bluuebunny/crossref_metadata_2025 huggingface dataset:
Microstructure and dielectric parameters of epitaxial SrRuO3/BaTiO3/SrRuO3 heterostructures
Authors: Boikov Yu. A. , Claeson T.
Published: July 2002

Epitaxial films of ferroelectric barium titanate are desirable in a number of applications but their properties are inferior to those of bulk material. Relations between microstructure and dielectric properties may give better understanding of limitations. Trilayer heterostructures SrRuO3/BaTiO3/SrRuO3 were grown by laser ablation on (100)LaAlO3 and (100)MgO substrates. The BaTiO3 layer was granular in structure. When grown on (100)SrRuO3/(100)LaAlO3, it was preferentially a-axis oriented due to tensile mechanical stress. Using (100)MgO as a substrate, on the other hand, produced a mixture of about equal value of a-axis and c-axis oriented grains of BaTiO3. The dielectric permittivity, ε, of the BaTiO3 layer was almost twice as large, at T>200 K and f=100 kHz, for the LaAlO3 substrate as compared to the MgO one. Its maximum value (ε/ε0≈6200) depended on temperature of growth, grain size, and electric field and compares well with optimal values commonly used for ceramic material. The maximum in the ε(T) shifted from about 370 to 320 K when the grain size in the BaTiO3 film decreased from 100 to 40 nm. At T<300 K, hysteresis loops in polarization versus electric field were roughly symmetric. The BaTiO3 films grown on (100)SrRuO3/(100)MgO exhibit the largest remnant polarizations and coercive fields in the temperature range 100–380 K.
A three-dimensional statistical mechanical model of folding double-stranded chain molecules
Authors: Zhang Wenbing , Chen Shi-Jie
Published: July 2002

Based on a graphical representation of intrachain contacts, we have developed a new three-dimensional model for the statistical mechanics of double-stranded chain molecules. The theory has been tested and validated for the cubic lattice chain conformations. The statistical mechanical model can be applied to the equilibrium folding thermodynamics of a large class of chain molecules, including protein β-hairpin conformations and RNA secondary structures. The application of a previously developed two-dimensional model to RNA secondary structure folding thermodynamics generally overestimates the breadth of the melting curves [S-J. Chen and K. A. Dill, Proc. Natl. Acad. Sci. U.S.A. 97, 646 (2000)], suggesting an underestimation for the sharpness of the conformational transitions. In this work, we show that the new three-dimensional model gives much sharper melting curves than the two-dimensional model. We believe that the new three-dimensional model may give much improved predictions for the thermodynamic properties of RNA conformational changes than the previous two-dimensional model.
Transmission electron microscopy studies of the effect of A-site cation size mismatch and disorder on charge ordering behavior in (La1−xYx)0.5(Ca1−ySry)0.5MnO3
Authors: Wang Y. Q. , Duan X. F. , Wang Z. H. , Shen B. G.
Published: July 2002

The effects of A-site cation mismatch and disorder on the charge ordering (CO) behavior in the manganites (La1−xYx)0.5(Ca1−ySry)0.5MnO3 have been studied by transmission electron microscopy. The presence of the size mismatch and disorder suppresses the CO transition. Incommensurate CO modulations are observed in three samples with 0⩽σ2⩽0.003. Structural models, based on the selected area electron diffraction and high-resolution electron microscopy observation, are suggested for such kinds of incommensurate modulations.
Correlation between dark spot growth and pinhole size in organic light-emitting diodes
Authors: Lim Shuang Fang , Ke Lin , Wang Wei , Chua Soo Jin
Published: July 2002

Our in situ experimental observations of dark spot growth in organic light-emitting diodes using optical microscopy show a linear rate of growth for the area of all the dark spots. We used uniformly sized silica micro particles to intentionally create size-controllable pinholes on the cathode protective layer. Subsequently, we observed initial formation of dark spots as a result of these pinholes and then monitored their growth. Due to usage of particles of various diameters, we were able to linearly correlate the growth rate with pinhole size. This allows us to estimate the original pinhole sizes that gave rise to the dark spots, which we believe were initiated by “dust” particles. Our studies verify that dark spot formation is due to pinholes on the protective layer that creates pathways for water or oxygen permeation, and that dark spot growth is dependent on the pinhole sizes.
Results from helical axis stellarators
Authors: Blackwell B. D.
Published: July 2002

Helical axis stellarators produce magnetic surfaces of high rotational transform and moderate shear solely by means of external currents, with the promise of high β. Several machines with quite different toroidal, helical, and “bumpy” Fourier components of magnetic field are now producing results, including high quality magnetic surfaces, confinement mode transitions, configuration studies, and confinement consistent with International Stellarator Scaling (ISS95) scaling up to Te∼2 keV. These devices permit concept evaluation over an unprecedented configuration space, and allow basic comparisons with those designed by established and alternative optimization strategies.
Vibrational spectra of ammonia clusters from n=3 to 18
Authors: Beu Titus A. , Buck Udo
Published: July 2002

