Session F36 - General Poster Session I.
POSTER session, Tuesday morning, March 21
Exhibit Hall, MCC

[F36.001] Effect of pH on Growth of Zinc (tris) Thiourea Sulfate

Zinc (tris) Thiourea Sulfate (ZTS) is an organometallic non-linear optical crystal and has been recently started to become a subject of immense research. It has an orthorhombic structure with point group mm2. Single crystals of ZTS were grown from aqueous solution method at room temperature. The effect of pH on the bulk growth of this crystal was studied at room temperature. In particular the relative growth along different crystal direction and growth rate were found to be strongly dependent on the pH of the solution. The pH was varied from 2.5 to 4.2.The clearest crystal with lowest absorption coefficient were obtained at pH 3.2. The mass increase per day was found to be highest for pH 2.5. The growth rate along c-direction was faster than a-direction for pH 4.2. The dislocation density was found to decreases with increase in pH.

[F36.002] A Novel Quinary Alloy Dielectric Crystal

We studied a quinary crystalline mixture, growth by Czochralski method, RbI:RbBr:KI:KBr:RbCl of equal molar fraction in each component, 20 % of each one, using optical absorption and diffractometry techniques. It is obtained the F absorption optical band, coloring a sample by additive coloration, this band is compared with that of a quaternary crystal 1). Is very recently the finding of ternary 2) and quaternary systems, which are a single phase crystals. The X ray difractometry spectra of the quinary crystal shows a crystal with three phases, two unckown. We are considering here that one of it, concerns to a crystal with a lattice constant given by the Generalized law of Vegard 2), obtained averaging the lattice constants of the five pure components, that implies the existense of a quinary crystal inside a crystalline matrix, with three kind of crystalls, one of it, the quinary. 1)R. R. Mijangos, H. G. Riveros, E. Camarillo, R. Guerrero, M. Atondo, E. Alvarez and A. Rodriguez, Physica Status Solidi, Proceedings XV SLAFES, Nov. 1999. In Press. 2)R. R. Mijangos, A. CorderoBorboa, E. Camarillo, H. Riveros and V. M. Casta�o, Phys Letters A, 245, 123, 1998.

[F36.003] The Electric Field from the Thermoelectric Effect in a Cylindrical Ice Crystal

One possible explanation for the charging of a thunderstorm involves the thermoelectric effect on the ice crystals present in the cloud. An equal number of H^+ and OH^- ions are present, making the ice electrically neutral, but a temperature gradient across the length of the crystal causes more of the ion pairs to be located at the warmer end. The H^+ ions diffuse faster allowing the warm end to be weakly negative and the cooler end to be positive. This creates a potential difference. The dynamics of this system will be modeled numerically.

[F36.004] Second Harmonic Generation and Laser Damage Studies in Zinc (tris) Thiourea Sulfate Crystals

Zinc (tris) Thiourea Sulfate (ZTS) is an organometallic non-linear optical crystal. ZTS crystals possess an orthorhombic crystal structure with point group mm2. ZTS crystals used in this work were grown from aqueous solution using the solution method. Previous studies have reported on cw Second Harmonic Generation (SHG) mainly at a fundamental Nd:YAG wavelength of 1064 nm. This study reports on both cw and femtosecond SHG over a wide range of fundamental wavelengths in the near infrared from about 800-1000 nm. SHG in ZTS is compared for both cw and femtosecond fundamental inputs with SHG in other standard crystals such as KDP. Damage studies with \sim 10 ns pulses from a nanosecond Nd:YAG laser indicated high damage threshold in these crystals approaching 1 GW/cm2. Damage microstructure reveals the symmetry of the crystals with the laser damage propagating along the softest c-direction in these crystals.

[F36.005] The complete optical spectrum of liquid water measured by inelastic x-ray scattering

A number of photochemical and photobiological processes take place in aqueous solutions and hence optical properties of liquid water play a crucial role in a variety of phenomena. In spite of its importance, however, absorption spectrum or optical oscillator strength distribution (OSD) of liquid water over wide energy (E) range has never been obtained accurately because of several experimental difficulties inherent in vacuum ultraviolet (VUV) spectroscopy. In a previous study [1], we have experimentally substantiated that inelastic x-ray scattering (IXS) spectroscopy can provide the OSD of water as long as the momentum transferred (q) is less than 0.3 a.u. In this work, for q = 0.28 a.u, IXS of liquid water was measured at 0.5 eV-resolution for E < 30 eV to resolve fine structures if any, and at 1.3 eV-resolution for 30 < E < 164 eV to achieve high S/N ratio. Then by making use of sum rule, we have succeeded in determining the OSD in absolute scale. After checking the data accuracy, the phase effects on the OSD were discussed.

[1] H. Hayashi et al., J. Chem. Phys. 108(3), 823 (1998)

[F36.006] Electron-Phonon Interactions in the Elpasolites Cs_2NaAlF_6:Cr^3+

The analysis of site-symmetry and electron-phonon coupling in guest:host materials is important to the understanding of the excited state properties of the material. The influence of site-symmetry and electron phonon coupling in Cs_2NaAlF_6:Cr^3+ (0.5 percent) is probed by correlation of optical and structural measurements. Based on neutron and X-ray diffraction analysis the structure remains hexagonal. Although the guest Cr^3+ ion does not significantly perturb the host lattice, vibrational analysis indicates that the Cr^3+ ion is strongly coupled to the first coordination sphere of the Al-F lattice, as evidenced by the observation of a local defect mode in the resonant Raman spectroscopy, which is associated with a Al-F A_1g lattice vibration. This result is analogous to observations in other doped elpasolites and lanthanides where strong el-ph coupling arguments have been invoked to explain the low quantum yields encountered in these materials. Work supported by FAPERJ, FINEP, CNPq, UCSB-LANL and DOE-BES under contract No W-31-109-Eng-38.

[F36.007] Spectroscopic probing of site symmetry in Cs_2NaGaF_6:Cr^3+ single crystals

Two different Cr^3+ crystal field sites are observed by excitation and emission spectroscopy in the elapsolite Cs_2NaGaF_6:Cr^3+. The zero-phonon lines of the ^2E - ^4A_2 transition are clearly observed on time-resolved emission spectra and associated with each type of site by high-resolution emission measurements. Indeed the different fluorescence lifetimes at 4.2K confirm the identification of the sites. In addition, the Raman spectra reveals a side band associated with the bulk (F-Ga) A_1g mode that is believed to arise from the appearance of a local mode associated with the occupation of Cr^3+ in the Ga^3+ site. The A_1g local mode can be assigned as a F stretching mode in the first coordination sphere of the Cr^3+ ion sitting in an Ga^3+site. This assignment is consistent with observations made in similar elpasolites structures and assigned to local modes arising from electron-phonon coupling. Work supported by FAPERJ, FINEP, CNPq, UCSB-LANL and DOE-BES under contract No W-31-109-Eng-38.

[F36.008] Structural and electronic properties of Al defects and centers in \alpha-quartz

One of the most important defects in both natural and synthetic quartz is Al^3+ substituting for Si^4+. Such defects are usually compensated by an interstitial alkali ion M^+ (where M=Li,Na,K), or a proton H^+, or (in irradiated samples) by an hole h^+ trapped at adjacent oxygen atoms. The resulting Al^3+-M^+ , Al-OH^-, Al^3+-h^+ centers have been detected by various experimental techniques and are believed to be responsible for the ionic conductivity observed at high temperatures (>500 C).

We present DFT results, obtained using large supercells, for the structure, electronic states, and energetics of Al^3+-h^+ , Al-OH^-, and Al^3+-Na^+ centers. We find that the spin density in the Al^3+-h^+ center is evenly distributed on the four Oxygen nearest-neighbors to Al, in contrast to results from phenomenological model and from semiempirical or small clusters calculations. The results for the Al-OH^- complex confirm previous results and assignements for the observed IR absorptions bands.

[F36.009] Self-Consistent Calculation for the Interface Between the Metal and Semimetal

We apply the density-functional theory to calculate the junction characteristics between the metal and semimetal. We obtain the self-consistent solution of the charge distribution both when Fermi levels of the metal and semimetal are set equal and when they are set unequal. The differential junction capacitance is obtained by differentiating the areal charge density with respect to the shift of Fermi levels. The electronic structure, potential, differential junction capacitance for the interface are reported. Our calculations for the metal with the conduction electron densities r=2, 6, and 12 a.u., and the semimetal with r=24 a.u.

[F36.010] A Mossbauer Investigation of ^57Fe Doped Nd_0.7Sr_0.3MnO_3

The magnetic structure of Nd_0.7Sr_0.3Mn_1-xFe_xO_3 (x = 0.02, 0.05, .10, 0.15) has been studied by Mossbauer spectroscopy in magnetic fields up to 6 Tesla. ^57Fe ions appear to occupy two magnetic sites. The first remains ferromagnetic for all values of x, but the second changes from canted to antiferromagnetic with increasing x. For x = 0.02 and 0.05, the latter magnetic phase is very sensitive to temperature and strongly influenced by the applied fields.

[F36.011] Specific Heat of La_0.65Ca_0.35MnO_3: Conduction-Electron and Spin-Wave Contributions; Spin Ordering at T_C

Specific heat of polycrystalline La_0.65Ca_0.35MnO_3 was measured in magnetic fields (H) to 9T from 1 to 300K. The low-temperature measurements permit a definitive separation of the hyperfine, conduction-electron, magnon, and phonon contributions. The magnon contribution has the H and T dependencies expected for ferromagnetic spin waves and agrees in magnitude with an extrapolation of high-temperature neutron data(J. W. Lynn et al., Phys. Rev. Lett. 76, 4046 (1996)). The conduction-electron contribution is compatible with band-structure calculations(D. J. Singh and W. E. Pickett, Phys. Rev. B 57, 88 (1998)). Consistent with recent theoretical predictions(A. S. Alexandrov and A. M. Bratkovsky, J. Phys.: Condens. Matter 11, 1989 (1999)), the specific-heat anomaly at T_C(0) = 265K includes only a small fraction of the expected magnetic entropy and shows unusual H and T dependencies.

[F36.012] Charge,orbital and magnetic order in La_0.5Sr_1.5MnO_4

Recent experiments on La_0.5Sr_1.5MnO_4 have indicated a real space ordering of manganese atoms leading to charge and orbital ordering below a critical temperature. We have analysed these results within the framework of ab-initio band structure calculations based on the pseudopotential formalism. Our results suggest that the CEAF-type magnetic order in the system leads to anisotropic hopping between different Mn sites resulting in Mn sites with distinct charge distributions which can be identified with the Mn^3+ and Mn^4+ sites. The charge imbalance between the Mn sites is however much less than what has been found experimentally. Our results for orbital and charge ordering will be presented alongwith the implications of the orbital ordering on the crystal structure.

