bluuebunny/crossref_metadata_2025 huggingface dataset:
Hydration of a structured excess charge distribution: Infrared spectroscopy of the O2−⋅(H2O)n, (1⩽n⩽5) clusters
Authors: Weber J. Mathias , Kelley Jude A. , Robertson William H. , Johnson Mark A.
Published: July 2002

To explore how a structured excess charge distribution affects the hydration of an anion, we report mid-IR, argon predissociation spectra for the hydrated superoxide cluster anions, O2−⋅(H2O)n, 1⩽n⩽5. This size range was chosen to establish the evolution of the structures through the putative shell closing [Weber et al., Science 287, 2461 (2000)] for superoxide hydration at the tetrahydrate. Whereas the observed bonding motifs for n⩽4 are those of single water molecules and dimeric subclusters bound to the ion, the pentahydrate spectrum displays strong bands in the region typically associated with ring modes of the water trimer. The present results reinforce the conclusion that the tetrahydrate adopts an especially robust structure in which each water molecule forms a single ionic H bond to one of the lobes of the π* highest occupied molecular orbital in superoxide.
Structure factors effects in small-angle scattering from block copolymer micelles and star polymers
Authors: Pedersen Jan Skov
Published: July 2002

Expressions for the form factor of a block copolymer micelle model with a spherical core and Gaussian polymer chains attached to the surface have previously been given by Pedersen and Gerstenberg [Macromolecules 29, 1363 (1996)]. In the present work the inclusion of particle interference effects in the intensity expression in terms of a structure factor is described. The results are derived assuming that the effective interaction between the particles is known, i.e., the structure factor related to the center–center distribution is assumed to be known. The derived expression for the intensity is not a simple product of the form factor and the structure factor, which has the important consequence that the effective structure factor depends on the relative scattering contrast of the core and the corona of polymer chains. The structure factor effects for Gaussian star polymers are described by the same expression for a vanishing core radius. The influences of chain self-avoidance and chain–chain interactions are discussed.
100 nm period silicon antireflection structures fabricated using a porous alumina membrane mask
Authors: Kanamori Y. , Hane K. , Sai H. , Yugami H.
Published: July 2002

An ordered anodic porous alumina membrane has been used as a lithographic mask of SF6 fast atom beam etching to generate a 100 nm period antireflection structure on a silicon substrate. The antireflection structure consists of a deep hexagonal grating with 100 nm period and aspect ratio of 12, which is a fine two-dimensional antireflection structure. In the wavelength region from 400 to 800 nm, the reflectivity of the silicon surface decreases from around 40% to less than 1.6%. The measured results are explained well with the theoretical results calculated on the basis of rigorous coupled-wave analysis.
Energy-dependent anisotropic deformation of colloidal silica particles under MeV Au irradiation
Authors: van Dillen T. , Polman A. , Fukarek W. , van Blaaderen A.
Published: July 2002

Spherical silica colloids with a diameter of 1.0 μm, made by wet chemical synthesis, were irradiated with 2–16 MeV Au ions at fluences in the range (2–11)×1014 cm−2. The irradiation induces an anisotropic plastic deformation turning the spherical colloids into ellipsoidal oblates. After 16 MeV Au irradiation to a fluence of 11×1014 cm−2, a size aspect ratio of 5.0 was achieved. The size polydispersity (∼3%) remains unaffected by the irradiation. The transverse diameter increases exponentially with ion fluence. By performing measurements as a function of ion energy at a fixed fluence, it is concluded that the transverse diameter increases linearly with the average electronic energy loss above a threshold value of ∼0.6 keV/nm. Nonellipsoidal colloids are observed in the case where the projected ion range is smaller than the colloid diameter. The data provide strong support for the thermal spike model of anisotropic deformation.
Structure and stability of tilted-chain phases of alkanethiols on Au(111)
Authors: Barrena E. , Ocal C. , Salmeron M.
Published: July 2002

We present evidence for the existence of ordered tilted-chain phases of self-assembled monolayers of C18H37SH alkylthiols (C18) on Au(111). Tilted phases have been observed before under compressive stress in thiol and silanes monolayers. In the present experiments with C18, tilted phases have been observed to exist also in the absence of applied pressure. Atomic force microscopy (AFM) has been used to determine the thickness and lattice periodicity of the films. Two phases have been found to coexist: a hexagonal one with (3×3)R30° periodicity (tilt angle 35°), which is characteristic of complete monolayers, and a less dense one with (2×∛)rect periodicity. This corresponds to a 50° tilted phase, predicted by a chain-interlocking model. The new phase can be irreversibly transformed into the hexagonal phase by mechanical perturbation with the AFM tip.
A photopolymerizable glass with diffraction efficiency near 100% for holographic storage
Authors: Cheben Pavel , Calvo Marı́a L.
Published: July 2002

Permanent holographic storage has been demonstrated in a photopolymerizable organically modified silica glass. The glass was prepared by dispersing a titanocene photoinitiator and a high refractive index acrylic monomer in a porous silica matrix. This glass exhibits unprecedented sensitivity and refractive index change upon a moderate exposure to green light and can be fabricated in thickness up to several millimeters. A photopolymerizable storage medium of such a thickness with good holographic properties is needed for practical holographic storage devices. Lack of such medium has been considered the main obstacle in development of write-once holographic memories. In our glass, we have stored permanent volume holograms of diffraction efficiency approaching 100% and refractive index modulation up to 4.5×10−3, making this photopolymerizable material suitable for use in holographic data storage.
Electromigration in eutectic SnPb solder lines
Authors: Huynh Q. T. , Liu C. Y. , Chen Chih , Tu K. N.
Published: July 2002

