bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of interfacial layers on dielectric properties in very thin SrBi2Ta2O9 capacitors
Authors: Moon Bum-Ki , Isobe Chiharu , Hironaka Katsuyuki , Hishikawa Shinichi
Published: July 2002

The effect of interfacial layers on the dielectric properties in very thin SrBi2Ta2O9 (SBT) capacitors has been investigated using static measurements. Total permittivity (εt) decreased as the film thickness was reduced in both Pt/SBT/Pt and Ir/SBT/Pt capacitors. The contribution of the interfacial capacitance (Cint) and bulk capacitance to the total capacitance indicates that Cint of the Ir/SBT/Pt structure was lower than that of the Pt/SBT/Pt structure, while the bulk permittivity (εb) was essentially the same. The dispersion of all capacitors followed the power law, while the Ir/SBT/Pt capacitor showed a larger dispersion of Cint. These results suggest that the Pt/SBT/Pt capacitor is preferred for obtaining the high performance with less effect of the interfacial layers on the dielectric properties.
On the observables describing a quantum reference frame
Authors: Mazzucchi S.
Published: July 2002

A reference frame F is described by the element g of the Poincaré group 𝒫 which connects F with a given fixed frame F0. If F is a quantum frame, defined by a physical object following the laws of quantum physics, the parameters of g have to be considered as quantum observables. However, these observables are not compatible and some of them, namely the coordinates of the origin of F, cannot be represented by self-adjoint operators. Both these difficulties can be overcome by considering a positive-operator-valued measure on 𝒫, covariant with respect to the left translations of the group, namely a covariance system. We develop a construction procedure for this kind of mathematical structure. The formalism is also used to discuss the quantum observables measured with respect to a quantum reference frame.
The Euler equations on Lie algebra so(4): An elementary approach to integrability condition
Authors: Maciejewski Andrzej J. , Popov Sasho Ivanov , Strelcyn Jean-Marie
Published: July 2002

We give a new derivation of the Manakov and the product case integrability conditions of the Euler equations on Lie algebra so(4). Fourth first integral functionally independent of the three already known integrals is obtained explicitly for all values of the parameters by an algorithmic approach.
A uniqueness theorem for entanglement measures
Authors: Rudolph Oliver
Published: July 2002

We obtain a mathematically simple characterization of all functionals coinciding with the von Neumann reduced entropy on pure states based on the Khinchin–Faddeev axiomatization of Shannon entropy and give a physical interpretation of the axioms in terms of entanglement.
Inclusion of quantum-mechanical vibrational energy in reactive potentials of mean force
Authors: Garcia-Viloca Mireia , Alhambra Cristobal , Truhlar Donald G. , Gao Jiali
Published: July 2002

Classical molecular dynamics and Monte Carlo simulations typically exclude quantum effects on the vibrations of reactants and transition states, and this may lead to significant errors in the computed potential of mean force. To correct this deficiency, a simple approximate procedure is proposed for the inclusion of quantum-mechanical vibrational energy in the computation of reactive potentials of mean force in condensed phases. The method is illustrated by a hydrogen atom transfer and a proton transfer reaction in water, in particular, the 1,5-sigmatropic shift in malonaldehyde and the intermolecular proton shift between ammonium ion and ammonia in an encounter complex. In both cases, quantum-mechanical vibrational energy makes significant contributions by reducing the free energy of activation by 2 to 3 kcal/mol. This finding has important implications in developing empirical potential functions for the study of enzyme reactions, and it is essential to quantize vibrational energy in the computed potential of mean force and free energy of activation in order to compare simulations quantitatively with experiment.
Dilational rheology of Langmuir polymer monolayers: Poor-solvent conditions
Authors: Monroy Francisco , Rivillon Sandrine , Ortega Francisco , Rubio Ramón G.
Published: July 2002

The viscoelastic moduli (elasticity and dilational viscosity) of monolayers of P4HS have been studied over a broad frequency range (0.1 mHz–200 kHz) using a combination of relaxation and capillary waves techniques. The analysis of the surface pressure, the elasticity and the viscosity in the semidilute regime show that the air–water interface is a poor (near-Θ) solvent for the monolayer. The results of viscoelastic moduli show that there is a broad relaxation process in the low-frequency range (ω<1 Hz), and another very intense relaxation process centered in the kHz region. This behavior contrasts with the one previously found for PVAc, a polymer for which the interface is a good solvent. For PVAc the relaxation found at low frequencies is much narrower, and two processes are clearly distinguished at higher frequencies: one centered at 500 Hz and another one at 40 kHz.
Modified epitaxy in Co/S/GaAs(001) and comparison with Co/GaAs(001)
Authors: Nath Krishna G. , Maeda Fumihiko , Suzuki Satoru , Watanabe Yoshio
Published: July 2002