We have calculated the vibrational spectra of the umbrella (ν2), the symmetric (ν1), and the asymmetric (ν3) N–H stretch mode for ammonia clusters from n=3–18. The results are based on recent structure calculations and a molecular perturbation approach that includes the anharmonicities. Clusters with high symmetry exhibit few lines only and show the expected blue shifts for the umbrella and red shifts for the two N–H stretch modes. The calculated frequencies of the umbrella mode agree very well with experimental results for n=2–5 as far as the general shape is concerned, but overestimate the absolute shifts.
Stochastic diffusion of energetic ions in optimized stellarators
Authors: Beidler C. D. , Kolesnichenko Ya. I. , Marchenko V. S. , Sidorenko I. N. , Wobig H.
Published: July 2002

It is shown that the collisionless transformation of locally trapped and passing particle orbits in the optimized stellarators of the Wendelstein line results in stochastic diffusion of energetic ions. This diffusion can lead to the loss of a significant fraction of the energetic ion population from the region where the characteristic diffusion time is small compared to the slowing down time. The loss region and losses can be minimized by shaping the plasma temperature and density profiles so that they satisfy certain requirements. The predictions of the theory developed here are in agreement with the results of numerical modeling of α-particle confinement in a Helias reactor, which has been carried out with the use of an orbit following code.
Nonequilibrium structure and rheology of concentrated colloidal suspensions: Linear response
Authors: Szamel Grzegorz
Published: July 2002

We propose a simple theory for the pair distribution function of a sheared colloidal suspension. The starting point is an exact equation for the stationary state pair distribution. We assume that in a shear flow the higher order distributions can be expressed through the pair distribution in the same way as in an equilibrium state. To get explicit results we use a simple decoupling approximation for the higher order equilibrium correlations. For moderate volume fractions our theory predicts low-shear viscosity that agrees well with Brownian Dynamics simulations results. Low-shear distortion of the pair structure is underestimated by the theory but its shape agrees qualitatively with the simulations.
Dielectric loss and defect mode of SrTiO3 thin films under direct-current bias
Authors: Ang Chen , Cross L. E. , Yu Zhi , Guo Ruyan , Bhalla A. S. , Hao Jian Hua
Published: July 2002

The dielectric behavior of SrTiO3 thin films prepared by the pulsed-laser deposition technique on SrTiO3 single-crystal substrates is studied under dc electric field. A high dielectric constant maximum εmax(∼2280) and a low-loss tan δ (∼0.001) are obtained. Compared with the observation in SrTiO3 single crystals, an additional dielectric loss peak with frequency dispersion is observed around 150 K (at 1 kHz). With increasing dc bias, the peak is suppressed and finally disappears at ∼350 kV/cm; however, the temperature at which the peak occurs is independent of electric field. The possible physical mechanism of the peak is briefly discussed.
Accelerating the calculation of energy levels and wave functions using an efficient preconditioner with the inexact spectral transform method
Authors: Poirier Bill , Carrington Tucker
Published: July 2002

In an earlier paper [J. Chem. Phys. 112, 8765 (2000)] our group introduced a preconditioned inexact spectral transform method for calculating energy levels and wave functions. Although we could calculate high-lying levels with far fewer matrix–vector products than with the filter diagonalization method of Mandelshtam and Taylor, even better performance can be achieved with a better preconditioner. In this paper, we develop an extremely efficient preconditioner consisting of two components: (1) transformation to an optimal separable basis, in which off-diagonal elements of the Hamiltonian matrix are minimized; and (2) removal of all off-diagonal coupling near the energies of interest. The new preconditioner works extremely well; it enables us to calculate high-lying vibrational states of H2O with orders of magnitude fewer matrix–vector products than for all other known methods. The new preconditioner should also accelerate the calculation of other quantities, such as photodissociation cross sections and rate constants.
Statistical equilibrium theory for axisymmetric flows: Kelvin’s variational principle and an explanation for the vortex ring pinch-off process
Authors: Mohseni Kamran
Published: July 2002

Thermodynamics of vorticity density fields (ω/r) in axisymmetric flows are considered, and the statistical equilibrium theories of Miller, Weichman, and Cross [Phys. Rev. A 45, 2328 (1992)], Robert and Sommeria [J. Fluid Mech. 229, 291 (1991)], and Turkington [Comm. Pure Appl. Math. 52, 781 (1999)] for the two-dimensional flows in Cartesian coordinates are extended to axisymmetric flows. It is shown that the statistical equilibrium of an axisymmetric flow is the state that maximizes an entropy functional with some constraints on the invariants of motion. A consequence of this argument is that only the linear functionals of vorticity density, e.g., energy and total circulation, are conserved during the evolution of an axisymmetric inviscid flow to the statistical equilibrium. Furthermore, it is shown that the final equilibrium state satisfies Kelvin’s variational principle; the mean field profiles maximize the energy compatible with the resulting dressed vorticity density. Finally, the vortex ring pinch-off process is explained through statistical equilibrium theories. It appears that only a few invariants of motion (the kinetic energy, total circulation, and impulse) are important in the pinch-off process, and the higher enstrophy densities do not play a significant role in this process.
Laser-induced fluorescence of nascent CH from ultraviolet photodissociation of HCCO and the absolute rate coefficient of the HCCO+O2 reaction over the range T=296–839 K
Authors: Carl Shaun A. , Sun Qian , Peeters Jozef
Published: July 2002

