bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of experiments and a nonlinear model equation for spatially developing flame instability
Authors: Searby G. , Truffaut J.-M. , Joulin G.
Published: July 2002

We extend the Michelson–Sivashinsky equation, for arbitrary gas density ratio, to model the dynamics of an inclined anchored flame front. A flow velocity component tangential to the flame front transforms temporally developing structures into spatially developing ones. Closer investigation shows that a transition from absolute to convective instability should occur for a finite flow velocity. Using pole decomposition, we give a particular solution to this equation in the limit of high flow velocity compared to laminar flame speed. The analytical results are compared to measurements on a two-dimensional laminar flame. The growth rate, shape, and amplitude of the saturated structures, as well as the width of the crossover region are all well described. However, we experimentally observe the presence of a large scale transverse gas circulation not predicted by the analysis. We attribute this effect to the streamwise variation of the pressure jump across the flame brush.
Monte Carlo simulation of homogeneous binary vapor–liquid nucleation: Mutual enhancement of nucleation in a partially miscible system
Authors: Yoo S. , Oh K. J. , Zeng X. C.
Published: July 2002

By means of a constrained canonical-ensemble Monte Carlo technique the equilibrium physical cluster distribution {nij} in a supersaturated vapor mixture is computed. From this the intensive Gibbs free energy of ij-cluster formation ΔGij is obtained via the relation nij=Ne−ΔGij/kBT, where N is the number of molecules in the system, T is temperature, and kB is the Boltzmann constant. The saddle point on the free energy surface ΔGij versus i and j provides a measure of the activation barrier to nucleation ΔG*. A statistical mechanical formula of ΔGij is derived based on the physical-cluster theory of nucleation, and from which a molecular interpretation of ΔGij is given. We applied the Monte Carlo method to investigate the mutual enhancement of nucleation in a binary model system which is composed of spherical Lennard-Jones monomers and rigid Lennard-Jones dimers consisting of a monophilic atom and a monophobic atom. This model system bears some qualitative similarity to the partially miscible water/higher alcohol systems in which the mutual enhancement of nucleation has been observed experimentally. The simulation confirms the existence of mutual enhancement of nucleation in monomer-rich vapors, as originally predicted by Napari and Laaksonen from a density-functional theory [Napari and Laaksonen, Phys. Rev. Lett. 84, 2184 (2000)]. The nucleation enhancement is also seen in dimer-rich vapors at high supersaturations.
Rb–He exciplex formation on helium nanodroplets
Authors: Brühl F. R. , Trasca R. A. , Ernst W. E.
Published: July 2002

Rubidium atoms have been deposited on helium nanodroplets and optical excitation and emission spectra of the 5 2P–5 2S transition have been measured and interpreted. After laser excitation of the 5 2P3/2 state, fluorescence from a Rb*He exciplex is observed while pumping the D1 (5 2P1/2–5 2S) line yields only emission from free rubidium atoms. This observation is in agreement with the predictions from a recent model by Reho et al. [J. Chem. Phys. 113, 9694 (2000)] about the extraction of an alkali–helium exciplex from the doped helium nanodroplet surface. A high barrier along the Hen–He–Rb* axis of the 1 2Π1/2 potential prevents desorption of Rb*He within the excited state lifetime, whereas the 1 2Π3/2 potential permits the exciplex extraction. The excitation spectrum, on the other hand, reflects the structure of the excited states 1 2Π1/2, 1 2Π3/2, and 2 2Σ1/2 of the HenRb complex whose potential surfaces will depend strongly on the alkali–He interaction dynamics near the droplet surface. For a heavy surface dopant like Rb or Cs the droplet surface will be strongly distorted upon vibrational excitation of the dopant. Some of the consequences for the potentials are discussed for the example of the 1 2Σ1/2 ground state.
Molecular dynamics simulation of polymer ordering. II. Crystallization from the melt
Authors: Yamamoto Takashi
Published: July 2002

Detailed microscopic image of polymer crystallization from the melt is searched for by molecular dynamics simulation. In order to accelerate crystallization, a simple model of polymethylene chain is devised; the polymer chain is made of 100 beads (CH2 united atoms) connected by harmonic springs and the lowest energy conformation is a linear stretched sequence of the beads with a slight bending stiffness being imposed. A system of polymer melt, made of 80 chains of C100, is placed between two parallel substrates that represent the growth surface of the lamellae. Initial melt at 600 K is rapidly cooled down to various crystallization temperatures, and the molecular process of crystallization is investigated. We first notice a marked layer structure in the melt near the substrate. We find that the layer structure leads to growing lamellae when cooled below the melting point. It is shown that the growing lamellae have a definite tapered shape and show thickening growth along the chain axis as well as normal growth. The molecular trajectory of the crystallizing chain at the growth front is demonstrated to be a sequence of processes of local adsorption to the growth front followed by stretching along the chain axis and the final fold into crystalline conformation. The chains are found to be very mobile showing active diffusion in the melt and also in the crystal.
Photochemistry of O2 on atomically stepped Pt surfaces: Enhanced O2 photodepletion at step sites
Authors: Tripa C. Emil , Yates John T.
Published: July 2002