Due to the S passivation, a modified growth of Co on GaAs(001) has been found. Using reflection high-energy electron diffraction and transmission electron microscopy, we observed the formation of a hcp Co overlayer of approximately 5 nm thickness on S/GaAs(001). In contrast, a similar 5 nm Co film on GaAs(001) shows a bcc structure. The metal-semiconductor interfaces in both systems were found to be different, where Co/S/GaAs(001) showed a relatively more abrupt interface. This epitaxial modification is explained on the basis of the morphology of the initial substrate surface, chemical compositions, the nature of the chemical reaction between adatoms and substrate atoms, and the effect of atomic segregation.
Neutralization and negative ion conversion of low-energy proton scattered from Ar, Kr, and Xe condensed on the Pt(111) surface
Authors: Souda Ryutaro , Kato Masahiko
Published: July 2002

Low-energy H+ ions (E0=100 eV) are scattered from Ar, Kr, and Xe layers condensed on Pt(111) to investigate the mechanism of neutralization and negative ion conversion of H+. The surface peak of H+ scattered from Xe is absent in a submonolayer coverage regime and increases in intensity as the multilayer grows. The H+ ion survives neutralization on Ar at any coverage. The Xe 5p orbital has covalency with the valence-band orbitals of Pt(111) through which the H+ 1s hole is delocalized, whereas Ar is basically physisorbed on Pt so that the H+ 1s hole (or the Ar+ 3p hole) is localized during the ion scattering time (around 5 femtoseconds). For the thick rare-gas solids, the H+ yield from Xe is almost two orders of magnitude as small as that from Ar since the H+ 1s hole is not perfectly localized in the former due to the hybridization between the Xe 5p orbitals. The H− yield from the solid Xe (Ar) surface is highly increased (decreased) relative to that from the Pt(111) surface. The H− ions are formed during a close encounter with a target atom and the existence of the band gap quenches the decay channels of H− on the outgoing trajectory from the surface. The experimental results of neutralization, electron-hole excitation, and negative ion conversion of H+ are elucidated consistently on the basis of the formation of the quasimolecule and the localization/delocalization of the valence holes in it.
Effects on the structure of monolayer and submonolayer fluid nitrogen films by the corrugation in the holding potential of nitrogen molecules
Authors: Hansen F. Y.
Published: July 2002

Molecular dynamics simulations have been used to study the effects of the corrugation in the holding potential of nitrogen molecules on the structure of fluid monolayer and submonolayer films of the molecules on a solid substrate. Structures of monolayer and submonolayer fluid films of nitrogen molecules adsorbed on graphite and on a model uncorrugated “smooth” graphite surface are compared. For films on the “smooth” graphite surface the melting temperature is lowered by 7 K. Contrary to what is found for films on the corrugated surface, the simulations show that there is a region of liquid–gas coexistence, demonstrating that this is a normal triple point system. A discrepancy between calculated and experimental melting temperatures of submonolayer films was traced to the intermolecular potentials. These have been tested by comparing molecular dynamics simulations of isosteric heats of adsorption in fluid films with experimental measurements. The establishment of the effects of the corrugation in the holding potential on the structure provides a basis for the study of the effects on the dynamical excitations in the films.
Laser deposition from a nanostructured YBaCuO target: Analysis of the plume and growth kinetics of particles on SrTiO3
Authors: Huhtinen H. , Järvinen J. , Laiho R. , Paturi P. , Raittila J.
Published: July 2002

The plume generated by a pulsed XeCl laser from a novel nanostructured YBaCuO target (n target) is investigated by methods of optical emission spectroscopy and atomic force microscopy. While the spectral positions of the emission lines are the same, stronger line intensities, pertinent to higher kinetic energy of the particles, are observed in the plumes generated from the n target than from a target having micron size grains (m target). The size of small clusters captured on Si plates assembled inside the plume grows in directions perpendicular to the axis of the plume. As shown by x-ray photoelectron spectroscopy investigations, in the particles deposited on a SrTiO3 substrate at Ts=700 °C in oxygen the correct 1-2-3 composition is achieved. The average ratio of the heights of the particles deposited from the n target and from the m target is hn/hm=0.6, both in the plume and on SrTiO3. This can explain the smoothness of YBaCuO films prepared by laser deposition from the n targets.
1/f
noise of Bi2Sr2CaCu2Oy intrinsic Josephson junctions
Authors: Saito A. , Hamasaki K. , Irie A. , Oya G.
Published: July 2002

We have measured the low-frequency voltage noise properties across mesa stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2Oy (BSCCO) single crystals as a function of bias voltage. The BSCCO mesas with area of 160×40 μm2 were fabricated on the surface of cleaved BSCCO single crystals using standard photolithography and Ar ion-milling techniques. The measured noise voltage spectrum density SV(f ) had a 1/f dependence on frequency. The magnitude of the 1/f noise spectra was found to increase monotonically with the branch number in the I–V characteristic at fixed bias current, and no anomalously enhanced 1/f noise was observed.
Atomic-scale graded structure formed by sedimentation of substitutional atoms in a Bi–Sb alloy
Authors: Mashimo Tsutomu , Ikeda Taiki , Minato Ichiro
Published: July 2002

