bluuebunny/crossref_metadata_2025 huggingface dataset:
Interaction of short-chain alkane thiols and thiolates with small gold clusters: Adsorption structures and energetics
Authors: Krüger Daniel , Fuchs Harald , Rousseau Roger , Marx Dominik , Parrinello Michele
Published: July 2002

Gold–sulfur bonding is investigated theoretically using a variety of electronic structure methods, including the Becke–Perdew semilocal density functional, the B3LYP hybrid approach, the Hartree–Fock method, and the post Hartree–Fock approaches MP2 and QCISD(T). Particular emphasis is given to adsorption structure and energetics in the case of weak and strong interactions of this general type, using up to five gold atoms and up to three carbon atoms in the aliphatic chain. It is found that all methods which take into account electron correlation, including the density functional methods, lead to quite similar structures. Concerning the energetics, the Becke–Perdew functional is found to overbind typically by about 5–15%. Quasiglobal structural relaxation based on ab initio simulated annealing clearly shows that the adsorption of thiolates onto gold clusters results in a dramatic distortion of the cluster framework. From a structural point of view the thiolate sulfur–gold bond has a distinctive directional (covalent) character which results in a clear preference for Au–S–C bond angles in the range of 103.5° and 108.7°. In general, dissociation into open-shell species is preferred against the formation of the ionic closed-shell counterparts if the sulfur–gold bond is forced to break. However, neutral closed-shell products can be favored if fragmentation of the gold cluster is allowed for as a dissociation channel. Finally, it is demonstrated that using ethyl or n-propyl chains instead of the methyl group leads to only small changes of the binding energies.
Experimental and theoretical studies of the near-infrared spectrum of bromomethylene
Authors: Yu Hua-Gen , Gonzalez-Lezana Tomas , Marr Andrew J. , Muckerman James T. , Sears Trevor J.
Published: July 2002

New measurements in the Ã 1A″–X̃ 1A′ band system of monobromomethylene (HCBr) at near-infrared wavelengths are reported. Rotationally resolved spectra of HCBr and DCBr for both naturally occurring Br isotopes are analyzed and provide accurate energies for the low-lying bending vibrational levels in both the Ã and X̃ states. The experimental results are compared to extensive ab initio calculations of these two states and the low-lying ã 3A″ state. The ab initio T00 value for the Ã–X̃ system is calculated within 500 cm−1 of the experimental results. For the triplet state, T00 (α̃–X̃) was calculated to be 1833 cm−1. The measurements of the bending intervals in the X̃ 1A′ state suggest that this is about 250 cm−1 too low. With this shift and inclusion of spin–orbit coupling between the ã and X̃ states, the calculated bending vibrational levels in the X̃ state reproduce the observations, and the calculations make testable predictions of the position of the triplet and other singlet state vibronic levels. Combining the experimental and computational results, we estimate the triplet, T00(ã–X̃), excitation energy to be 2028 cm−1.
The disorder deviation in the deconfined phase
Authors: Kozlov G. A.
Published: July 2002

A systematic study of the strongly interacting matter under extreme conditions via the form of the thermal ratio of the disorder deviation is presented. The evolution of Fermi and Bose particles (quarks and gluons) is studied in the framework of multi-particle correlation and distribution functions to predict the size of the finite-temperature deconfined phase.
Optical and electrical properties of Al1−xInxN films grown by plasma source molecular-beam epitaxy
Authors: Lukitsch M. J. , Danylyuk Y. V. , Naik V. M. , Huang C. , Auner G. W. , Rimai L. , Naik R.
Published: July 2002

Epitaxial Al1−xInxN thin films with 0⩽x⩽1 have been grown by plasma source molecular beam epitaxy on sapphire (0001) substrates at a low temperature of 375 °C. Both reflection high-energy electron diffraction and x-ray diffraction measurements confirm the c-plane growth with the following epitaxial relations: nitride [0001] ∥ sapphire [0001] and nitride 〈011̄0〉 ∥ sapphire 〈21̄1̄0〉. However, the degree of crystalline mosaicity and the compositional fluctuation increase with increasing x. The observed direct energy band gap, determined using optical transmission and reflection measurements show relatively less bowing compared to some earlier studies. Electrical resistivity and Hall effect measurements show n-type electrical conductivity in these alloys with carrier concentrations n⩾1019 cm−3 for In-rich alloys and n⩽1010 cm−3 for Al-rich alloys.
Theoretical analysis of the diffusive ion in biased plasma enhanced diamond chemical vapor deposition
Authors: Saito R. , Sano J. , Ishigaki N. , Kimura T. , Yugo S.
Published: July 2002

The role of the bias pretreatment for the enhanced nucleation of the diamond nuclei in the chemical vapor deposition of diamond is theoretically investigated. We obtain the kinetic energy of ions impinging on the surface of the substrate as a function of bias voltage. The experimentally obtained optimum bias voltage of −100 V for the enhanced nucleation of diamond nuclei corresponds to the C ion energy of ∼5 eV at the surface of the substrate. The reduction of the ion energy is caused by the scattering in the ion sheath region of the plasma. The ion sheath width and the ion energy on the surface of the substrate are given as functions of the gas pressure and the bias voltage. The results are compared with the experimental results.
Femtosecond response time in beryllium-doped low-temperature-grown GaAs/AlAs multiple quantum wells
Authors: Okuno Tsuyoshi , Masumoto Yasuaki , Sakuma Yasushi , Hayasaki Yuuichi , Okamoto Hiroshi
Published: July 2002