Photodepletion efficiencies of O2(a) on stepped Pt(335) and Pt(779) surfaces have been measured for ultraviolet light in the range 3.87–4.77 eV, using temperature programmed desorption to separate photodepletion processes from step and terrace sites. The O2 photodepletion at the (001) step sites is faster by a factor of 2 or 3 than at the (111) terrace sites. The larger cross section for O2 photodepletion at Pt step sites compared to smooth (111) Pt terrace sites may be due to chemical differences between O2 on the two types of sites or to the presence of an enhanced lifetime for photoexcited electron states at step sites.
Rules of pair electron correlation in MF(M=H, Li, Na)
Authors: Zhuo Shuping , Wei Jichong , Ju Guanzhi
Published: July 2002

In this paper, the transferable property of pair correlation energies of F centered orbitals is discussed for a series F containing compounds MF(M=H, Li, Na). Comparing our two calculated results using 6-311++G(d) and cc-PV5Z* basis sets, it is found that the absolute values of the correlation energy of these systems increase with the increase of the size of the basis set, but the following relative changing regularities remain the same. In F containing compounds, from F neutral atom through HF, LiF, NaF to F−, both the intra-, interpair correlation energies and intra-, intershell correlation energies of the inner orbital electrons differ little. The correlation effects of 1sF2 are very much alike in all the above F containing systems and such pair correlation is transferable. However, the valence intrapair correlation and intrashell correlation energies are large and differ a lot in all systems. In MF molecules, the F correlation energy contribution increases with the increase of the ionic bond strength of the compound and this contribution is always between the correlation energy values of F atom and F− anion. For strong ionic compounds, we present a very simple method to estimate the correlation energy of the compound by adding the correlation energies of its component ions and this simple method can reach the chemical accuracy (2 kcal/mol) but require much less computing power.
Si
3
N
4
/
AlGaN/GaN
–metal–insulator–semiconductor heterostructure field–effect transistors
Authors: Hu X. , Koudymov A. , Simin G. , Yang J. , Khan M. Asif , Tarakji A. , Shur M. S. , Gaska R.
Published: July 2002

We report on a metal–insulator–semiconductor heterostructure field-effect transistor (MISHFET) using Si3N4 film simultaneously for channel passivation and as a gate insulator. This design results in increased radio-frequency (rf) powers by reduction of the current collapse and it reduces the gate leakage currents by four orders of magnitude. A MISHFET room temperature gate current of about 90 pA/mm increases to only 1000 pA/mm at ambient temperature as high as 300 °C. Pulsed measurements show that unlike metal–oxide–semiconductor HFETs and regular HFETs, in a Si3N4 MISHFET, the gate voltage amplitude required for current collapse is much higher than the threshold voltage. Therefore, it exhibits significantly reduced rf current collapse.
Anomalous orientational relaxation of solute probes in binary mixtures
Authors: Bhattacharyya Sarika , Bagchi Biman
Published: July 2002

The orientation of a solute probe in a binary mixture often exhibits multiple relaxation times at the same solvent viscosity but different compositions [Beddard et al., Nature (London) 294, 145 (1981)]. In order to understand this interesting observation, we have carried out (NPT) molecular dynamics simulation study of rotation of prolate ellipsoids in binary mixtures. The simulations show that for a broad range of model parameters the experimental behavior can be reproduced. The plot of orientational relaxation time versus the solvent viscosity, the latter changed by varying the composition, shows a nonmonotonic viscosity dependence. The nature of the plot is found to depend on the system parameters, especially on the solute–solvent interactions. A mode coupling theoretical analysis of this complex dependence of the rotational relaxation time τR on the binary viscosity η is presented. The theory can qualitatively explain the origin of the multiple relaxation time at the same viscosity.
Modeling of complex plasmas under micro-gravity conditions
Authors: Morfill G. , Tsytovich V. N.
Published: July 2002

Computer modeling of complex (dusty) plasmas under micro-gravity conditions is performed. It is shown that several requirements are necessary to obtain almost homogeneous self-confining micro-particle structures: (a) The ion and electron densities at the center of the structure should exceed certain critical values; (b) the rate of ionization should be less than a certain critical value; and (c) the micro-particle structures surrounded by walls should be separated from the wall by a dust void. It is found that in the optimum case up to 6×106 micro-particles can be confined in the structure. For that the ion flux in the center part of the structure should be almost convective. Less than 103 micro-particles can be confined without optimal tuning of the parameters. The void surrounding the micro-particle structure has specific properties determined both by the particle cloud and the wall conditions. The void’s properties are found to be crucial for the properties of the particle cloud.
Wet electrons and how to dry them
Authors: Beyer Martin K. , Fox Brigitte S. , Reinhard Björn M. , Bondybey Vladimir E.
Published: July 2002

We present the formation of hydrated electrons by laser vaporization, and investigate in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer their destruction by the 300 K black body infrared background radiation. While clusters above n⩾32 decay almost exclusively by fragmentation and loss of ligands, the smaller species also detach electrons, with the relative rates of the two processes exhibiting an interesting alternation. Below n=15 they almost exclusively detach electrons, and for n⩽12 the detachment rate is apparently so fast that no clusters are observed in the ICR-experiment. From n=19 to n=24 a pronounced odd–even alternation between electron detachment and fragmention is observed, which is tentatively attributed to entropic rather than energetic effects.
Optical gain at 1.54 μm in erbium-doped silicon nanocluster sensitized waveguide
Authors: Han Hak-Seung , Seo Se-Young , Shin Jung H.
Published: July 2002