We have prepared eutectic SnPb solder lines for electromigration study by a process of solder reflow into V-grooves etched on (001) Si wafer surfaces. They are thick lines and are highly reproducible. We report here results of lines of 100 μm in width and 150 to 800 μm in length, stressed by a current density of 2.8×104 A/cm2 at 150 °C in ambient. The accumulation of a large lump of solder, rather than hillocks of Sn and Pb, was observed at the anode, and depletions and voids were observed at the cathode. By measuring the volume of the lump, we have calculated the average effective charge number of electromigration in the eutectic solder to be 33, which is close to the reported value of 47 for self-electromigration in bulk Pb. Using x-ray dispersive analysis, we found that Pb is the dominant diffusing species.
Raman and infrared modes of hydrogenated amorphous carbon nitride
Authors: Rodil S. E. , Ferrari A. C. , Robertson J. , Milne W. I.
Published: July 2002

Features in the Raman and infrared (IR) spectra of highly sp3 bonded hydrogenated amorphous carbon nitride films are assigned. The Raman spectra show three main features all found in a-C itself, the G and D peaks at 1550 and 1350 cm−1, respectively, and the L peak near 700 cm−1. The intensity ratio of the D and G peaks, I(D)/I(G), is found to scale as (band gap)−2, which confirms that nitrogen induces carbon to form sp2 graphitic clusters. The intensity of the L mode is found to scale with the D mode, supporting its identification as an in-plane rotational mode of sixfold rings in graphitic clusters. A small feature at 2200 cm−1 due to C≡N modes is seen, but otherwise the Raman spectra resembles that of a-C and shows no specific features due to N atoms. The hydrogen content is found to have a strong effect on the IR spectra at 1100–1600 cm−1 making this band asymmetric towards the 1600 cm−1 region.
Generating all Wigner functions
Authors: Curtright Thomas , Uematsu Tsuneo , Zachos Cosmas
Published: July 2002

In the context of phase-space quantization, matrix elements and observables result from integration of c-number functions over phase space, with Wigner functions serving as the quasiprobability measure. The complete sets of Wigner functions necessary to expand all phase-space functions include off-diagonal Wigner functions, which may appear technically involved. Nevertheless, it is shown here that suitable generating functions of these complete sets can often be constructed, which are relatively simple, and lead to compact evaluations of matrix elements. New features of such generating functions are detailed and explored for integer-indexed sets, such as for the harmonic oscillator, as well as continuously indexed ones, such as for the linear potential and the Liouville potential. The utility of such generating functions is illustrated in the computation of star functions, spectra, and perturbation theory in phase space.
On the nature of the D1-defect center in SiC: A photoluminescence study of layers grown by solid-source molecular-beam epitaxy
Authors: Fissel A. , Richter W. , Furthmüller J. , Bechstedt F.
Published: July 2002

Undoped and boron-doped SiC layers are grown on hexagonal SiC(0001) substrates by means of solid-source molecular-beam epitaxy. Hexagonal 4H– and 6H–SiC layers are grown homoepitaxially via step-controlled epitaxy, whereas the cubic 3C–SiC is grown pseudomorphically via nucleation and subsequent step flow. The low-temperature photoluminescence spectra only show the well-known emission lines of the so-called D1 center. The line positions are compared with results of first-principles calculations. The growth conditions, the line shape, and the line shift with the polytype support an interpretation as bound-exciton recombination at a native-defect complex that contains a Si vacancy.
Field emission from cylindrical carbon nanotube cathodes: Possibilities for luminescent tubes
Authors: Bonard Jean-Marc , Stöckli Thomas , Noury Olivier , Châtelain André
Published: July 2002

We show that the field of application of cold electron film emitters can be extended to nonplanar geometries by demonstrating a cylindrical field emission diode. The cathode is a metallic wire on which multiwall carbon nanotubes are grown by the catalytic decomposition of acetylene over a Fe catalyst. The emitter shows excellent performances and can be used to realize a luminescent, mercury-free, tube.
Synthesis and magnetic properties of CoFe2O4 spinel ferrite nanoparticles doped with lanthanide ions
Authors: Kahn Myrtil L. , Zhang Z. John
Published: July 2002

Lanthanide ions have been doped into cobalt spinel ferrites using an oil-in-water micellar method to form CoLn0.12Fe1.88O4 nanoparticles with Ln=Ce, Sm, Eu, Gd, Dy, or Er. Doping with lanthanide ions (LnIII) modulates the magnetic properties of cobalt spinel ferrite nanoparticles. In particular cases of Gd3+ or Dy3+ ions, a dramatic increase in the blocking temperature and coercivity is observed. Indeed, the introduction of only 4% of Gd3+ ions increases the blocking temperature ∼100 K and the coercivity 60%. Initial studies on the magnetic properties of these doped nanoparticles clearly demonstrate that the relationship between the modulation of magnetic properties and the nature of doped LnIII ions is interesting but very complex.
Hyperbolic theory of the “shallow water” magnetohydrodynamics equations
Authors: De Sterck H.
Published: July 2002

