prepared in an Ultra-High Vacuum system with a base pressure of 1.2\times10^-10Torr. Cesium is dosed with commercial SAES getters, and saturation coverage is determined via Auger Electron Spectroscopy.

[S36.156] ^1H-NMR Transversal Relaxation and Mean Field Theory of PDMS Networks with Pendant Chains

In this work we have estimated the mass fraction of elastic and pendant chains of model polydimethylsiloxane (PDMS) networks using transversal proton relaxation in nuclear magnetic resonance (^1H-NMR). These experiments were compared with theoretical estimations of the mass fraction of pendant chains predicted by mean field calculations. Preliminary results showed that proton relaxation measurements underestimate the mass fraction of pendant material. We speculate that trapped entanglements, in which long pendant chains are involved, may act as temporary cross-linking points in the time scale of the ^1 H-NMR experiments. In this condition only portions of the pendant chains would be detectable by the experiments justifying the observed differences between proton relaxation experiments and the mean field results. In order to verify this presumption we formulated a modification of the recursive calculations to estimate the amount of entanglements in which pendant chains are involved. If entanglements are taken into consideration, a very good agreement between theoretical mass fraction of pendant chains calculated by the modified mean field theory and experimental values determined from proton relaxation is obtained.

[S36.157] Ground State Structure of Spin Glasses

The nature of the spin glass phase is still controversial. In this talk, we investigate numerically the ground state (GS) structure of Ising spin glasses by determining how the GS changes in a fixed block far from the boundaries when the boundary conditions are changed. Both in 2D and 3D, the probability of a change in the block GS configuration tends to zero for large volumes. This indicates a trivial GS structure, as predicted by the droplet theory.

We also study the effect of a bulk perturbation that increases the GS energy by a volume--independent amount. Both in 3D and 4D, the overlap between the perturbed GS and the unperturbed GS tends to one for large volumes. This confirms a trivial GS structure in finite dimensions. By contrast, in two mean field models, the Sherrington--Kirkpartick and the Viana--Bray model, the overlap tends to a value smaller than one, as predicted by replica symmetry breaking. Therefore, the GS structure of mean field models is different from that in finite dimensions. (M. Palassini and A.P. Young, Phys. Rev. B 60, R9919, (1999); Phys. Rev. Lett. 83, 5216 (1999)).

[S36.158] Structural Study of the Smectic-I and Smectic-F Phases in Freely Suspended Films

The Smectic-I (S_I) and Smectic-F (S_F) phases of terephthal-bis-(4n)-decylaniline (TB10A) have been examined to determine the significance of conformational changes in the phase transition. X-ray diffraction measurements of thick, single domain, freely suspended films are used to to classify the phases. A multi-wire area detector and a three-dimensional mapping software is used to obtain the reciprocal space

representation of the structures. We obtain the molecular tilt by analyzing the effect of the molecular form

factor on the X-ray scattering structure factor. The tilt obtained this way is inconsistent with the tilt estimated from published layer spacing measurements, suggesting that a comprehensive model of the hexatic phases of TB10A may need to consider conformational changes in the molecules. Analysis of the form factor indicates that the S_I and S_F phases have different tilts when coexisting and implies that conformational changes drive the transition.

[S36.159] X-ray Mesurements of Correlation Lengths in Hexatic Phases Using an Area Detector

Positional correlation lengths in TB10A and C10 liquid crystals are measured using an area detector and reciprocal space mapping of the structure factor. We investigate the resolution of the area detector by analyzing crystalline peaks in 7O.7 and C10. Gaussian curves are used to approximate the resolution function at several locations in reciprocal space. The widths of the Gaussian fits are compared with estimates obtained from a theoretical analysis of the instrument. We use the resolution function to extract temperature dependent correlation lengths for TB10A and C10.