[F36.013] Local Electronic and Atomic Structure of CMR Oxides Under High Pressure

The application of pressure has been known to suppress the peak resistivity of CMR oxides in a manner analogous to the suppression by magnetic fields . The coupling of the transport and structural changes which occurs under high pressure has not been well explored. It is know that at x=0.875 the systems La _1-xCa_xMnO3 and Bi _1-xCa_xMnO_3 exhibit a maximum magnetic moment and show canted spin behavior. The low temperature resistivity is a minimum at this doping with a value typical of semiconductors. Hence, the possibility of a metal to insulator transition exists. High pressure x-ray absorption measurements have been used to determine the structural changes which occur with pressure. The changes are correlated with the change in resistivity with pressure. The origin of the minimun in the resistivity is explored. This work is supported by National Science Foundation Career Grant DMR-9733862 and by DOE Grant DE-FG02-97ER45665 [1] (a) J. J. Neumeier et al., Phys. Rev. B 52, R7006 (1995). (b) Y. Moritomo, A. Asamitsu, and Y. Tokura, Phys. Rev. B 51, 16491 (1995). (c) S. Tamura, J. Mag. Mag. Mat. 31-34, 675 (1982). (d) Z. Arnold et al., Appl. Phys. Lett. 67, 2876 (1995)

[F36.014] Physical Properties of Cr Doped Pr_0.5Ca_0.5MnO_3 Thin Films

Strain in epitaxial perovskite thin films can be very large, giving rise to the artificial control of the ground state for the La_0.5Sr_0.5MnO_3 films depending on the substrate latticeconstant via modulation of the orbital ordering pattern.^(1) We extend this strain induced�@phase control to the Pr_0.5Ca_0.5MnO_3 films. Charge ordered(CO) phase in this compound is very stable, but can be destabilized by a few percent of Cr doping at the Mn sites to turn it into ferromagnetic-metal.^(2) Cr doping can be a new parameter to judge the stability of CO phase for strained epitaxial films. The films with biaxial tensile strain show very suppressed magnetization even with 10% Cr. Actually, the in-plane lattice parameters are the almost same as those of CO bulk crystal at low temperatures. However, magnetization increases to 1.5\mu_B/B-site for the compressive strained films. The results indicate that CO phase is stabilized via orbital ordering induced by tensile strain.

This work was partly supported by NEDO.

(1) Y. Konishi et al., J. Phys. Soc. Jpn., in press

(2) B. Raveau et al., J. Solid State Chem. 130, 162 (1997)

[F36.015] Spin Wave Resonances in CMR Manganite Thin Films

We have observed spin wave resonances (SWR) in La_0.7Sr_0.3MnO_3 (LSMO) thin films grown by MOCVD using ferromagnetic resonance. The SWR's depend on crystalline orientation as well as the degree of crystallinity. Elastic strain in magnetic thin films can change the magnetoelastic coupling, which can be related to the observed direction of magnetization. Here we will present our results in terms of lattice mismatch, induced strain and orientation in the magnetic field for these LSMO thin films.

[F36.016] Magneto-Resistance Anisotropy in Uncompensated Ca:YIG Films

Abstract:

A magneto-resistance (MR) in an uncompensated p-type Ca:YIG film having a bulk resistivity of 400 ohm.cm at 300K has been observed using an AC signal modulating method. For the geometry of the magnetic field (H) in the plane of the film and parallel to the current, the resistivity increases by 4 parts in 10000 where H varies from 0 to �10G without hysteresis. The dependence of the MR as a function of the angle (theta) between the direction of the current and the applied H varies as cos2theta. At the perpendicular orientation the MR initially increases up to 6 pts in 10000 as H varies from 0 to �300G, and then reverses sign and changes by -12 parts per 10000 at H=�2000G where the film magnetization is saturated. For higher H the resistance increases by 4 pts in 10000 at H=�4000G. The origin of the conductivity in these Ca doped garnets is due to a hopping of small polarons along various spin channels in the garnet structure [1,2]. The MR is then consistent with a hopping mechanism being more favorable when the magnetization is oriented perpendicular to the current. The temperature dependence between 240 K and 320 K will be reviewed. Any magneto-resistance present in n-type Si:YIG or Ge:YIG was too small to detect. A small variation in the amplitude and phase of the MR response with the AC frequency will be discussed.

[1] G.B. Turpin, Ph.D. Thesis, The Ohio State University (1996).

[2] S. Batra, Andrea Lehman-Szweykowska, and P.E. Wigen, J. Appl. Phys., 61, 3274 (1987).

[F36.017] Second harmonic generation signal from La_1-xCa_xMnO_3 films grown on LaAlO_3 substrates

We report on a second harmonic generation signal measured for epitaxial La_1-xCa_xMnO_3 films grown on LaAlO_3 substrates. The films are grown under compressive strain, as previously reported for two of us, [1] and exhibit a Curie temperature of \sim250K. The samples were studied using a Ta:sapphire laser with an exciting wavelength of 800 nm. The light from the sample, both the scattered 800 nm and other, was collected and measured using a monochromator. The power(P_i )of the incident exciting laser was varied. We found that the second harmonic generation (SHG) signal varied (P_i ) ^2 as expected for a SHG signal. The monochromator results indicate that there is no "burning" (additional light) beyond the exciting and SHG signal. The SHG signal exhibits two qualitatively different behaviors. In one circumstance, the signal increases as \sim250K and \sim150K. In the other circumstance, the signal increases monotonically with decreasing temperature, but there are two abrupt drops in intensity, at \sim250K and \sim150K. We discuss possible origins of the SHG signal and its temperature dependence.

[F36.018] Study of the Colossal Magnetoresistance Property of La_0.7Sr_0.3Cr_yMn_1-yO_3

We report the change in colossal magnetoresistance property of La_0.7Sr_0.3MnO_3 whose Mn sites are replaced by Cr. The magnetization versus temperature curve for La_0.7Sr_0.3Cr_yMn_1-yO_3, y = 0.05-0.20 is similar to the undoped compound but the ferromagnetic behavior of the curves decreases as the Cr content increases and the curves become broad. This broadening is ascribed to the presence of Cr that acts as a magnetic impurity and hinders the ferromagnetic interaction between the Mn^3+-Mn^4+ ions. The value of the magnetization per unit mass and also the transition temperature decreases with increasing Cr concentration. The electrical resistivity shows that increase of Cr content causes the samples to become insulator although the parent compound has a distinct metallic property. The effect of Cr is maximum at Cr concentration of 20%. From our data we can conclude that at high Cr concentrations the antiferromagnetic interaction between the Mn^3+-O-Cr^3+ dominates as compared to the ferromagnetic interaction between the Mn^3+-O-Mn^4+ and this results in an increased resistivity of the samples. However there is not an appreciable change in the magnetoresistance.

[F36.019] Magnetotransport properties of Sr2FeMoO6 based multilayer thin films

Heteroepitaxial thin films containing half-metallic ferrimagnet, Sr_2FeMoO_6 (SFMO), were fabricated by pulsed laser deposition. SrMoO_3 (SMO), which is a paramagnetic metallic perovskite, and SrFeO_2.5 (SFO), which is a G-type antiferromagnetic insulator with having an oxygen-deficient-perovskite (brownmillerite) structure were chosen as the `buddy' compounds to be involved in heterostructures with SFMO. Both SFO and SMO could be epitaxially grownon SrTiO_3 substrates under similar conditions optimum for SFMO eputaxy. SFO thin films have atomically flat surfaces with showing atomic-scale step-and-terrace structures. The resistivity of SMO thin films are 40 - 80 \muØmega cm at 300 K and about 0.2 \muØmega cm at 5 K. These values are smaller by two orders of magnitude than those reported for polycrystalline samples. The magnetism and electrical properties of multi-layered films will be discussed with taking the interface and strain effects into consideration.

This work was partly supported by NEDO.

[F36.020] Grain Boundary groove on ice surfaces

The grain boundary groove formed at the ice surface of a bicrystal with \langle 10\bar10\rangle/60^o misorientation is studied. Samples are thermostatized in a transparent box, sealed so as to attain the vapor - ice equilibrium pressure at the ice surface. The time evolution of the groove width is registered for a lapse of 24 hs and sample temperatures of -2 and -12^o C. The results obtained are analyzed taking into account the different theories developed to explain the groove growth. It is found that the obtained results can be satisfactorily explained considering that the groove is formed by surface diffusion. At -9^o C an abrupt decrease in the rate of groove growth is observed. This change shows a variation on the surface structure which can be related to the vanishing of the quasi-liquid layer normally found on ice surfaces at high temperatures

[F36.021] FLAPW determinations of the magnetic structure of double perovskite LaMO_3/LaM'O_3(M,M'=Cr,Mn,Fe) superlattices

We present results based on bulk FLAPW(H.J.F.Jansen and A.J.Freeman Phys.Rev.B 30,561(1984)) local-spin-density approximation(LSDA) calculations of artificial double perovskite structure oxides LaMO_3/LaM'O_3 (M,M'=Cr,Mn,Fe) with (100) stacking periodicity. The stable magnetic structure strongly depends on the occupancy of e_g and t_2g orbitals and direction of stacking periodicity. The total energy calculations with various spin configurations show that LaCrO_3/LaMnO_3 has a stable antiferromagnetic(AF)/ferromagnetic(FM) magnetic structure, while the LaMnO_3/LaFeO_3 and LaCrO_3/LaFeO_3 superlattices have stable FM/FM and AF/AF magnetic structures, respectively. The calculated magnetic structures are well matched with experimental results.

[F36.022] Combined cluster expansions and density functional approach to the studies of thermodynamics and inhomogeneous states in magnets

We outline a cluster expansion approach for the description of thermodynamic properties of systems consisting of localized magnetic moments. A method for combining this approach with density functional theory is suggested, and several test application for SC, BCC and FCC structures are presented. Comparison with alternative Monte-Carlo, stochastic and deterministic methods and the general applicability of this approach will be discussed. We also demonstrate the generalization of the technique for the studies of stationary inhomogeneous states.

[F36.023] ELECTRONIC STRUCTURE CALCULATIONS ON QUASIBINARY MnBi-MnSb SYSTEM

We have performed self-consistent electronic structure calculations on NiAs-type compounds in the MnBi, MnSb and quasibinary MnBi-MnSb systems. We have used the scalar relativistic, spin-polarized Green's function technique based on the KKR-ASA method within frozen core and ASA with MT and electrostatic multipole corrections within a minimal spd basis. In the MnBi-MnSb alloy, disorder on the As-type sublattice was treated by means of the multisublattice generalization of the CPA. The valence electrons were treated self-consistently within the GGA using the exchange-correlation functional of Perdew-Burke-Ernzerhof. The equilibrium lattice parameter of a given alloy were obtained on the basis of self-consistent calculations of the total energy and a subsequently fit to a Morse-type equation of state. There is minimal exchange splitting for Bi (Sb) states. Mn states exhibit substantial splitting, consistent with the large Mn local moment. Integrated densities of states are used to infer a Mn moment of 3.83 mB and 3.41 mB for MnBi and MnSb compounds, respectively, that is in excellent agreement with experimental data. The equilibrium lattice parameter and magnetic properties of NiAs-type compounds in the quasibinary MnBi-MnSb alloys show a slight deviation from linear behavior. Support by the DARPA under ONR contract No. N14-98-C-0268, the AFOSR, AFMC, USAF under grant No. F49620-96-1-0454 and the Danish National Research Foundation.