An atomic-scale graded structure was formed by the sedimentation of component atoms in an all-proportional miscible alloy under a strong gravitational field of 1 million g in maximum acceleration at high temperature. Continuous composition gradients of Sb (about 20–0 wt. %) and Bi (about 80–100 wt. %) and the resultant continuous change in the lattice constants were observed in the ultracentrifuged Bi–Sb (70:30 in mol %) alloy bulk body. Crystal growth in the direction of acceleration field was also observed. It must be noted that the substitutional solute atoms with component level rate acted in the sedimentation of this system, while interstitial solutes (Au) at an impurity level rate acted in some elemental metals with low melting temperature in the previous studies. It is expected that the sedimentation of substitutional atoms can be used to control the compositions and structure of alloys or compounds.
Alloy formation of Y with Pd investigated by photoemission and electron diffraction
Authors: Borgschulte A. , Rode M. , Jacob A. , Schoenes J.
Published: July 2002

The structure of Pd capped Y films for an application as a switchable mirror was studied. The surface structures are investigated by reflecting high energy electron diffraction and Auger electron spectroscopy. The alloy formation between Y and Pd leads to different structural and electronic properties of the surface, which are compared to the switching behavior of the underlying Y film when exposed to hydrogen. The electronic structure of the intermetallic compound YxPd1−x(0<x<0.25) was studied by angular-resolved ultraviolet photoemission spectroscopy. The samples were prepared by Y deposition on Pd (111) surfaces of epitaxial films. The electronic structure of the alloy is discussed in the framework of a qualitative tight-binding approach. The reactivity of the Y–Pd alloy surfaces is estimated using the chemisorption model of Hammer and Norskov [Nature 376, 238 (1995)]. We show that exposure to air leads to an oxygen induced surface segregation of the Y–Pd alloy, which hinders hydrogen adsorption.
Particle fluxes in an electron cyclotron resonance plasma discharge
Authors: González-Damián J. , Gutiérrez-Tapia C.
Published: July 2002

In electron cyclotron resonance (ECR) plasma reactors, it is extremely important to control critical parameters such as the flux and energy distribution of ions impacting surfaces. These parameters are influenced by physical construction, magnetic field geometry, and chemical kinetics. In this article we present a model that accounts for the magnetic field geometry in the reactor region. The model is based on the drift kinetic equation approach and it is used to calculate the axial and radial ion fluxes in the materials processing zone of an ECR plasma source. Analytical expressions for the radial and axial fluxes, the mean energy, and the ion density are derived. An acceptable approximation is obtained from the described model and that reported in the literature from experimental results.
Chain stretching effect on the morphology and kinetics of microphase separation of diblock copolymer under simple shear flow
Authors: Luo Kaifu , Yang Yuliang
Published: July 2002

By incorporating the information of chain conformation into the free-energy calculation, the chain stretching effect on the morphology and kinetics of microphase separation of symmetric diblock copolymers under simple shear flow is investigated based on the time-dependent Ginzburg–Landau model. We first showed that the order–disorder transition (ODT) temperature increases with τ1γ̇, where τ1 is the Rouse terminal relaxation time and γ̇ is the shear rate. The simulation results show that τ1 and γ̇ play the central role in the morphology of microphase separated diblock copolymers. It is found that, for the case without chain stretching effect (τ1γ̇=0), perpendicular (with the lamella normal perpendicular to the flow direction and the velocity gradient direction) and parallel (with the lamella normal parallel to the velocity gradient direction) alignments are obtained for shallow and deep quenches, respectively. However, when the chain stretching effect is strong, i.e., high τ1γ̇, the undulated lamella transverse alignment (with the lamella normal parallel to the flow direction) can be observed. The processes of morphology forming and the lamella undulation are discussed based on the anisotropic growth of the correlation lengths.
Ex situ spectroscopic ellipsometry investigation of the layered structure of polycrystalline diamond thin films grown by electron cyclotron resonance-assisted chemical vapor deposition
Authors: Gupta S. , Weiner B. R. , Morell G.
Published: July 2002

Polycrystalline diamond thin films deposited by electron cyclotron resonance-assisted chemical vapor deposition on Si (111) were investigated using spectroscopic phase-modulated ellipsometry from the near IR to UV range (830–270 nm). Analysis of the raw ellipsometry data [ψ(λi), Δ(λi)] by applying the conventional Bruggeman effective medium theory and linear regression analysis provided details about the film microstructure: (i) the multilayer structure and the component layer thickness of the films; (ii) the volume fraction of the constituents (sp3- and sp2- bonded carbon) and of voids (fv) in the bulk layer (L2); (iii) the inhomogeneity of the structure along the growth axis and its variation with the seeding density; and (iv) the surface roughness layer thickness (dS). A simplified three-layer structural model consisting of an interfacial layer, an intermediate (or bulk) layer, and a top surface roughness layer has been proposed that simulates the ellipsometry data reasonably well. The results obtained through ellipsometry modeling, such as surface roughness and overall film thickness, were compared with those from atomic force microscopy and profilometry, respectively, in order to validate the model employed. Typically, high surface roughness values around 60 nm were found for films grown under different substrate temperatures and oxygen-to-carbon ratios. It was also found that a combination of relatively high substrate temperature and O/C ratio can be used to reduce the surface roughness to around 25 nm. In general, the void fraction (fv) of the bulk layer decreases as a function of seeding density, indicating the formation of a denser film. The sp2-bonded carbon fraction (fsp2 C) also varies with the process parameters. These results (fv and fsp2 C) for the bulk layer and its behavior with respect to process parameters are discussed.
Monte Carlo analysis of the noise behavior in Si bipolar junction transistors and SiGe heterojunction bipolar transistors at radio frequencies
Authors: Martı́n-Martı́nez M. J. , Pérez S. , Pardo D. , González T.
Published: July 2002