The absolute rate coefficient of the gas-phase reaction HCCO+O2 was determined over the temperature range 296–839 K and at a pressure 7±1 Torr helium. The experiments were performed in a slow-flow kinetic apparatus employing pulsed photolysis of CH2CO at 193 nm as a source of HCCO radicals. Reaction time profiles of [HCCO] were constructed using a newly developed, sensitive spectroscopic technique in the visible spectral region to detect this radical: laser—induced fluorescence of nascent CH(X 2Π) photofragments following HCCO photodissociation at 266 nm. Photodissociation of HCCO at this wavelength was found to produce rotationally excited CH(X) populated to N″⩾26. The rate coefficient for the title reaction was found to be described by k(T)(HCCO+O2)=(2.6±0.3)×10−12 exp[−(325±80)K/T] cm3 s−1 molecule−1 (2σ errors). The absorption cross section of HCCO at 266 nm, σHCCO(266 nm), was also determined relative to that of CH2CO at 193 nm as σHCCO(266 nm)=0.07−0.05+0.20σCH2CO(193 nm).
HgCdTe photodetectors with negative luminescent efficiencies >80%
Authors: Bewley W. W. , Jurkovic M. J. , Felix C. L. , Lindle J. R. , Vurgaftman I. , Meyer J. R. , Aifer E. H. , Butler J. E. , Tobin S. P. , Norton P. W. , Hutchins M. A.
Published: July 2002

We have characterized the negative luminescent properties of photovoltaic HgCdTe detector structures with a room-temperature cutoff of 4.2 μm. Using an optical modulation method to directly measure the emittance as a function of applied bias and temperature, the blackbody signal at 295 K is found to be suppressed by a factor of 5.6. The negative luminescence efficiency of 82% and the apparent blackbody temperature change of ≈−35 K are nearly independent of heatsink temperature over the range 265–305 K.
The effect of the cooling rate on the fictive temperature in some model glassy systems
Authors: Halpern V. , Bisquert J.
Published: July 2002

In order to investigate the importance of cooperative effects in the cooling of systems into a supercooled liquid or glassy state, a kinetic Ising model is considered with all transition rates thermally activated, so that cooling can lead to the system being trapped in a long-lived nonequilibrium glassy state. A difference in energy between the states with up and down spin permits the definition of a fictive temperature Tf that defines the state of the system, and in this paper the effects of different cooling rates on the evolution of Tf are examined. Two types of model system are considered, one in which the transitions of the different spins are independent and the other in which they need to be facilitated by the presence of adjacent up spins (the Fredrickson–Andersen model), so that cooperative effects are involved. It is found that the requirement of facilitation not only slows down the transition rates but also affects the apparent activation energy deduced from the variation of the final fictive temperature with the cooling rate, and a theoretical analysis of the results is presented.
DNA-mediated electrostatic assembly of gold nanoparticles into linear arrays by a simple drop-coating procedure
Authors: Sastry Murali , Kumar Ashavani , Datar Suvarna , Dharmadhikari C. V. , Ganesh Krishna N.
Published: July 2002

The assembly of nanoparticles in topologically predefined superstructures is an important problem in the area of nanoscale architecture. In this letter, we demonstrate the electrostatic assembly of lysine-capped colloidal gold particles on drop-coated DNA films. Electrostatic interaction between the positive charges on the gold nanoparticles and the negative charges on the phosphate groups of the DNA template molecules leads to the assembly of the gold nanoparticles in linear superstructures. The use of DNA as templates for the assembly of nanoparticles shows promise for extension to more complex geometries through rational design of the DNA base sequences as well as in the realization of nanowires by stringing together metal nanoparticles.
Quadratic Brownian-oscillator model for solvation dynamics in optical response
Authors: Chernyak Vladimir , Mukamel Shaul
Published: July 2002

The optical response of a chromophore coupled to a collective Gaussian solvation coordinate with a different curvature of the ground- and excited-state surfaces is studied. Exact expressions are derived for the time-dependent Stokes shift assuming an arbitrary solvent spectral density. Deviations from Onsager’s linear fluctuation–dissipation relations between the Stokes shift and equilibrium fluctuations are discussed.
Modifications of human lymphoblastoid cells induced by low-frequency magnetic field: A three-dimensional atomic-force microscopy study
Authors: Girasole M. , Cricenti A. , Generosi R. , Congiu-Castellano A. , Pozzi D. , Pasquali E. , Lisi A. , Grimaldi S.
Published: July 2002

Air operating atomic force microscopy has been used to study human B lymphoblastoid (Raji) cells exposed to a 50 Hz 2 mT sinusoidal magnetic field (MF), up to 64 h. A statistical analysis performed on mean height and membrane roughness of the cells showed a loss of microvilli and several modifications of the overall shape and membrane surface at increasing MF exposure.
Critical conditions for structure formation in semidilute polymer solutions induced under continuous shear flow
Authors: Saito Shin , Hashimoto Takeji
Published: July 2002

Critical conditions for shear-induced structure formation in semidilute polystyrene solutions with dioctyl phthalate as a solvent are studied by light scattering and rheological measurement. The critical shear rate γ̇cx for the onset of the nonlinear enhancement (defined in the text) of the concentration fluctuations are investigated as functions of temperature and concentration. γ̇cx increases with increasing temperature or decreasing concentration, reflecting the temperature and concentration dependence of the thermodynamic and viscoelastic properties. This temperature and concentration dependence of γ̇cx was found to be in parallel to that of the critical shear rate for the onset of the linear enhancement (also defined in the text) of the concentration fluctuations, which can be explained in terms of a linear theory proposed by Onuki. The critical shear stress and the critical first normal stress difference corresponding to γ̇cx are also explored, the physical interpretation of which will be discussed in detail in conjunction with the stress components arising from anisotropic concentration fluctuations, based on the calculation by van Egmond.
Large eddy simulation of decaying magnetohydrodynamic turbulence with dynamic subgrid-modeling
Authors: Agullo O. , Müller W.-C. , Knaepen B. , Carati D.
Published: July 2002