We have investigated optical nonlinearity in beryllium-doped low-temperature (LT) molecular-beam-epitaxy-grown GaAs/AlAs multiple quantum wells (MQWs). The response time of the nonlinearity is reduced by Be doping in the MQW. While the undoped LT MQW shows a 0.7–0.9 ps response, the response time of the Be-doped LT MQW is as short as 0.25 ps. The saturation density of the Be-doped MQW is almost the same as that of the undoped MQW, and is smaller than that of bulk GaAs. These results demonstrate that the Be-doped LT MQW exhibits a faster response than the undoped LT MQW, and a faster response as well as larger nonlinearity than LT bulk GaAs.
Evidence for the precipitation of the Fe2Pr phase in the Fe–Pr binary system
Authors: Santos I. A. , Coelho A. A. , Ribeiro C. A. , Gama S.
Published: July 2002

Directional solidification experiments were employed on Fe–Pr alloys at several low cooling rates in order to observe the solidification sequence displayed along the length of the samples. Measurements of the transition temperatures around the eutectic temperature of this system were also performed using a Calvet-type calorimeter. The results show the precipitation of the Fe2Pr phase and its peritectic formation, described as Fe17Pr2+L→Fe2Pr. The calorimetric measurements give the temperature of this reaction as being 669 °C. The system presents only one eutectic reaction at 664 °C, described as L→Fe2Pr+Pr, with a feathered morphology under metallographic analysis. Thermomagnetic measurements give the Curie temperature of the Fe2Pr phase as 44 °C. A sequence of reactions during solidification of the alloys around the eutectic point for the Fe–Pr binary system is proposed.
Avoided crossings in mesoscopic systems: Electron propagation on a nonuniform magnetic cylinder
Authors: Exner P. , Joye A.
Published: July 2002

We consider an electron constrained to move on a surface with revolution symmetry in the presence of a constant magnetic field B parallel to the surface axis. Depending on B and the surface geometry the transverse part of the spectrum typically exhibits many crossings which change to avoided crossings if a weak symmetry breaking interaction is introduced. We study the effect of such perturbations on the quantum propagation. This problem admits a natural reformulation to which tools from molecular dynamics can be applied. In turn, this leads to the study of a perturbation theory for the time dependent Born–Oppenheimer approximation.
Relation between the solitons of Yang–Mills–Higgs fields in 2+1 dimensional Minkowski space–time and anti-de Sitter space–time
Authors: Zhou Zixiang
Published: July 2002

The Yang–Mills–Higgs–Bogomolny equations in both 2+1 dimensional Minkowski space–time and 2+1 dimensional anti-de Sitter space–time are known to be integrable and their soliton solutions have already been obtained. In this article, we show that there is a natural relation between the Lax pairs and soliton solutions in these two space–times when the curvature changes from 0 to −1. The changes of the asymptotic behaviors of the solitons are also discussed.
Nanosprings
Authors: McIlroy D. N. , Zhang D. , Kranov Y. , Norton M. Grant
Published: July 2002

The transition from linear to helical growth of amorphous boron carbide nanowires has been examined with high-resolution transmission electron microscopy. Based on the observed changes of the iron catalyst at the transition point from linear to helical growth, a model of helical growth of solid nanowires has been developed based on the work of adhesion of the metallic catalyst to the tip of the nanowire. This model demonstrates that contact angle anisotropy at the catalyst/wire interface drives helical growth. The anisotropy is introduced when the radius of the droplet (R) exceeds the radius of the nanowire (ρ), and the center of mass of the metal droplet is displaced laterally from the central axis of the nanowire.
Reduction of nonradiative recombination centers in V-grooved AlGaAs/GaAs quantum wires grown using tertiarybutylarsine
Authors: Liu Xing-Quan , Wang Xue-Lun , Ogura Mutsuo
Published: July 2002

We report our comparative study on the luminescence recombination processes of V-grooved AlGaAs/GaAs quantum wires (QWRs) grown using tertiarybutylarsine (TBAs) and AsH3 as arsenic sources. Constant integrated photoluminescence (PL) intensity up to 120 K, as well as radiative efficiency of about 3000 times higher at room temperature, were observed for the TBAs sample compared with the AsH3 sample. Time-resolved PL measurements show a maximum decay time at temperature of as high as 240 K for the TBAs sample, which is about 100 K higher than that of similar samples grown using AsH3 as the arsenic source. These results suggest a dramatic reduction of nonradiative centers in QWR regions grown using TBAs as the arsenic source.
Controlled oxygen doping of GaN using plasma assisted molecular-beam epitaxy
Authors: Ptak A. J. , Holbert L. J. , Ting L. , Swartz C. H. , Moldovan M. , Giles N. C. , Myers T. H. , Van Lierde P. , Tian C. , Hockett R. A. , Mitha S. , Wickenden A. E. , Koleske D. D. , Henry R. L.
Published: July 2002

