bluuebunny/crossref_metadata_2025 huggingface dataset:
Time-dependent measurement of the trapped charge in electron irradiated insulators: Application to Al2O3–sapphire
Authors: Belhaj M. , Odof S. , Msellak K. , Jbara O.
Published: July 2002

A method is described which uses a scanning electron microscope for the investigation of charge trapping in insulators under electron bombardment. The technique commonly used to deduce the amount of trapped charge and its spatial extent is based on the mirror effect, while in the present approach the electron-beam deflections are measured during the primary irradiation. We have performed measurements of the trapped charge during time in an Al2O3–sapphire sample under electron irradiation. Furthermore, the effects of the electron-beam energy and current on charging are also examined and the errors concerning the method are discussed in detail.
Designability of a protein chain in an off-lattice heteropolymer model
Authors: Liang Haojun
Published: July 2002

In a three-dimensional off-lattice model, Shakhnovich and Gutin’s method of minimizing the Hamiltonian was applied to protein design. The results reveal that the number of the type of amino acids is one of important factors that determines the designability of a protein. For a chain made of 16 residues, the designed protein could fold into its native state from an arbitrary initial random coil conformation under appropriate conditions only when the number of type of amino acid increased to four.
A reaction-path Hamiltonian described with quasirectilinear vibrational coordinates constructed from a nonlinear combination of curvilinear internal coordinates: Application to examination of the reaction CH4+F→CH3+HF
Authors: Okuno Yoshishige , Yokoyama Shiyoshi , Mashiko Shinro
Published: July 2002

The reaction-path Hamiltonian formulation reported in the preceding paper, where the Hamiltonian is described with quasirectilinear vibrational coordinates related nonlinearly to curvilinear internal coordinates, was applied to the examination of the reaction CH4+F→CH3+HF. For this reaction we made ab initio calculations and determined the harmonic vibrational frequencies along the reaction path by each of (1) a method using the new formulation, (2) the method of Miller et al. [J. Chem. Phys. 72, 99 (1980)], and (3) that of Truhlar et al. [J. Chem. Phys. 102, 3188 (1995)]. We found that the harmonic vibrational frequencies determined by the new method differ significantly from those determined by the other two methods in the region away from the stationary states. This difference is attributed to the limitations of the latter two methods. We concluded that the reaction-path Hamiltonian determined by the new method should be used to obtain an accurate picture of the reaction-path dynamics under the zero-angular-momentum assumption.
Alternating current losses in stacked Bi2Sr2Ca2Cu3O10/Ag tapes in perpendicular magnetic fields
Authors: Suenaga M. , Chiba T. , Ashworth S. P. , Welch D. O. , Holesinger T. G.
Published: July 2002

Alternating current losses in stacks of powder-in-tube-processed Bi2Sr2Ca2Cu3O10/Ag tapes were measured in perpendicular magnetic fields and compared with calculated hysteresis losses based on the critical-state model. Four main factors had to be taken into account for the calculations of the losses: (1) the inclusion of the area of the Ag sheath in determining the critical-current densities of the stacks, (2) the zero-applied-field critical currents Ic⊥(0) with truly perpendicular fields only, not the standard self-field critical current IcS(0), and (3) the field-dependent critical current Ic(H), e.g., the Kim’s model, and (4) the subtraction of the frequency-dependent losses at very high fields. With these considerations, the agreement between the calculated and the measured values of the losses is excellent for fields beyond the full-penetration fields. However, these factors did not totally account for the low-field losses.
Polarization self-modulation of speckle patterns in a bundle of photorefractive crystals
Authors: Nazhestkina Nadezhda , Nippolainen Ervin , Kamshilin Alexei A. , Prokofiev Victor V. , Jaaskelainen Timo
Published: July 2002

The experimental and theoretical studies of a polarization self-modulation effect in photorefractive Bi12TiO20 crystals are presented. We developed an approximate theory that allows optimizing the performance of the adaptive sensor based on the polarization self-modulation effect. To diminish external voltage simultaneously keeping the aperture of the sensor large enough, we constructed a sensor consisted of several Bi12TiO20 crystals of different orientations. High sensitivity and adaptability to slow environment changes of the proposed sensor make it useful for various applications, particularly for laser ultrasound detection.
Characteristics of transmission resonance in a quantum-dot superlattice
Authors: Joe Yong S. , Ikeler David S. , Cosby Ronald M. , Satanin Arkady M. , Sub Kim Chang
Published: July 2002