[F36.024] Spectroscopic study of ablation plasma plume during KrF pulsed laser deposition of diamond-like carbon films

Laser induced fluorescence (LIF) and optical emission spectroscopy have been employed to diagnose the ablation plasma plume during pulsed laser deposition of diamond-like carbon films, and to correlate the plasma condition with the properties of deposited films. A KrF excimer laser was used to irradiate an amorphous carbon target with an incident angle of 45^o. A dye laser beam tuned at 516.52 nm corresponding to the excitation of C_2 molecules ((0,0), a^3 \Pi _u - d^3 \Pi _g) was introduced into the ablation plume along with parallel to the target surface. At a distance of 1 mm from the target surface, the LIF signal of 563.49 nm ((0,1), d^3 \Pi _g - a^3 \Pi _u) was successfully detected at a right angle of both laser beams by a photomultiplier tube and an image intensified CCD camera. The LIF intensity increased with increase of the probing laser delay until 200 ns, then decreased with the further delay. The LIF was observed about 1.2 \mus after the ablation laser irradiation, while spontaneous optical emission of C_2 was observed only for first 200 ns. It suggests that C_2 molecules are formed near the target surface as a result of recombination of carbon atoms and/or dissociation of heavier particles.

[F36.025] Study of Pressure-induced Molecular Dissociation in Solid Hydrogen

We have calculated the enthalpies of compressed hydrogens for some of candidate structures in both molecular and atomic phases at megabar pressures using the band theoretical treatment in the local density approximation (LDA), and discussed the pressure of the molecular dissociation. Our results have shown that the Cmca structure has fairly low enthalpy over 200GPa and transforms into the Cs-IV structure around 380GPa. We have also performed the calculation for the same quantities in the generalized gradient approximation (GGA) and compared those with the results in the LDA. The GGA lowers the energy of the molecular phase more than it does the energy of the atomic phase, so that the dissociation pressure is increased by \sim70GPa. In addition, we have studied the ionic zero-point energy (ZPE) by the use of the frozen-phonon method in the harmonic approximation. Our ZPE's are much smaller than those by the quantum Monte Carlo calculation (Natoli et al. 1993 and 1995). The ZPE's are larger in the molecular phase than in the atomic phase, which leads to the decrease of the dissociation pressure. The decrease is \sim60GPa when we assume the molecular dissociation from the Cmca structure to the Cs-IV structure. The \beta-Sn and diamond structures have lots of imaginary-frequency modes, which may suggest the instability of these structures.

[F36.026] NMR Study of Single Wall Carbon Nanotubes

^13C NMR experiments have been carried out for single-wall carbon nanotubes (SWNTs), which were produced by using non-ferromagnetic Rh-Pt mixed catalysts. Hydrogen peroxide was used to remove amorphous carbon particles in the raw soot almost perfectly. From the line shape analysis of the ^13C spectrum measured at 100.1MHz(9.4 T), the shift tensor was evaluated to be (\delta_11,\delta_22,\delta_33)=(192,186,132) ppm. Small anisotropic value(\delta\delta= -57 ppm) compared with that reported for MWNTs suggests that this SWNTs sample contains metallic tubes with larger electronic density of states at the Fermi level than that of multi-wall carbon nanotubes(MWNTs) sample. We also performed ^13C spin lattice relaxation time(T_1) . It is found that SWNTs follows a Korringa-like behavior(T_1^o~T=940^o60(sec.K)) in the temperature region between 4.2 K and 100 K. Alkali metal-doping effects and hydrogen uptake effects will be discussed.

[F36.027] Engineering of Carbon Nanotube Actuators and Tools for Their Characterization

We report comparative data on electromechanical response as a function of voltage for carbon actuators based on single-wall and multi-wall nanotubes, graphite and nanotube-polymer composites. The measurements were carried out in aqueous NaCl electrolyte, under variable load, in broad voltage and temperature ranges. A specially designed tensile tester provided isometric conditions for sample characterization. Data on stress-strain generation, elastic constants, and thermal expansion was obtained from our apparatus. Measurements show that isometric stress generation in an actuator based on low-surface- area graphite is less than 0.01 MPa, whereas for single-wall nanotubes this stress readily exceeds 0.7 MPa. Experimental data are compared with predictions for the dimensional changes in carbon nanotubes caused by charge injection and temperature variation. Tensile, cantilever and helical actuation mechanisms in single-wall nanotubes are discussed.

[F36.028] Diameter Dependent Electron Scattering in Vapor-Grown Graphite Fibers*

The resistivity of highly graphitic vapor-grown carbon fibers has a strong dependence on diameter^1^,^2 attributed to electron scattering from the boundaries of macroscopic crystallites formed during the growth process.^1 To obtain more information, we measured the magnetoresistance of annealed (3000 ^oC) fibers (diameter 4-30 \mum) at 4.2 K over the field range 0-2.2 T, thus varying the carriers� cyclotron orbit size. Magnetoresistance increases monotonically with fiber diameter, consistent with diameter-dependent mobility and with previous results^2 at 77 K and 1 T. However, the relative field dependence of the magnetoresistance is independent of fiber diameter even though the cyclotron orbit sizes in much of the sample volume at our highest field strength are considerably smaller than estimated crystallite size.^1

*Supported by NASA Cooperative Agreements NCC3-486, 3-522 and 3-740. Fibers supplied by Applied Sciences, Inc., Circleville, OH ^1M. Z. Tahar, M. S. Dresselhaus, and M. Endo, Carbon Vol. 24, 67 (1986). ^2Munehiro Ishioka et al, J. Mater. Res. Vol. 8, 1866 (1993).

[F36.029] Local Density of States of Kinked Multiwalled Nanotubes

Mechanical deformation related changes in the local density of states (LDOS) of kinked multi-wall carbon nanotubes (MWNT) is studied using scanning tunneling microscopy and spectroscopy (STM)/(STS). The measured STS data in the kinked region indicates that the LDOS and the differential conductivity (DC) are asymmetric about the Fermi level, while the corresponding behaviors away from the kink region remain to be the usual symmetric LDOS and DC for straight tubes. In the kinked region, the distribution of unoccupied states compared to the occupied states is found to be drastically suppressed. This suppression of the conduction band states becomes noticeable with in 1nm of the kinked center. Moving away from the kink center the perfect tube LDOS features are recovered with in 1.5 nm from the kink center.

[F36.030] Controlled Molecular Device Engineering

Single wall carbon nanotubes are a particularly promising class of molecules for nanoscale electrical devices. "Flat" electrical contacts (i.e., flush to the substrate to within 1 nm) for carbon nanotube circuitry were manufactured using optical lithography, reactive ion etching, and thermal evaporation of the contact metal. We are able to manipulate nanotubes using a tapping-mode Atomic Force Microscope and will present the results of first attempts to manipulate tubes onto these leads to form complex circuits consisting of metallic and semiconducting nanotubes. We will also report on first experiments to locally dope individual tubes and controllably induce rectifying behavior [1]. Together, these methods provide a technique for controlled manufacture of electrical devices from single molecules.

[1] R.D. Antonov and A.T. Johnson, Subband population in a single wall carbon nanotube diode, Phys. Rev. Lett. 83, 3274-3276 (1999).

[F36.031] Magnetic field dependence of tunneling spectroscopy in carbon nanotubes

Tunneling spectroscopy of electrons through multi-walled carbon nanotubes electrostatically trapped between two metal electrodes is studied as a function of the magnetic field. Clear Coulomb blockade and energy-level splitting were observed in differential conductance-voltage characteristics. The magnetic field dependence of conductance peaks could be understood within a model based on single-particle states.

[F36.032] Electronics of strained nanotubes and multiple nanotube junctions

We present results of experiments studying the transport of multiple nanotube samples. Through the use of an advanced interface for an atomic force microscope, we are able to manipulate nanotubes into specific geometries before making electrical contact. This allows us to study the behavior of well defined junctions, including crossed tubes, and tubes lined up either end to end or side by side. Using the same interface, we are also able to study the effects of strain on individual tubes, or the aforementioned junctions.

[F36.033] Theoretical Investigations of the Interaction of Silver Clusters with Ethylene

The interaction of Ag_3 and Ag_5 clusters with the ethylene molecule is described within the density functional framework, using the Becke exchange and Perdew-Wang correlation functionals. Geometric structures and electronic properties of the cluster-molecule complexes are characterized. The binding energies and vertical and adiabatic ionization potentials are correlated with the details of the bonding and charge transfer in these systems.

[F36.034] New low-energy models for Si clusters in the N=13-25 atom range

We have used a genetic algorithm search strategy with a hierarchy of electronic structure methods to identify low-energy isomers for Si clusters in the N=15 to N=23 atom size range. Our GA is a variant of earlier approaches, utilizing genetic operations on a single parent cluster. A fast tight binding method is used in the GA to identify low-lying structures. The candidate structures are then re-optimized using accurate density-functional theory methods. We present a number of new structures for both cation and neutral clusters in the intermediate size range that are lower in energy than previously published structures. In addition to being lower in energy, the new clusters appear to give better agreement with ion mobility measurements for the cations and ionization potential measurements for the neutrals.

[F36.035] MAGNETIC AND CALORIMETRIC MEASUREMENTS ON THE LAYERED III-VI DILUTED MAGNETIC SEMICONDUCTORS Ga1-xMnxS AND Ga1-xMnxSe

We have investigated single crystalline Ga1-xMnxS and Ga1-xMnxSe. This material is in the new class of diluted magnetic semiconductors (DMS) based on III-VI semiconductors. The magnetization for both systems is linear with field in fields up to 6 T at 10 K but show signs of saturation by 30 T. The broad peak from 119 to 195K in the magnetization of Ga1-xMnxSe, ascribed to direct Mn-Mn pairs, is absent in the Ga1-xMnxS data. In this temperature range, the magnetization of Ga1-xMnxS is Curie-Weiss like with Jeff/kB=-51K, suggesting there are no direct Mn-Mn pairs in Ga1-xMnxS. Heat capacity measurements will also be presented in this temperature range. The magnetization of Ga1-xMnxS does show a sharp cusp at 10.9 K similar to the II-VI DMS spin-glass transition, but at a substantially higher temperature.

[F36.036] Cathodoluminescence of LEO GaN: Dependence on Excitation Conditions

We have found that cathodoluminescence (CL) for LEO GaN samples is affected when the scanned area is increased or decreased, for example by changing the magnification, although the electron beam voltage and current are held constant and similar regions of GaN are being examined. We have also observed effects of beam dwell time on CL spectra when areas are scanned pixel by pixel rather than line by line. Both CL intensities and spectral distributions are affected. These studies have employed a JEOL JSM-6400 scanning electron microscope equipped with a Gatan Digiscan system, an Oxford Instruments CF302 cathodoluminescence system and an ISA/SPEX monochromator. These phenomena will be discussed in term of kinetics of different recombination paths and effects of local charging.

[F36.037] Organic Light Emitting Diodes Using Calamitic Electron Transport Liquid Crystals

Start your abstract by replacing this line with your text.

Recently liquid crystals that exhibit efficient electron transport and high fluorescence yield have become available. These properties made them appealing for applications in organic light emitting diodes. Sandwiched-type devices with polyimide alignment layers were fabricated. By rubbing the polyimide layer, orientation of the liquid crystalline materials was achieved, leading to large mono-domain liquid crystalline layers. The efficiency and its dependence on the morphology of the LC layer were studied.