High-quality (0001) and (0001̄)-GaN films were grown by plasma-assisted molecular-beam epitaxy to study the dependence of oxygen incorporation on polarity and oxygen partial pressure. Oxygen incorporates at a rate ten times faster on nitrogen-polar GaN than on the Ga polarity. Oxygen doping is controllable, reproducible, and produces low compensation material up to concentrations of at least 1018 cm−3 with higher levels showing significant compensation. Layers containing oxygen at levels above 1022 cm−3 exhibit severe cracking while oxygen concentrations less than 1021 cm−3 do not introduce significant strain. The oxygen incorporation rate has a weak dependence on Ga overpressure during Ga-stable growth but dramatically increases for conditions approaching N-stable growth.
Fabrication of strained Si on an ultrathin SiGe-on-insulator virtual substrate with a high-Ge fraction
Authors: Tezuka T. , Sugiyama N. , Takagi S.
Published: July 2002

A promising fabrication method for a Si1−xGex-on-insulator (SGOI) virtual substrate and evaluation of strain in the Si layer on this SGOI substrate are presented. A 9-nm-thick SGOI layer with x=0.56 was formed by dry oxidation after epitaxial growth of Si0.92Ge0.08 on a silicon-on-insulator substrate. During the oxidation, Ge atoms were rejected from the surface oxide layer and condensed in the remaining SGOI layer, which was partially relaxed without introducing a significant amount of dislocations. It is found from the analysis of the Raman spectra that the strained Si layer grown on the SGOI layer involves a tensile strain of 1%. This strained Si on the SGOI structure is applicable to sub-100-nm metal–oxide–semiconductor field-effect transistors.
Axially uniform resonant cavity modes for potential use in electron paramagnetic resonance spectroscopy
Authors: Mett Richard R. , Froncisz Wojciech , Hyde James S.
Published: July 2002

This article is concerned with cylindrical transverse electric TE011 and rectangular TE102 microwave cavity resonators commonly used in electron paramagnetic resonance (EPR) spectroscopy. In the cylindrical mode geometry considered here, the sample is along the z axis of the cylinder, dielectric disks of 1/4 wavelength thickness are placed at each end wall, and the diameter of the cylinder is set at the cutoff condition for propagation of microwave energy in a cylindrical waveguide at the desired microwave frequency. The microwave magnetic field is exactly uniform along the sample in the region between the dielectric disks and the resonant frequency is independent of the length of the cylinder without limit. The rectangular TE102 geometry is analogous, but here the microwave magnetic field is exactly uniform in a plane. A uniform microwave field along a line sample is highly advantageous in EPR spectroscopy compared with the usual sinusoidal variation, and these geometries are called “uniform field” modes. Extensive theoretical analysis as well as finite element calculation of field patterns are presented. The perturbation of field patterns caused by sample insertion as functions of the overall length of the resonator and diameter of the sample is analyzed. The article is intended to provide a basis for design of practical structures in the range of 10 to 100 GHz.
Acoustic excitation of vortex instabilities
Authors: Leblanc Stéphane
Published: July 2002

The asymptotic solution of Ford and Llewellyn Smith [J. Fluid Mech. 386, 305 (1999)] which describes the dynamics of a Kelvin vortex disturbed by a single large wavelength acoustic excitation, is shown to be unstable to three-dimensional short-wave nonlinear perturbations. This result might have interesting physical applications such as the destabilization of vortices in geophysics or engineering.
Internal waves generated by the wake of Gaussian hills
Authors: Dupont Pascal , Kadri Yassine , Chomaz Jean-Marc
Published: July 2002

Depending on their structure and dynamics, the wakes of various obstacles can generate different kinds of internal waves in stratified fluids. Experiments on waves emitted by three-dimensional Gaussian models towed uniformly in a linearly stratified fluid were carried out. Beyond a critical value Frc of the Froude number Fr, the developed wake was observed to radiate an internal wave field shorter than the lee waves of the hill. The emergence of such waves is correlated with the periodic shedding of three dimensional vortical structures at Fr>Frc. Measurements show that the wavelengths of these short waves are constant in space and time, and proportional to Fr. Their phase and group velocities are proportional to the coherent structure velocity which is estimated from velocity measurements inside the wake. All those spatio-temporal characteristics prove that these short waves are generated by the displacement of the coherent structure inside the wake.
The floating potential of cylindrical Langmuir probes
Authors: Chen Francis F. , Arnush Donald
Published: July 2002

The floating potential of a cylindrical probe is computed numerically, and the results are fitted to analytic functions. They differ significantly from the plane approximation.
Nonlinear waves in a low-β plasma with cylindrical symmetry
Authors: Shi J. K. , Xu B. Y. , Torkar K. , Liu Z. X.
Published: July 2002

The “Sagdeev potential” is derived from the magnetohydrodynamic equations in a cylindrical coordinate system, and nonlinear electrostatic density waves propagating along the magnetic field in a low-β plasma with cylindrical symmetry are studied. The results show the existence not only of periodic density waves, solitons with a density hump and solitons with a density dip, but also of density shock waves.
Stationary equilibria of self-gravitating quasineutral dusty plasmas
Authors: Rao N. N. , Verheest Frank , Čadež V. M.
Published: July 2002

Self-consistent stationary equilibrium states of a quasineutral, self-gravitating dusty plasma have been investigated by avoiding the usual “Jeans swindle” assumption. The analysis has been carried out for the Cartesian one-dimensional, cylindrical, as well as spherical symmetric cases. It is shown that allowed equilibria permit steady but inhomogeneous dust flows governed by a nonlinear differential equation, which has a singularity at the dust-acoustic speed. The qualitative nature of the admissible solutions of the latter equation have been discussed. The corresponding results for the case of a self-gravitating neutral fluid have also been pointed out.
Three-dimensional instability of a two-layer Dean flow
Authors: Gelfgat Alexander Yu. , Yarin Alexander L. , Bar-Yoseph Pinhas Z.
Published: July 2002