We investigate phase-coherent electron transport through height-varying potential barriers in a quantum-dot superlattice. Due to the aspect ratio variations of two alternating potential heights in the quantum channel, well-arranged resonant peaks in the first miniband of each plateau are divided into the paired peaks of two groups, which produce an extra gap inside each miniband. In addition, for a five barrier case, the second and third resonant peaks in the miniband are no longer distinguishable at a critical aspect ratio, and the amplitude of this degenerate peak becomes smaller than one and eventually approaches zero. The mean lifetimes of the resonant peaks whose amplitudes remain unity are studied. We also examine the resonant tunneling with under-unity transmission in the one-dimensional superlattice system with alternating potential barriers. Finally, it is found that the “quasi-resonance” appears in a quantum-dot superlattice with 13 barriers consisting of 2 alternating potential heights.
Raman study of the ordering in Sr(B0.5′Nb0.5)O3 compounds
Authors: Ratheesh R. , Wöhlecke M. , Berge B. , Wahlbrink Th. , Haeuseler H. , Rühl E. , Blachnik R. , Balan P. , Santha N. , Sebastian M. T.
Published: July 2002

Strontium based complex perovskites are potential candidates for microwave integrated circuit applications. In the present article, we report on Raman scattering studies of cubic and noncubic structures of Sr(B0.5′Nb0.5)O3 [B′=Ga, In, Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Yb] based complex-perovskite materials for an improved understanding of structure-property relations. The spectral results are compared to some tantalum analogues of known crystal structure. The present study reveals a higher degree of ordering for the tantalum compounds compared to those of the niobium analogues. Vibrational studies show a correlation between the tolerance factor and symmetry of these materials.
Optical second-order nonlinearity of poled borosilicate glass containing CuCl
Authors: Sasai Jun , Tanaka Katsuhisa , Hirao Kazuyuki
Published: July 2002

The effect of surface etching on second-harmonic (SH) intensity and Maker fringe pattern have been examined for poled borosilicate glasses containing CuCl. A drastic decrease in the SH intensity was observed with anode-side surface etching, while such a decrease did not appear when the cathode-side surface etching was carried out. However, after 20 μm etching of the anode-side surface, a decrease in SH intensity with the cathode-side etching was observed. For some samples, the theoretical Maker fringe pattern derived on the assumption that the SH wave is generated from a thin layer near the anode-side surface is in good agreement with the experimental one. On the other hand, other samples show oscillation in the fringe, which originates from the interference of SH waves generated from both surfaces. These results suggest that the nonlinear layer is present at both surfaces, although it is much weaker near the cathode. It is thought that the origin of the nonlinearity is attributable to migration of Cu+ and Cl−.
Photoluminescence measurements from the two polar faces of ZnO
Authors: Sherriff R. E. , Reynolds D. C. , Look D. C. , Jogai B. , Hoelscher J. E. , Collins T. C. , Cantwell G. , Harsch W. C.
Published: July 2002

The crystal structure of ZnO is wurtzite and the stacking sequence of atomic layers along the “c” axis is not symmetric. As a result, a ZnO crystal surface that is normal to the c axis exposes one of two distinct polar faces, with (0001̄) being considered the O face and (0001) the Zn face. Photoluminescence (PL) measurements on the two faces reveal a striking difference. Two transitions are observed in PL that are dominant from the O face and barely observed in PL from the Zn face. These lines are identified as phonon replicas of a particular D0,X transition using energy separations, excitation dependence, and time-resolved PL measurements. In addition, PL emission from free excitons is found to be more intense from the O face than from the Zn face.
First-order transition of a homopolymer chain with Lennard-Jones potential
Authors: Liang Haojun , Chen Hanning
Published: July 2002

The thermodynamics of a homopolymer chain with the Lennard-Jones (LJ) potential was studied by the multicanonical Monte Carlo method. The results confirm there indeed exists a liquid–solid-like first-order transition at lower temperatures for a free-joint chain, revealing that the transition is a characteristic of a homopolymer chain, independent of the algorithms and potential used in simulation.
Elastic stiffnesses of an Nb–Ti/Cu-composite superconductive wire
Authors: Kim Sudook , Ledbetter Hassel , Ogi Hirotsugu
Published: July 2002

Elastic-stiffness coefficients were determined on a 1.4-mm-diameter wire consisting of superconducting Nb–Ti fibers in a copper matrix, with a polyvinyl-resin coating. The matrix contained 324 Nb–Ti fibers. An electromagnetic-acoustic-resonance method was used to obtain five independent elastic-stiffness coefficients assuming transverse-isotropic symmetry. From these we calculated Young moduli, bulk modulus, and principal Poisson ratios. As a check, we used a mechanical-impulse-excitation method to directly measure the Young modulus in the fiber direction. The three-phase composite wire showed a 10% anisotropy in the Young modulus.
Ballistic electronic transport in quantum cables
Authors: Zeng Z. Y. , Xiang Y. , Zhang L. D.
Published: July 2002