The numerical large eddy simulation (LES) technique is tested on decaying magnetohydrodynamic (MHD) turbulence. The LES approach allows for a strong reduction in computational cost compared to direct numerical simulations by modeling the effects of the smallest turbulent scales instead of computing them directly. Two small-scale models of eddy-viscosity type are presented for this purpose in combination with a procedure for the self-consistent calculation of the model parameters in the course of the simulation. The method is successfully tested by comparing the obtained results to a high-resolution direct numerical simulation of decaying three-dimensional MHD turbulence.
Absorption and emission spectroscopy of matrix-isolated benzo[g,h,i]perylene: An experimental and theoretical study for astrochemical applications
Authors: Chillier Xavier , Boulet Pascal , Chermette Henry , Salama Farid , Weber Jacques
Published: July 2002

The absorption and emission spectra of benzo[g,h,i]perylene, a six ring polycyclic aromatic hydrocarbon molecule (C22H12), embedded in a rare gas matrix are reported. Time dependent emission shows that this molecule exhibits sharp phosphorescence in the red. Supporting theoretical calculations using the recently developed time-dependent density-functional response theory formalism (TD–DFRT) allow a tentative assignment for the observed transitions. The astrochemical significance of the results is briefly discussed.
Time-dependent quantum wave packet study of the C+CH reaction
Authors: Tang Bi-Yu , Chen Mao-Du , Han Ke-Li , Zhang John Z. H.
Published: July 2002

The quantum dynamics of the C+CH reaction has been studied by means of time-dependent quantum wave packet calculation on 1A′ potential energy surface. Initial state-specific total reaction probabilities and integral cross section are calculated, and the rate constants obtained are in reasonably good agreement with experimental measurement. Although a deep well is present in the potential surface, no long-lived resonances are found from the dynamics calculation and the reaction proceeds by a direct mechanism, similar to that of O(1D)+H2 reaction. Stereodynamics study of the reaction is carried out by transforming the scattering result to stereodynamics (SD) representation at zero total angular momentum. The SD result shows that the C+CH reaction proceeds from almost all attack angles with a preference for attacking at the C side of CH (molecule or radical) by the carbon atom.
Mass analyzed threshold ionization of deuterium substituted isotopomers of aniline and p-fluoroaniline: Isotope effect and site-specific electronic transition
Authors: Lin Jung Lee , Tzeng Wen Bih
Published: July 2002

Two-color resonant two-photon mass analyzed threshold ionization (MATI) spectroscopy was used to record the threshold ion spectra of deuterium-substituted isotopomers of aniline and p-fluoroaniline. The respective adiabatic ionization energies of C6H5NH2, C6H5NHD, C6H5ND2, C6D5NH2, C6D5NHD, C6D5ND2, p-FC6H4NH2, p-FC6H4NHD, and p-FC6H4ND2 were determined to be 62 271, 62 253, 62 233, 62 258, 62 237, 62 214, 62 543, 62 520, and 62 507 cm−1 with an uncertainty of about 5 cm−1. Analyses on the shifts in the electronic transition and ionization energies prove that the S1←S0 transition mainly occurs around the aromatic ring whereas the transition from the neutral S1 to the cationic state corresponds to the removal of one of the lone-pair electrons of nitrogen. The present results provide the first experimental evidence for the site-specific electronic transition in aniline as well as the isotope effects on the vibrations of the aniline cation. These findings are well supported by ab initio and density functional calculations.
Study of the effect of grain boundary migration on hillock formation in Al thin films
Authors: Kim Deok-kee , Nix William D. , Vinci Richard P. , Deal Michael D. , Plummer James D.
Published: July 2002

We have studied the effect of grain boundary migration on hillock formation in unpassivated Al thin films during thermal cycling. Hillocking occurs more frequently in Al films that experience grain growth during thermal cycling than in films with stabilized grain structures. The hillocking frequency is at least four times greater in the films that experience grain growth, as judged by the number of hillocks observed per initial grain boundary triple junction. This latter measure takes account of the smaller initial grain size in the film that experiences grain growth and shows that grain boundary migration itself must enhance the hillocking frequency.
Image charge forces inside conducting boundaries
Authors: Tinkle Mark D. , Barlow S. E.
Published: July 2002

The common description of the electrostatic force, F(x)=−q∇φ(x), provides an incomplete description of the force on the charge q at a point x when the charge itself induces additional fields, e.g., image charges, polarizations, etc. The equation may be corrected through the introduction of a “pseudopotential” formalism. Exploration of some of the elementary properties of the pseudopotential demonstrates its essential simplicity. This simplicity allows it to be incorporated directly into dynamics calculations. We explicitly evaluate the pseudopotential in a number of simple but important cases including the sphere, parallel plates, the rectangular prism, and the cylindrical box. The pseudopotential formalism may be expanded to include extended charge distributions; in this latter form we are able to directly apply the results to experimental measurements.
Atomic-scale characterization of a Co/AlOx/Co magnetic tunnel junction by scanning transmission electron microscopy
Authors: Plisch M. J. , Chang J. L. , Silcox J. , Buhrman R. A.
Published: July 2002