[S36.160] To Study the Phase Transition of Self-Assembled Nanostructures: (1-x)PZN-xPT using Inelastic Light Scattering

Relaxors are one of the important classes of self-assembled nanostructure composite materials. The interesting features associated with the nanoregions/nanodomains; required to describe these relaxors, give rise to the most relevant device related characteristics and peculiar properties in these materials. In addition, they possess astronomical property coefficients by themselves or when modified with lead titanate (PT). In the present work, a detailed temperature dependent study has been carried out on (1-x)PZN-xPT relaxors with compositions x =0.02 and x = 0.05 using Raman scattering under optical and E-field variables and inferred the structure-property relations in order to obtain information to characterize the material for matching the application criteria. Besides, the phase transitions associated with the relaxors have also been investigated to understand the polarization mechanism for the unpoled (x =0.02) and poled (x = 0.05) specimens. The difference in the case of poled specimen is accountyed for by the influence of residual electric field. Poling also suggested an enhanced local ordering and the increase in the volume of the polar nanoregions or self-assembled nanostructures. Thanx are due to U.S National Science Foundation Grant No. NSF-DMR-9801759.

[S36.161] Coexistence of Magnetism and Ferroelectricity in a New Pseudocubic Perovskite Phase of Thin-Film BaFe_xTi_1-xO_3 (0.5\leqx\leq0.75)

The properties of a new pseudocubic perovskite phase of thin-film BaFe_xTi_1-xO_3 (0.5\leqx\leq0.75) is reported. The films are grown epitaxially by pulsed laser deposition on MgO (various orientations) and SrTiO_3 substrates. This material is novel because the corresponding bulk compound has hexagonal structure for comparable x. The films are characterized by the coexistence of magnetism (T_C > 500~^\circC) and ferroelectricity (T_C \sim 200-300~^\circC). The ferroelectric transition is evidenced by temperature dependent x-ray diffraction and measurements of the electrical resistivity (PTCR-effect). Magnetization measurements indicate a saturation moment of ~0.3 \mu_B/f.u. suggesting ferrimagnetic ordering.

[S36.162] DILUTED QUANTUM HEISENBERG MODEL FOR THE Fe-Al ALLOYS

In this work we present a theoretical study for the magnetic behavior of the structural disordered Fe-Al alloys on the basis of a simple site-diluted quantum Heisenberg model. With the assumption that the exchange interaction between nearest-neighbor spins depends on the Al concentration q we have calculated, by using the mean field renormalization group method, the critical temperature and critical exponents of the model. An acceptable fit to the experimental phase diagram for Al concentration in the range 0.30

[S36.163] Characterisation of Diblock Copolymer Blends

Two diblock copolymers, namely poly(isoprene)-b-poly(ethylene oxide) (PI-PEO) and poly(isoprene)-b-poly(styrene) (PI-PS), are synthesized by anionic polymerisation. The two diblock copolymers are blended in 30% solution in the common solvent chloroform, and dried afterwards. The binary blends are characterized by small angle X-ray and small angle neutron scattering experiments, by transmission electron (TEM) and light microscopy, and by rheology experiments. The order-disorder-transition temperatures and ordered structures can be determined by scattering experiments. Real space pictures are taken by TEM, and light microscopy supports statements about the miscibility. Ordered structures are found at low temperatures. Upon heating the diblock copolymers become homogeneously mixed. There is an intermediate phase where only the PI-PEO diblock copolymer is ordered. The experimental phase diagrams are in excellent agreement with theoretical predictions of a random phase approximation.

[S36.164] X-Ray Diffraction Study on the Structural Phase Transitions in CsPbCl_3

CsPbCl_3 crystal undergoes successive phase transitions at 320, 315 and 310 K. Besides these transitions, another phase transition was also reported to exist at 200 K. In our recent calorimetric and X-ray measurements using "mK-stabilized cell", a superfine multistage transition was observed in a small temperature range of 0.1 K at the 320 K phase transition. In order to make clear the nature of the superfine multistage transition, we have reexamined the structural transitions over the temperature range from 340 to 90 K. On heating from the room temperature, structure changes at 310, 315 and 320 K were in good agreement with the previous studies. On cooling, however, an abrupt increase of intensity of the X point superlattice reflections which had appeared at 310 K transition was again observed at about 265 K. With further lowering temperature to 200 K, another abrupt increase of intensity was observed at the R point superlattice reflections which had appeared at the 315 K transition. The results obtained in this study indicate that there are two series of successive phase transitions on cooling starting from the two anomalies at the 320 K transition on cooling, respectively: One is 320-315-310 K transition series well explained by the soft phonon condensation theory, and the other is 320-265-200 K transition series which is dominant in the specimen.