Optical gain at 1.54 μm in erbium-doped silicon-rich silicon oxide (SRSO) is demonstrated. Er-doped SRSO thin film was fabricated by electron-cyclotron resonance enhanced chemical vapor deposition of silicon suboxide with concurrent sputtering of erbium followed by a 5 min anneal at 1000 °C. Ridge-type single mode waveguides were fabricated by wet chemical etching. Optical gain of 4 dB/cm of an externally coupled signal at 1.54 μm is observed when the Er is excited via carriers generated in the Si nanoclusters by the 477 nm line of an Ar laser incident on the top of the waveguide at a pump power of 1.5 W cm−2.
Proton dynamics in supercritical water
Authors: Andreani C. , Colognesi D. , Degiorgi E. , Ricci M. A.
Published: July 2002

An inelastic neutron scattering experiment has been performed on supercritical water at high momentum transfer, up to 90 Å−1, in order to study single proton dynamics. The value of the proton mean kinetic energy has been extracted in the framework of the impulse approximation. The anisotropy of the proton momentum distribution inside a single water molecule is discussed. The extracted experimental mean kinetic energy is found in good agreement with the predictions of a harmonic model, under the assumptions of decoupling between translational, rotational and vibrational degrees of freedom. Differences emerge between our results and those obtained in a recent inelastic neutron scattering experiment on water in sub- and supercritical conditions. These differences are pointed out and examined in detail.
Structural and thermodynamic properties of spin-polarized fluid hydrogen
Authors: Xu Hong , Hansen Jean-Pierre
Published: July 2002

The “orbital-free” two-component density functional theory introduced previously [H. Xu and J. P. Hansen, Phys. Rev. E 57, 211 (1998)] is extended to the study of the proton–proton and proton–electron pair correlations in a spin-polarized hydrogen plasma as a function of density and temperature. An approximate formula is suggested to compute the free energy of the plasma phase, and applied to the investigation of the plasma-insulator transition of spin-polarized fluid hydrogen.
Single-particle orbit equation in crystalline beams
Authors: Okamoto Hiromi
Published: July 2002

This paper presents a set of equations that describe the closed orbits of individual particles in a crystalline beam. A mean-field model is employed under the condition that single-particle trajectories in a crystalline ground state are proportional to each other. It is shown that the solution to the orbit equations can almost uniquely be determined once the lattice design of a storage ring is made. The theoretical predictions are compared with molecular dynamics simulation results in order to confirm the validity of the present formalism. The orbit equations can be used, e.g., to study the intrinsic stability of crystalline beams in an arbitrary storage-ring lattice and to calculate the optimum tapering factor of a beam cooling force.
Modeling pinchoff and reconnection in a Hele-Shaw cell. II. Analysis and simulation in the nonlinear regime
Authors: Lee Hyeong-Gi , Lowengrub J. S. , Goodman J.
Published: July 2002

This is the second paper in a two part series in which we analyze two diffuse interface models to study pinchoff and reconnection in binary fluid flow in a Hele-Shaw cell with arbitrary density and viscosity contrast between the components. Diffusion between the components is limited if the components are macroscopically immiscible. In one of the systems (HSCH), the binary fluid may be compressible due to diffusion. In the other system (BHSCH), a Boussinesq approximation is used and the fluid is incompressible. In this paper, we focus on buoyancy driven flow and the Rayleigh–Taylor instability. In the fully nonlinear regime before pinchoff, results from the HSCH and BHSCH models are compared to highly accurate boundary-integral simulations of the classical sharp interface system. In this case, we find that the diffuse interface models yield nearly identical results and we demonstrate convergence to the boundary-integral solutions as the interface thickness vanishes. We find that the break-up of an unstably stratified fluid layer is smoothly captured by both models. The HSCH model seems to be more diffusive than the BHSCH model and predicts an earlier pinchoff time which causes subtle differences between the two in the pinchoff region. Further, in the limit of zero interface thickness, we find that the effect of compressibility does not vanish at pinchoff. This distinguishes the HSCH model from all others in which compressibility effects are neglected. It may turn out, for example, that characterizing the limiting effect of compressibility at pinchoff may suggest a physically based selection mechanism for cutting and reconnecting sharp interfaces. Varying the gravitational force and viscosities of the fluids yields different pinchoff times and numbers of satellite drops. Moreover, using the analysis of the linear growth rates from our first paper [paper I, Phys. Fluids 14, 492 (2002)], we confirm that the modified HSCH/BHSCH parameters suggested in that work lead to improved agreement with sharp interface results at finite interface thicknesses. Last, we also consider a case in which the fluid components are miscible. We find competition between buoyancy, viscous, diffusional and, at very early times, surface tension-like forces.
The Faraday resonance of interfacial waves in weakly viscous fluids
Authors: Hill D. F.
Published: July 2002

The Faraday resonance of interfacial waves in a two-layer, weakly-viscous system in a rectangular domain is presented. A perturbation analysis is pursued and, at the second-order, the scaling of the viscosity results in boundary layer corrections at the solid walls and at the interface. Special attention is paid to the damping in the meniscus region where the interface contacts the side-walls. As a result of the presence of both destabilizing effects (vertical oscillation) and stabilizing effects (viscosity), a threshold condition for instability is determined. The derived analytic results are quite general and prove useful in elucidating the influences of the various boundary layers, as well as the threshold for growth. In an effort to describe the maximum amplitude attained by the resonated wave, a third-order analysis is then presented for the idealized case of equal-depth, inviscid layers, with a rigid-lid condition at the free surface. A balance between cubic nonlinearity and the vertical shaking yields a Landau-type equation for the interfacial wave amplitude. Comparisons with some existing experimental data are made at both orders and indicate very good agreement.
Development of a radio-frequency-driven ion source of the diagnostic neutral beam for the Hanbit device
Authors: Yang H. L. , Yoo S. J. , Hwang S. M. , Kwon M. , Hong I. S. , Hwang Y. S. , Chung K. H.
Published: July 2002