Stability of a two-layer Dean flow in a cylindrical annulus with respect to three-dimensional perturbations is studied by a global Galerkin method. It is shown that for large inner radius of the annulus (i) the instability becomes three-dimensional if one of the fluid layers is thin, (ii) its onset is not affected by possible small deformations of the interface, and (iii) multiple three-dimensional flow states are expected in a slightly supercritical flow regime. Stability diagrams and patterns of the three-dimensional perturbations are reported. It is concluded that even when the axisymmetric perturbation is the most dangerous, the resulting supercritical flow is expected to be three-dimensional. Possible multiplicity of supercritical three-dimensional states is predicted. The basis functions of the global Galerkin method are constructed so as to satisfy analytically the boundary conditions on no-slip walls and at the liquid–liquid interface. A modification of the numerical approach, accounting for small deformations of the interface which is subject to the action of the capillary force, is proposed. The results are of potential importance for development of novel bioseparators employing Dean vortices for enhancement of mass transfer of a passive scalar (say, a protein) through the interface. The developed numerical approach can be used for stability analysis in other two-fluid systems.
On the scalar probability density function transport equation for binary mixing in isotropic turbulence at supercritical pressure
Authors: Lou Hong , Miller Richard S.
Published: July 2002

Effects of Soret cross-diffusion, whereby concentration diffusion occurs in the presence of either temperature or pressure gradients, are investigated in the context of the binary isotropic turbulent mixing problem at supercritical pressure with emphasis on results relevant to probability density function (PDF) modeling. Direct numerical simulations (DNS) are conducted for compressible isotropic turbulent binary mixing of heptane with nitrogen at mean pressure and temperature equal to 45 atm and T=700 K, respectively. The formulation is based on a cubic real gas state equation, and includes generalized forms for heat and mass diffusion derived from nonequilibrium thermodynamics and fluctuation theory. Results from two simulations at 1923 resolution are compared. One case is based on the complete diffusion formulation, whereas in the second simulation only “standard” Fickian and Fourier mass and heat flux terms are considered as a basis for comparisons. The evolutions of the two mixing processes are shown to be nearly the same at early times; however, at long times the pressure gradient based Soret diffusion acts as a forcing function resulting in a statistically stationary scalar fluctuation distribution. In this case the scalar variance achieves a stationary value, in contrast to the exponential decay associated with purely Fickian diffusion. The mechanical to scalar time scale ratio, CY=τu/τY, is increased by a factor of approximately 3 by Soret effects during this latter mixing regime. Soret diffusion is shown to substantially alter the forms of the conditional expected diffusion and the conditional expected dissipation at these long times. A commonly used model based on an assumed proportionality between the conditional diffusion and the scalar fluctuation is tested, and is found to perform poorly in the presence of cross diffusion at long mixing times. Several new conditional expectations resulting from cross diffusion are measured. Implications of the results for extension of PDF models to the high pressure regime are discussed.
Magnetic helicity reversal of a whistler vortex transmitted through a three-dimensional magnetic null point
Authors: Griskey M. C. , Stenzel R. L.
Published: July 2002

The transmission of a magnetic vortex through a magnetic null point on a separatrix surface is studied experimentally in a large laboratory plasma. The plasma is in the electron magnetohydrodynamic parameter regime and the vortex is an antenna-produced magnetic field perturbation propagating in the whistler mode. Topologically, the background field is separated into two regions; a closed field line region and an open field line region. The two regions are separated by a surface of magnetic field lines with two cusp null points referred to as the separatrix. The vortex propagates into one of the null points. Its energy is partially transmitted through the separatrix and partially spreads away from the null along curving field lines. The self and mutual-helicity of the transmitted vortex reverses, thus the total magnetic helicity is not conserved. Helicity conservation breaks down because the field lines are not frozen to electron flows in the unmagnetized plasma region around the magnetic null point.
Localization, the clue to fast magnetic reconnection
Authors: Biskamp D. , Schwarz E.
Published: July 2002

The effect of localization of the resistivity is studied in the coalescence of two magnetic flux bundles. The reconnection process is consistent with the Petschek configuration, and the reconnection rate E depends only weakly on the value η0 of the resistivity at the X-point, E∝1/|log η0|.
Theoretical description of H behavior in GaN p-n junctions
Authors: Myers S. M. , Wright A. F.
Published: July 2002

The diffusion and reactions of hydrogen in GaN are described by applying differential equations for the concentration profiles of H species, charged dopants, and carriers with simultaneous solution of Poisson’s equation. This approach dispenses with the simplifying assumptions of local equilibrium among states and local charge neutrality that were employed previously by us to treat high-temperature H behavior in uniform layers. The result is a more general modeling capability which encompasses nonequilibrium conditions and space-charge effects such as are encountered in devices. Density-functional theory, previously used by us to treat equilibrium H energies, is employed herein to examine activation barriers and wave-function overlaps affecting the rates of relevant H and carrier reactions, thereby guiding the selection of mechanisms to be included and influencing the evaluation of some rate parameters. The model is applied to H-containing p-n junctions, with detailed consideration of the reversible, metastable electrical activation of H-passivated Mg acceptors that has been observed experimentally under forward bias. The calculations point to interstitial H2 as the state of the H resulting from such activation, and this conclusion is supported by good agreement between the predicted and observed onset temperatures for repassivation under open-circuit annealing. In modeling the more complex activation process, experimentally observed qualitative features are reproduced by choosing relative carrier-capture cross sections in accord with ab initio theoretical considerations. In other model calculations, H is shown to be expelled from the carrier-depleted zone of p-n junctions, causing H redistribution under reverse bias.
Emission uniformity and emission area of explosive field emission cathodes
Authors: Shiffler D. , Ruebush M. , LaCour M. , Golby K. , Umstattd R. , Clark M. C. , Luginsland J. , Zagar D. , Sena M.
Published: July 2002