Analytical electron microscopy has been employed to characterize the localized physical and electronic structure of a Co/AlOx/Co magnetic tunnel junction. The tunnel barrier is amorphous alumina with an extensive conduction band tail due to disorder. Both barrier edges are Al terminated and an Al-rich layer exists at the bottom Co/AlOx interface. sp-d hybridization between interfacial Co and Al atoms is observed and it is likely that the interfacial Al is metallic. All of these features are expected to be important to the magnetoresistance behavior of the junction.
Predicting chaotic dispersion with Eulerian symmetry measures: Wavy Taylor-vortex flow
Authors: King G. P. , Rowlands G. , Rudman Murray , Yannacopoulos A. N.
Published: July 2002

In a recent investigation of particle transport in numerically computed wavy Taylor-vortex flow, Rudman estimated an effective axial diffusion coefficient, Dz, to characterize the enhanced mixing due to chaotic advection [AIChE J. 44, 1015 (1998)]. We find that Dz is proportional to the product of two measures of symmetry deviation. The first is a measure of the average deviation of the flow from rotational symmetry, and the second is a measure of the average deviation from flexion-free flow (a flow where the curl of the vorticity is zero). Because these quantities are obtained directly from the velocity field, we call them Eulerian symmetry measures. Thus, we show that the macroscopic transport behavior in a flow can be quantified directly in terms of the velocity field and its gradients, and hence provides a connection between Eulerian and Lagrangian pictures of transport—a problem of fundamental and widespread interest.
Modeling a collision-dominated space-charge sheath in high-pressure arc discharges
Authors: Benilov M. S. , Coulombe S.
Published: July 2002

A model of collision-dominated space-charge sheaths is developed using the approximation of mobility-controlled ion motion. Results of numerical and asymptotic solutions are given. The asymptotic solution has exponential accuracy with respect to the ratio sheath voltage/electron temperature and provides for near-cathode sheaths results which can be considered as practically exact. On the basis of the asymptotic analysis, a simple exponential-accuracy analytical model of the ion layer is given. Voltage drops in different zones (the ionization layer, the ion-electron layer, and the ion layer) are identified. The voltage drop in the ion-electron layer is found to be quite close to kTe/e. Results of numerical integration for conditions typical for high-intensity discharge lamps are presented.
Interaction potentials of LiH, NaH, KH, RbH, and CsH
Authors: Geum N. , Jeung G.-H. , Derevianko A. , Côté R. , Dalgarno A.
Published: July 2002

Quantum-mechanical calculations of the potential energy curves of the singlet and triplet states of LiH, NaH, KH, RbH, and CsH formed by the approach of ground state alkali–metal atoms and hydrogen atoms are presented. Precise values are determined for the coefficients of the van der Waals interaction and estimates are made of the contribution of the exchange interaction at large distances. Together with empirical data, they are used to assess and improve the accuracy of the ab initio potentials.
High temperature x-ray diffraction studies on antiferroelectric and ferroelectric phase transitions in (Pb1−xBax)ZrO3 (x=0.05,0.10)
Authors: Pokharel Bhadra P. , Pandey Dhananjai
Published: July 2002

We have carried out high temperature x-ray diffraction studies on (Pb1−xBax)ZrO3(PBZ) to correlate the large thermal hysteresis (∼100 °C for x=0.05) and irreversibility (for x=0.10) of the antiferroelectric (AFE)–ferroelectric (FE) phase transition observed in dielectric measurements with structural changes. It is shown that for both the compositions, the sequence of phase transitions during heating is orthorhombic antiferroelectric (AO) to rhombohedral ferroelectric (FR) and then to cubic paraelectric (PC). The wide phase coexistence region (∼80 °C for x=0.05 and ∼160 °C for x=0.10) and the arrest of the FR to AO transition for x=0.10 during cooling strongly indicate first order character of the AO–FR transition. It is shown that the transformation strains associated with the AO to FR transition increases with Ba2+ concentration from a value of 0.6% for x=0 to 0.9% for 0.10. Similarities of the AO–FR transition in PBZ with nonthermoelastic martensitic transformations are pointed out. The FR to PC transition is also shown to be first order but with a small thermal hysteresis (∼10 °C) and a small discontinuous change in the cell volume (∼0.5%).
Depth distributions of hydrogen and intrinsic stress in a-Si:H films prepared from hydrogen-diluted silane
Authors: Danesh P. , Pantchev B. , Grambole D. , Schmidt B.
Published: July 2002

The thickness dependencies and depth distributions of hydrogen and intrinsic mechanical stress are studied for a-Si:H films prepared with 10% silane in hydrogen. Nuclear reaction analysis has been used to establish the total concentration of the incorporated hydrogen. It has been shown that the hydrogen distribution in the films is uniform and does not depend on the film thickness. On the contrary, the intrinsic stress depends on the film thickness and has a nonuniform depth distribution, as the stress increase linearly in the direction from the substrate/film interface to the film surface. The obtained results are discussed in view of the hydrogen-related processes and structural improvement of the silicon network during the film growth.
Hard sphere perturbation theory for thermodynamics of soft-sphere model liquid
Authors: Mon K. K.
Published: July 2002