Recently the shallow water magnetohydrodynamic (SMHD) equations have been proposed for describing the dynamics of nearly incompressible conducting fluids for which the evolution is nearly two-dimensional (2D) with magnetohydrostatic equilibrium in the third direction. In the present paper the properties of the SMHD equations as a nonlinear system of hyperbolic conservation laws are described. Characteristics and Riemann invariants are studied for 1D unsteady and 2D steady flow. Simple wave solutions are derived, and the nonlinear character of the wave modes is investigated. The ∇⋅(h B)=0 constraint and its role in obtaining a regularized Galilean invariant conservation law form of the SMHD equations is discussed. Solutions of the Rankine–Hugoniot relations are classified and their properties are investigated. The derived properties of the wave modes are illustrated by 1D numerical simulation results of SMHD Riemann problems. A Roe-type linearization of the SMHD equations is given which can serve as a building block for accurate shock-capturing numerical schemes. The SMHD equations are presently being used in the study of the dynamics of layers in the solar interior, but they may also be applicable to problems involving the free surface flow of conducting fluids in laboratory and industrial environments.
Structure and magnetic properties of yttrium–iron–garnet thin films prepared by laser deposition
Authors: Popova E. , Keller N. , Gendron F. , Guyot M. , Brianso M.-C. , Dumond Y. , Tessier M.
Published: July 2002

We report on a study of thin (500–4000 Å) and ultrathin (100–500 Å) yttrium–iron–garnet (YIG) films deposited onto amorphous quartz substrates by the pulsed laser deposition technique. The growth conditions of well-formed polycrystalline films have been determined. The crystalline structure and the magnetic behavior of the films are strongly influenced by the processes occurring on the film–substrate interface. Primarily, a strain due to the difference in thermal expansion coefficients of the film and substrate induces a uniaxial anisotropy. Another source of strain are lattice distortions due to oxygen vacancies. The results obtained from x-ray diffraction analysis, magneto-optical, superconducting quantum interference device, vibrating sample magnetometer, and ferromagnetic resonance (FMR) measurements indicate that films thicker than 200 Å can be approximated by a two-layer model. One of the layers with a highly distorted structure and low magnetization is located near the surface. The other one, the upper layer, can be estimated as an almost perfectly formed YIG. Their relative thickness determines the magnetic and FMR behavior of the samples. An equilibrium direction of the magnetization along the film normal has been found to occur in the ultrathin films with the lowest magnetization where the perpendicular anisotropy energy exceeds the dipolar energy.
Comparison of four methods to compute the dielectric permittivity of liquids from molecular dynamics simulations
Authors: Heinz Tim N. , van Gunsteren Wilfred F. , Hünenberger Philippe H.
Published: July 2002

Four methods to compute the dielectric permittivity ε of a liquid from molecular simulations are compared in the context of the simple point charge (SPC) water model. In the first method (unrestrained method), ε is evaluated from the fluctuations of the box dipole moment M, monitored during a single equilibrium simulation. In the three other methods, ε is evaluated from the probability distribution p(M) of the dipole moment norm. This distribution is itself evaluated in three different ways: (i) from multiple simulations involving a M-dependent biasing potential (umbrella-sampling method), (ii) from multiple simulations involving a constrained dipole moment norm (M-constraint method), or (iii) from fitting of incomplete p(M) estimates to a Maxwell distribution (fitting method). The four methods are shown to converge to an identical estimate of ε=61±1 for SPC water (256 molecules, reaction-field electrostatics). The convergence properties, advantages, and drawbacks of the different methods are analyzed in detail.
Ab initio rovibrational spectroscopy of hydrogen sulfide
Authors: Tarczay György , Császár Attila G. , Polyansky Oleg L. , Tennyson Jonathan
Published: July 2002

Potential energy hypersurfaces (PES) have been constructed for the ground electronic state of H2S utilizing results from state-of-the-art ab initio quantum chemical methods, most notably higher-order coupled cluster theory employing (core-polarized) correlation-consistent basis sets. Small corrections due to extrapolation to the complete basis set and full configuration interaction limits, core correlation, and relativistic corrections, as well as effects beyond the Born–Oppenheimer approximation have been investigated and incorporated into the final PES. Using the exact rovibrational kinetic energy operator rovibrational energy levels have been computed with the different PESs. The final converged ab initio PES of this study reproduces the available vibrational band origins of H2 32S, HD32S, D2 32S, and H2 34S with maximum deviations, gradually increasing for increased vibrational excitation, of 29(14 300), 10(3800), 7(4600), and 12(6400) cm−1, respectively, where the maximum energy above the zero-point energy is given in parentheses. The errors are considerably larger for the bending states than for the stretching states. Reproduction of rotational term values, given explicitly for J=17 of the vibrational ground state, shows remarkable agreement between experiment and the purely ab initio approach of this study.
Elastic properties of zinc tris(thiourea) sulphate single crystals
Authors: Alex A. V. , Philip J.
Published: July 2002