Explosive field emission cathodes have been used extensively in high power microwave tubes. These cathodes emit electrons without the use of cathode heaters. Recently, some theoretical and simulation work has been performed to gain further understanding of the physics of these cathodes. The purpose of this letter is to provide the experimental background and justification for the theoretical work. The general idea of how explosive field emission cathodes operate is that plasma is rapidly formed, which provides the sea of electrons for space charge limited flow. However, recent theoretical and experimental work suggests edge effects, rather than plasma formation across the entire emission area, can also provide the same effect. In this letter we review three types of cathodes which have been tested. We provide optical data on the cathode emission uniformity as well as the electrical data for the same devices. In particular, we find that a large percentage of the cathode can fail to take part in the emission process and yet the voltage and current can appear identical from the case in which the entire cathode contributes electrons to the emission process.
Influence of alloy stability on the photoluminescence properties of GaAsN/GaAs quantum wells grown by molecular beam epitaxy
Authors: Pinault M.-A. , Tournié E.
Published: July 2002

We have investigated the influence of both the growth rate and the growth temperature on the structural and optical properties of GaAs0.972N0.028/GaAs single quantum wells grown by solid-source molecular beam epitaxy. The results are analyzed in light of the surface phase diagram obtained from in situ reflection high energy electron diffraction. We show that the best quality is achieved at the highest temperature below the onset of alloy decomposition. The use of high growth rates allows one to significantly increase the growth temperature. Our results demonstrate that it is the GaAsN alloy stability which governs the sample properties.
Electro-optic and electromechanical properties of poled polymer thin films
Authors: Shi Wei , Ding Yujie J. , Mu Xiaodong , Yin Xin , Fang Changshui
Published: July 2002

A simple interferometric technique has been developed to sequentially measure the piezoelectric and electro-optic (EO) coefficient of the poled polymer films. This technique, similar to the Michelson interferometer, is based on compensating the change of the optical path due to the piezoelectric and electric-optic effect of the poled polymer films by opposite piezoelectric effect of a quartz crystal. Our technique has advantages of simplicity, high sensitivity, and improved accuracies over the previous ones. The experimental values of the EO coefficients agree with the values predicted by the two-level model.
Vibrationally-rotationally inelastic cross sections for H+SiO collisions
Authors: Palov Alexander P. , Jimeno Pedro , Gray Malcolm D. , Field David , Balint-Kurti G. G.
Published: July 2002

Vibrationally-rotationally inelastic cross sections have been computed for H+SiO collisions over the energy range 200–9600 K. The calculations use a previously calculated ab initio potential energy surface which was fitted to an analytic form [P. Jimeno, M. D. Gray, and G. G. Balint-Kurti, J. Chem. Phys. 111, 4966 (1999)]. Potential optimized discrete variable representation techniques were used to compute vibrational matrix elements of this potential and the infinite order sudden method was then used to compute the required cross sections. The potential is very anisotropic and complicated. This is mirrored in the form of the vibrational matrix elements and in their behavior as a function of the scattering coordinate and of the Jacobi angle. The properties of the potential are markedly different from those of the He+SiO system which has previously been used to model the behavior of both H+SiO and H2+SiO in interstellar and stellar media. Selected vibrationally elastic and vibrationally inelastic cross sections are presented and discussed. Tables of cross sections covering a wide range of vibrational and rotational quantum numbers, as well as a large energy range, are made available through the journal’s EPAPS service. The inelastic cross sections computed here are needed in the modeling of circumstellar SiO maser radiation. Significant structure is observed in the computed inelastic cross sections. This structure is of a complicated nature and might well give rise to the vibrational-rotational quantum state population inversions which in turn leads to the observed circumstellar maser action.
Band gap bowing and exciton localization in strained cubic InxGa1−xN films grown on 3C-SiC (001) by rf molecular-beam epitaxy
Authors: Chichibu S. F. , Sugiyama M. , Kuroda T. , Tackeuchi A. , Kitamura T. , Nakanishi H. , Sota T. , DenBaars S. P. , Nakamura S. , Ishida Y. , Okumura H.
Published: July 2002

Spontaneous emission mechanisms in InGaN alloys were studied by determining the effective band gap energies using photoluminescence excitation spectroscopy and static and time-resolved photoluminescence (PL) measurements on fully strained cubic (c-) InxGa1−xN films on c-GaN templates, which were grown by rf molecular-beam epitaxy on smaller lattice-mismatched 3C-SiC (001) substrates prepared on Si (001). The c-InxGa1−xN alloys exhibited large band gap bowing. The PL decay dynamics showed that the emission is due to recombination of localized excitons, the same as in the case of hexagonal InGaN. The c-InxGa1−xN exhibited a larger Stokes-like shift and a larger localization depth, showing that the material’s inhomogeneity is much enhanced compared to that of the hexagonal polytype.
Characterization of CrBN films deposited by ion beam assisted deposition
Authors: Aouadi S. M. , Namavar F. , Tobin E. , Finnegan N. , Haasch R. T. , Nilchiani R. , Turner J. A. , Rohde S. L.
Published: July 2002