[S36.165] Multilayer water condensation and desorption on hydrophobic and hydrophilic surfaces

We are studying water condensation and desorption on hydrophilic (e.g. Pt (111)) and hydrophilic (e.g. octane films on Pt(111) and graphite (0001)), at temperatures around 100-150K. On hydrophilic surfaces the behavior is well known from a manifold of earlier studies. On hydrophobic surfaces there are two qualitatively new behaviors; (i) The effective sticking coefficient can be much less than one because of the much weaker monomer H2O interaction with the surface, compared to hydrophilic surfaces. As a consequence the initial nucleation and growth rate of the film varies strongly with temperature and water vapor pressure (on hydrophobic surfaces) for the studied temperature range. (ii) The required mass of water to reach a coherent and completely covering film is much larger on the hydrophobic surface, i.e. the film morphology differs on the two types of surfaces. These differences are also reflected in differences in the phase transition from amorphous to crystallineof the ice film upon heating through the transition temperature. The measurements were primarily done by temperature programmed desorption. Theoretical modeling can semiquantitatively reproduce the nucleation and growth behavior on the hydrophobic substrate.

[S36.166] Velocity dependent nonlinearity in the kinetic roughening of combustion fronts

Recent experiments have shown^1,2 that KPZ interface scaling can be found in different physical systems. In addition, interesting short-scale dynamics is observed.^3 Despite the work by Barabasi et al.^4 not much is known about the depinning transition. By refining the methods reported by us earlier,^1,3 the average velocity of slow combustion fronts can now be controlled such that the pinning limit can be approached. We find that the KPZ type nonlinearity is decreased with a decreasing front velocity. \par ^1Maunuksela et al., PRL 79, 1515 (1997). \par ^2Surdeanu et al., PRL 83, 2054 (1999). \par ^3Myllys et al., to appear in PRL. \par ^4Reka et al., PRL 81, 2926 (1998).

[S36.167] A Space-Time Adaptive Method for Simulating Complex Cardiac Dynamics

A new space-time adaptive mesh refinement algorithm (AMRA) is presented and analyzed which, by automatically adding and deleting local patches of higher-resolution Cartesian meshes, can simulate quantitatively accurate models of cardiac electrical dynamics efficiently in large domains. We find in two space dimensions that the AMRA is able to achieve a factor of 5 speedup and a factor of 5 reduction in memory while achieving the same accuracy compared to a code based on a uniform space-time mesh at the highest resolution of the AMRA method. We summarize applications of the code to the Luo-Rudy 1 cardiac model in large two- and three-dimensional domains and discuss the implications of our results for understanding the initiation of arrhythmias.

[S36.168] Fluctuations and correlations in population models with age structure

We study the population profile in a simple discrete time model of population dynamics. Our model, which is closely related to certain ``bit--string'' models of evolution, incorporates competition for resources via a population dependent death probability, as well as a variable reproduction probability for each individual as a function of age. We first solve for the steady--state of the model in mean field theory, before developing analytic techniques to compute Gaussian fluctuation corrections around the mean field fixed point. Our computations are found to be in good agreement with Monte--Carlo simulations.

[S36.169] Influence of Seeding over the Nature of Stress, Defects and its relationship with the Growth of Diamond Thin Films

We investigated diamond thin films grown by electron cyclotron-resonance assisted chemical vapor deposition (ECR-CVD) at low pressure (1.0 Torr) and low temperatures (550 and 700 ^o C). These films were grown on pretreated Si (111) substrates with diamond particles of varying seeding densities :0.01, 0.1, 1.0, 10.0 densities of 0.223, 1.5, 2.3 and 3.1 x10 10^9/cm2. We employed scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy (RS) to investigate the influence of seeding densities on the crystalline quality, diamond yield and intrinsic stress developed in these films during growth. The thermal interfacial stress, interactions across grain boundaries, and internal stress were considered to account for the total stress observed from the representative Raman band. A correlation was presented among seeding density, relative amount of non-sp ^3 phase, O/C ratio and total intrinsic stress.