It is a long-standing consensus in the literature that hard sphere perturbation theory (HSPT) is not accurate for dense soft sphere model liquids, interacting with repulsive r−n pair potentials for small n. In this paper, we show that if the intrinsic error of HSPT for soft sphere model liquids is accounted for, then this is not completely true. We present results for n=4, 6, 9, 12 which indicate that, even first order variational HSPT can provide free energy upper bounds to within a few percent at densities near freezing when corrected for the intrinsic error of the HSPT.
Structural study of a thermally diffused Al/Ni bilayer via x-ray absorption spectroscopy and x-ray photoelectron spectroscopy
Authors: Chevrier France , Delobbe Anne , Traverse Agnés , Zanghi Didier
Published: July 2002

Ni thin films deposited on (111) Al thick samples have been submitted to annealing treatment. The goal is to identify the phases formed in the early stage of mixing between Ni and Al. Combining complementary techniques such as x-ray absorption spectroscopy and x-ray photoelectron spectroscopy, we characterized two different systems depending on the temperature conditions. For a thermal treatment at 110 °C, a system formed of small AlNi3 clusters embedded in the Al matrix was identified. This was interpreted as due to the existence of a spontaneous Al/Ni interface mixed layer acting as a seed for the AlNi3 cluster formation. After annealing at 300 °C, the Al3Ni intermetallic compound was detected, in agreement with results in the literature.
Effect of S and O on the growth of chemical-vapor deposition diamond (100) surfaces
Authors: Tamura Hiroyuki , Zhou Hui , Takami Seiichi , Kubo Momoji , Miyamoto Akira , N.-Gamo Mikka , Ando Toshihiro
Published: July 2002

Sulfur and oxygen are known to improve the crystal quality of the chemical vapor deposition (CVD) diamond. In the CVD process, the sulfur is incorporated into the diamond crystal, while the oxygen is not incorporated. In the present study, first-principle calculations have been performed to investigate the effect of sulfur and oxygen on the growth mechanisms of CVD diamond (100) surfaces. The S and O atoms in the vapor are spontaneously inserted into the dimer bond on the diamond (100). The S and O insertions induce a compressive stress along the dimer bond and enhance the CH2 bridging across the trough. In the case of oxygen, the on-top C=O structure is spontaneously formed and it is considered to be desorbed from the surface during the CVD process. The S atom is considered to be incorporated into the diamond (100) lattice via the formation of 3-and 4-coordinated S on the surface.
Impact of texture-enhanced transmission on high-efficiency surface-textured light-emitting diodes
Authors: Windisch R. , Rooman C. , Meinlschmidt S. , Kiesel P. , Zipperer D. , Döhler G. H. , Dutta B. , Kuijk M. , Borghs G. , Heremans P.
Published: July 2002

The transmission properties of semiconductor surfaces can be changed by surface texturing. We investigate these changes experimentally and find that an enhancement of the angle-averaged transmission by a factor of 2 can be achieved with optimum texturing parameters. This enhanced transmission provides an additional light extraction mechanism for high-efficiency surface-textured light-emitting diodes. External quantum efficiencies of 46% and 54% are demonstrated before and after encapsulation, respectively.
Quantitative C lattice site distributions in epitaxial Ge1−yCy/Ge(001) layers
Authors: D’Arcy-Gall J. , Gall D. , Petrov I. , Desjardins P. , Greene J. E.
Published: July 2002

Epitaxial metastable Ge1−yCy alloy layers with y⩽0.035 were grown on Ge(001) from hyperthermal Ge and C atomic beams at deposition temperatures Ts of 250 and 300 °C. The use of hyperthermal beams allows us to controllably vary the concentration of C incorporated as Ge–C split interstitials. Ge1−yCy layers grown with incident Ge-atom energy distributions corresponding to ⩽0.14 lattice displacement per incident atom (dpa) are in a state of in-plane tension and contain significant concentrations of C atoms incorporated in substitutional sites. Increasing the dpa to 0.24 yields layers in compression with C incorporated primarily as Ge–C split interstitials. Ab initio density functional calculations of the formation energies and strain coefficients associated with C atomic arrangements in Ge show that configurations containing multiple C atoms, referred to collectively as C nanoclusters, are energetically more favorable than substitutional C and Ge–C split interstitials and yield a nearly zero average strain. In contrast, substitutional C and Ge–C split interstitials produce large tensile and compressive strains, respectively. Using the calculated strain coefficients, measured layer strains obtained from high-resolution reciprocal lattice maps, and substitutional C concentrations determined by Raman spectroscopy, we obtain the fraction of C atoms incorporated in substitutional, Ge–C split interstitial, and nanocluster sites as a function of the total C concentration y and Ts. We find that at low y and Ts values, all C atoms are incorporated in single-C configurations: substitutional C and Ge–C split interstitials. Their relative concentrations are controlled by the dpa through the production of near-surface Ge self-interstitials which are trapped by substitutional C atoms to form Ge–C split interstitials. Increasing y and Ts, irrespective of the dpa, leads to an increase in the fraction of C nanoclusters, while the fractions of substitutional C and Ge–C split interstitials decrease, due to the higher C–C encounter probability at the growth surface.
Nonlinear optical properties of chalcogenide glasses: Observation of multiphoton absorption
Authors: Bindra K. S. , Bookey H. T. , Kar A. K. , Wherrett B. S. , Liu X. , Jha A.
Published: July 2002