This article reports on the growth and analysis of CrBN nanocrystalline materials using an ion beam assisted deposition process. In addition, this article addresses the utilization of spectroscopic ellipsometry for in situ analysis of ternary nitrides. Coatings, with a total thickness of 1.5±0.2 μm, were deposited at low temperatures (<200 °C) on silicon substrates using ion beam assisted deposition. These coatings were characterized postdeposition using x-ray diffraction (XRD), atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), visible-light spectroscopic ellipsometry (VIS-SE), infrared spectroscopic ellipsometry (IR-SE), and nanoindentation. The primary phases in the films were investigated using XRD. The surface morphology and nanocrystalline nature of the coatings (grain size of 5–7 nm) were deduced using AFM. The elemental composition and phase composition of the samples were determined from XPS and AES measurements and were subsequently deduced from the analysis of the VIS-SE data, and these correlated well. XPS, AES, and IR-SE revealed the crystal structure of the BN phase in the ternary compounds. The correlation of the results from these various techniques indicates that in situ SE may be a potential technique to control the growth of ternary nitride coatings in the future. The mechanical properties of the coatings were evaluated using nanohardness testing. The hardness and elastic modulus were measured to be 19–22 GPa and 250–270 GPa, respectively.
Higher-order hydrodynamics: Extended Fick’s Law, evolution equation, and Bobylev’s instability
Authors: Jou David , Casas-Vázquez José , Madureira Justino R. , Vasconcellos Áurea R. , Luzzi Roberto
Published: July 2002

A higher-order hydrodynamics for material motion in fluids, under arbitrary nonequilibrium conditions, is constructed. We obtain what is a generalized—to that conditions—Fick-type Law. It includes a representation of Burnett-type contributions of all order, in the form of a continuous-fraction expansion. Also, the equation includes generalized thermodynamic forces, which are characterized and discussed. All kinetic coefficients are given as correlations of microscopic mechanical quantities averaged over the nonequilibrium ensemble, and then are time- and space-dependent as a consequence of accounting for the dissipative processes that are unfolding in the medium. An extended evolution equation for the density of particles is derived, and the conditions when it goes over restricted forms of the type of the telegraphist equation and Fick’s diffusion equation are presented.
Specific heat and thermodynamic properties of undercooled liquid cobalt
Authors: Wang N. , Han X. J. , Wei B.
Published: July 2002

Co is undercooled to 227 K (0.128 Tm) and its specific heat has been determined as 40.6 Jmol−1 K−1 by electromagnetic levitation drop calorimeter, whose good accuracy is verified by the measured specific heat data of pure nickel. On the basis of measured specific heat data, the thermodynamic properties, such as thermodynamic driving force and relative nucleation rate, are calculated. Moreover, the theoretical models of Turnbull and Dubey–Ramachandrarao are also used to calculate the thermodynamic properties for a comparison. It is found that both the thermodynamic driving force and relative nucleation rate have large deviation compared with the calculated results based on experimental data. This indicates that, since the deviation of specific heat value has a drastic influence on thermodynamic driving force and nucleation rate for the undercooled liquid metals, it is necessary to measure the specific heat quantitatively by experiments.
Enhanced photoluminescence from GaN grown by lateral confined epitaxy
Authors: Zamir S. , Meyler B. , Salzman J. , Wu F. , Golan Y.
Published: July 2002

Lateral confined epitaxy (LCE) of GaN on Si substrates results in a reduction of thermal crack density with decreasing the lateral dimensions of the growth pattern. Below a critical size, crack-free GaN on Si is obtained. The intensity of band-to-band photoluminescence (PL) peak in LCE GaN is strongly enhanced with respect to uniformly grown GaN on Si. The present study rules out the effect of crack density, internal reflections (microcavity effects), as well as enhanced light extraction efficiency, and excitation or emission through preferred facets (shape effects) as the main factor in PL enhancement. It is shown that the reduction in threading dislocation density (TDD) along the edges of the LCE patterns improves the luminescence efficiency. The relative increase in high quality material (low TDD) with the reduction of LCE unit size is, thus, the main reason for the enhanced PL intensity.
Acoustic study of dislocation rearrangement at later stages of fatigue: Noncontact prediction of remaining life
Authors: Ogi Hirotsugu , Minami Yoshikiyo , Hirao Masahiko
Published: July 2002

This study is devoted to clarifying the mechanism of the surface-shear-wave attenuation peak observed during rotating bending fatigue of carbon steels. We have developed electromagnetic acoustic resonance to make a contactless monitoring of the attenuation throughout the fatigue test. The attenuation peak occurs at a fixed fraction to lifetime, being independent of the bending stress (0.49–1.20 of the yield stresses) and the carbon content (0.22–0.45 mass %). Low-temperature heat treatment reduces the peak attenuation back to the previous value, which indicates a dominant contribution of dislocations. Microstructure observations with transmission electron microscopy, surface crack study with replicas and the acoustic measurements show that a large-scale change occurs in the dislocation structure (persistent slip bands to cells) at the attenuation peak and that it is triggered by the inward growth of cracks. This change is completed in a short time, a few percent of the total lifetime. The acoustic-resonance technique can be an important means for the exact prediction of the remaining life of fatigued steels.
Enhanced anisotropic magnetoresistance in Co/Pt multilayers due to the interface effect of inserted Ni layers
Authors: Wang Hai , Zhao HongWu , Zhu Tao , Li Xiang , Zhan WenShan
Published: July 2002