[S36.160] To Study the Phase Transition of Self-Assembled Nanostructures: (1-x)PZN-xPT using Inelastic Light Scattering

Relaxors are one of the important classes of self-assembled nanostructure composite materials. The interesting features associated with the nanoregions/nanodomains; required to describe these relaxors, give rise to the most relevant device related characteristics and peculiar properties in these materials. In addition, they possess astronomical property coefficients by themselves or when modified with lead titanate (PT). In the present work, a detailed temperature dependent study has been carried out on (1-x)PZN-xPT relaxors with compositions x =0.02 and x = 0.05 using Raman scattering under optical and E-field variables and inferred the structure-property relations in order to obtain information to characterize the material for matching the application criteria. Besides, the phase transitions associated with the relaxors have also been investigated to understand the polarization mechanism for the unpoled (x =0.02) and poled (x = 0.05) specimens. The difference in the case of poled specimen is accountyed for by the influence of residual electric field. Poling also suggested an enhanced local ordering and the increase in the volume of the polar nanoregions or self-assembled nanostructures. Thanx are due to U.S National Science Foundation Grant No. NSF-DMR-9801759.

[S36.171] Electronic and impurity states in triple quantum wells

A theoretical study of electronic and impurity states in triple coupled GaAs/GaAlAs quantum wells is presented. We have adopted a variational scheme in which ground and excited impurity states are obtained simultaneously. We consider different physical regimes for a donor impurity by modeling distinct confinement potentials for the structure. The particular case of In layers immersed in GaAs/GaAlAs matrix is described as a narrow and deep central well connected to the lateral ones via thin barriers. The effects of magnetic and electric fields, both applied along the heterostructure, on the impurity spectrum are also discussed.

[S36.172] Dipolar Interaction in Multilayers with Rough Interfaces

Calculations of the dipolar interaction between two slabs with rough interfaces are presented. An analytical model is developed in order to calculate the dipolar interaction energy between multilayers of any size and different rough configurations at the interface. We found that in the absence of roughness, the dipolar coupling is negligible. However, in the presence of topographic inhonogenities, the magnetoestatic coupling gives rise to non-negligible ferromagnetic or antiferromagnetic coupling.

[S36.173] The role of single-ion excitations in the mixed-spin quasi-one-dimensional quantum antiferromagnet Nd_2BaNiO_5.

Inelastic neutron scattering experiments on Nd_2BaNiO_5 single-crystals and powder samples are used to study the dynamic coupling of one-dimensional Haldane-gap excitations in the S = 1 Ni^2+-chains to local crystal-field transitions, associated with the rare earth ions. Substantial interference between the two types of excitations is observed even in the 1-dimensional paramagnetic phase. Despite that, the results provide solid justification for the previously proposed "static staggered field" model for R_2BaNiO_5 nickelates. The observed behavior is qualitatively explained by a simple chain-Random-Phase-Approximation (chain-RPA) model.

[S36.174] Non-Fermi-Liquid behavior and incipient superconductivity in the heavy- fermion compound CeNi_2Ge_2

CeNi_2Ge_2 is a clean stoichiometric heavy-fermion compound which shows Non-Fermi-liquid (NFL) behavior already at ambient pressure [1]. Recently the observation of incipient superconductivity (sc) , i.e. a drop in \rho(T) below T \leq 0.1 K in high-quality crystals (\rho_0 < 1 \muØmegacm) was reported [1,2]. Here we present a detailed investigation of the low-temperature resistivity \rho(T,B) at 0.01 K \leq T \leq 6 K for various slightly off-stoichiometric Ce_1+xNi_2+yGe_2+z polycrystals. Particular attention is paid to the composition-dependence of the lattice constants, the residual resistivity \rho_0, and the occurrence of incipient sc. NFL behavior in the resistivity, i.e. \rho(T) - \rho_0 \sim T^\epsilon, with 1.32 \leq \epsilon \leq 1.5 depending on the composition, is analyzed in terms of an antiferromagnetic quantum critical point. We examine the effect of impurity scattering for samples with \rho_0 varying between 0.2 and 10 \muØmegacm and compare it with recent theoretical predictions [3]. \ [1] P. Gegenwart et al., Phys. Rev. Lett. 82, 1293 (1999) \ [2] F.M. Grosche et al., cond-mat/9812133 \ [3] A. Rosch, Phys. Rev. Lett. 82, 4280 (1999).