[F36.038] The atomic structure and magnetic properties of ultrathin Ni films on Cu(001) substrate

Atomic structure and magnetic properties of ultrathin Ni films on the Cu(001) substrate are studied by using the FLAPW method with GGA for exchange correlation interactions. Unlike the uniform structure relaxations found in a recent LEED measurement, the Ni-Ni inter-layer distances strongly vary in depth with relaxations of -8% , 1.1% and -1.7% from the surface to the interface in Ni_4/Cu(001) compared to that in the bulk Ni(1.76ÅInterestingly, uniform relaxations are found in Cu_2/Ni_4/Cu(001), indicating a possible subsurface growth mode in these systems. The spin and orbital magnetic moments of Ni atoms enhance at the surface and decrease at the interface.

[F36.039] M\ddotossbauer study of nano-crystalline thin films of Fe-Zr-O.

Transmission ^57Fe M\ddotossbauer spectroscopy is used to study Fe-Zr-O, high resistive soft magnetic thin films, deposited on polycrystalline alumina substrate (Al_2O_3) by PVD (magnetron sputtering). The atomic composition (stochiometry) of the as deposited film using ESCA is found to be Fe_54Zr_10O_36. The as deposited sample's spectrum is a doublet with a quadrupole splitting of 0.8 mm/s and isomer shift of 0.4 mm/s. The M\ddotossbauer spectra of films annealed at several temperatures yielded a superimposed magnetic and non-magnetic sub-spectra. The intensity of the magnetic sub-spectrum characterized by a hyperfine split sextet became preponderant with annealing temperature. TMS, x-ray, and magnetization measurements will be presented.

[F36.040] Roughness induced in-plane uniaxial anisotropy of off-normal-direction growth in thin Fe films on MgO(001)

It is shown that atomic steps on a vicinal (100) surface could induce in-plane uniaxial magnetic anisotropy due to the rotational symmetry breaking at the step edges. Recently it has been proposed that roughness in a preferential orientation also could induce in-plane uniaxial anisotropy due to the dipole-dipole interaction. To realize the latter situation experimentally, Fe films (~30�) were grown on MgO (001) with off-normal-direction and measured in situ by SMOKE. With the evaporation beam normal to the MgO (001) substrate, square shape hysterisis loops were obtained for magnetic fields along [100] and [010] directions, indicating the absence of the uniaxial anisotropy. Fe films were then grown at 45o from the normal direction in the [011] plane. For magnetic fields applied along the [100] direction , square shape loops were observed, whereas along the [010] direction, we observed split loops, indicating a hard magnetic axis. It was then verified with STM that there is preferential roughness in the [010] direction. This result clearly shows the roughness induced in-plane uniaxial magnetic anisotropy.

[F36.041] Ferrimagnetic Ordering in FeMnO_3 Produced by Mechanical Alloying.

The stability of various compounds formed in the Fe-Mn-O system depends not only on Fe/Mn/O ratios but also on the method of preparation and on the calcination temperature. These compounds are of interest as ferrites in electronics and as catalysts. In this work mechanical alloying of \alpha-Fe_2O_3 with Mn_2O_3 is shown to produce FeMnO_3. The x-ray diffraction pattern fits the cubic structure (space group Ia3, lattice constant \approx 9.40 Åidentical to that of Mn_2O_3, with Fe replacing some of Mn atoms. Temperature variation of its magnetic susceptibility \chi shows a phase transition near T_c \approx 40 K and the \chi vs T data for T > T_c fits the variation expected for a ferrimagnet with a magnetic moment of 2.8 \mu_B per formula unit. The M\ddotossbauer spectrum for T > T_c is a doublet with quadrupole splitting \approx 1 mm/s and isomer shift \approx 0.4 mm/s, changing to a sextet for T < T_c with a hyperfine field of 445 kOe at 16 K.

*The author would like to acknowledge partial support from HUD special project grant #B 98 Sp MD 0074 and a grant from NASA, NASA-NAG-5-3508.

[F36.042] Space-based simulation of structural evolution in crosslinked polymers

Chain polymerization of multifunctional monomers results in highly crosslinked networks with many structural complexities. In particular, diffusion-controlled reactions and unequal functional group reactivities lead to the formation of microgels and a distribution of crosslinking densities that are not readily characterized. However, this information is critical for understanding the influence of heterogeneity on the physical and mechanical properties of the final network. In this work, we developed an advanced off-lattice model to study the structural evolution during multifunctional monomer polymerizations. The model removes the constraint of lattice-based approaches and allows for more realistic species mobility and monomer structure. Molecules interact, move, and react via a Lennard-Jones potential energy function, the Metropolis criteria for move acceptance, and simple polymerization rules for initiation, propagation, and termination. Variations in monomer reactivity and initiation mechanisms were explored. Simulation results are compared to physical and mechanical data as well as nano-scale images of polymer features obtained with atomic force microcopy.

[F36.043] ATOMISTIC SIMULATIONS OF SCREW DISLOCATIONS IN COPPER

Properties of dissociated and constricted screw dislocations in Cu are simulated using molecular dynamics. The calculations are done using the Parinello-Ray-Rahman isothermal constant stress method and a potential based on the second moment approximation (SMA). The core structure of the dislocation and its response to external shear stress and temperature are described. The threshold stresses required for the activation of dynamic processes are identified. Qualitative and quantitative characteristics of the dislocation structure and its dynamics are compared to theory and to experimental results.

[F36.044] Secondary electron emission characteristics of a thermally grown SiO2 thin layer on Si

The secondary electron emissions of thin SiO2 layers prepared by dry thermal oxidation of doped Si substrates were measured as a function of the oxide layer thickness and the dopant element. The oxide layer thickness was varied between 200 A and 1150 A by changing the oxidation time at 930 C. We found that secondary electron emission yield curves for the samples with a relatively thick oxide layer revealed two local maxima (one near 300 - 500 eV, the other near 1000 eV) regardless of the dopant type, while those for the samples with a relatively thin layer showed one maximum like many other previous results. When the oxide layer was formed by vapor deposition, the two peaks were not observed. This leads us to interpret the secondary electron emission data for the thermally oxidized samples in terms of the dopant accumulation and the electron tunneling through the narrow barrier at the Si/SiO2 interface. Sample characterization data and a simple phenomenological model will be presented.

This work was supported by the Korean Ministry of Science and Technology through the Creative Research Initiative program.

[F36.045] Distorted Wave Born Approximation for scattering from rough magnetic surfaces

Resonant x-ray magnetic scattering has been used to elicit the magnetic properties of surfaces and multilayer interfaces. Recently, it was found that the magnetic roughness of a single Co layer was larger than its chemical roughness.^1,2 Calculations using first-order perturbation theory and the vector wave equation (the Born Approximation) found that, in the absence of any correlation between the magnetic and structural roughness, there is no contribution to the difference in the diffuse magnetic scattering from x-rays circularly polarized in opposite senses relative to the direction of magnetization.^3 The diffuse magnetic resonant x-ray scattering from Fe/Gd multilayers has been measured;^4 fits to the data (using the Born Approximation) result in longer correlation lengths for charge-magnetic roughness than for charge roughness. We go beyond the current model to the Distorted Wave Born Approximation and simulate the diffuse magnetic scattering from various experimental systems studied to date.

^1 J.F. MacKay, C. Teichert, D.E. Savage, M.G. Lagally, Phys. Rev. Lett. 77, 3925 (1996). ^2 J.W. Freeland, K. Bussmann, Y.U. Idzerda, C.-C. Kao Phys. Rev. B 60, R9923 (1999). ^3 R.M. Osgood III, S.K. Sinha, J.W. Freeland, S.D. Bader, J. Appl. Phys. 85, 4619 (1998). ^4 C.S. Nelson et al. Phys. Rev. B 60, 12234 (1999).

[F36.046] Electron phase coherence and the electron-electron interaction in tantalum thin films

Weak localization and high field resistance measurements were carried out on 16 tantalum thin films ranging in thickness from 35 to 600 Angstroms. The temperature dependence of the inelastic scattering rate is in good agreement with predictions for spin-spin, electron-electron and electron-phonon scattering; however, the magnitude of the electron-electron scattering rate exceeds the theoretical value by a factor of 10. The temperature and thickness dependences of the electron-electron interaction in the diffusion channel agree with a weighted sum of 2D and 3D contributions, with the coefficient of the 3D term directly proportional to the film thickness.

[F36.047] Monolayer-dependent surface structures of Pb thin film on Si(111)/3x/3-Ag

We show complicated changes in the surface structures of Pb thin films dependent on the monolayer difference of the film. The Pb film was formed by a two-step process: deposition of Pb onto the Si(111)/3x/3-Ag surface at low temperature then warming up to room temperature. The film exhibits three different phases in the film thickness of 1-3 monolayers. The 1 monolayer (ML) film shows a stripe structure along the 3-fold axis; distances between the neighbor stripes are about 2.0 nm. The 2 ML film exhibits a hexagonal reconstruction with a period of 2.3 nm. For the 3 ML film, a moire pattern with a period of 7.1 nm was observed. This thickness-dependent structure is preliminarily attributed to the strain relaxation at the interface due to balance between the energies of in-plane elastic strain and interlayer misfit strain.

[F36.048] Heat transport in the structure of DVD, which consists of Ge_2Sb_2Te_5, ZnS:SiO_2 and Al-alloy layers

We report the thermal conductivity of some DVD materials, amorphous/crystalline-Ge_2Sb_2Te_5(a/c-GST), ZnS:SiO_2 and Al-alloy layers. Ge_2Sb_2Te_5 alloy is a phase-change material proper to rewritable digital versatile disk(DVD), which is a promising storage media for multimedia application. The thermal conductivity of these sputtered films has been measured in the temperature range 50 K - 300 K using the 3ømega method - an AC method. We also measured the thermal boundary resistance between these films; a-GST/ZnS:SiO_2, c-GST/ZnS:SiO_2 and Al-alloy/ZnS:SiO_2. The interface resistance effect was amplified by depositing multilayer films having 40 interfaces. The results support that the thermal boundary resistance effect should be considered in the practical design of the optically rewritable DVD's.

[F36.049] Simulation of transient photocurrent of disordered multilayer

Transient photocurrents as measured by the canonical technique of time of flight (TOF) were simulated for a sample of many disordered layers. The main characteristics of photocurrents were reproduced, considering that the conduction edge of the disordered structure forms a multiple barrier potential. Thus charge carriers move by two different mechanisms in the sample: they move mainly by hopping between the localized states of the entire disordered system, but eventually they must cross the energy barriers formed by the multiple potential well. The former conduction mechanism should provide a power time decaying current, as stated for anomalous diffusion in disordered systems, but the presence of the barriers lead to a peak-shape photocurrent. This work is mainly focused on the discussion of the physical origin of the double-peak-shape photocurrents for multiple layers of disordered material.