We report observation of four- and five-photon absorption in the chalcogenide glasses at the telecommunication wavelengths. The nonlinear refractive index is sufficiently large that the optical switching criterion is satisfied.
Analytical theory of pulsed relativistic electron beams entering a vacuum
Authors: Fill Ernst E.
Published: July 2002

An electron beam with a high-current density, passing from a conductor into a vacuum, generates strong electric fields which significantly influence further propagation of the electrons. An analytical solution for a one-dimensional pulsed electron beam with an arbitrary energy distribution is found which allows one to calculate electric field distributions, electron density distributions and potential distributions in the vacuum region. The analytical solution is fully relativistic and is applicable up to the time at which the interpenetration of electron paths occurs. It may be useful in research with fs lasers relating to generation of relativistic electron beams, ion beams, and the pumping of x-ray lasers.
Asymptotic analysis of state-to-state tetraatomic reactions using row-orthonormal hyperspherical coordinates
Authors: Museth Ken , Kuppermann Aron
Published: July 2002

The state-to-state asymptotic analysis of tetraatomic reactions is presented. It is assumed that the four-atom time-independent partial wave Schrödinger equation has been solved subject to the condition that in the limit of very compact geometries the wave function vanishes. These solutions are initially obtained in body-fixed row-orthonormal hyperspherical coordinates and transformed in the asymptotic arrangement channel regions of nuclear configuration space to Jacobi body-fixed coordinates. From the latter, compact explicit expressions for the reactance (R) and scattering (S) matrices, useful for accurate numerical calculations, are obtained. The different systems of coordinates used and their interrelations are given. The approach described is particularly well suited for implementation on massively parallel architectures and is appropriate for the calculation of benchmark-quality state-to-state integral and differential cross sections on currently available computers.
Modeling of nonstationary vacuum arc plasma jet interaction with a neutral background gas
Authors: Gidalevich E. , Goldsmith S. , Boxman R. L.
Published: July 2002

A plasma beam formed by a stationary vacuum arc source with a copper cathode is considered as a supersonic hydrodynamic jet interacting by collisions with a neutral atmosphere of background gas (argon). The plasma jet propagation in the neutral atmosphere is calculated numerically using a one-dimensional approximation. The values assumed at the entrance cross section of the plasma jet were ion temperature 104 K, electron temperature 3×104 K, ion concentration ni=1019 m−3 plasma jet mass velocity 1.5×104 m/s, and neutral concentration of (0.1,1,2,5)×ni. Two cases were analyzed:(1) isothermal gas motion and (2) gas motion with energy transfer from the ions to the neutrals. It is found for the isothermal case that the plasma front expands with decreasing velocity, but it never decreases down to the speed of sound. A snowplow effect is found with respect to the neutral gas. The neutral gas is accelerated by the plasma jet up to supersonic velocity. With ion–neutral energy transfer the plasma jet deceleration leads to shock discontinuities for all studied neutral densities.
Testing for scaling behavior dependence on geometrical and fluid parameters in the two fluid drop snap-off problem
Authors: Cohen Itai , Nagel Sidney R.
Published: July 2002

We present experimental results on the snap-off dynamics of a drop with viscosity λη dripping through a fluid of viscosity η. This paper focuses on the Stokes regime where both the inner and outer fluid viscous stresses are balanced by the pressure gradients arising from the interfacial curvature. We track the time dependence of the drop profiles near snap-off and find that successive profiles can be rescaled onto a single curve. We explore the dependence of this scaling on the nozzle diameter, surface tension, density mismatch, and viscosity ratio λ. We find that only λ affects the rescaled profile. Finally we investigate the dependence of the breaking rate on λ.
Theory of thermotropic polymer liquid crystals
Authors: Brostow Witold , Walasek Janusz
Published: July 2002

The thermal nematic system consisting of semiflexible polymer liquid crystal chains is considered. Orientational order for liquid crystal particles and for chain as a whole in addition to the chain anisotropy parameters at the nematic-to-isotropic transition and in the stable nematic phase are considered. Results are obtained by extension of the Maier and Saupe theory of thermotropic liquid crystals on systems of semiflexible macromolecules.
Improved mobilities and resistivities in modulation-doped p-type AlGaN/GaN superlattices
Authors: Waldron Erik L. , Graff John W. , Schubert E. Fred
Published: July 2002

The transport properties of modulation, shifted modulation, and uniformly doped Al0.20Ga0.80N/GaN superlattices are presented. The modulation-doped sample is doped only in the AlGaN barriers. The shifted-modulation-doped sample has its dopants shifted by one-quarter period. Measurements reveal a strong improvement in mobility and resistivity for the modulation-doped and shifted-modulation-doped structures versus the uniformly doped structure. The modulation-doped sample has a mobility of 9.2 and 36 cm2/V s at 300 and 90 K respectively and a very low resistivity of 0.20 and 0.068 Ω cm at 300 and 90 K, respectively. Capacitance–voltage profiling shows multiple two-dimensional hole gases. The results are consistent with a reduction of neutral impurity scattering for modulation-doped structures as compared to uniformly doped structures.
Characterization of patterns, oscillations, and chaos in chemical systems
Authors: Yoshida Nobuo
Published: July 2002