Zinc tris(thiourea) sulphate (ZTS) single crystals possess excellent nonlinear optical properties and are nearly 1.2 times more efficient than potassium dihydrogen phosphate in optical harmonic generation. Large single crystals of this material have been grown from aqueous solution. All nine second order elastic stiffness constants of this orthorhombic crystal have been determined by measuring the velocity of propagation of ultrasonic waves of longitudinal and transverse polarizations along specific symmetry directions. The McSkimin Δt correction has been applied for better accuracy of velocity values. The corresponding elastic compliance constants and Poisson’s ratios have also been evaluated. Planar plots of phase velocity, group velocity, slowness, Young’s modulus, and linear compressibility have been drawn to display the anisotropy of wave propagation in the crystal. The bulk modulus and volume compressibility of this crystal have been evaluated from the elastic constant data. Variation of a selected number of elastic constants with temperature in a limited range have also been measured and reported.
Fluid description of electron temperature gradient driven drift modes
Authors: Singh R. , Tangri Varun , Nordman H. , Weiland J.
Published: July 2002

The electron temperature gradient (ETG) driven drift mode is studied using an advanced fluid model retaining effects of nonadiabatic ions, Debye shielding and the electron diamagnetic heat flow. The derived eigenmode equation is solved analytically in the strong ballooning limit. Both the toroidal and the slab branch of the ETG mode are included and the fluid growth rates are compared with gyrokinetic results. The role of nonadiabatic ion response is found to have a stabilizing effect on ETG-mode in the lower-hybrid regime. Strong stabilization is also found due to Debye shielding effect for λDe2/ρe2>1. In particular, it is shown that nonadiabatic ion response can result in inward flows of particles for peaked density profiles.
White light emission from blends of blue-emitting organic molecules: A general route to the white organic light-emitting diode?
Authors: Thompson J. , Blyth R. I. R. , Mazzeo M. , Anni M. , Gigli G. , Cingolani R.
Published: July 2002

We show that all possible binary combinations of molecules from four different families of organics—a diamine derivative, N,N′-bis(3methylphenyl)-N,N′-diphenylbenzidine, an oxidiazole derivative, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, a substituted thiophene dioxide, 2,5-bis(trimethylsilyl thiophene)-1,1-dioxide, and poly(9-vinylcarbazole)—produce white or near-white emission. We suggest that this is due to exciplex formation, and that this is likely to be a general phenomenon for blends of blue-emitting aromatic organics. This implies that films of spin-coated blends of blue-emitting organics represent a general, simple, and cheap route to white-emitting organic light-emitting diodes.
Hybrid model for the calculation of ion distribution functions behind a direct current or radio frequency driven plasma boundary sheath
Authors: Kratzer Matthias , Brinkmann Ralf Peter , Sabisch Winfried , Schmidt Harald
Published: July 2002

A hybrid fluid dynamic/kinetic model is presented which describes the sheath and the presheath regions of dc or rf driven low pressure gas discharges in a realistic and self-consistent way. The model assumes an infinite extended sheath parallel to the electrode, allowing a one-dimensional spatial description. It provides for the presence of multiple positive ion species and their collisional interactions with the neutral background, and takes into account the possibility of a nonharmonic modulation of the sheath potential and the application of an external dc bias; in this work, the model is applied to a two-species capacitively coupled argon and oxygen plasma. The input required by the model consists of the fluxes of the incoming ions, of the modulating current, and of the pressure, the composition, and the temperature of the background gas. On output, the model provides the values of the electric field and of the particle densities within the sheath and the presheath, the total voltage drop across the sheath, and also the energetically and angularly resolved distributions of the positive ions and the energetic neutrals which impinge the material substrate at the boundary. In general, the model is able to treat dc discharges as well as capacitively and/or inductively coupled rf discharges, it thus covers most of the plasmas used in very large scale integration microelectronics manufacturing and other surface modification techniques. Using the model, studies of the energy distributions of the incoming ions have been performed for a wide range of parameters, and the effects of varying process conditions have been investigated. At low and intermediate pressures (p<50 mTorr), the distribution functions of rf driven discharges exhibit a characteristic bimodal structure; this structure disappears with increasing pressure as ion-neutral collisions become significant. A comparison of calculated ion energy distributions with experimental measurements on capacitively coupled argon and oxygen discharges shows excellent quantitative agreement. In addition to the ion energy distribution, the angular distributions of the incident ions at various energies are also discussed as a function of the neutral gas pressure. It turns out that the details of the angular distribution not only depend on the field structure of the sheath itself but also on that of the presheath. The results of the presented model are therefore more reliable than those of previous models which restricted themselves to the sheath region. This high physical accuracy of the presented model, together with its flexibility and its high execution speed, allows its use as a tool for technology-oriented computer-aided design in the microelectronics industry.
Modification of the optical and photorefractive properties of Ce-doped strontium–barium niobate by co-doping with a nonphotorefractive La impurity
Authors: Volk T. , Ivleva L. , Lykov P. , Isakov D. , Osiko V. , Wöhlecke M.
Published: July 2002