[F36.050] Numerical Determination of Radiation Torque on Dielectric Micro-Gears

Recent experiments have employed optical tweezers to trap and rotate miniature gear geometries\footnote J.Appl.Phys.,vol.82,no.6 (1997)., which may be employed in a micro-opto-mechanical system (MOMS). In order to predict performance, ray optics is usually employed. However, ray optics is inapplicable when the dimensions of the scattering object are on the order of the beam wavelength. Therefore, an EM wave technique was explored, in which the E and H field components were numerically\footnote Field components calculated using program FDTD3D, provided by Prof. A. Taflove of Northwestern University. determined on the surface of the micro-gears, with the results incorporated in Maxwell�s stress tensor to determine torque. In all cases, the incident wavelength was chosen to be 1.064 \mu m, with the gear diameter a maximum of 5 \mu m. Although the gears investigated were smaller than those considered in the literature^1, rotational behavior consistent with the larger gears was observed. Furthermore, the observed linearity of beam power versus rotational speed^1 is easily explained with the stress tensor. The effects of varying beam focal point, spot size, gear dimensions and refractive index will be discussed.

[F36.051] Computer Simulation of MicroSystems (MEMS) Devices to Correlate Materials with Performance

An L-Edit ( Tanner Associates ) CAD design* of a microaccelerometer to be fabricated of SiC was imported into commercial Memcad ( Microcosm Inc.) software. The purpose was to create a 3D solid model and then use the Memcad software simulation capabilities to compare the performance of devices made separately of SiC and polysilicon. After describing the simulations the performance difference between each material will be compared. Usually the material property is used as a variable to fit simulated performance to experimental results. This seems unreasonable, but the variations in processing can result in large material property variations between devices. Ultrafast pulses of laser generated ultrasound is being performed in our laboratory to determine Young's modulus and will be used for comparison.

* from Professor Mehran Mehregany and his group at CWRU.

[F36.052] Modeling of ion drift in an applied electric field in ambient pressure air for laser ionization detection applications.

We have previously demonstrated sub parts per billion detection of aromatics in ambient air by laser ionization. A model has been developed to describe the behavior of the laser-produced ions in an applied electric field and for the subsequent induced current flow in an external circuit between the biased electrodes. The numerical solutions provide a "motion picture" of the laser generated charge distribution moving between the parallel plates as well as the measured induced current behavior with time. First, a numerical model is established to describe the distributions and dynamics of positive and negative ions between parallel plate electrodes. This initial model was greatly simplified by considering an ideal situation of a low pressure reduced electric field (E/P) between infinite parallel plates and neglecting the influence of the electric fields created by the charged particles in the ion swarm itself; however, it showed good agreement with experimental results. We further add the space charge effect and also consider the case of cylindrical electrodes.

[F36.053] A New Resonant Tunneling Theory for Double barrier Quantum Well Systems

Based upon time-dependent numerical simulation results, a new tunneling theory on double barrier quantum well systems is presented. The origin of hysteresis and plateau-like behavior of the I-V characteristics and the intrinsic high frequency current oscillation of resonant tunneling diods are systematically expounded for the first time.

[F36.054] Variable Temperature Current-Voltage Measurements of CdTe Solar Cells

We have used a 2" x 2" Peltier heat pump chip powered with 24 V from a computer power supply to build a variable temperature stage for current voltage measurements of solar cells. A voltage divider was used to achieve several different set point temperatures from 25 oC to -24 oC. This system was used with a halogen lamp to study the electrical performance of polycrystalline thin-film solar cells fabricated in our group. These cells have the superstrate structure glass/SnO2:F/CdS/CdTe/metal.(1) The I-V characteristic shows evidence of a blocking back-diode which sets in below room temperature. This behavior will be related to the diffusion into the CdTe of the metals used for our back contact.(2) 1. M. Shao, A. Fischer, D. Grecu, U. Jayamaha, E. Bykov, G. Contreras-Puente, R.G. Bohn, and A.D. Compaan, Appl. Phys. Lett. 69, 3045-3047 (1996). 2. D. Grecu and A.D. Compaan, Appl. Phys. Lett. 75, 361-363 (1999).

[F36.055] COMPARISON OF FLUCTUATIONS IN FIRST AND SECOND STOKES ORDERS IN FIBER-BASED RAMAN GENERATION

We present measurements of the pulse energy statistics of red-shifted light generated by Raman scattering of Q-switched pump pulses in optical fiber. The light scattered directly from the pump is known as the first Stokes pulse, which grows exponentially due to stimulated Raman scattering as it propagates through the fiber. Once the first Stokes becomes sufficiently strong it may pump a second Stokes order, which is red shifted from the first order. Even though the pump laser has a relatively stable pulse energy, we observe large fluctuations in the Stokes order energies. These fluctuations are due to the amplification of classical pump noise and quantum noise from the spontaneous start-up of the Stokes orders. We will compare probability distributions for the first and second Stokes orders to probe the relative importance of classical and quantum noise in the pulse-energy fluctuations of these orders, since the pump noise for the first Stokes is substantially less than the pump noise for the second Stokes.

[F36.056] MODELING STOKES RAMAN SCATTERING

We present a computer model of Stokes Raman scattering in optical fiber. The main focus is on how the pump pulse shape affects the first-order Stokes power at the output of the fiber. Using the experimental pump pulse envelope, we examine the effect of modulations that result from the multi-mode operation of the pump. By changing the amplitude and frequency spacing of the spectral modes of the pump pulse, we produce a variety of different input pulses to be analyzed. We vary the input power of the pump and study the relative growth of the generated Stokes pulses by plotting the fractional energies of the pump and Stokes pulses. A comparison of simulation results with experimental data demonstrates the necessity of incorporating the pump pulse temporal shape in modeling Raman scattering in optical fiber.

[F36.057] New Phase Condition for Lasers in Subharmonic Hybrid Mode-Locking

In a conventional laser cavity, the resonance condition requires that the phase change for one round trip must be 2k\pi. In monolithic semiconductor hybrid mode-locked lasers, the saturable absorber shares the same structure of the gain region, so the modulation signal changes the absorption coefficient and the refractive index at the same time. Under subharmonic modulation, the conventional resonance condition is no longer valid. A new resonance condition is proposed, such that the phase change for n-round trips is 2k\pi, where n is the order of the subharmonic.

A direct consequence of this new phase condition is the decrease of the longitudinal mode spacing. Suppose the spacing in the optical spectrum of an unmodulated laser is \Deltaf, it will be changed to \Deltaf/n under the n-th order subharmonic modulation. This has been observed in experiment with the third order subharmonic modulation. Detailed analysis also explains the relative intensity of the split peaks.

In the experiment, shifts of the spectrum are also detected. In the case of first and third order subharmonic modulations red shifts are observed, while a blue shift appears in the case of second order subharmonic modulation. Those phenomena can be well explained with this new phase condition combined with the optoelectronic properties of the device.

[F36.058] A Novel Infrared Gas Monitor

In the paper a novel non-dispersive infrared(IR) gas monitor is described.It is based on the principle that certain gases absorb IR radiation at specific(and often unique) wavelengths.Conventional devices typically include several primary components:a broadband source, usually an incandescent filament,a rotating chopper shutter,a narrow-band filter,a sample tube and a detector. We have developed a number of IR light emitting diodes(LED) having narrow optical bandwidths and which can be intensity modulated by electrical means,for example InAsSbP(4.2 micron)LED.The IR LED can thus replace the thermal source,narrow-band filter and chopper assembly of the conventional IR gas monitor,yielding a solid state,low- powered,compact and almost maintenance-free instrument with high sensitivity and stability and which free of the effects of mechanical vibration too. The detector used in the IR gas monitor is the solid-state detector,such as PbS,PbSe, InSb,HgCdTe,TGS,LT and PZT detector etc. The different configuration of the IR gas monitor is designed.For example,two-path version for measuring methane concentration by monitoring the 3.31 micron absorption band,it can eliminate the interference effects,such as to compensate for LED intensity changes caused by power and temperature variations,and for signal fluctuations due to changes in detector bias. we also have designed portable single-beam version without the sample tube.Its most primary advantage is very cheap(about cost USD 30 ).It measures carbon dioxide concentration by monitoring the 4.25 micron absorption band.Thought its precisions is low,it is used to control carbon dioxide concentration in the air in the green houses and plastic houses(there are about twenty millon one in the China).Because more carbon dioxide will increase the quanity of vegetable and flower production to a greatextent. It also is used in medical,sanitary and antiepidemic applications,such as hospital, store,hotel,cabin and ballroom etc. Key words:infrared gas monitor LED

[F36.059] Fabrication and characterization of an low-amplifying alumina based MCP

A new kind of an electron amplification device was fabricated by the simple method, i.e., solution based coating and evaporator, with alumina. The aim of this device with microsize holes, called a microchannel plate (MCP), is to amplify electrons to a few of magnitude without complicated fabrication processes and with low manufacturing cost. Cu_2O (1 \mu m thick) was used as a resistive layer by oxidation of evaporated Cu at 1040 ^oC in the air environment. Solution based MgO and SiO_2 (300 Åthick) was adapted as a secondary electron emissive layer on the resistive layer. Cu electrical contact layers were deposited on the both sides of an MCP by an electron beam evaporator. In order to investigate the amplification characteristics of an MCP, the MCP was placed between the cathode and anode plates of a field emission display (FED). It was found that the FED which incorporated a MCP exhibited high brightness compared with a conventional FED. Furthermore, the uniformity and focusing behavior of the MCP-FED were also improved. The dynamic behaviors of MCPs with the MgO and SiO_2 emissive layer were examined by changing the width of the FED pulse signal. The interface between the emissive layer and the SiO_2 resistive layer is considered to be a primary cause for the different dynamic behavior between MgO and SiO_2. This result suggests promising application for this low amplifying MCP, such as cathode electron amplification in a FED.

[F36.060] Influence of Low Intensity Laser Irradiation on Oxygen-carrying Ability of Blood

Low intensity laser irradiation has been used clinically to treat some diseases related to the lack of oxygen, such as mountain sickness. In this work, experimental research has been conducted to understand the mechanism of this treatment. Venous blood samples taken from rabbits were diluted and irradiated by Low intensity diode laser. Oxygen was injected into the blood in test tubes at different rates and times after the irradiation and the oxygen concentration in the blood was determined. The results indicate that the saturated oxygen concentration increases significantly when the injection is performed at about 20 minutes after the irradiation. Statistical analysis on the results will be presented also.

[F36.061] A Novel Method for Measuring the Optical Activity of Chiral Molecules

Optical Activity is defined as a property, which a substance will absorb incident (optical) radiation and/or change its polarization state. Molecules of this type are known as chiral (its mirror image cannot be superimposed upon itself, i.e. the molecule has a handedness). Optical Rotatory Dispersion (circular birefringence) occurs when a material exhibits a difference in its index for right (n_R) and left (n_L) circularly polarized light. In terms of the indices of refraction, the observed rotation is defined as \alpha =\frac\pi d\lambda_0(n_L - n_R), where n_L is the index of refraction for left-circular polarization, n_R is the index of refraction for right-circular polarization, d is the path length (sample thickness) and \lambda_0 is the wavelength of incident radiation measured in vacuum. We have developed and tested a new instrument that measures the optical rotatory properties of chiral molecules. The system consists of a frequency stabilized, Zeeman split He-Ne laser, with independent left and right circularly polarized output beams. Optical activity is measured by the relative phase shift between the beams exiting through the sample. The advantage of this instrument is that it allows a real time measurement of the optical activity of chiral substances; which can be used to monitor samples for changes of state. By using a differential measurement scheme, system errors are minimized and resolution is increased over current measurement techniques.