Open chemical systems far from equilibrium may give rise to spatial (Turing) patterns, oscillations, and chaos. States displaying such phenomena generally coexist with other states, either stationary, or oscillatory, or chaotic, under the same boundary conditions. A local function is defined here as a sum of products of the differences between two such states of thermodynamic forces and flows that appear in an expression for entropy production. It is averaged over the volume of a system when the state under consideration is spatially nonuniform, and over time when oscillatory or chaotic states are involved. It is found that this function is always zero for systems with fixed boundary conditions and negative for other systems including continuous-flow, stirred tank reactors and continuously fed unstirred reactors. These results indicate that on average, all thermodynamic forces and flows never increase or decrease simultaneously between two states, placing some restrictions on the difference in entropy production between them becoming too large.
Rovibrational analysis of molecular collisions using coherent states
Authors: Blass Anatol , Deumens Erik , Öhrn Yngve
Published: July 2002

A generalization of the Prony method, which fits a set of complex exponentials to a time dependent signal, is presented. This generalized Prony method is applied to analyze rovibrational dynamics of products in a theoretical treatment of molecular collisions. Employing suitable coherent states an a posteriori quantum analysis can be performed on molecular trajectories with classical nuclei obtained with electron nuclear dynamics theory. Illustrative calculations are presented for state resolved cross sections of proton collisions with water molecules and compared to results from crossed beam experiments.
Three-dimensional simulations of reversible bimolecular reactions: The simple target problem
Authors: Popov Alexander V. , Agmon Noam
Published: July 2002

We report three-dimensional simulations of the reversible reaction A+B↔C for a single static A molecule and a uniform initial concentration of noninteracting B-molecules. The results are compared with various analytic approximations for the time-dependence of the binding probability. They are in excellent agreement with a recent theory of Sung and Lee [J. Chem. Phys. 111, 796 (1999)] for all times and rate parameters. The second-order term in the long-time expansion of this theory is incorrect, yet it explains an apparent kinetic transition observed when the B-concentration increases. We also investigate the concentration profiles near the reversible trap.
Gas phase and surface kinetics in plasma enhanced chemical vapor deposition of microcrystalline silicon: The combined effect of rf power and hydrogen dilution
Authors: Amanatides E. , Stamou S. , Mataras D.
Published: July 2002

A gas phase and surface simulator of highly diluted silane in hydrogen rf discharges used for the deposition of microcrystalline silicon has been developed. The model uses the spatial density distribution of SiH (X 2Π) radicals measured using laser induced fluorescence and the total silane consumption for estimating the primary electron induced silane dissociation, thus avoiding fluid or statistical approaches commonly used for the prediction of electron impact rate coefficients. A critical analysis is made for the relative importance of all the parameters involved either in the gas phase chemistry or in the surface processes. The model results are compared to experimental data concerning disilane production and film growth rate over a wide range of rf power densities in 2% and 6% SiH4 in H2 discharges. The good agreement between experimental and model results allows for the extension of the discussion to the composition of the radical flux reaching the substrate, the relative contribution of each of the radicals to the film growth, and the most probable mechanism of microcrystalls formation under typical conditions of low and high microcrystalline silicon deposition rate.
Optimization of matrix chemical ratio for high flux pinning in ternary (Nd–Eu–Gd)Ba2Cu3Oy
Authors: Muralidhar M. , Jirsa M. , Sakai N. , Murakami M.
Published: July 2002

We prepared (Nd, Eu, Gd)Ba2Cu3Oy samples with various Nd: Eu: Gd ratios in the rare earth site. It was found that the three elements contributed to flux pinning in different ways. Nd mainly enhanced flux pinning at low magnetic fields, Eu controlled the second peak position and the irreversibility field, while Gd slightly enhanced intermediate and high-field Jc values. Scaling analyses for the pinning force density as a function of the reduced field h=Ha/Hirr (where Hirr denote the irreversibility field) showed that the highest peak was achieved at h=0.56. This value is even higher than the theoretically predicted highest value of h=0.5. We also show that a maximum flux pinning can be achieved in the whole magnetic field when very fine secondary phase particles are dispersed in a superconducting (Nd, Eu, Gd)Ba2Cu3Oy matrix with an optimum Nd: Eu: Gd ratio.
Coupling of codoped In and N impurities in ZnS:Ag: Experiment and theory
Authors: Kohiki Shigemi , Suzuka Takayuki , Oku Masaoki , Yamamoto Tetsuya , Kishimoto Seiichi , Iida Seishi
Published: July 2002

A vapor-phase-grown epitaxial ZnS:Ag layer simultaneously codoped with In and N on GaAs substrate exhibited a 436-nm light emission and p-type conduction with a low resistivity. X-ray photoemission spectroscopy revealed that the In 3d5/2 electron binding energy of the codoped ZnS:In,N layer was smaller by 0.5 eV than that of the ZnS:In independently doped layer, although the 2p3/2 electron binding energies of Zn and S of the codoped layer agreed well with those of the independently doped layer, respectively. The reduction of binding energy was ascribed to an increase in the electronic relaxation energy for core-hole states in photoemission and reflects a large charge transfer between the In and N atoms at the first neighbor sites through covalent sp3 bonding orbitals. An increase of the spectral intensity at around 4 eV relative to the valence band maximum observed for the codoped layer corresponds to a new state at −3.67 eV from the valence band maximum due to a strong coupling between the In 5s and N 2p orbitals at the first neighbor sites, derived from a first-principle band structure calculation for ZnS:(In,2N).