A radio-frequency-driven hydrogen-ion source of the diagnostic neutral beam has been developed for the Hanbit magnetic mirror device at the Korea Basic Science Institute [S. M. Hwang et al., Trans. Fusion Technol. 35, 99 (1999)]. The three-electrode system with 37 apertures extracts the hydrogen-ion beam with 29.8 keV beam energy and 1.2 A beam current at a 9.6 cm diam. The ion source has the capability of varying the beam pulse width from 10 to 100 ms and also is capable of beam modulation at 50 Hz with a 10 ms flattop pulse width. The beam power and the ratio of the mixed ion species were measured by infrared thermography and Doppler shift spectroscopy, respectively. Most recent experimental results will be presented in this article.
Resistive pressure-gradient-driven instabilities in the transition regime to fully developed turbulence
Authors: Garcia L. , Carreras B. A. , Lynch V. E.
Published: July 2002

Numerical calculations of resistive pressure-gradient-driven turbulence in toroidal geometry for tokamak plasma edge parameters have been carried out in the unstable regime below the threshold for fully developed turbulence. This threshold is at a β value of about three times the linear stability threshold. In this regime, the toroidal mode number spectrum in the stationary state is dominated by a single toroidal mode. The dominant mode may intermittently fluctuate among a narrow range of possible values.
Thermodynamic signature of the onset of caged dynamics in glass-forming liquids
Authors: Kamath Sudesh , Colby Ralph H. , Kumar Sanat K. , Baschnagel Jörg
Published: July 2002

We examine the thermodynamics of supercooled liquids focusing on the immediate vicinity of an onset temperature, TA, where system dynamics just begin to experience caging effects. Simulation data from a polymer model and from small molecule mixtures show, in agreement with experiment, that the configurational entropy becomes strongly temperature dependent below TA. Since the diffusion coefficient follows the Adam-Gibbs relationship, our results clearly establish a thermodynamic connection to the localized dynamics of glass-forming liquids.
Effects of ion streaming and propagation direction on ion-acoustic solitary waves
Authors: Mahmood S. , Saleem H.
Published: July 2002

It is found that ion streaming and the direction of propagation of the nonlinear structure are the factors which can determine the amplitude of the density hump in fully nonlinear ion-acoustic solitary waves.
Nonlinear kinetic regime of the Weibel instability in an electron–ion plasma
Authors: Califano F. , Cecchi T. , Chiuderi C.
Published: July 2002

Kinetic numerical simulations of the evolution of the Weibel instability during the full nonlinear regime including ions dynamics are presented. The formation of strong density inhomogeneity and its influence on the resulting electrostatic and electromagnetic wave modes are shown.
Simulations of electron transport in laser hot spots
Authors: Brunner S. , Valeo E.
Published: July 2002

Simulations of electron transport are carried out by solving the Fokker–Planck equation in the diffusive approximation. The system of a single laser hot spot, with open boundary conditions, is systematically studied by performing a scan over a wide range of the two relevant parameters. (1) Ratio of the stopping length over the width of the hot spot. (2) Relative importance of the heating through inverse Bremsstrahlung compared to the thermalization through self-collisions. As for uniform illumination [J. P. Matte et al., Plasma Phys. Controlled Fusion 30, 1665 (1988)], the bulk of the velocity distribution functions (VDFs) present a super-Gaussian dependence. However, as a result of spatial transport, the tails are observed to be well represented by a Maxwellian. A similar dependence of the distributions is also found for multiple hot spot systems. For its relevance with respect to stimulated Raman scattering, the linear Landau damping of the electron plasma wave is estimated for such VDFs. Finally, the nonlinear Fokker–Planck simulations of the single laser hot spot system are also compared to the results obtained with the linear nonlocal hydrodynamic approach [A. V. Brantov et al., Phys. Plasmas 5, 2742 (1998)], thus providing a quantitative limit to the latter method: The hydrodynamic approach presents more than 10% inaccuracy in the presence of temperature variations of the order ΔT/T≳1%, and similar levels of deformation of the Gaussian shape of the Maxwellian background.
Jacobson generators of the quantum superalgebra Uq[sl(n+1|m)] and Fock representations
Authors: Palev T. D. , Stoilova N. I. , Van der Jeugt J.
Published: July 2002

As an alternative to Chevalley generators, we introduce Jacobson generators for the quantum superalgebra Uq[sl(n+1|m)]. The expressions of all Cartan–Weyl elements of Uq[sl(n+1|m)] in terms of these Jacobson generators become very simple. We determine and prove certain triple relations between the Jacobson generators, necessary for a complete set of supercommutation relations between the Cartan–Weyl elements. Fock representations are defined, and a substantial part of this paper is devoted to the computation of the action of Jacobson generators on basis vectors of these Fock spaces. It is also determined when these Fock representations are unitary. Finally, Dyson and Holstein–Primakoff realizations are given, not only for the Jacobson generators, but for all Cartan–Weyl elements of Uq[sl(n+1|m)].
Electron Bernstein wave emission from an overdense reversed field pinch plasma
Authors: Chattopadhyay P. K. , Anderson J. K. , Biewer T. M. , Craig D. , Forest C. B. , Harvey R. W. , Smirnov A. P.
Published: July 2002