[F36.062] Resonance widths and curvature-corrected Fresnel formulas for dielectric cylinders

Of great importance in photonics are microresonators based on waveguiding by refraction - an effect that can be described within ray optics. Wave corrections arise because Fresnel's formulas are modified at curved dielectric interfaces. For a dielctric cylinder of circular cross section, such corrections permit a ray-based understanding of how numerically calculated resonance widths depend on parameters such as frequency, angular momentum, polarisation and refractive index n. For n > 2, we derive uniform analytical generalisations of Fresnel's formulas from Maxwell's wave equations. In the limit of small n, we instead use the classical Fresnel formulas as a starting point and identify two essential corrections: tunneling and the Goos-Hänchen shift. From these ray optics considerations, universal predictions are obtained: with sharp interfaces and for polarisation perpendicular to the plane of incidence, the resonance widths are bounded from above; whereas no such bound exists when the polarisation is in the plane of incidence and hence allows Brewster transmission.

[F36.063] Plasmons of an N-Quantum-Wire System in a Semi-infinite Semiconductor

We have examined the spectrum of N-quantum-wire plasmons coupled with those of a semi-infinite plasma-like host medium in which the wires are embedded, in particular, the surface and bulk plasmons of the host. Explicit numerical results have been obtained for N values up to N=5, and the coupled collective modes are analyzed as functions of z_0, the distance between the first quantum-wire and the bounding surface of the host semiconductor. We have also determined the dispersion of the coupled mode frequencies in their dependence on wavenumber parallel to the wire direction.

[F36.064] Electronic structure and magnetic properties of Ni_n and Al_n clusters

We have studied the electronic structure and magnetic properties of Ni_n and Al_n clusters within the density functional theory. All the atomic structures of the cluster are obtained by the generalized simulated annealing with Sutton-Chen interatomic potentials. The calculated ionization potentials and the general trend of magnetic moments changing with the cluster size are in good agreement with experimental data. For Al_n cluster, we find that the magnetic moments are close to zero when the number of atoms is larger than 10. We have also proposed a tight binding model to describe the properties of cluster.

[F36.065] Intrasubband and Intersubband Relaxation of Electrons in Q1D Doped Semiconductors

We have calculated the intersubband and intrasubband inelastic Coulomb scattering rates of electron in two-subband quantum wires at zero temperature. By assuming a symmetric potential in the confinement direction, we were able to analyze the inter- and intrasubband charge-density excitations separately in the phase space. We showed that the intersubband (intrasubband) charge-density excitations are responsible for intersubband (intrasubband) inelastic-scattering rates. We exactly identified all sort of contributions to the inelastic scattering rate coming from the emission of both the single-particle and the collective excitations in the Fermi sea. We discussed such contributions as the second subband is empty. We also presented the total lifetime of hot electrons, injected in each subband, as a function of the total charge density in the wire. These quantities represent the electron relaxation processes occurring in the conduction band. We figured out the role of each charge-density excitations in the intra- and intersubband relaxation processes.

[F36.066] Magnetoacoustic transport in narrow electron channels

We develop a theory of the effect due to a small perpendicular magnetic field on the quantized acoustoelectric current induced by a surface acoustic wave (SAW) in a narrow electron channel. The quasi one-dimensional channel is formed in a piezoelectric GaAs/AlGaAs semiconductor structure by a split gate technique with the gate voltage beyond pinch-off. The current is the result of the trapping of electrons in the SAW induced moving quantum dots and the transfer of electrons residing in these dots through the channel. It has been observed recently (J. Cunningham, et al., Phys. Rev.B, 1999) that in small magnetic fields the acoustoelectric current oscillates as a function of magnetic field. Based on a simple model for the quantized acoustoelecric transport in a narrow channel (G. Gumbs et al., Phys. Rev.B, Rapid Commun., 60, N20, R13954, 1999) we develop a theory for these oscillations. The case when one electron is captured in the dot is considered, and the period, the amplitude, and the phase of the current oscillations as a function of the system's parameters are obtained and analyzed.

[F36.067] Phonon Scattering Effects on Quantum Cell Dynamics with Tunneling, Coulomb Coupling

We analize a simple model of a quantum cell composed of two Coulomb-coupled double dot systems with one electron in each double dot.

The interaction of electrons with acoustic phonons is considered as the principal mechanism which relaxes the cell to the ground state. Non-Markovian stochastic equations for population differences and dipole moments of the constituent double dots are derived. It is found that in order for the bistable state of the quantum cell to exist, the effective energy of Coulomb repulsion between the double dot systems must exceed the individual tunnel splitting energy, as well as the phonon temperature. The behavior of the cell polarization near the critical temperature is described analytically. We calculate the damping rates determining cell relaxation to its minimum-energy state and discuss limitations on the speed of cell response to external fields caused by the finite relaxation time.

[F36.068] Investigation of Local Structure Around Ge in MBE-Grown SiGe Multilayers with ``Groove Islands" of SiGe Quantum Dots

Local structures around Ge in SiGe multilayers grown by MBE with the formation of ``groove-islands" of SiGe quantum dots have been investigated by using extended x-ray absorption fine structure (EXAFS) techniques. For comparison, the local structures around Ge in a bulk Ge and a SiGe/Si quantum well samples have also been investigated. It is found that the nearest neighbor atoms around Ge in the "groove-islands" sample are predominantly Ge with a Ge-Ge bond length shorter than that in bulk Ge (2.45ÅOn the other hand, a Si nearest neighbor shell was found in the quantum well sample containing 20 percent of Ge in the SiGe layers.

[F36.069] Parametric resonance in quantum dots

It is shown theoretically that nonlinear interaction between electrons in quantum dots and a strong electromagnetic pumping field can cause parametric instability of electron motion resulting in a transition of the electronic system to a highly exited state that is far away from equilibrium. The instability threshold (critical intensity of external pumping) is found as a function of the intrinsic and external parameters of the system. At the resonance conditions, the threshold amplitude of the pumping field is inversely proportional to the relaxation time, which provides a direct and explicit method for determining the relaxation time from experimental data. This should give a rise to new method for optical (far-infrared) investigation of quantum dots. It is also shown that the threshold is independent of the number of electrons in the dot when the constraining potential is parabolic, and that the obtained results are stable with respect to small, nonlinear perturbations of the parabolic potential.

[F36.070] Three particle exciton problem in type II semiconductor quantum wires

We construct a model to describe the interaction between three particles constrained in parallel quantum wires (QWs). Our formalism can be applied to the interaction of an exciton in a QW with another particle moving freely in the other QW or with an external impurity or charged quantum dot. We expand the three particle interaction potential in terms of multipoles by assuming that two of the particles experience transverse harmonic confinements in the x and y direction while the third particle is harmonically constrained in all directions. For the resulting excitonic Schrodinger equation we calculate eigenenergies and eigenfunctions for the exciton ground and first excited states.

[F36.071] Excitation Spectrum of Cyclotron Resonance in Single Quantum Dots

Cyclotron resonance (CR) excitation of single quantum dots (QD) is studied via transport through the QD (mechanical size of 0.7\mum with about 300 electrons) fabricated by lithography technique in 2DEG of GaAs/AlGaAs hetero-structure crystal. In magnetic field range from 3.4 to 4.2 T (the filling factor of Landau levels (LL's) from 2 to 3 in the QD), single events of photon absorption due to the CR inside the dot are distinguished as conductance switches through the QD. Excitation spectrum of the CR, studied through the photon counting, shows the upper branch of magneto-plasma resonance of the QD, which is shifted from the bulk CR frequency. The lifetime of the excited QD varies in wide range and strongly depends on the number of electrons on the LL2. It exhibits tooth-like behavior as a function of magnetic field, reaching 20 minutes when the last electron remains on the LL2.

[F36.072] Quantum Dot Plasmons

We have employed closed-form thermodynamic Green's functions for a harmonically confined quantum-dot in a magnetic field to determine its plasmon spectrum. Due to confinement and Landau quantization this system is fully quantized, with a large number of collective modes. The RPA integral equation for the inverse dielectric function is analyzed using Fredholm theory to determine the relative excitation amplitudes with which the plasmons participate in response to an external potential, as well as their frequencies.

[F36.073] Effect of the lined-up quantum-well states in a resonant tunneling triple-barrier structure

The effect of the properly lined-up quantum-well states under the external bias on the electron resonant tunneling is investigated in an InAlAs/InGaAs(001) triple-barrier structure. The degree of alignment of two quantum-well confined ground states at a resonant voltage is analyzed and confirmed with the low-temperature measurement. The experimental data shows the enhanced resonant tunneling effects in the triple-barrier structure, and proves that the added second quantum-well structure to the InGaAs/InAlAs double-barrier heterostructure can act as an effective tool for probing and extracting the resonant tunneling properties deep in a quantum well.

[F36.074] Electron interference in InAs/AlGaSb mesoscopic devices

We report on the magneto-transport in InAs/AlGaSb quantum wires (QWRs) and open quantum dots, in which electron interference should be an important process. The InAs QWRs with and without a periodic modulation along the wire have been fabricated by electron beam lithography and oxidation technique by atomic force microscope. Oscillations of magnetoresistance appeared in the wide range of magnetic fields were compared for different shaped InAs devices. The amplitude of the aperiodic oscillations observed at fields below 2 T in the narrow QWRs with a periodic potential modulation was considerably large even at 4.2K. These aperiodic oscillations have not been observed with increasing the sample current. From the analyses of correlation functions of these oscillations, interference of electron waves in the InAs QWRs will be discussed.

[F36.075] SULFUR NANOWIRES

We have synthetized sulfur nanowires by a template approach using nanoporous anodic alumina. High resolution electron microscopy shows that isolated sulfur nanowires (15 nanometers of diameter) present crystalline structure different to that observed in the stable bulk allotrope (orthorhombic alfa-sulfur). Melting behavior of the sulfur nanowires embedded into the nanoporous alumina matrix was studied by differential scanning calorimetry, showing again very different behavior of the nanowires compared to that of the bulk sulfur. On the other hand, in order to study the bonding configuration of the sulfur atoms in the nanowires, we will present infrared spectroscopy characterization of the nanowires confined into the nanoporous alumina. Finally, on the base of the experimental observations, we will present a structural model for the sulfur nanowires.

[F36.076] The metal-insulator transition in Ba-Si clathrate compounds : ab-intio study

Recently, silicon based clathrates draw much attention with discovery of superconductivity, containing alkali (Na) and alkaline-earth (Ba) atoms which locate at the center of silicon cluster cages. The materials have been known as having an insulator-metal transition as a function of sodium content by experiment as well as ab-initio calculation. However, there is no report on the transitions for doping of alakaline-earth (Ba) atoms in the clathrate. We clarify the electronic structures and their band structure of the clathrates by the first principle norm-conserving pseudopotentials with planewave basis and supercell approximation. We will discuss the relationship between barium content and electronic conductivity in terms of a band structure calculations.