We present a comparative analysis of the current spectral densities in a Si bipolar junction transistor (BJT) and a SiGe heterojunction bipolar transistor (HBT) of identical geometry performed by means of an ensemble Monte Carlo simulator self consistently coupled with a two-dimensional Poisson solver. We focus on the physical origin of the different noise sources in the transistors at rf when varying the injection level conditions. At low injection the spectral density of base current fluctuations, SJB(0), is governed by thermal noise related to the base resistance, while the collector spectral density, SJC(0), reaches a typical shot noise response. At high current density the onset of high injection in the base and the base push-out play an important role in the noise behavior of both transistors. Thus, SJC(0) deviates from the typical shot noise response. Hot carrier effects are also present. In the HBT, these effects are less important than in the BJT due to the SiGe/Si heterointerface, and SJB(0) can be neglected in the overall noise analysis because of the Ge content benefits when the structure enters the high-injection regime.
Second-order elastic constants for pentaerythritol tetranitrate single crystals
Authors: Winey J. M. , Gupta Y. M.
Published: July 2002

Existing sound speed measurements for various orientations of pentaerythritol tetranitrate single crystals were reanalyzed to obtain corrected values for the second-order elastic constants. Previous analysis in the literature had resulted in an inconsistency in the elastic constant values. The source of the inconsistency is identified. The corrected analysis restores consistency to the results and yields a significantly different value for the elastic constant c13.
Rate-determining moves in protein folding
Authors: Dimitrievski K. , Kasemo B. , Zhdanov V. P.
Published: July 2002

We present lattice Monte Carlo simulations of protein folding for the three main types of 3-D native protein conformations, including globular structures, bundles of α helices, and β sheets. Scrutinizing the relative role of the end, crankshaft, and corner monomer moves, we show that the latter moves are rate-determining in all the cases.
Double-diamond phase in amphiphilic systems confined between parallel walls
Authors: Babin V. , Ciach A.
Published: July 2002

Effects of confinement on the cubic bicontinuous phase with double-diamond symmetry (D) are studied. We focus on a slit geometry and on highly selective external walls. We show that the period of the solvation force in different systems containing amphiphilic molecules can be found on simple symmetry grounds. The simple symmetry-based predictions are verified by explicit calculations in a lattice model for oil–water–surfactant mixtures. The explicit results obtained within the model allow for a detailed description of the effects of the confinement on the structure of the D phase. The structure of the D phase with the face of the unit cell parallel to the external wall is shown and discussed for slits ranging from very narrow to very large ones.
Application of a one-port dielectric resonator for measurement of the intermodulation in high temperature superconducting films
Authors: Vopilkin E. A. , Panin A. N. , Parafin A. E. , Pavlov S. A.
Published: July 2002

A technique for characterization of intermodulation performance of unpatterned high TC superconducting (HTS) films, based on the application of the resonator with a single coupling probe is proposed. It allows us to sufficiently increase the sensitivity in comparison with the conventional method based on the resonator with two coupling probes. The measurements of the power at mixed frequency as a function of input power are performed for epitaxial HTS films of various surface resistance and thickness. From the experimental data an intermodulation coefficient α is determined, which provides the film intermodulation properties independently on the measuring system.
Microscopic nature of the Er3+ emission in mixed amorphous-nanocrystalline Si:H films
Authors: Aldabergenova S. B. , Strunk H. P. , Taylor P. C. , Andreev A. A.
Published: July 2002

ErO 6 complexes, where every Er3+ ion is surrounded by six oxygen atoms forming an octahedron with C3v point symmetry, are found to best describe the strong Stark splitting of the characteristic Er3+ emission in the 1460–1610 nm range. An a-Si:H matrix serves as an ideal semiconductor host to permit codoped O atoms to form an optimal octahedral ligand field around the Er3+ ions. The observation of sharp intense Stark peaks for temperatures between 153 and 300 °C and of strong enhancement of the Er3+ emission after 350 °C annealing can be understood first by diffusion and then outdiffusion of weakly bound hydrogen atoms in the a-Si matrix.
Insulator issues in alternating-current thin-film electroluminescent devices
Authors: Hitt J. C. , Wager J. F.
Published: July 2002