Blackbody levels of emission in the electron cyclotron range of frequencies have been observed from an overdense (ωpe∼3ωce) Madison Symmetric Torus [Dexter et al., Fusion Technol. 19, 131 (1991)] reversed field pinch plasma, a result of electrostatic electron Bernstein waves emitted from the core and mode converted into electromagnetic waves at the extreme plasma edge. Comparison of the measured radiation temperature with profiles measured by Thomson scattering indicates that the mode conversion efficiency can be as high as ∼75%. Emission is preferentially in the X-mode polarization, and is strongly dependent upon the density and magnetic field profiles at the mode conversion point.
Investigation of a density modulated electron beam emitted by a ferroelectric plasma cathode
Authors: Chirko K. , Krasik Ya. E. , Dunaevsky A. , Felsteiner J.
Published: July 2002

We present experimental results of the generation of a high-frequency modulated electron beam in a diode with a ferroelectric plasma cathode. The energy of electrons in the modulated beam is significantly higher than that obtained in previous experiments, reaching 150 keV. The maximum amplitude of the modulated beam current reaches 65 A, which corresponds to a beam power of 4.5 MW. Data concerning the influence of the diode geometry and the accelerating voltage on the electron energy and the amplitude of the modulated beam current are presented. It was found that the beam modulation is caused by the variation of the beam electron density, and that it has uniform cross-sectional electron beam current density distribution. Efficient transport of the modulated electron beam in a guiding external magnetic field without distortion of its waveform is demonstrated. The electron beam modulated at 190 MHz with an amplitude of 25 A was transported in the magnetic field (0.3–3 kG) along a distance of 50 cm. In addition, it was shown that the modulated beam could be efficiently transported through a slow wave structure without the application of the external magnetic field.
Quantum Clifford algebra from classical differential geometry
Authors: Vargas Jose G. , Torr Douglas G.
Published: July 2002

We show the emergence of Clifford algebras of nonsymmetric bilinear forms as cotangent algebras of Kaluza–Klein (KK) spaces pertaining to teleparallel space–times. These spaces are canonically determined by the horizontal differential invariants of Finsler bundles of the type, B′(M)→S(M), where B′(M) is the set of all the tangent frames to a differentiable manifold M, and where S(M) is the sphere bundle. If M is space–time itself, M4, the “geometric phase space,” S(M4), has dimension seven. This reformulation of the horizontal invariants as pertaining to a KK space removes the mismatch between the dimensionality of the tangent frames to M4 and the dimensionality of S(M4). In the KK space, a symmetric tangent metric induces a cotangent metric which is not symmetric in general. An interior covariant derivative in the sense of Kaehler is defined. It involves the antisymmetric part of the cotangent metric, which thus enters electrodynamics and the Dirac equation.
Electronic and structural properties of Fe3Pd–Pt ferromagnetic shape memory alloys
Authors: Stern R. A. , Willoughby S. D. , Ramirez A. , MacLaren J. M. , Cui J. , Pan Q. , James R. D.
Published: July 2002

Ferromagnetic shape memory (FSM) alloys are scientifically and technologically interesting materials that combine ferromagnetism with a reversible martensitic phase transformation. Fe70Pd30 has recently been shown to display a FSM effect at usable temperatures and low fields. Reported here are results of experimental studies on Fe70Pd30 and electronic structure calculations on Fe70Pd30−xPtx. The calculations show that additions of Pt by 6 at % to Fe70Pd30 can triple the magnetocrystalline anisotropy. There is, however, a large discrepancy between the measured and calculated anisotropy values of Fe70Pd30, suggesting the presence of significant disorder in the measured samples. Other calculated structural and magnetic properties are in close agreement with experimental values.
Converged five-dimensional quantum calculations for OH+CO→H+CO2
Authors: McCormack Drew A. , Kroes Geert-Jan
Published: July 2002

We perform five-dimensional quantum wave packet calculations of initial-state-resolved reaction probabilities for the reaction OH+CO→H+CO2, with OH and CO initially in the rovibrational ground state, and total angular momentum J=0. In essence, the dynamics are treated exactly for all molecular degrees of freedom except the CO reactant bond, for which a vibrational-averaging approximation is adopted. Comparison of reaction probabilities to those obtained in an earlier, similar study [D. H. Zhang and J. Z. H. Zhang, J. Chem. Phys. 103, 6512 (1995)] show that the previously obtained results were not well converged, primarily because too few rotational basis functions were used in the calculations. The resonances found in the current study are also more abundant and narrower than in the earlier study. Reaction probabilities from calculations on an updated potential energy surface (PES) [K. S. Bradley and G. C. Schatz, J. Chem. Phys. 106, 8464 (1997)] do not differ significantly from those for the PES used in the rest of the calculations [K. Kudla, G. Schatz, and A. Wagner, J. Chem. Phys. 95, 1635 (1991)], though there is a severe effect on resonance structure.
Fine crystalline grain model for the determination of the morphology of ultrathin amorphous silicon films
Authors: Hazra Sukti , Sakata Isao , Yamanaka Mitsuyuki , Suzuki Eiichi
Published: July 2002