We demonstrate a possibility to modify photorefractive properties of a ferroelectric crystal by means of controlling the ferroelectric phase transition. This is shown by an example of co-doping a photorefractive SBN-0.61:Ce crystal with a nonphotorefractive La impurity, which strongly lowers the ferroelectric phase transition and in turn drastically enhances the linear electrooptic coefficients. The observed low coercive fields enable us to build in SBN:(Ce+La) a reproducible fast switching of the energy transfer direction. This is done by an electric field switching which changes the sign of the two-beam coupling gain factor Γ.
Parametric study of single-axis acoustic levitation
Authors: Xie W. J. , Wei B.
Published: July 2002

Remarkable enhancement of the single-axis acoustic levitation force is achieved by properly curving the surface and enlarging the section of the reflector so as to levitate high density material like tungsten (ρs=18.92 g/cm3). A two-cylinder model incorporating the boundary element method simulations is presented for systematic study of the relationship between levitation capabilities and geometric parameters. The model proves to be successful in predicting resonant modes and explaining deviation of the levitated samples near the reflector and driver. The dependence of levitation force on resonant mode, reflector section radius Rb and curvature radius R is revealed and summarized, which agrees with the experiment in principle and suggests that a reflector with large Rb and small R (when Rb/λ⩾0.982) working under mode 1 assures better levitation capabilities.
Interactions between passivated nanoparticles in solutions: Beyond the continuum model
Authors: Rabani Eran , Egorov S. A.
Published: July 2002

We apply the integral equation theory to study the solvent-induced potential of mean force between two passivated nanoparticles in dilute solution. This approach explicitly accounts for the molecular structure of the solvent and the anisotropy of its density profile induced by the pair of nanoparticles. We discuss the implications of our theoretical results for the problem of self-assembly of nanoparticles, and point out significant differences from the commonly used continuum solvent model.
Comparison of Poly(methylmethacrylate) and Novolak waveguide coatings for an acoustic biosensor
Authors: Rasmusson Anne , Gizeli Electra
Published: July 2002

In order to optimize the geometry of the acoustic waveguide biosensor, the performance of the device was assessed for two different guiding layers: a photeresist (Novolak) and a polymer Poly(methylmethacrylate) (PMMA) one. Initially, the effect of the thickness of each layer on the insertion loss of the device and frequency of the wave was monitored. The 1.5 and 1.7 μm Novolak and PMMA, respectively, coated devices were used to monitor the binding of Immunoglobulin G to the protein A activated device surface. Both devices were found to exhibit the same phase sensitivity to protein deposition. However, Novolak waveguides showed a higher stability in contact with water, making them more suitable for biosensing applications than PMMA waveguides. Finally, the reproducibility of the Novolak-coated waveguide was studied during the formation of the polymer layer, addition of buffer, and antibody binding.
Magnetic properties of the MnBi intermetallic compound
Authors: Yang J. B. , Kamaraju K. , Yelon W. B. , James W. J. , Cai Q. , Bollero A.
Published: July 2002

A MnBi alloy containing over 90 wt % low-temperature phase (LTP) has been obtained by high-temperature sintering and magnetic purification. The coercivity of the bonded MnBi magnet increases with increasing temperatures. A coercivity of 2.0 T has been achieved at 400 K. The maximum energy product (BH)max of the magnet is 7.7 MGOe (61 kJ/m3) and 4.6 MGOe (37 kJ/m3) at room temperature and 400 K, respectively. Neutron diffraction and magnetic data reveal a spin reorientation, which gives rise to low anisotropy fields and coercivity at lower temperatures for the LTP MnBi alloy.
Gravitational instability of miscible fluids in a Hele-Shaw cell
Authors: Martin J. , Rakotomalala N. , Salin D.
Published: July 2002

We revisit the Rayleigh–Taylor instability when the two fluids are miscible and in the geometry of a Hele-Shaw cell. We provide analytical dispersion relations for the particular cases of either a sharp front between the two fluids or of a uniform density gradient stratification and for various fluid flow models, including an unbounded geometry, a two-dimensional gap-averaged Navier–Stokes–Darcy equation, and an effective porous medium. The results are compared to three-dimensional lattice BGK simulations, based on which the relevance of the various models in different wavelength regimes is discussed.
Effect of confinement by porous materials on chemical reaction kinetics
Authors: Turner C. Heath , Brennan John K. , Johnson J. Karl , Gubbins Keith E.
Published: July 2002

A methodology for including the effects of nonidealities, such as confinement in a porous solid or solvation, into the calculation of bimolecular reaction rate constants is presented. The method combines the transition-state theory formalism with the Reactive Monte Carlo simulation method. The approach is computationally efficient and accurate, within the approximations imposed by transition-state theory and the intermolecular potentials. Several applications of the method are presented for the decomposition reaction, 2HI→H2+I2, including effects due to confinement within carbon micropores and due to inert solvents. The method can be readily extended to other chemical reaction rate calculations in which the structure and the activation energy of the transition state is known a priori.
Semiclassical calculations of energy level widths for the unimolecular dissociation of HN2 and DN2
Authors: Guo Yin , Thompson Donald L.
Published: July 2002