An n-sheet, state-space model is employed for the assessment of insulator issues of relevance to the operation of alternating-current thin-film electroluminescent (ACTFEL) devices. The model presented herein has been refined by the inclusion of luminance, which is modeled as arising from impact excitation of luminescent impurities. Simulation demonstrates that ACTFEL devices which have large insulator capacitances and no space charge present in the phosphor are optimal in terms of providing maximum luminance and efficiency. A method for estimating the phosphor space charge density from a single transferred charge curve is presented. Simulation indicates that the use of a leaky insulator in an ACTFEL device can result in an increase in luminance, but a decrease in efficiency and that measured charge–voltage and internal charge–phosphor field curves are misleading when one or more of the constituent insulator or phosphor layers is leaky.
Dielectric relaxation and conduction in SrTiO3 thin films under dc bias
Authors: Ang Chen , Yu Zhi , Cross L. E. , Guo Ruyan , Bhalla A. S.
Published: July 2002

The dielectric and conduction behavior of SrTiO3 thin films deposited on a SrTiO3 single-crystal substrate is studied. Without dc bias, an obvious dielectric “defect mode” in the dielectric loss is observed in the temperature range of ∼100–200 K; however, no noticeable corresponding dielectric constant peak is observed. By applying a high dc bias (⩾40 kV/cm), a dielectric constant peak with frequency dispersion appears in the same temperature range, the dielectric loss is increased, and simultaneously high dc conduction is observed. The induced dielectric constant peak is related to dc conduction and attributed to the coupling effect of the mobile carriers with the dielectric defect mode.
Lighting up erbium with water: Observation of substantial crystalline matrix element enhancements
Authors: Thomas G. A. , Bubb D. M. , Koo T.-Y. , Werder D. J. , Cheong S.-W. , Federici J. F.
Published: July 2002

We have found substantial enhancements of the optical matrix elements of Er atoms when water molecules augment the symmetry-breaking electric fields in their crystalline environment. We measure enhancement factors of order 5 for the set of Er energy levels with wavelengths near 1550 nm. These transitions are technologically important because they are used in optical amplifiers for communications.
Multipole response of doped He3 drops
Authors: Garcias Francesca , Serra Llorenç , Casas Montserrat , Barranco Manuel
Published: July 2002

The multipole response of HeN3 drops doped with very attractive impurities, such as a Xe atom or an SF6 molecule, has been investigated in the framework of the Finite-Range Density Functional Theory and the Random Phase Approximation. We show that volume (L=0) and surface (L=1, 2) modes become more fragmented, as compared with the results obtained for pure HeN3 drops. In addition, the dipole mean energy goes smoothly to zero when N increases, indicating that for large N values these impurities are delocalized in the bulk of the drop.
Models for quantitative charge imaging by atomic force microscopy
Authors: Boer Elizabeth A. , Bell L. D. , Brongersma Mark L. , Atwater Harry A.
Published: July 2002

Two models are presented for quantitative charge imaging with an atomic-force microscope. The first is appropriate for noncontact mode and the second for intermittent contact (tapping) mode imaging. Different forms for the contact force are used to demonstrate that quantitative charge imaging is possible without precise knowledge of the contact interaction. From the models, estimates of the best charge sensitivity of an unbiased standard atomic-force microscope cantilever are found to be on the order of a few electrons.
Heterogeneous relaxation in supercooled liquids: A density functional theory analysis
Authors: Murarka Rajesh K. , Bagchi Biman
Published: July 2002

Recent time domain experiments which allow selective study of the relaxation of slower subpopulations among the distributions of local, inhomogeneous regions, have shown the existence of a length scale (∼2–3 nm) beyond which the liquid behaves like a homogeneous liquid. Here we use the density functional theory to calculate the probability of creating a soft localized density fluctuation (density droplet). Theoretical calculation shows that the free energy penalty for creating a local inhomogeneity of small size is much less than that for a large size and that a dense supercooled system is unlikely to sustain inhomogeneity of a length, lf, which is larger than 5σ, where σ is the molecular diameter. We have calculated both the equilibrium and the nonequilibrium (subsequent to photobleaching) orientational correlation functions with the theoretically obtained inhomogeneous distributions. The nonequilibrium distribution relaxes at a slower rate. A simple two state exchange model has been used to mimic the relaxation of the slow regions to equilibrium; the model shows that the diffusional exchange cannot be the mechanism for the extremely slow relaxation process very near to the glass transition temperature. These results have been compared with recent experimental results.
Predictions of the change in extinction coefficient for prolate particles under shear
Authors: Gason Samuel J. , Lester Daniel , Yeow Leong , Boger David V. , Dunstan Dave E.
Published: July 2002

The following study investigates the microstructural changes induced by shear in dilute suspensions of colloidal Fe2O3 rods (aspect ratio=8.4) using rheo-optic techniques. The large extinction coefficient for the hematite system at the wavelength used (λmax=392 nm) enables rheo-optic experiments on truly dilute suspensions to be undertaken. Optical absorbance spectra have been measured over a range of shear rates for unpolarized light and for light polarized relative to the shear direction. The measured absorbance changes may be attributed to the shear-induced anisotropy in the suspension. Numerical methods are used to obtain solutions of the convective-diffusion equation, which are used to predict the measured change in extinction coefficient with shear for colloidal hematite rods measured using optical rheometry.
Estimating equilibrium properties from non-Hamiltonian dynamics
Authors: VandeVondele Joost , Rothlisberger Ursula
Published: July 2002