To obtain the morphology of ultrathin hydrogenated amorphous silicon (a-Si:H) films with thicknesses of 2–10 nm grown by thermal chemical-vapor deposition, we have utilized spectroscopic ellipsometry and introduced a model, called the fine crystalline grain model. This model resolves that the growth of the amorphous silicon matrix starts with the formation of dense deformed crystallites or paracrystallites. A better fit to the experimental data has been obtained with this model when the a-Si:H film thickness is below 20 nm. The fine grain component gradually decreases with the film growth. It has been clarified that the presence of deformed crystallites in the starting a-Si:H films acts as a constraint for their crystallization by rapid thermal annealing.
Diffusion coefficient for reptation of polymers with kinematic disorder
Authors: Willmann Richard D.
Published: July 2002

We give a lower bound on the diffusion coefficient of a polymer chain in an entanglement network with kinematic disorder, which is obtained from an exact calculation in a modified Rubinstein–Duke lattice gas model with periodic boundary conditions. In the limit of infinite chain length we show the diffusive motion of the polymer to be slowed down by kinematic disorder by the same factor as for a single particle in a random barrier model.
Fast development of high coercivity in melt-spun Sm(Co,Fe,Cu,Zr)z magnets
Authors: Yan A. , Bollero A. , Müller K. H. , Gutfleisch O.
Published: July 2002

A simple method was developed for magnetic hardening melt-spun Sm(Co,Fe,Cu,Zr)z alloys. The as-spun ribbons reached a coercivity of 2.8 T only by slow cooling from 850 to 400 °C, without the standard solid solution and isothermal aging treatments which are required for a bulk precipitation hardened 2:17 SmCo alloy. A single 1:7 phase, identical to that found in as-solubilized ribbons, was obtained in the as-spun state. At the same time, Cu and Zr are supersaturately dissolved in the 1:7 matrix by melt spinning due to its very high cooling rate. Thus, solid solution treatment can be avoided for melt-spun materials. After aging, more lamellar phase with larger width was observed by transmission electron microscope in ribbons without treatment in solution. This leads to faster development of a uniform finer cellular microstructure with a Cu-rich 1:5 cell boundary phase, which gives rise to stronger domain-wall pinning and therefore to higher coercivity.
Isotopic effect on thermal mobility of atomic hydrogen in solid xenon
Authors: Khriachtchev Leonid , Tanskanen Hanna , Pettersson Mika , Räsänen Markku , Feldman Vladimir , Sukhov Fedor , Orlov Aleksei , Shestakov Alexander F.
Published: July 2002

We have studied thermal mobility of atomic hydrogen in solid Xe using decomposition of water molecules as a source for hydrogen atoms. The formation of various isotopomers of HXeH and HXeOH is monitored at temperatures from 37 to 42 K by using infrared absorption spectroscopy, and the activation energy of this diffusion-controlled process is found to be ∼110 meV. Most importantly, the different mobility for hydrogen isotopes is demonstrated, H being faster than D, and the difference between the corresponding activation energies is estimated to be ∼4 meV. The electron paramagnetic resonance measurements of the thermal decay of H atoms and OH radicals show that the formation of HXeH and HXeOH is controlled by hydrogen mobility. The modeling of thermally activated jumps of hydrogen atoms in a relaxed Xe lattice agrees reasonably with experiment with respect to the isotopic effects but it underestimates the jump rate.
Geometry-independent intensity correction factors for grazing-incidence diffraction
Authors: Smilgies Detlef-M.
Published: July 2002

A geometry-independent approach to a unified description of intensity corrections factors for the two most common surface scattering geometries is presented. Simple analytical formulas relate the representations of the Lorentz factor, polarization factor, area factor, and rod interception factor in a surface reference frame to the actual diffractometer circles. An analytical formula for the area correction for very large exit angles, as can be achieved on modern diffractometers, is derived. A test data set shows that the derived formulas provide a good description of the scattering intensity up to the edge of the Ewald sphere.
A floating potential method for measuring ion density
Authors: Chen Francis F. , Evans John D. , Arnush Donald
Published: July 2002

A simple method for analyzing cylindrical Langmuir probe curves in a cold-ion plasma is described which yields the ion density in weakly collisional plasmas for which purely collisionless theories give erroneous results. The method is based on an extrapolation to the floating potential of the saturation ion current raised to the 4/3 power. This procedure is not supported by theory but apparently works because effects neglected in the theory tend to cancel.
Reduction of ordering temperature of an FePt-ordered alloy by addition of Cu
Authors: Maeda Tomoyuki , Kai Tadashi , Kikitsu Akira , Nagase Toshihiko , Akiyama Jun-ichi
Published: July 2002

We have found that the addition of Cu to an FePt alloy film is an effective approach for reducing the ordering temperature of FePt. The coercivity of the FePtCu film is around 5 kOe after annealing at 300 °C, whereas that of FePt shows several hundred Oe. In the FePtCu film annealed at 700 °C, the coercivity is almost the same as for the FePt films. Therefore, the FePtCu film displays a hard-magnetic property similar to that of the FePt film. The results of x-ray diffraction indicate that a ternary FePtCu alloy is formed. Thus, the formation of the ternary FePtCu alloy is considered to play an important role in reducing the ordering temperature.
Self-organized nanodot formation on MgO(100) by ion bombardment at high temperatures
Authors: Lu M. , Yang X. J. , Perry S. S. , Rabalais J. W.
Published: July 2002