We further examine the semiclassical approach for treating tunneling within classical trajectory simulations by applying it to the dissociation of HN2 and DN2. The calculated level widths of vibrational states are in good accord with the corresponding quantum-mechanical results for a wide range of energies from the tunneling to classical regimes, demonstrating the accuracy of the method. Comparisons of results obtained with normal-mode and efficient microcanonical samplings show that proper sampling of initial states is crucial for the low-lying states in the deep tunneling region, but becomes less important for the states at higher energies. The present work provides further evidence of the applicability of this semiclassical approach to tunneling problems in polyatomic systems.
Thermal stability and performance reliability of Pt/Ti/WSi/Ni ohmic contacts to n-SiC for high temperature and pulsed power device applications
Authors: Cole M. W. , Joshi P. C. , Hubbard C. , Demaree J. D. , Ervin M.
Published: July 2002

Pt/Ti/WSi/Ni ohmic contacts to n-SiC, initially annealed at 950 and 1000 °C for 30 s, were evaluated for thermal stability via pulsed/cyclic thermal fatigue and aging experiments at 650 °C. Modifications of material properties in response to cyclic thermal fatigue and aging tests were quantitatively assessed via current–voltage measurements, field emission scanning microscopy, atomic force microscopy, and Rutherford backscattering spectrometry. Negligible changes in the electrical properties, microstructure, and surface morphology/roughness were observed for both annealed ohmic contacts in response to 100 cycles of acute cyclic thermal fatigue. Aging of the 950 °C annealed contact for 75 h at 650 °C resulted in electrical failure and chemical interdiffusion/reaction between the contact and SiC substrate. The 1000 °C annealed contact retained ohmicity after 100 h of aging and was found to be chemically and microstructurally stable. These findings indicate that the 1000 °C annealed Pt/Ti/WSi/Ni ohmic contact to n-SiC is thermally stable and merits strong potential for utilization in high temperature and pulsed power devices.
Drop of magnetocaloric effect related to the change from first- to second-order magnetic phase transition in La2/3(Ca1−xSrx)1/3MnO3
Authors: Mira J. , Rivas J. , Hueso L. E. , Rivadulla F. , López Quintela M. A.
Published: July 2002

From data of initial magnetization isotherms, the magnetic entropy change ΔSM of the series of ferromagnetic perovskites La2/3(Ca1−xSrx)1/3MnO3, with x=0, 0.05, 0.15, 0.25, 0.50, 0.75 and 1, has been measured near their Curie temperatures. The results go from 3.7 J kg−1 K−1 for La2/3Ca1/3MnO3 to 1.5 J kg−1 K−1 for La2/3Sr1/3MnO3. Nevertheless, the evolution of ΔSM with x is not monotonic, it shows a steep decrease between x=0.05 and x=0.15. This point corresponds to a tolerance factor t=0.92, at which the system changes from orthorhombic (Pbnm) to rhombohedral (R3̄c) structure, and the magnetic phase transition from first to second order. Provided that rhombohedral symmetry forbids static long-range cooperative Jahn–Teller distortions, present in orthorhombic samples, we interpret our results as the strong increase of lattice effects in them. The anomalous thermal expansion at the Curie temperature, due to these lattice effects, is thought to underlie the jump in ΔSM with x.
Characterization of pit formation in III-nitrides grown by metalorganic chemical vapor deposition
Authors: Cho H. K. , Lee J. Y. , Yang G. M.
Published: July 2002

Pit formation in III-nitride heterostructures such as InGaN/GaN and AlGaN/GaN grown by metalorganic chemical vapor deposition was characterized by transmission electron microscopy. The pit formation related with V defects has been reported in the InGaN/GaN multiple quantum well with high In composition [Appl. Phys. Lett. 79, 215 (2001)]. In this letter, we found that the mechanism of pit formation strongly depends on the indium and aluminum compositions in InxGa1-xN and AlxGa1-xN layers, respectively. By increasing the indium composition, the origin of pits is changed from the vertex of threading dislocations to the stacking mismatch boundaries induced by stacking faults and the three-dimensional island growth at the initial stage due to the large lattice mismatch. By increasing the aluminum composition, the origin of the pits also varied from the surface undulation due to the elastic misfit strain to the vertex of threading dislocations. In addition, several inversion domains observed in III nitrides result in pit formation in the surface of films.
Effect of nonadiabatic dust charge variations on nonlinear dust acoustic waves with nonisothermal ions
Authors: Ghosh Samiran , Sarkar Susmita , Khan Manoranjan , Gupta M. R.
Published: July 2002

The effect of nonadiabatic dust charge variation on the nonlinear dust acoustic wave in a dusty plasma consisting of warm adiabatic dust grains, isothermal electrons and nonisothermal ions have been investigated by reductive perturbation technique. It has been shown that due to the nonadiabatic dust charge variation and also for the presence of nonisothermal ions, nonlinear dust acoustic wave is governed by a modified Korteweg–de Vries Burger (mKdVB) equation. The Burger term arises due to the nonadiabatic dust charge variation, whereas the nonlinear terms are modified due the different effects of nonisothermal ions. Numerical integration of mKdVB equation shows that the nonlinear dust acoustic wave admits negative potentials with the oscillatory (dispersion dominant) or monotonic (dissipation dominant) shock transition and exhibits compressional shock wave.
Photochemistry in phosphorus-doped silica glass by ArF excimer laser irradiation: Crucial effect of H2 loading
Authors: Hosono Hideo , Kajihara Koichi , Hirano Masahiro , Oto Masanori
Published: July 2002