[S36.175] Direct fifth-order non-resonant Raman scattering from CS_2

Direct fifth-order non-resonant Raman scattering from CS_2

We present the direct fifth-order electronically non-resonant intermolecular signal from liquid CS_2.We recently demonstrated that previous attempts to measure the direct fifth-order spectrum were dominated by third order cascaded signals. (J. Chem. Phys. 111, 3105 (1999)) Here, we show that phase matching considerations can be used to provide substantial discrimination against the cascaded third-order signals, and the direct fifth-order signal can be measured. The measured spectra indicate that the intermolecular motions of liquid CS_2 at room temperature are strongly coupled, i.e. they exchange energy on a timescale comparable to or faster than those associated with these intermolecular motions. Thus, the liquid is dynamic on these timescales, and the intermolecuar motions can be well described as homogeneous.

[S36.176] Dependence of the intermixing of InGaAs/InGaAsP quantum well in impurity free vacancy disordering on the NH_3 flow rate for the growth of SiN_x capping layer

The dependence of impurity free vacancy disordering (IFVD) of InGaAs/InGaAsP QW structure on the characteristics of dielectric capping layer was studied using SiN_x film as capping layers. The characteristics of the SiN_x capping layer were varied by changing the NH_3 flow rate from 0 sccm to 40 sccm at fixed SiH_4 flow rate during the SiN_x deposition by plasma enhanced chemical vapor deposition (PECVD). The result showed that the amount of a blue-shift was controlled not only by annealing time but also by the flow rate of NH_3 gas supplied during the growth of SiN_x capping layer. SiN_x capping layer grown at higher NH_3 flow rate induced larger blue-shift of bandgap at the same SiN_x thickness. The maximum differential blue-shift obtained by changing the NH_3 flow rate is larger than that obtained with SiN_x capping and SiO_2 capping. This result implies that SiN_x film can be used to get spatially selective quantum well intermixing (QWI) on the same substrate by simply changing its characteristics with the reactant gas ratio, instead of using different dielectric capping materials

such as SiN_x film and SiO_2 film.

[S36.177] Implementation of the refined Deutsch-Jozsa algorithm on a three-bit NMR quantum computer

We implemented the refined Deutsch-Jozsa algorithm on a three-bit nuclear magnetic resonance quantum computer. All of the balanced and constant functions were realized exactly. The results agree well with theoretical predictions and clearly distinguish the balanced functions from constant functions. Efficient refocusing schemes were proposed for the soft z-pulse and J-coupling and it is proved that the thermal equilibrium state gives the same results as the pure state for this algorithm.

[S36.178] Percolative phase separation induced by nonuniformly distributed excess oxygens in La_0.8Ca_0.2MnO_3+\delta

We carried out ^139La and ^55Mn nuclear magnetic resonance measurements in La_0.8Ca_0.2MnO_3+\delta with different oxygen stoichiometry \delta. From the signal intensity, peak frequency and line broadening of ^139La NMR spectrum, we found that excess oxygens have a tendency to concentrate and establish the ferromagnetic ordering around themselves. The newly formed ferromagnetic clusters connect the previous ferromagnetic clusters which are embedded in the antiferromagnetic host, to establish the percolative conduction path in the sample with excess oxygens. According to ^55Mn NMR results, the increment of the hole density in the forromagnetic regions due to excess oxygens is higher than the average value, 2\delta. This phase separation is not a charge segregation type, but a electroneutral type. As temperature increases, the percolative phase separation disappears at a temperature well below phase transition temperature, and the magnetoresistance curve shows a peak at that temperature. This fact indicates that the phase separation is one of the important sources of the colossal magnetoresistance.

[S36.179] Constructing Quantum Combinational Logic Circuits

The evaluation of a binary function is required in the quantum algorithms such as Deutsch and Grover algorithms. A binary function can be implemented by combinational logic circuits. Several methods have been developed which help to construct combinational logic circuits easily for classical computing. For quantum computing, however, there has been no efficient method of constructing quantum combinational logic circuits. We describe a method of constructing quantum combinational logic circuits efficiently with basic quantum logic gates. A Karnaugh map method is suggested as a useful tool for this purpose. This method helps us find several different quantum circuits for one given binary function out of which we can choose the one suitable for implementation.