The production of self-organized, spatially dense nanodots on a MgO(100) surface by Ar+ bombardment at elevated sample temperatures (700 and 1000 °C) and ion energies in the range 0.5–5 keV is reported. The results show that at elevated temperatures, the size of the nanodots can be modified by varying the ion energy in a well-defined manner. The nanodot diameter is found to increase with increasing temperature and decrease with increasing ion flux and increasing ion energy. The dot diameter is several tens of nanometers and the height is several nanometers. These results can be interpreted in terms of a recently developed theoretical model.
Negative thermal expansion behavior in single crystal and ceramic of Nb2O5-based compositions
Authors: Choosuwan H. , Guo R. , Bhalla A. S. , Balachandran U.
Published: July 2002

The thermal expansion coefficients of a single crystal and ceramic of Nb2O5 are measured in the temperature range of −200–500 °C by the dilatometer technique. Both single crystals and ceramics of Nb2O5 and Nb2O5(1−x):xTiO2 show negative thermal expansion in this temperature range. Some contribution to the result could be due to the presence of the Magneli phases. The main phase transition temperature, which also matches with the dielectric anomaly, occurs at ≈150 °C.
Orientation-controlled nonepitaxial L1 CoPt and FePt films
Authors: Zeng H. , Yan M. L. , Powers N. , Sellmyer D. J.
Published: July 2002

We report results on highly oriented, face-centered tetragonal ordered CoPt and FePt thin films grown nonepitaxially by directly depositing films on thermally oxidized Si substrates and subsequent annealing. By controlling the thickness, composition, and annealing conditions, and/or depositing a proper underlayer, nearly perfect (001)-oriented CoPt and FePt films can be obtained. Magnetic measurements reveal large perpendicular anisotropy for such films.
Does a sheared flow stabilize inversely stratified fluid?
Authors: Benilov E. S. , Naulin V. , Rasmussen J. Juul
Published: July 2002

We examine the stability of sheared flows in an inversely stratified fluid (where the density increases upward). As demonstrated by Kuo (1963), if the shear and Väsälä frequency are both constant (i.e., if the velocity and density profiles are both linear), the shear suppresses the Rayleigh–Taylor instability that would affect the fluid in the absence of the flow. Our main goal is to reexamine this problem for a wider class of velocity and density profiles. Using the standard linear normal-mode analysis, we consider two types of flows: jets (for which asymptotic solutions were found), and currents with a monotonic profile (which were examined numerically). It turns out that virtually any deviation from the linear profiles examined by Kuo (1963) triggers off instability. This instability, however, is restricted either spectrally or spatially, which makes it different from the usual Rayleigh–Taylor instability (in the absence of the flow, inversely stratified fluids are unstable at all points and all wavelengths). The conclusions of the paper are verified by simulation of the governing (nonlinear) equations.
Mg-rich precipitates in the p-type doping of InGaN-based laser diodes
Authors: Hansen M. , Chen L. F. , Lim S. H. , DenBaars S. P. , Speck J. S.
Published: July 2002

Uniformly distributed precipitates have been observed by transmission electron microscopy in the p-type layers of laser structures. The precipitate density decreases with decreasing flow of biscyclopentadienyl-magnesium (Cp2Mg), and the hole concentration in the laser structure was higher for a lower precipitate density. The higher hole concentration reduces the threshold current density and improves the internal quantum efficiency of the laser because of the higher number of holes available for radiative recombination. The lasers with higher precipitate density also exhibit a higher resistance. The threshold voltage was reduced 30% from 20.8 V for lasers with a high precipitate density to 14.3 V for lasers with a lower precipitate density due to the lower resistance.
Low leakage current Cu(Ti)/SiO2 interconnection scheme with a self-formed TiOx diffusion barrier
Authors: Liu C. J. , Chen J. S.
Published: July 2002

Electrical and material properties of Cu(0.02 wt % Ti) alloy and pure Cu films deposited on SiO2/Si are explored. Current–voltage measurement using metal–oxide–semiconductor (MOS) capacitor structure reveals low leakage current (10−8 A/cm2) for capacitors with as-deposited Cu(0.02 wt % Ti) and pure Cu metal gates. However, after annealing at 700 °C in a vacuum, leakage current of MOS capacitors using a pure Cu gate shows a dramatic rise of leakage current at a low electrical field, while the leakage current of capacitors with Cu(0.02 wt % Ti) gate stays at ∼10−7 A/cm2. Concurrently, the resistivity of annealed Cu(0.02 wt % Ti) is reduced to 2.5 μΩ cm, which is only slightly greater than the resistivity of as-sputtered pure Cu films. X-ray photoelectron spectroscopy indicates that a TiOx layer has formed at the Cu(0.02 wt % Ti)/SiO2 interface after annealing and Auger electron spectrometry depth profiles show less interdiffusion at the Cu(0.02 wt % Ti)/SiO2 interface than the Cu/SiO2 interface. The correlation between leakage current reliability and the interfacial reaction upon annealing is discussed.
Molecular dynamics study of the photodissociation of OClO in bulk liquids
Authors: Chorny Ilya , Vieceli John , Benjamin Ilan
Published: July 2002