We derive an expression that enables the accurate estimation of equilibrium properties using non-Hamiltonian dynamics. The major advantage of our scheme is that a time average over a single non-Hamiltonian trajectory can be employed instead of an ensemble average. Hence, it can directly be used in standard molecular dynamics simulations. The connection between non-Hamiltonian dynamics and equilibrium properties is established by assigning to the individual frames of the trajectory a weight that is based on the fluctuations of the phase space compression factor. Additionally, a simple scheme that takes into account only fluctuation of a given maximum duration is introduced to reduce the statistical error. By systematically extending the duration of the allowed fluctuations, increasingly accurate results can be obtained. Non-Hamiltonian dynamics schemes that are capable to enhance sampling efficiency are applied to two model systems in order to demonstrate the practical performance of our approach for the calculation of equilibrium free energy differences and probability density profiles.
Precise ranging by emitting long laser pulses
Authors: Stoyanov D. V. , Mechev I. G. , Naboko V. N.
Published: July 2002

A precise pulsed ranging method using long laser pulses was developed and tested experimentally. It is based on direct analog–digital sampling of both the reference and received pulses and their software processing by digital filtering, pulse shape retrieving, selections, pulse center calculation, and averaging. The method is effective for pulse widths longer than the sampling step, including the case when only one sample exceeds the threshold. A temporal accuracy on the order of 10−2 of the sampling step and better can be realized in the optimal receiving regime. The method offers an approach for the creation of precise ranging instruments by using relatively long laser pulses. This was demonstrated on a 10.6 μm range-finding system emitting 500 ns laser pulses with time instability with respect to the triggering pulses. A ranging accuracy on the order of (3–5) cm was achieved using traditional 10 MHz/8 bit analog/digital converters. The method enlarges the set of prospective laser sources suitable for precise ranging and can be applied in different instruments based on the analog photon detection regime.
Differentiation of biological hydroxyapatite compounds by infrared spectroscopy, x-ray diffraction and extended x-ray absorption fine structure
Authors: Chassot E. , Oudadesse H. , Irigaray J. , Curis E. , Bénazeth S. , Nicolis I.
Published: July 2002

Pure hydroxyapatite (HAP) and HAP doped with 800 ppm of zinc were implanted in cortical bone of femur diaphysis of ovines [J. L. Irigaray et al., Mater. Clin. Appl. 28, 399 (1999)]. We observed that the doped HAP was better resorbed than pure HAP. The first hypothesis is that zinc acts as a stimulator on macrophage cells and improves quantity and quality of osteoblast cells. The second hypothesis is that zinc yields HAP structure that is better resorbed in biological field. For our experiment we used HAP doped with 3000 ppm of zinc in order to have a good sensitivity. In the present work, chemical studies by inductively coupled plasma absorption emission spectrometry, x ray diffraction, and infrared were carried out to determine the composition of major and trace elements in the doped hydroxyapatite, and the crystallographic structure. These studies can indicate possible modifications induced by the insertion of zinc. We used the extended x-ray absorption fine structure experimental station of LURE (Orsay, France) to try to clarify the atomic surroundings of zinc in doped HAP structure and transformations induced in initial lattice. Despite the low zinc concentration, we got good quality fluorescence mode spectra. These spectra showed medium range order of the material that is consistent with its crystalline form. To perform the analysis, we compared the result obtained with another models like β tricalcium phosphate and we created theoretical models of zinc in substitution of calcium in order to reproduce as well as possible the experimental spectrum. After this study, only two models are coherent with experimental spectrum, zinc in substitution of calcium in site I and zinc in the interstice between the two hydroxydes.
DNA mutation detection via fluorescence imaging in a spatial thermal gradient, capillary electrophoresis system
Authors: Crane B. , Hogan C. , Lerman L. , Hunter I. W.
Published: July 2002

To improve the speed and efficiency of genetic screening, we are developing an instrument to detect mutations via a high throughput, automated system. Detection is based on changes in the melting temperature induced by single point mutations as in denaturing gradient gel electrophoresis. This instrument measures the migrating position of test and wild-type molecules in a spatial thermal gradient during capillary electrophoresis. In this work, the concept is described. Important aspects of the design of the instrument are discussed focusing on controlling thermal expansion, creating a stable, predictable temperature gradient, and optimizing software and hardware. Finally, the concept is proved in an example experiment successfully identifying the presence of a mutation in the test sample.
Novel electronic design for double-modulation spectroscopy of semiconductor and semiconductor microstructures
Authors: Ho Ching-Hwa , Hsieh Chang-Hsun , Chen Ying-Jui , Huang Ying-Sheng , Tiong Kwong-Kau
Published: July 2002

The double-modulation reflectance technique has proven to be a powerful characterization tool of semiconductors applied in high-background–light-interference systems. In this article, we present a novel and inexpensive electronic-circuit design for implementing the double-modulation measurements using only one lock-in amplifier. The electronic design is fabricated in an accessory hardware of modulation spectroscopy which can easily select the double- or single-modulation mode via a manual switch. Detailed design diagrams of the electronic hardware are described. Measurements of some representative samples of semiconductors and actual device structures are carried out. Experimental results demonstrated the well-behaved performance of this design.
A triclinic phase of relaxor La-modified Pb(Zr0.65Ti0.35)O3 and its structure at 40 K by high-resolution neutron diffraction
Authors: Liu Hongchao , Harrison Richard , Putnis Andrew
Published: July 2002