[S36.180] The effect of a DC bias and a top gate voltage on the transmission in a single mode symmetric ballistic ring

It still remains a challenging problem to build an interference device whose transmission could be switched between the highest (constructive interference) and the lowest (destructive interference) values by a simple adjustment of the gate voltage. So far it has been assumed that in order to serve the purpose the conventional ring interferometer should have some asymmetry in its design. In the present work we investigate the coherent electron transport in a symmetrical AlGaAs/GaAs ballistic ring uniformely covered by a top metal gate. We find that as the gate voltage is varied, the phase of the Aharonov-Bohm oscillations near zero magnetic field switches between 0 and pi. We have also demostrated for the first time that the phase of the Aharonov-Bohm oscillations behaves in a similar fashion in response to a variation of a DC bias. We give a qualitative explanation of our observations on the baisis of a weakly coupled model in which the transmission of the ring interferometer as a function of the Fermi energy is determined by the energy spectrum of an isolated ring.

[S36.181] High Performance Implementation of a Carbon Molecular Dynamics Code on a Cluster of Workstations

We recently ported and optimized a Carbon Molecular Dynamics code developed at Michigan State University on a cluster of workstations connected via a gigabit SCI network. Using an extension of Amdahl's Law as an instrumentation tool we analyzed the primary factors influencing the performance of the parallel code: the sequential component, load balance, and communication overhead. Significant improvement in the speedup was obtained by modifying the algorithm to reduce the sequential portion of the code to less than 0.1load-balancing techniques further improved the performance. Finally, we demonstrate that the MPI collective communications implementation on the SCI network dramatically reduces the communication overhead for our code. An almost ideal speedup was obtained using 18 processing elements (PEs). These results open the prospect of simulating the dynamics of 1,000-atom nanotubes in the microsecond regime using affordable clusters of workstations.

[S36.182] New High Resolution Positron Annihilation Induced Auger Electron Spectroscopy (PAES) with Time-Of-Flight Energy Analyzer

Positron annihilation induced Auger electron spectroscopy (PAES) has been shown to have unique advantages over conventional Auger techniques, including the ability to eliminate the secondary electron background under low energy Auger peaks and selectively probe the top-most atomic layer on the surface. However, the signal rates and energy resolution of first-generation PAES apparatus are relatively low. Suzuki et al. have demonstrated that the time-of-flight (TOF) technique, in which the entire energy spectrum is collected simultaneously, can be used in PAES measurements to significantly increase the data acquisition rate in comparison with a single channel energy analysis. We present the design of a new magnetically guided TOF-PAES spectrometer which should be capable of acquiring data at a rate ~100 times greater than the existing single energy channel PAES systems at UT Arlington, while maintaining an energy resolution of 3system of Suzuki et al. in that it will make use of a DC positron beam and rely on the detection of annihilation gamma rays with a BaF2 detector to provide a timing signal. We present the results of system parameter optimization based upon this method along with the trajectories calculated. The system performance was modeled with the SIMION ion optics analysis program.

[S36.183] Phase Synchronization of Chaotic Plasmas

Experimental phase synchronization of chaos is demonstrated for a plasma discharge tube subject to a high voltage, and paced with a low amplitude wave generator. Similarities with the paced Roessler system are pointed out and a mathematical model is presented for the physical system under consideration. The experimental and the theoretical results are analysed.

[S36.184] First-Principles Study of Ge Adsorption on the H/Si(001) surface

Ge/Si heterostructure system has attracted much attention due to not only the technological importance such as optoelectronic applications but also the scientific interest. Hydrogen is known to act as the surfactant for the growth of this system. It eliminates the undesired features such as a rough surface due to the SK growth and the interface mixing. These effects are reported by the macroscopic experimental investigation. But there is no report from the microscopic point of view. STM is known as the powerful tool for the atomic level investigation. But hydrogen is difficult to investigate by using STM. In this paper, we theoretically investigate the adsorption and diffusion of the Ge adatoms on the H/Si(100)-(2x1) surface by using first-principles total-energy calculation techniques. The Ge atom adsorbs on the H/Si(100) surface by spontaneously capturing one nearest neighbor H atom without any activation barrier, forming the Ge monohydride. Then it captures one more H atom, resulting in the Ge dihydride. These features are similar to the case of the Si adsorption. There are, however, extremely different geometries from the Si adsorption. The Ge adatom exchanges the site with the surface Si atom. It forms the substrate Ge-Si dimer and the Si adatom. Instead of the Ge atoms, the Si atoms may become the diffusion species.