A striking difference in photochemistry induced by ArF excimer laser irradiation was found between H2-unloaded and H2-loaded P-doped SiO2 glasses. Although a pair of phosphorus-electron center and phosphorus-oxygen-hole center (POHC) were produced in the former, PHO2 (pyramidal shape) and PO2 (planar) centers were created instead of the POHC in the latter. The present finding strongly suggests that H2 loading treatment of P-doped SiO2 at ambient temperature induces a drastic structural alternation around phosphorus, leading to new precursor structures which account to the photosensitivity of the materials.
CH
4
dissociation on Ru(0001): A view from both sides of the barrier
Authors: Mortensen H. , Diekhöner L. , Baurichter A. , Luntz A. C.
Published: July 2002

This paper reports measurements of both dissociative adsorption on and associative desorption from CH4 on Ru(0001). We consider the former a view of dissociation from the front side of the barrier, while the latter is considered as a view of dissociation from the back side of the barrier. A combination of both previous and new molecular beam measurements of dissociative adsorption shows that S0 depends on all experimental variables (E, Tn, Ts and isotope) in a manner similar to other close-packed transition metals. The interpretation of this behavior in terms of a theoretical description of the dissociation is discussed critically, with special emphasis on insights from new theoretical studies. The energy-resolved desorption flux Df(E,Ts) is obtained in associative desorption experiments using the technique of laser assisted associative desorption (LAAD). Measurements at several Ts allow both a direct determination of the adiabatic barrier V*(0) and considerable insight into the dynamics of dissociation. The V*(0) obtained from Df(E,Ts) is in excellent agreement with density functional theory (DFT) calculations and with the value indirectly inferred from molecular beam experiments. The chief dynamic conclusion from an analysis of Df(E,Ts) is that both bending and stretching coordinates must be produced in associative desorption, although they are not populated statistically. The absence of an isotope effect in the shape of Df(E,Ts) argues against the importance of tunneling in the desorption/adsorption. When reactive fluxes are compared via detailed balance, both the molecular beam experiment and the LAAD experiment are in good agreement.
The dependence of nonadiabatic couplings on the origin of electron coordinates
Authors: Belyaev Andrey K. , Dalgarno Alexander , McCarroll Ronald
Published: July 2002

It is shown both analytically and numerically for a number of examples that both radial and rotational nonadiabatic couplings within the standard adiabatic approach depend on the origin of the light-particle coordinates and the ambiguity in the nonadiabatic couplings does not lead to ambiguity in the coupled channel equations. The examples considered are the nH, nD, npμ− quasimolecules, for which the nonadiabatic couplings can be calculated analytically, and the HeH molecule, for which ab initio calculations are carried out. Analytical formulas for couplings calculated with the shifted origin are derived. The coupled equations take their simplest form in Jacobi coordinates for which many nonadiabatic couplings are nonzero, even for such noninteracting systems as nH, nD, and npμ−. These couplings are a fundamental feature of the adiabatic approach.
Study of circumferential waves and their interaction with defects on cylindrical shells using line-source laser ultrasonics
Authors: Gao Weimin , Glorieux Christ , Thoen Jan
Published: July 2002

An application of laser ultrasonics to investigate circumferential waves and their interaction with defects on cylindrical structures is presented. Due to the high effectiveness of the laser line-source generation regime and the broadband nature of the system, circumferential waves could be generated and detected with a wide bandwidth and an excellent signal-to-noise ratio. A clear recognition of multimode wave forms is achieved by two-dimensional Fourier transform signal processing. Measurements were also made on a cylindrical shell with a crack-like defect which was simulated by an artificial groove on the surface of the specimen. The interaction of the circumferential waves with a defect in the inner or outer surface of the thin cylindrical shell has been studied. The results may lead to an application for defect identification at the inner or outer surface of thin cylindrical shells.
Hamiltonian and symmetric hyperbolic structures of shallow water magnetohydrodynamics
Authors: Dellar Paul J.
Published: July 2002

Shallow water magnetohydrodynamics is a recently proposed model for a thin layer of incompressible, electrically conducting fluid. The velocity and magnetic field are taken to be nearly two dimensional, with approximate magnetohydrostatic balance in the perpendicular direction. In this paper a Hamiltonian description, with the ubiquitous noncanonical Lie–Poisson bracket for barotropic magnetohydrodynamics, is derived by integrating the three-dimensional energy density in the perpendicular direction. Specialization to two dimensions yields an elegant form of the bracket, from which further conserved quantities (Casimirs) of shallow water magnetohydrodynamics are derived. These Casimirs closely resemble the Casimirs of incompressible reduced magnetohydrodynamics, so the stability properties of the two systems may be expected to be similar. The shallow water magnetohydrodynamics system is also cast into symmetric hyperbolic form. The symmetric and Hamiltonian properties become incompatible when the relevant divergence-free constraint ∇⋅(hB)=0 is relaxed.
Development of 〈110〉 texture in copper thin films
Authors: Wei H. L. , Huang Hanchen , Woo C. H. , Zheng R. K. , Wen G. H. , Zhang X. X.
Published: July 2002