[F36.077] Transmission Anomalies of Photonic Crystals Having Near Unit Index Contrast

Calculations are presented that reveal anomalous characteristics in the transmission spectra of photonic crystals having near unit index contrast. The anomalies present a limitation for devices demanding photonic crystals with very narrow band gaps. We find that opaque regions of the transmission spectra through finite length crystals may be substantially broader than the band gap of the corresponding infinite crystals. The opaque spectral width decreases gradually with increasing thickness. Many hundreds of layers may be needed to obtain a transmission null that approaches the width of the band gap for the infinite crystal. The analysis is done for two configurations of interest for optical applications due to their relative ease of manufacture. The first is a two-dimensional triangular array of dielectric rods for which finite difference time domain simulations are done. The second is a one-dimensional dielectric slab.

[F36.078] Using Liquid Crystals to Detect Biomolecules Bound to Nanostructured Substrates

Recent advances in the capability to control the structure of organic surfaces over a wide range of length scales offers new opportunities to design the interactions between liquid crystals and surfaces, and thus to orient liquid crystals through these interactions. This poster will report on the balance of intermolecular forces acting between nematic liquid crystals and nanostructured interfaces formed by using x-ray lithography, nano-scale methods of replication, and molecular self-assembly. This understanding will be exploited to design surfaces that permit simple acid-base reactions and complex biospecific interactions (antigen-antibody) to be amplified and transduced at surfaces by observation of the orientations of liquid crystals.

[F36.079] Qualitative features of 1D Schrodinger equation with an asymmetric Coulomb potential in the study of an impurity at the boundary of different quantum wires.

In the momentum space we find the bound eigenenergies and eigenfunctions of 1D Schrödinger equation for an asymmetric Coulomb potential. Our model can be used as a qualitative guideline for the calculation of the electronic states of an impurity located between two coaligned quantum wires characterized by different screening dielectric constants. In the configuration space eigenfunctions are expresed in terms of fractional derivatives.

[F36.080] SERS activity and micro-structure of silver colloids

We have investigated the relation between the microscopic structure and SERS activity of silver colloidal particles toward single molecule detection with Raman spectroscopy. Silver particles (diameter (d) >200 nm) aggregated by adding dyes and NaCl into solution showed a promising SERS activity, while isolated nanoparticles (d <50 nm) did not give a detectable Raman signal. Even if the differences in observed surface area for these particles are taken into account, Raman spectra from the isolated particles should be observed assuming the same SERS activity as the aggregated particles. This is probably due to the extended LSPP (localized surface plasmon polariton) absorption from ca. 400 nm to longer wavelength by aggregation, and thereby it become in resonance with excitation wavelength (488 nm). Moreover similar Raman enhancement factor for crystal violet and adenine was obtained, although the latter one is not in resonance with the excitation wavelength. Thus, the electronic resonance of adsorbed molecules is not always necessary to yield a prominent SERS activity from the aggregated silver particles.

[F36.081] Fabrication of metallic nanowires via photon-assisted selective chemical vapor deposition on self-assembled calcium fluoride/silicon (111)

We explore the possibility of manufacturing metallic nanostructures on a large scale. We have focused on enhancing the site selectivity for metallocenes adsorption. This is achieved by creating a template with well defined local chemical reactivity [1] using self-assembled CaF2 stripes on a stepped Si(111). A N2 laser is employed to drive the chemical reaction on the adsorption site, mainly along edges of the CaF2 stripes. The photon-assisted surface chemical process removes organic ligands, leaving metal atoms behind on the substrate. This makes it particularly attractive in developing ferromagnetic materials, such as Ni, Fe, and Co, without carbon and oxygen contamination. Scanning tunneling microscopy shows that ordered arrays of metal wires on the scale of single digit nanometer are formed under the low coverage. We ascribe the formation of such nanostructures to the regular patterns of the adsorption site on a CaF2/Si(111) substrate. Studies on electronic and magnetic properties of the structures will be discussed. This work is supported by NSF. [1] J.-L. Lin et al., JAP 86, 5492 (1999).

[F36.082] Wannier exciton with an electron in a quantum wire and a hole in a perpendicular 2D quantum layer

Within the spirit of investigating systems that exhibit spatial separation between electron and hole, we calculate the states of a Wannier exciton formed by an electron confined in a quantum wire interacting with a hole confined in a 2D quantum layer. Since one quasi-particle provides two degrees of freedom and the other provides one degree of freedom, our solutions are expressed in terms of the bulk 3D exciton states.

[F36.083] Transverse elastic waves in superlattices: The Brewster acoustic angle

We use a plane wave basis to study the transverse acoustic waves in elastic superlattices - artificial binary structures of periodic mass density and periodic transverse elastic velocity. The bulk frequency bands for oblique propagation support mini-gaps that can close due to the edge crossing of subsequent bands. Some of these crossings are explained in terms of the acoustic Brewster angle that is a function of the material parameters only. A second condition for the complete shrinking of the gaps that involves the structural parameters is also discussed. The surface waves in the corresponding truncated superlattices are obtained by use of the supercell method. The dispersion curves of these waves are strongly dependent on the material of the layer at the surface - the surface waves that appear for a selected layer at the surface can completely disappear when the last layer is the other one. Specific applications are given for Zn/Fe, Pb/Ge and Si/Au superlattices.

[F36.084] Diffusion and Localization in a One-Dimensional Superlattice with a Phonon Bath

The study of localization has been a focus of fundamental research for many years. There are many varieties of this phenomenon, for example, Anderson localization in one-dimensional disordered systems, Wannier-Stark localization in a superlattice in the presence of a constant electric field, and dynamic localization in this system in the presence of a time-dependent electric field. The coupling of carriers to a dissipative environment destroys all these types of localization. As is shown in [1], localization is restored for electrons coupled to acoustic phonons in a one-dimensional superlattice at the points of electrophonon resonance E=2\pi n\hbar u/ed^2, where E is the applied electric field, u is the sound velocity, d is the superlattice period, and n is integer. In the present report we consider diffusion in a one-dimensional superlattice. Our analysis, based on the general theory of open quantum systems applied to a superlattice, yields results for the diffusion coefficient, which show that the diffusion coefficient also vanishes at the points of electrophonon resonance, indicating that the localization is complete.

Furthermore, we have considered the dynamics of a Gaussian wave packet in a one-dimensional superlattice with phonon scattering at finite temperature and have shown the existence of Bloch oscillations and electron wave function coherence when electrophonon resonance occurs.

1. A.Yu.Smirnov and L.G.Mourokh, Phys.Lett.A 231, 429 (1997).

[F36.085] A Cantor film set showing low reflection of electro-magnetic waves

A Cantor fractal film set has been designed. The set shows a good absorption and very low reflection of electro-magnetic waves. The reflectivity of the film set has been calculated analytically based on a simplified model. A group of criterions for the selection of suitable materials for the set layers have been suggested according to the analytic results. Some example sets that are formed by ideal or practical materials with the criterions have been discussed. Our numerical computation has shown that the set averaged reflectivity decreases and its reflection spectrum becomes more even when the total degree number of the fractal increases. The lowest averaged reflectivity obtained is 0.01.

[F36.086] Study of Te film adhesion at ZnSe crystal surface

The effect of Te film adhesion to ZnSe crystal surface is investigated. It is well known that heterostructures of Te-A_2B_6 type are of great interest for light diode fabrication . The Te films are traditionally precipitated at a substrate surface in vacuum by a target evaporation method. Before the process of putting the crystal surface underwent a chemically-dinamical treatment. According to the data of electron microscopy studies of film-substrate interboundary the method has essential drawback. The quality of interboundary can not satisfy necessary requirements because of transition region existence. Transition layers make worse the film adhesion to the surface, therefore with the time the film exfoliation is observed. Authors of the proposed work have tested various conditions of Te film precipitation at ZnSe surface and determined that the drawback can be avoided when before the process of Te film putting the ZnSe single crystal surface is irradiated by coherent laser light or heated in vacuum. The essence of the idea consists in the fact that in the process of laser irradiating and annealing the phenomenon of non-metal atom evaporation out of the surface region is observed. Hence ZnSe crystalline surface is metallized. When later Te film is put its adhesion to ZnSe surface is highly improved that is confirmed by electron-microscopy studies of film-substrate cross section as well as studies of adhesion properties after manifold thermocircling under the scheme 300 K � 77 K � 300 K.

[F36.087] Magnetic and resistance measurements on boron-doped and undoped Ni(3)Al thin films*

We report preliminary results of magnetization and I-V measurements of the effects of boron doping on the magnetic and electron transport properties of Ni3Al thin films. Magnetization and resistance measurements in magnetic fields up to 5 T were performed on 500 Ånominal) thick films that were fabricated by ion beam sputtering of compound targets. Both a doped (\sim200 ppm B) and undoped film were investigated. For the boron-doped film, the magnetization is enhanced with a broad transition that occurs in several stages over the temperature range from 27 K to 56 K. Further, as the temperature is increased through the transition range dM/dT fluctuates between negative and positive values, and the magnetization changes from positive to negative near T = 52 K. Results of I-V measurements performed on the samples with the current in the plane of the film, and an applied magnetic field parallel to the plane of the film, are consistent with these results. *Work supported by the LEQSF and the Dept. of Physics, Southern U. and Aamp;M College, Baton Rouge campus.

[F36.088] Surface tension and the internal pressure of a spherical condensed matter particle

Burgeoning interest in nanometer-sized condensed matter particles stems from the fact that their physical properties can differ greatly from the same material in bulk form. We offer an argument that the currently-accepted relation between the surface tension and internal pressure of a spherical condensed matter particle is inaccurate by a factor of 3/2. The currently-accepted relation is P=2g/r, where P is the amount by which the pressure within the particle exceeds that of its surroundings. This relation is derived by considering force balance on a spherical cap between the forces due to the surface tension and the pressure [1]. For a sphere, we conclude that the force balance argument is invalid because if one divides the sphere into two with a plane, the forces due to the surface tensions of the two parts balance, as do the forces due to the pressures acting on the two parts. Force balance between the surface tension and the internal pressure then need not apply. Integrating the Gibbs free energy in differential form for the case of a sphere, we obtain the result P=3g/r. Given that P=3g/r follows from energy conservation, P=2g/r therefore violates energy conservation in the case of a sphere. However, P=2g/r remains valid for a spherical cap.

[1] R. Defay, I. Prigogine, A. Bellemans, and D. H. Everett, Surface Tension and Adsorption (Longmans, London, 1966) pp. 6-7

[F36.089] Quantum confinement in GaAs nanoparticles incorporated in SiO_2 matrix.

Using the Empirical Pseudopotential Method and applying the Ramakrishna and Friesner approximation [1], the band gap is calculated for GaAs nanoparticles. Theoretically calculated results are compared with the experimentally obtained band gap values extracted from the optical absorption measurements on the GaAs nanoparticles embedded in SiO_2 matrix. The nanocomposite materials were grown by r. f. sputtering. Transmission electron microscope images were used to determine the size distribution of GaAs nanoparticles. Size distribution broadening and the discretization of energy levels are applied to correlate the theoretical results with the experimentally obtained data values. A good agreement between theory and experiment is obtained.

[1] Ramakrishna, M. V. and Friesner, R. A. Phys. Rev. Lett. 67 (1991) 629.