[S36.185] Tip induced Shift of the Diffusion Saddle Point

Using total energy calculations, based on interaction potentials from the embedded atom method, we show that for an adatom placed along a (100) microfacetted step-edge on Cu(111), the barrier to diffusion, in the direction of a tip is reduced considerably, but not completely. For a tip with (111) orientation, we find the barrier to be reduced by 55% in the direction of the tip. The disagreement between this result and a recent study (U. Kuerpick and T.S. Rahman, Phys Rev Lett. 83, 2765 (1999)) is traced to the placement of the saddle point. Our calculated potential energy surface shows a shift in the saddle point by few tenths of an angstrom. The diffusion barriers, towards and away from the tip are calculated with respect to the shifted saddle point. The barrier to diffusion was found to reach a minimum at a lateral separation between adatom and tip, of about one lattice constant. The influence of the tip was found to disappear at a height of about 4.5A above the adatom. We will discuss the effect of changing the orientation of the tip to (100). The cases of blunt versus sharp tips will also be discussed.

[S36.186] Dynamics of a vortex in a trapped Bose-Einstein condensate

We consider a large condensate in a rotating anisotropic harmonic trap. Using the method of matched asymptotic expansion, we derive the velocity of the vortex line as a function of the local gradient of the trap potential, the line curvature and the angular velocity of the trap rotation. We investigate normal modes and stationary configurations of the vortex line for 2D and 3D condensates. Including the effects of trap rotation allows us to find the angular velocity that makes the vortex locally stable.

[S36.187] Use of a High-flux Atomic Oxygen Source for MBE growth of the di- and tri- oxides of Cr, Mo, and W

The MBE growth of several of the highly oxidized phases of the Group IIB elements (Cr, Mo, and W) are of great current interest. In particular, CrO_2, a theorized half-metallic ferromagnet, has yet to be grown in a form that allows high quality tunneling measurements to be performed. Also, thin films of WO_3 for controlled Na doping studies are of interest to help understand the recently published results on possible superconductivity in this material(S. Reich and Y. Tsabba, EUROPEAN PHYSICAL JOURNAL B v. 9(1) pp. 1-4 MAY 1999 and Shengelaya A, Reich S, Tsabba Y, and Muller KA EUROPEAN PHYSICAL JOURNAL B , v. 12(1) pp. 13-15 NOV 1999). Using a new high-flux atomic oxygen source and detection scheme, RHEED, and in-situ core-level photoemission we present the atomic oxygen-temperature phase diagrams indicating the conditions under which the di- and tri- oxide phases of Cr, Mo, and W can be grown.

[S36.188] Composite Media where Maxwell's Equations Obey a Left Hand Rule

We demonstrate that a composite medium, based on interwoven periodic arrays of conducting split ring resonators (SRRs) and continuous wires, can exhibit a microwave frequency region where the effective permeability and the effective permittivity, are simultaneously negative. Since the product of the permeability and the permittivity is positive, Maxwell's equations permit wave propagation, but the material must be "left handed" (LH). That is, the direction of wave propogation and the direction of energy propagation are antiparallel.

In 1968 Veselago examined Maxwell's equations for LH media and concluded that phenomon such as the Doppler effect, Cerenkov radiation, and even Snell's Law will be reversed! These effects have never been explored, as there has not been demonstrated any material for which the permeability could become negative in any frequency region. Building upon the suggestion of J. B. Pendry et al., we have been able to construct both a medium exhibiting negative permeability media, and also a LH media, for which microwave experiments validate the interpretation in terms of simultaneously negative permittivity and permeability.

We will discuss the non-intuitive behavior of electromagnetic wave propagation from a RH host to a LH medium, and the potential of LH meta materials and devices.

[S36.189] Molecular Dynamics Studies of Ni/Al Shape Memory Alloys

Shape memory alloys demonstrate the ability to return to some previously defined shape or size, after plastic deformation, when subjected to appropriate thermal procedure. We have studied these characteristics of shape memory alloys using computer simulations. The simulations were developed using molecular dynamics techniques based on the embedded atom model. The results display austenitic/martensitic transitions leading to shape memory effects in Ni/Al alloys. Temperature induced transitions were observed at different concentrations of Ni and Al. Plots between temperature and strain, show hysteresis during the cycle of heating and cooling over a range of temperature. From these curves, different values of transition temperatures were calculated with different compositions of Ni/Al alloys. Isotherms of Ni/Al alloy (Al 40 percent) were also obtained during loading and unloading at different temperatures. In summery our molecular dynamics simulations were able to reproduce shape memory behavior and structural transitions in Ni/Al alloys.