Apart from the scientific interest, texture development in copper thin films is of crucial importance to their applications as interconnects or corrosion resistant coating. We report here a dominant 〈110〉 texture of copper thin films—preferred for oxidation-resistant applications—deposited by direct current magnetron sputtering. Scanning electron microscopy shows that the copper films go through a transition from 〈111〉 columns to 〈110〉 hillocks as the deposition proceeds. Cross-sectional transmission electron microscopy (TEM) indicates that the 〈110〉 grains nucleate at boundaries of 〈111〉 grains. Further, we have proposed a stress-driven nucleation and growth model of 〈110〉 grains based on the x-ray diffraction characterization and the TEM observations.
Phase diagrams and dielectric response of epitaxial barium strontium titanate films: A theoretical analysis
Authors: Ban Z.-G. , Alpay S. P.
Published: July 2002

We develop phase diagrams for single-domain epitaxial barium strontium titanate films on cubic substrates as a function of the misfit strain based on a Landau–Devonshire phenomenological model similar to the one developed by Pertsev et al. [Phys. Rev. Lett. 80, 1988 (1998)]. The biaxial epitaxy-induced internal stresses enable phase transformations to unusual ferroelectric phases that are not possible in single crystals and bulk ceramics. The dielectric response of the films is calculated as a function of the misfit strain by taking into account the formation of misfit dislocations that relieve epitaxial stresses during deposition. It is shown that by adjusting the misfit strain via substrate selection and film thickness, a high dielectric response can be obtained, especially in the vicinity of structural instabilities. Theoretical estimation of the dielectric constant of (001) Ba0.7Sr0.3TiO3 and Ba0.6Sr0.4TiO3 films on (001) Si, MgO, LaAlO3, and SrTiO3 substrates as a function of misfit strain and film thickness is provided. An order-of-magnitude increase in the dielectric constant with increasing film thickness is expected for films on LaAlO3 and SrTiO3 substrates. A structural instability around 40 nm is predicted in films on MgO substrates accompanied by a substantial increase in the dielectric constant. For films on MgO substrates thicker than 40 nm, the analysis shows that the dielectric constant decreases significantly. We show that the theoretical approach not only predicts general trends but is also in good quantitative agreement with the experimental data reported in literature.
The effects of continuous size distributions on the rapid flow of inelastic particles
Authors: Dahl S. R. , Clelland R. , Hrenya C. M.
Published: July 2002

Molecular-dynamics simulations are employed to investigate the stresses and granular energy in granular materials with Gaussian and lognormal size distributions. Specifically, smooth circular disks of uniform material density engaged in unbounded two-dimensional shear flow are simulated using an event-driven algorithm. Particle collisions are treated as hard-sphere collisions and all collisions have the same coefficient of restitution. The resulting stresses, when nondimensionalized with the root-mean-square diameter, are found to remain relatively constant as the widths of the particle size distributions are increased away from the monodisperse limit. As a consequence, the stresses predicted by monodisperse kinetic theory (using the root-mean-square diameter) are reasonably accurate in the Gaussian and lognormal systems studied herein. This width-independent nature of the total stresses is traced to an effective balancing of the stresses between the larger particles, which generate relatively high stresses, and smaller particles, which generate lower stresses. Moreover, similar to binary-sized systems, the granular energy in Gaussian and lognormal systems is found to be unequally distributed among the various sizes of particles, with large particles possessing more granular energy than their smaller counterparts (i.e., an equipartition of energy is not observed). This difference in granular energy between two particles increases with both inelasticity and the size difference.
Phase transformation studies of ceramic BaTiO3 using thermo-Raman and dielectric constant measurements
Authors: Baskaran Natesan , Ghule Anil , Bhongale Chetan , Murugan Ramaswamy , Chang Hua
Published: July 2002

Ferroelectric phase transformation characteristics of ceramic BaTiO3 have been studied by combined thermo-Raman and dielectric measurements. The temperature dependence of Raman bands at 311 and 721 cm−1 shows that tetragonal to cubic (T–C) phase transformation occurs over a range of temperature. The increase in the bandwidths of these bands indicates that this transformation is an order-disorder transformation. The differential thermo-Raman intensity thermograms show a dip at a temperature corresponding to the maximum rate of phase transformation and this temperature can be defined as phase transformation temperature Tp. The ferroelectric transformation temperature Tm corresponding to the peak in the ε(T) obtained from the temperature dependence of dielectric constant is found to coincide with Tp. This indicates that maximum in dielectric constant occurs at the maximum rate of T–C phase transformation. Both Tp and Tm exhibit thermal hysteresis. The Curie temperature TC obtained from the intersection of tangential of inverse dielectric constant to the temperature axis and is found to be less than Tm and Tp in agreement with O. G. Vendik and S. P. Zubko [J. Appl. Phys. 88, 5343 (2000)] calculations. Thermo-Raman spectroscopy coupled with dielectric constant measurement is shown to be an effective technique to investigate the phase transformation in ferroelectric materials.
Experimental observation of complete chaos synchronization in semiconductor lasers
Authors: Liu Y. , Takiguchi Y. , Davis P. , Aida T. , Saito S. , Liu J. M.
Published: July 2002

We experimentally demonstrate the complete synchronization of a semiconductor laser to the injection of a chaotic oscillating optical signal that is generated by a similar semiconductor laser with external optical feedback. The synchronization is characterized by sensitive dependencies on frequency detuning and injection strength and a time lag that varies reversely with the variation of the delay time in the external optical feedback of the master laser.