The electronic spectra and the photodissociation dynamics of OClO on the excited state in bulk water, acetonitrile, and ethanol are computed using classical molecular dynamics computer simulations. The trajectories are run on an ab initio potential energy surface of Peterson [J. Chem. Phys. 109, 8864 (1998)], which is fit to a global three-dimensional analytical surface. The calculated cage escape probability in these liquids seems to correlate with the vibrational relaxation rate of the parent molecule and is in reasonable agreement with experiments in water and acetonitrile, but somewhat overestimates the experimental probability in the case of ethanol.
Submillimeter-wave spectra of HCS and DCS
Authors: Habara Hideta , Yamamoto Satoshi , Amano Takayoshi
Published: July 2002

The a-type R-branch K−1=0 rotational transitions of the HCS and DCS radicals have been measured in the frequency range of 161 to 644 GHz using source modulation spectrometers. For DCS, the seven fine and hyperfine components of the 101–000 rotational transition are also measured at 35 GHz using a Fourier transform millimeter-wave spectrometer. The spectra are found to be perturbed by the K−1=1 state through the off-diagonal spin–rotation interaction (εab+εba)(NaSb+SbNa+NbSa+SaNb). In particular for DCS, strong perturbations are observed. The rotational constants, A, B+C, and B−C, of DCS are determined through an analysis of the perturbation. The r0 structure of HCS has been determined as follows: r0(CH)=1.079(3) Å, r0(CS)=1.562 28(3) Å, and α0(HCS)=132.8(3)°. The quasilinearity parameter, γ0, is evaluated to be 0.80 for DCS, indicating that HCS is not a simple bent molecule.
Electrostatic modes in (self-gravitating) dusty plasmas with charge and mass distributions
Authors: Verheest Frank , Yaroshenko Victoria V. , Hellberg Manfred A.
Published: July 2002

To obtain the mass and charge densities for a dusty plasma containing charged grains with individual identities, either an ensemble of discrete grains can be used, or a continuous distribution in charge and in mass, where the densities are determined from an appropriate kinetic theory. Using the electrostatic and gravitational Poisson’s equations the general electrostatic dispersion law is derived. There occur in a natural way two definitions of the dust plasma frequency, and hence of the dust-acoustic velocity. One is based on the average over all dust grains of the ratio of the square of the charge to the mass, whereas the other one uses the square of the average charge and the average mass. The difference between the two gives rise to a dust distribution mode, which together with the self-gravitational effects influences various dust-acoustic and Jeans modes and instabilities in dusty plasmas. This corroborates earlier results, but also indicates that in dusty plasmas containing grains of opposite charges the dust distribution mode could be much stronger than for mixtures with like dust charges, as previously studied.
Fragmentation path for hydrogen atom dissociation from methoxy radical
Authors: Petraco Nicholas D. K. , Allen Wesley D. , Schaefer Henry F.
Published: July 2002

Salient features of the potential surface for hydrogen atom dissociation from the methoxy radical (CH3O) have been investigated via high-level coupled-cluster methods using a TZ2P(f,d) basis set for geometry optimization and harmonic vibrational analyses and the correlation-consistent cc-pVXZ (X=2–6) series for final energetic determinations and extrapolations. Of central concern for continuing photofragmentation dynamics experiments is the Cs-symmetry A′2 transition state for dissociation, which TZ2P(f,d) RCCSD(T) theory locates at a critical C–H distance of 1.79 Å with a barrier frequency of 947i cm−1. Our zero-point-corrected focal-point extrapolations place this transition state 4.7 kcal mol−1 above the CH2O+H products and yield a dissociation energy of 20.1 kcal mol−1; the latter differs from the most reliable experimental values by only 0.2–0.3 kcal mol−1. A revised enthalpy of formation, ΔHf,0°(CH3O)=6.5 kcal mol−1, is proposed. Disappointingly, TZ2P(f,d) UB3LYP theory underestimates the CH2O+H association barrier by 2.3 kcal mol−1, missing about half the barrier height. The complete set of TZ2P(f,d) RCCSD(T) data for structures and frequencies coupled with final focal-point energetics provides definitive values for parameters essential to the analysis of experimental photofragmentation rate profiles.
Switched cavity: An approach to micromass-flow sensors
Authors: Torigoe Ippei
Published: July 2002

A method is proposed for measuring and/or controlling the mass-flow rate of gases. A small cavity is connected in series to a conduit via two valves that are alternately opened and closed. When the upstream valve is open and the downstream one is closed, the gas in the cavity is compressed to the upstream pressure. In the next phase, the cavity is switched downstream, i.e., the upstream valve is closed, the downstream valve is opened, and the gas in the cavity is rarefied to the downstream pressure. Thus, a small volume of gas is conveyed via the cavity from the upstream to the downstream in each switching cycle. The average mass-flow rate per unit time is proportional to the valve switching frequency, the pressure difference between the upstream and downstream regions, the inverse of the gas temperature, and the capacity of the cavity. The mass-flow rate can be determined from the pressure difference and gas temperature, provided that the cavity is switched at a fixed frequency. In addition, the mass-flow rate can be controlled by changing the frequency of the switching. An experimental device that employed piezoelectric bimorph actuators as switching valves and a cavity of capacity 1.33×10−6 m3 was constructed and experiments were performed with mass-flow rates ranging from 0.2×10−4 to 2.7×10−4 g/s. The results verified the applicability of the proposed method as a micromass-flow sensor/controller.