[S36.190] STM Investigation of Benzene Adsoprtion on Ag(110)

We have investigated the adsorption of benzene on Ag(110) using the scanning tunneling microscope. We found that the molecules preferentially adsorb above step edges at 66 K. The preference for step edge adsorption is attributed to the Smoluchowski effect enhancing the empty states to which charge is donated from the \pi orbitals of the benzene. However, there is no adsorption at the [001] steps. A lack of free charge due to a gap in the Ag Fermi surface along that direction reduces the Smoluchowski effect and thus the adsorption at these steps. After further deposition, we find that benzene forms a weakly adsorbed hexagonal monolayer. The monolayer is imaged at large tip-sample separations and is transparent upon closer approach. This transparency is reduced near steps and point defects.

[S36.191] Multifrequency High Field Quasi Optic Homodyne EMR Spectrometer

The NHMFL, in collaboration with Thomas Keating Ltd., is developing a new Quasi Optic EMR spectrometer. This spectrometer uses quasi optic techniques, including elliptical focusing mirrors and a corrugated wavetube, to propagate the millimeter waves in the frequency range from 180 to 360 GHz. The millimeter waves propagate in a broadband, low loss, polarization conserving mode. This permits the spectrometer to operate in a broad frequency range with increased sensitivity and allows for phase sensitive homodyne techniques to be used to selectively detect either the absorptive or dispersive component of the EMR signal.

[S36.192] Ab initio molecular dynamics study of topological defects in polyethylene strands

The mechanical resistance of polyethylene strands subject to tension and the way their properties are affected by the presence of knots(Nature 399), 46 (1999); JCP 111, 9434 (1999). and entanglements(JPCB, in press) (2000). are studied using first-principles molecular dynamics calculations. The presence of topological defects significantly weakens the chain strength, and the radicals generated at the rupture undergo a series of unexpected ``femto'' chemical reactions(JACS 121), 11827 (1999)..

[S36.193] Image Potential States of Electrons Interacting with Roughened Metal Surfaces Coated with Self-Assembled Alkanethiol Monolayers

Electrons in image potential states (IPS) [1] have been a topic of great interest at smooth dielectric/metal interfaces [2]. IPSs at roughened dielectric/metal interfaces have recently been reported [3]. The IPS model is extended to the case of a charge interacting with two intersecting metal planes. We discuss the role of the angle between intersecting metal planes (characteristic of roughened surfaces) on the IPS binding energies for electrons interacting with these structures. Our results are applied to roughened gold surfaces coated with self-assembled alkanethiol monolayers. [1] T. Fauster, Applied Phys. A 59, 479, 1994. [2] N.-H. Ge, C. M. Wong, R. L. Lingle, Jr., J. D. McNeill, K. J. Gaffney, and C. B. Harris, Science 279, 202, 1998, for example. [3] B. K. Clark, B. W. Gregory, A. Avila, T. M. Cotton, and J. M. Standard, J. Phys. Chem. B 103, 8201, 1999.

[S36.194] Electronic States and Optical Properties of Si/SiO_2 Superlattices

We present the electronic structures of \Si \_m \SiO_2 \_n superlattices (SLs) grown along the [001] direction, using tight-binding methods. Detailed atomic models of the Si/SiO_2 interface are considered. A clear feature of the results is the essentially direct band-gap structure with flat bands along the Z\Gamma symmetry line of the SL-Brillouin zone which has a blueshifted energy gap due to quantum confinement. The optical properties of the SLs are calculated using a parametrization of the imaginary part of the dielectric function of bulk Si. The strong confinement of the electron-hole pairs in the Si wells and their tendency to localize at the low-dielectric \ SiO_2 \ interfaces due to the mutual Coulomb attraction lead to strong electrostatic effects. These produce an interplay of several length scales in determining possible regimes of high radiative efficiency. Our results have implications for the understanding of the luminescence in porous Si and Si-based nanostructures like the amorphous Si/SiO_2 SLs studied recently.

Part S of program listing