The phase transitions and structure of 7% La-modified Pb(Zr0.65Ti0.35)O3 (PLZT 7/65/35) at 40 K were investigated using high-resolution powder diffraction. The high-resolution data suggest the existence of an alternate phase, which is not expected from the widely accepted phase diagram. By structural analysis and Rietveld refinement, the space group of this phase was determined to be F1 (the standard setting is P1 by adding face-centered generators), rather than the expected R3c or the reported Cm phase. The unit cell of this phase is deduced by doubling the primitive cubic cell along three main axes, cell parameters area a=8.14546(13) Å, b=8.18539(15) Å, c=8.16785(11) Å, and interaxial angles are α=90.1925(24)°, β=89.8785(26)°, and γ=90.1564(34)°. The tilting system of oxygen octahedra is āb̄c̄ instead of āāā in the R3c phase. The cation displacements and polarization are described in detail. The polarization direction was found to be along 〈112〉, not the main axes. The phase relationship and domain morphology are discussed in light of this triclinic phase, suggesting that this phase is important to understand the complex phase transitions and domain morphology in this series of materials. The experimental results suggest that this triclinic phase may extend to other compositions and temperature range.
Growth of epitaxial CoSi2 on 6H-SiC(0001)Si
Authors: Platow W. , Nemanich R. J. , Sayers D. E. , Hartman J. D. , Davis R. F.
Published: July 2002

Epitaxial growth of (111)-oriented CoSi2 has been achieved on a scratch-free 6H-SiC(0001)Si substrate. The surface was prepared using atmospheric hydrogen etching and ultrahigh vacuum Si cleaning. A high-quality CoSi2 thin film was obtained by a modified template method and co-deposition of Co and Si at 550 °C. The structure and morphology of the film is studied by means of reflection high electron energy diffraction, x-ray absorption fine structure, x-ray diffraction, and atomic force microscopy.
Modifications of c-Si/a-Si:H/indium tin oxide heterostructures upon thermal annealing
Authors: Losurdo M. , Giangregorio M. , Capezzuto P. , Bruno G. , Varsano F. , Tucci M. , Roca F.
Published: July 2002

c- Si /a- Si:H / indium tin oxide (ITO) heterojunctions have been prepared by electron-beam deposition of an (ITO) thin film on a plasma enhanced chemical vapor deposition grown c-Si/a-Si:H heterojunction. These heterostructures, which are the basis of solar cells, have been annealed in N2 atmosphere at temperatures in the range 250–650 °C. Thermal annealing effects on structural and optical properties of the ITO, the a-Si:H layer, and of the c-Si/a-Si interface have been detected by spectroscopic ellipsometry. The optical response of ITO is described in the energy range 1.5–5.0 eV, where a high transparency is required for ITO, by analyzing ellipsometric spectra in terms of a model which combines the Drude model and a double Lorentzian oscillator. Spectroscopic ellipsometry has shown that annealing at T>450 °C causes hydrogen out-diffusion from the a-Si:H layer into the ITO layer whose optical and electrical properties are modified. Additionally, damage of the c-Si/a-Si interface and of the ITO layer by hydrogen diffusion is detected and seen as a factor affecting performance of c-Si/a-Si/ITO stacked structure based solar cells. X-ray photoelectron spectroscopy and atomic force microscopy measurements have corroborated ellipsometric analysis.
Spectral density, memory function, and mean relaxation time for resonant subsystem-reservoir interactions
Authors: Wilkie Joshua
Published: July 2002

We show that the projection operators P usually employed to derive the Nakajima–Zwanzig master equation are non-Hermitian. As a consequence the operator QLQ which governs the dynamics of subsystem-reservoir interactions (here Q=1−P and L is the Liouville operator) is also non-Hermitian and possesses a complex temperature-dependent spectrum of resonances. We use the self-consistent Born approximation to derive a formula for the spectral density of QLQ. From this spectral density we calculate the memory function and mean relaxation time of subsystem-reservoir correlations. These formulas are compared with numerical results obtained from the spin–spin-bath model to test their accuracy. The memory function and mean relaxation time are essential ingredients for the SRA (statistical resonance approximation) theory of microscopic dissipation and decoherence. We show that the SRA master equation preserves positivity when the memory function derived in this paper is employed.
Numerical simulation of laser induced plasma during pulsed laser deposition
Authors: Zhang Zhaoyan , Han Zhen-Xue , Dulikravich George S.
Published: July 2002