A study of the anisotropic magnetoresistance (AMR) effect of Co/Pt multilayers and Ni layer inserted Co/Pt multilayers is presented. By comparing the two kinds of multilayers, an enhancement of the AMR has been found for the Ni inserted series, which could be attributed to the enhancement of interface scattering.
Electrical properties of pn junctions formed by plasma enhanced epitaxial growth
Authors: Reidy Sean G. , Varhue Walter J. , Adams Ed , Lavoie Mark
Published: July 2002

Properly functioning pn junction diodes have been fabricated by a low temperature plasma enhanced chemical vapor deposition (PECVD) technique. The diodes were constructed such that the metallurgical junction was coincident with the starting substrate surface. The electrical quality of the diodes was quantified by measuring their reverse bias leakage current. Contrary to popular opinion, it has been shown that the PECVD process is inherently capable of producing device quality material, and rather it is the in situ plasma cleaning technique typically associated with this method that is the cause of poor electrical performance. The chemical and physical nature of defects caused by the plasma cleaning step have been investigated by various experimental techniques including photoluminescence, secondary ion mass spectroscopy, and atomic force microscopy. The plasma cleaning step has been linked in certain cases to the production of a photoluminescence signal in Czochralski grown Si wafers. Results show that the plasma clean is neither required nor desired in the low temperature growth of device quality epitaxial Si thin films.
Quantitative evaluation of magnetocrystalline anisotropy of columnar grains and thickness of initial layer in CoCr-based perpendicular media
Authors: Saito Shin , Hasegawa Daiji , Hoshi Fumikazu , Djayaprawira David D. , Takahashi Migaku
Published: July 2002

We propose a quantitative analysis using perpendicular torquemetry to evaluate the thickness of the initial layer which consists of nanocrystalline grains, whose c axes are three-dimensionally randomly oriented, and the intrinsic magnetocrystalline anisotropy of the columnar grains with its c plane parallel to the film plane for CoCr-based perpendicular thin-film media. By applying this analysis to CoNiCrTa, CoCrPtTa, and CoCrPtB media, we found that the initial layer can be completely removed in the CoCrPtB medium epitaxially grown on an hcp-structured Co60Cr40 intermediate layer.
Raman scattering in GaN pillar arrays
Authors: Demangeot F. , Gleize J. , Frandon J. , Renucci M. A. , Kuball M. , Peyrade D. , Manin-Ferlazzo L. , Chen Y. , Grandjean N.
Published: July 2002

We present a detailed study of GaN pillar arrays by atomic force microscopy (AFM), Raman spectroscopy, and photoluminescence (PL) spectroscopy. AFM is used to characterize the shape of the GaN pillars and revealed a large roughness of etched pillar surfaces. Raman scattering spectra of the pillars are well described by angular dispersion of polar optical phonons induced by the three-dimensional shape of the pillar. Additional Raman scattering has been tentatively assigned to the activation of the high frequency B1 silent mode by defects introduced during the ion etching. This result is well correlated with the appearance of donor and acceptor-related PL of the GaN pillars. N vacancy or/and Ga interstitials would be likely candidates for donors in the nonstoichiometric GaN near the surface of the etched pillars.
Functionals of quantities other than the electron density: Approximations to the exchange energy
Authors: Ernzerhof Matthias , Maximoff Sergey N. , Scuseria Gustavo E.
Published: July 2002

A new method is proposed for constructing approximations to the electron–electron interaction. In this approach tools of density functional theory, such as the gradient expansion, are employed. Traditional gradient expansions depend on the electron density ρ. Here, however, we propose an alternative route to ρ functionals, we derive functionals that depend solely on the noninteracting kinetic energy density τ(r) defined by τ(r)=12∑iocc|∇φi(r)|2, where φi is a single-particle orbital. We demonstrate the potential of our approach by constructing an approximation to the exchange energy (EX) of electrons that rivals the accuracy of generalized gradient approximations of density functional theory. The strategy presented here is quite general and suitable to construct a variety of nonelectron-density functionals.
Controlling nucleation and growth of nanodroplets in supersonic nozzles
Authors: Streletzky Kiril A. , Zvinevich Yury , Wyslouzil Barbara E. , Strey Reinhard
Published: July 2002

We present the first results for a new supersonic nozzle that decouples nucleation and droplet growth, and closely controls the supersaturation and temperature during nucleation. We characterize the expansions using pressure trace measurements, and the aerosol properties using light scattering and small angle neutron scattering. We show that when nucleation and droplet growth are separated, the aerosol number density decreases, the average particle size increases, and the aerosol can be more monodisperse than that formed in a conventional nozzle. Under these conditions, we can estimate the nucleation rate J as a function of supersaturation S and temperature T directly from the experimental data. For D2O we find that the nucleation rate is 4.3×1015⩽J/cm−3 s−⩽6.0×1015 at 230.1⩽T/K⩽230.4 and 29.2⩽S⩽32.4.
Enhancement of low field magnetoresistance by chemical interaction in bulk composites La0.7Sr0.3MnO3/SrMeO3 (Me=Ti, Zr)
Authors: Shlyakhtin O. A. , Shin K. H. , Oh Young-Jei
Published: July 2002