[F36.090] A Monte Carlo Approach to Population Dynamics of Cell in an HIV Immune Response Model

A direct Monte Carlo method is used to study the growth and decay of celluar elements, macrophages (N_H), helper T-cells (N_H), cytotoxic cells (N_C), and antigens (N_V), with an HIV immune response model. A set of rule-based logical interactions among the cells are considered. Cells divide and decay on a discrete lattice as a result of immune mechanism implemented via the inter- and intra-cellular interactions. Viral mutation is considered probabilistically (P_mut). Cells are mobile with their local motility-bias and the overall mobility is controlled by cellular mobility (P_mob). Computer simulations are performed on different lattice sizes with a number of independent runs for each parameter for averaging. Cellular mobility (P_mob=1) enhances the viral growth and reduces the stimulative T-cell growth. As a function of viral mutation rate, the interplay between the steady- state density of helper T-cells (\rho_H) and the viruses (\rho_V), leads to interesting predictions regarding the degree of infection including AIDS. For example, below a mutation threshold, (P_mut \le P_c), while the relative T-cell count (- \Delta_0 = \rho_H-\rho_V > 0) is not as alarming, above the threshold, viral population increasingly dominates as a function of the mutation rate with the onset of a continuous transition \Delta \rho_0 \propto (P_mut - P_c)^\beta; \beta \simeq 0.574 \pm 0.016 as P_mut \to P_c in absence cellular mobility.

[F36.091] A biological junction with quantum-like characteristics

A model of chemical synapse as an electric junction is proposed. Estimations and analysis of the model show that the junction has unique physical characteristics reminding the Josephson junction. The basic assumption is made that the electric coupling across the synaptic gap is indirectly provided by means of approximately quantized portions of a chemical mediator, each the portion is content of a synaptic bubble. We suppose that effective quantum of charge is q, |q|\gg |e|. The synapse characteristics are dominated by electrostatic energy, Q^2/2C, Q=qN, N=0,1,2...; where C is electric capacity of membrane. Estimations show that the integer-valud character of N must be explicitly taken into account. The consistent theory of the junction is constructed on the basis of operator realization of number-phase canonical pair in the Hardy space. The charge passing from one side of the junction to other is decribed by the Toeplitz operators. The synapse state space is constructed explicitly. The unique physics of the model is investigated in detail. We do not exclude the possibility that the model is prototype of a molecular electronics device.

[F36.092] FT-IR Spectra of Antifreeze Glycoproteins in Heavy Water and D2O Ice.

This work presents FT-IR studies on the antifreeze glycoprotein (AFGP)/heavy water (D2O) mixtures during freezing and melting. AFGP in the blood serum of polar fish are known to prevent ice crystal growth by a non-colligative mechanism. There are 8 known fractions of AFGP (1 � 8) that range in molecular mass from 33.7 to 2.6 kD respectively, each composed of alanine-alanine-threonine repeats, with a disaccharide attached to the threonine residue. The smallest peptide (AFGP-8) is structurally different from fractions 1-5 in that it contains proline substituting for alanine in certain positions. Substantial linewidth change of the D20 bending mode (ca. 1210 cm-1) was measured with solutions containing fractions 2-5 during both freezing and thawing cycles, suggesting significant coupling between protein and water molecules. At the same time, the Amide I band between 1620 and 1675 cm-1 shows that 310 helix and random coils are the main conformations of fractions 2-5 and fraction 8 in the presence of ice. In liquid state, b-sheet dominates the secondary structure of AFGP 8, whereas b-sheet and random coil are the main conformations of AFGP 2-5. These results are discussed in terms of the ability of AFGP 2-5 to affect the surface states of ice.

[F36.093] The Sunscreen Octyl Methoxycinnamate Binds to DNA

Sunscreens are designed to prevent skin cancer by absorbing ultraviolet radiation from the sun before it gets to the DNA in skin cells. The purpose of this work is to determine whether or not octyl methoxycinnamate, an active ingredient in many sunscreens, will bind to DNA. If so, the sunscreen could transfer the energy it absorbed from the sun to the DNA and cause damage. To determine this, we prepared samples with varying concentrations of cinnamate added to herring sperm DNA, sonicating the mixture to disperse the hydrophobic sunscreen into solution. Absorption and fluorescence spectra of the mixtures showed (i) much more sunscreen was dispersed into solution when DNA was present, and (ii) the spectra of both DNA and sunscreen differed from those of the separate solutions. We conclude that the octyl methoxycinnamate can indeed bind to DNA in aqueous solution. Energy transfer experiments from DNA to sunscreen and from sunscreen to 2-aminopurine- (a fluorescent DNA base) labeled DNA will be presented.

[F36.094] Free Energy Change Upon Loop Formation in DNA Hairpins

Hairpin loops are ubiquitous in secondary structures of single-stranded DNA and RNA chains. Knowledge of the energetics and kinetics of loop formation, which is the nucleation step in hairpin structures, and how the rate of nucleation depends upon the loop size, is essential for understanding secondary structure formation in nucleic acids. The simple scaling of the free energy change with the logarithm of the loop size, which holds for wormlike chains, is expected to break down for small loops because of chain stiffness. Here we use a statistical mechanical model to describe the melting of hairpin loops. The free energy change upon loop formation is assumed a free parameter in the calculation of the order parameter that describes the melting. Our analysis shows that the change in free energy corresponding to loop formation decreases as the nucleation loop size decreases, as expected from purely entropic considerations, with an optimum loop size of about 12 bases. For loops smaller than that the free energy change increases sharply owing to the increase in enthalpy from steric hindrances among the bases in the loop and the loss of their stacking interactions.

[F36.095] Polarization-Modulated DIC Microscopy with a fast Variable Retarder Increases Image SNR

Differential interference contrast microscopy yields high-resolution images of unstained biological samples as well as semiconductor structures. Insertion of a fast liquid crystal variable retarder into a DIC microscope can switch (modulate) image highlights into shadows and \emvice versa in alternate frames. Synchronously computed and immediately displayed PM-DIC difference images exhibit both enhanced contrast and reduced fixed-position noise because background is automatically subtracted. The transmitted intensity for modulation amplitude \Gamma and sample phase difference \delta is obtained by a Jones optical calculus analysis for the central ray: I=2I \sin\Gamma \sin\delta/(1-\cos\Gamma \cos\delta). To select the optimum modulation amplitude of the retarder, experimental SNR values are determined from the cross-correlation between two images of an invariant object (diatom skeleton). The measured SNR is a maximum when \Gamma \simeq \delta. The measured dependence of SNR on \Gamma agrees with the Jones calculus prediction. A video clip of moving organelles in living cells, obtained by the new method, will be shown.

[F36.096] Tumor growth and its effect on Magnetic Resonance Imaging signal

The goal of this project is twofold. On one hand, we have developed computer code based on simple probabilistic rules to model the growth (or shrinking) of cancerigenous tissue. We assume that initially there exists a differentiated cell, which has a time- dependent probability of reproducing. If it did reproduce, then we assume that it has a finite probability of dying before reproducing again. This simple model falls into the Eden-type kind, and presents appropriate bulk growth characteristics, as it follows Gompert observational law. We propose new methods of geometrical characterization of the tumor. Besides its total mass, we also consider higher multipolar order of mass distribution and surface fractal dimension. In addition, we study how the geometrical properties of the tumor affect the Magnetic Resonance Imaging (MRI) signal. To this end, we consider a human brain in the presence of radiofrequency fields. We calculate the MRI image of this object. Then, we introduce a tumor in the white-gray matter region and reobtain the MRI image. We associate the signal changes with the geometrical properties of the tumor.

[F36.097] Magnet Healing?

Many people are convinced that static magnets�applied to their skin�will heal ills, and many businesses sell such magnets. The biophysics of such healing was reviewed [1] together with the general biophysics of static fields. Birds and insects do use the earth�s magnetic field for navigation. While insect and frog egg development can clearly be influenced by high fields (7 T and 17 T respectively), there is no experimental evidence that small magnetic fields (of less than 0.5 T) might heal, and much evidence that they cannot heal.

A puzzle to the physics community is: How to show laypersons that simple magnets (very probably) do not heal, however attractive that idea might be.

[1] L. Finegold, The Physics of "Alternative Medicine": Magnet Therapy, The Scientific Review of Alternative Medicine 3:26-33 (1999).

[F36.098] Two-Photon Excitation in Biological Material for Conventional and Long Working-Distance Objectives.

The application of laser two-photon excitation or nonlinear second-harmonic generation to imaging, spectroscopy, and light activated medical therapies, is an expanding field of research. When small feature sizes such as cells and their components are to be studied, high numerical aperture (NA) lenses are required to obtain the necessary lateral and axial resolutions. If one wishes to increase the depth of sample penetration, factors such as scattering and absorption quickly degrade the quality of the focused beam. The problem is further exacerbated by the short working distance of conventional high NA microscope objectives if they are used for light delivery and pickup. These lenses and their accompanying eyepieces, are designed to produce an exit pupil that can be accomodated by the human eye. Such a design will underfil detectors such as large CCD arrays. To simultaneously increase the working distance at the sample and the system exit pupil, larger scale objectives can be used. We will report the results of two-photon excitation and fluorescence investigations of several feature sizes as a function of penetration depth in homogeneous media and tissue samples, for conventional and long working distance objectives. The possible implications of these results to imaging and therapeutic dose delivery will also be presented.

[F36.099] Phase synchronization and noise-induced resonance in systems of globally coupled oscillators

We study the synchronization phenomena and the noise-induced resonance behavior in systems of globally coupled oscillators, each possessing finite inertia. The self-consistency equation for the order parameter, which measures collective synchronization of the system, is derived and behavior of the order parameter is investigated as the noise strength is varied. It is found that the hysteresis present in the system without noise disappears as the thermal noise comes into the system. The power spectrum of the phase velocity is also obtained and the possibility of noise-induced resonance is examined.

[F36.100] Kinetic Theory Calculation of Protein Unfolding Rates

Protein folding in the diffusion collision model is viewed as an hierarchical process of transient secondary structre formation, then interaction of secondary structures, called microdomains, to form the final state. The decrease in free energy associated with the coalescence of microdomains to form higher structures, called kinetic intermediates, is due to the burying of hydrophobic residues. The larger the buried area, the more stable the intermediate. The favorable hydrophobic interaction can be disrupted, and the secondary structural coalescence dissociated by energy fluctuations in the solvent of sufficient energy. The quantity of interest is the rate at which these fluctuations occur, which gives the backward folding, or unfolding rates of the kinetic intermediates. We model the process by considering the effect of collisions between volume elements of the solvent and microdomain pairs in the protein. This analysis yields the unfolding rates of microdomain pairs used in the diffusion collision model calculations of protein folding.

[F36.101] Non-Native Kinetic Intermediates in the Diffusion Collision Model of Protein Folding

The process of protein self assembly is governed by the aggregation of secondary structural elements called microdomains. These microdomains diffuse through the solvent relative to one another and collisions between them may result in coalescence or pair formation. The final native structure is given by a specific collection of pairings. Kinetic intermediates, on the other hand, are given by a collection of pairings occuring prior to the formation of the native structure. If an intermediate contains pairings which are not present in the final fold, the intermediate is considered non-native. Some criticsms of the diffusion collision model point out that using only native pairin