A numerical study of the laser induced evaporation and ionization process during pulsed laser deposition is presented. The process is separated into three domains: (i) conduction inside the solid, (ii) a discontinuity layer between solid and vapor, and (iii) expansion of high temperature vapor/plasma. A quasi-one-dimensional model is solved to predict the temperature field inside the solid. Mass, momentum, and energy are conserved across the discontinuity layer. Equations of mass, momentum, and energy conservation are solved simultaneously to provide boundary conditions for the expansion process. Euler equations are used to model the expansion of high temperature vapor/plasma. The Euler equations are integrated numerically using a Runge–Kutta scheme combined with flux vector splitting. The density, pressure, temperature, and velocity contours of the vapor phase are calculated and the results are analyzed.
π-electronic and electrical transport properties of conjugated polymer nanocomposites: Poly(p-phenylenevinylene) with homogeneously dispersed silica nanoparticles
Authors: Ho Peter K. H. , Friend Richard H.
Published: July 2002

The π-electronic and hole-transport properties of homogeneous composites of poly(p-phenylenevinylene) (PPV) with 5 nm diam colloidal silica have been characterized. When the interparticle distance becomes comparable to or shorter than the coherence distance of the polymer chains, the intrachain and interchain order in these nanocomposites gets disrupted. This disruption is reflected in a reduction of the mean conjugation length 〈Lc〉 and a broadening of the Lc distribution. These parameters may be estimated through a combination of optical absorption, Raman scattering, and fluorescence spectroscopies. The optical 〈Lc〉 measured at absorption band maximum decreases from 6.5 repeat units in neat PPV to 4.9 units in the 50 vol % composite. The Raman 〈Lc〉 measured with 633 nm excitation correspondingly decreases from 7 units to 5.6 units while the fluorescence 〈Lc〉 deduced from the 0–0 molecular transition remains nearly constant at 10–11 units. Therefore the bulk of the Lc distribution shifts by a small fraction toward shorter conjugation while retaining a tail of long conjugation segments thereby causing the distribution width to increase. This indicates PPV has a remarkable propensity to adopt extended conformations around the nanoparticles so that intrachain π-electron delocalization is only slightly effected. However, the electrical transport characteristics are strongly modified. The zero-field hole mobility is decreased by 1–2 orders of magnitude and its field activation increased by a factor of 2–3, even at 3 vol % particle loading. X-ray photoelectron spectroscopy and infrared spectroscopy rule out any increase in the concentration of chemical defects. Therefore the loss of mobility may be related to roughening of the hopping energy landscape. This reduction in electrical conductivity however can be mitigated through controlled chemical doping of the PPV chains. Interesting properties can thus be obtained by careful design of conjugated polymer–nanoparticle composites.
Structural and physical properties of Ag doped poly(dimethylsiloxane) modified silica xerogels
Authors: Chakrabarti K. , Whang C. M.
Published: July 2002

The purpose of the present work is to study the effect of silver (Ag) incorporation on the structural and physical properties of poly(dimethylsiloxane) (PDMS) modified silica xerogels. PDMS was chosen as the organic component because of its similarity of backbone structure (–Si–O–) with tetraethyl orthosilicate, the inorganic component of organically modified silicates. In the present work silver was incorporated by two routes: (i) directly in the sol and (ii) silver doping was done after preformed porous matrix was obtained. The microstructures, particle size distribution, infrared spectra, and optical coloration have been studied. The utility of two methods has been discussed. It was found that the particle size was much smaller compared with that of classical gels with dispersed Ag particles. For the indirect method of silver doping the particle size distribution was found to follow log-normal distribution function. Upon exposure to humid air the samples change optical state from transparency to opacity. However, this was found to be reversible when treated above 300 °C. This phenomenon may be utilized to fabricate humidity sensor and optical switches.
Physical phenomena in a coplanar macroscopic plasma display cell. II. Comparisons between experiments and models
Authors: Ganter R. , Ouyang J. , Callegari Th. , Boeuf J. P.
Published: July 2002

Measurements of infrared and visible emission in a macroscopic discharge cell similar to a plasma display panel (PDP) cell are analyzed using a two-dimensional fluid model of the discharge. The comparisons between experiments and models show a good qualitative agreement but the plasma spreading velocity above the cathode surface is much faster in the experiments. We find that including photoemission (in a simplified way) in the model considerably increases the agreement between experiments and models. With a well-chosen photoemission coefficient, the model reproduces the trends observed in the experiments when the gas mixture (between 2% and 10% of xenon in neon) or the applied voltage is changed. The influence of photoemission on the current rise time and on the velocity of plasma spreading above the dielectric surfaces is more important in the macrocell than in a similar (same dimension × pressure) PDP cell because resonant photon transport does not follow the similarity laws.
Model for Schottky barrier and surface states in nanostructured n-type semiconductors
Authors: Malagù C. , Guidi V. , Stefancich M. , Carotta M. C. , Martinelli G.
Published: July 2002

A semiclassical model for Schottky contacts to be applied to nanosized polycrystalline n-type semiconductors was developed. To this purpose we determined the density of surface states as a function of the mean grain radius, which establishes the Schottky barrier height. The intergranular potential shape was investigated in depletion approximation under spherical geometry and a critical revision of this method was proposed. The model was then extended to also include nanostructured materials, which could not be considered in the previous approach. Thus we were able to explain the flattening of the band bending and the decrease in the surface state density, which are experimentally observed when the granulometry is very fine.