In order to enhance low field magnetoresistance (LFMR) of bulk manganite materials, two series of composites, La0.7Sr0.3MnO3/SrTiO3 and La0.7Sr0.3MnO3/SrZrO3, have been prepared by thermal processing of mixed powders. For both series annealing of pelletized mixtures over certain critical temperature leads to the substantial enhancement of LFMR for compositions close to theoretical percolation threshold. At higher processing temperatures enhanced MR values are observed for composites with higher La0.7Sr0.3MnO3 content, while compositions with a lower amount of manganite become insulating. The magnitude of enhanced MR remains almost the same within the same series, but the value of enhancement is substantially different for composites, containing SrTiO3 and SrZrO3 (1.5% and 2.5% at H=100 Oe, respectively). Similar dependencies are observed for coercive field values. Observed shift of the enhancement boundary during thermal processing and significant dependence of SrZrO3 lattice parameters versus components ratio and composite processing temperatures (1100–1200 °C) show at substantial chemical interaction in these composites.
Magnetic and transport behaviors of La0.5(Sr0.53Ca0.47)0.5MnO3 under pressure, magnetic field, and Fe doping
Authors: Sun J. R. , Liu Z. X. , Yeung C. F. , Chan H. N. , Wong H. K. , Li R. W. , Shen B. G.
Published: July 2002

Effects of pressure, magnetic field, and Mn-site doping on the magnetic structure and resistive behavior of La0.5(Sr0.53Ca0.47)0.5Mn1−xFexO3 (0⩽x⩽0.10) have been experimentally studied. The system experiences two magnetic transitions subsequently with decreasing temperature: a paramagnetic to ferromagnetic transition and a ferromagnetic to antiferromagnetic (AFM) transition. The second magnetic transition coincides with a steep resistivity jump. In addition to reducing the critical temperature for the AFM transition (TN), the incorporation of Fe enhances the ferromagnetic order below TN as demonstrated by the gradual increase of magnetization with the content of Fe. Long-range AFM order is replaced by short-range AFM order when x exceeds 0.03, and disappears completely for x>0.06. The application of pressure or magnetic field depresses the AFM nature of the compounds, resulting in a downward shift of TN at a rate of ∼7 K/GPa or ∼4 K/T. The most interesting observation of the present study is the greatly enhanced resistivity jump at TN, which increase from 1 to 2 orders, after the sample undergoes a pressurizing and then a pressure relieving process. Although high pressure, magnetic field, and Fe doping produce similar effects on the AFM order, the underlying physics is different.
Spectroscopic ellipsometry study of ion-implanted Si(100) wafers
Authors: Tsunoda Kazuaki , Adachi Sadao , Takahashi Mitsutoshi
Published: July 2002

Optical properties of P+ ion-implanted Si(100) wafers have been studied using spectroscopic ellipsometry (SE). The P+ ions are implanted at 150 keV with fluences ranging from 1×1014 to 2×1015 cm−2 at room temperature. An effective-medium-approximation analysis suggests that the ion-implanted layer can be explained by a physical mixture of microcrystalline and amorphous silicon. The ε(E) spectrum of the microcrystalline component is found to differ appreciably from that of single-crystalline silicon, especially in the vicinity of the sharp critical-point features. This difference in ε(E) can be successfully interpreted by increasing the broadening parameter at each critical point. Considering these and previous data, we obtain an expression, A=(5.13×1011/EacM)1.872, which enables us to estimate the amorphization-threshold fluence A for silicon implanted with optional ion species of mass number M at energy Eac in keV. No clear change in the original structure of silicon surface after P+ ion implantation has been observed by atomic force microscopy. SE has been proven to be an easy, fast, and nondestructive technique which can be used to assess important ion-implantation parameters.
Dynamics of the aerosol particle photocharging process
Authors: Maisels Arkadi , Jordan Frank , Fissan Heinz
Published: July 2002

In the photocharging process, aerosol particles become electrically charged through interaction with high-energy photons, e.g., ultraviolet (UV) irradiation. Photon adsorption by particles leads to electron emission and, as a result, particles become positively charged. While maximum achievable charges have been described in previous studies of dependency on particle and irradiation parameters, the influence of photoemitted charges on the charging process was not taken into account. In this work it is shown that such charges interact with the particles, which heavily influences the entire process. This complex process (the charging of particles positively by photons and simultaneously negatively by ions) is described in this work by a set of differential equations. These differential equations are solved numerically and, with simplifying assumptions, analytically. Multicomponent polydisperse aerosol is considered. As was found by comparing the analytical and numerical solutions, analytical results coincide with numerical ones for a wide range of initial parameters (particle number concentration, diameter). The analytical expressions evaluated allow one to quantify the influence of guiding processes and the limits of the charging process, which is heavily influenced by the previously neglected free ions. Model results explain such phenomena as the experimentally observed decrease in particle charge through the increase in particle number concentration. As a result of this work, the particle charge distribution obtained by the photocharging process can now be considered a known function of aerosol and UV-irradiation parameters.