We studied theoretically ballistic electronic transport in a proposed mesoscopic structure—quantum cable. Our results demonstrated that quantum cable is a unique structure for the study of mesoscopic transport. With the increasing Fermi energy of electrons, the ballistic conductance of quantum cable increases in a series of steps with different height. Besides the steps of one and two quantum conductance units (2e2/h), conductance steps of three and four quantum conductance units can be also expected due to the accidental degeneracies (crossings) of the transverse subbands of quantum cable. As one of the cable structure parameters is varied, the width of conductance plateaus varies in the nonmonotonous way, which arises from the nonmonotonic variation of the separation between the adjoining transverse subbands. In the weak coupling limit, the first two conductance plateaus are both of just one quantum conductance unit 2e2/h for the quantum cable structure of two cylindrical wires with similar thicknesses.
Molecular dynamics of a dense fluid of polydisperse hard spheres
Authors: Sear Richard P.
Published: July 2002

Slow dynamics in a fluid are studied in one of the most basic systems possible: polydisperse hard spheres. Monodisperse hard spheres cannot be studied as the slow down in dynamics as the density is increased is preempted by crystallization. As the dynamics slow they become more heterogeneous, the spread in the distances traveled by different particles in the same time increases. However, the dynamics appears to be less heterogeneous than in hard-spherelike colloids at the same volume fraction. The particles which move least far in a characteristic relaxation time and, particularly, the particles which move farthest in the same time are clustered, not randomly distributed throughout the sample. The slow dynamics (but not the statics) show strong finite size effects, suggesting that slow dynamics are correlated over distances of several diameters. We study the dynamics at three different widths of the distribution of diameters of the hard spheres. For each width, the relaxation time is the same function of the compressibility factor, suggesting that this determines the relaxation time for hard spheres.
On the classical limit for electronic structure and dynamics in the orbital approximation
Authors: Remacle F. , Levine R. D.
Published: July 2002

The classical limit is shown to provide a description exactly equivalent to the quantum mechanical one in the approximation where each electron is assigned to an orbital. Strictly speaking it is therefore not a limit but an alternative way of solving the problem. There are some merits of this reformulation, most notably in that it brings the phase of the orbitals to the forefront, on equal footing as the occupancies. This allows one to discuss, e.g., electron localization, in a clearer manner. But computationally the classical description is not superior. There will be a definite advantage for more realistic electronic Hamiltonians, i.e., for implementing configuration interaction, and/or when the nuclear motion is coupled to the electronic dynamics. In this paper we limit attention to a derivation and discussion of the simple orbital approximation.
Low-loss Ba0.5Sr0.5TiO3 thin films by inverted cylindrical magnetron sputtering
Authors: Cukauskas E. J. , Kirchoefer Steven W. , Pond J. M.
Published: July 2002

The structural and electrical characteristics of Ba0.5Sr0.5TiO3 (BST) thin films deposited by inverted cylindrical magnetron rf sputtering have been investigated. This unconventional sputter deposition technique consisting of a hollow cylindrical composite target of BST, high argon/oxygen gas pressure 53.2 Pa (400 μm), and 750 °C substrate temperature was employed for depositing low-loss BST thin films. The films were postannealed in a tube furnace at 780 °C for 8 h in flowing oxygen. Atomic-force microscopy revealed anisotropic grain growth with a columnar grain structure protruding from the surface with a 0.25 μm grain size. X-ray diffractometry shows the films to be purely (h00) oriented for certain deposition parameters. The lattice parameter of the best film was slightly larger than that for bulk BST. Other deposition conditions yielded films having many of the BST powder peaks. Capacitance versus voltage characteristics have been measured from 50 MHz to 20 GHz. Device Q values >600, beyond the resolution of the device/measurement system, were realized with a 6.7% tunability at 10 GHz for the best films.
An Ewald summation based multipole method
Authors: Duan Zhong-Hui , Krasny Robert
Published: July 2002

We present a method for evaluating Coulomb interactions in periodic molecular systems. The real space term in Ewald summation is accelerated using a tree code in which interactions between clusters and distant particles are approximated by multipole expansions. The performance is reported for water systems.
Dielectric relaxation of supercritical water: Computer simulations
Authors: Skaf Munir S. , Laria Daniel
Published: July 2002

Dielectric relaxation times of supercritical SPC/E water from molecular dynamics simulations are found to be in good agreement with recent experimental data for densities ρ⩾0.4 g/cm3, but the sharp increase in the experimental Debye time as ρ decreases is not reproduced. Large discrepancies between experimental and simulation data in the dilute regime strongly suggest the need for additional measurements and/or theoretical work.
Vibronic interpretation of the low-energy absorption spectrum of the sexithiophene single crystal
Authors: Petelenz Piotr , Andrzejak Marcin
Published: July 2002

The classic model of vibronic coupling in dimers and tetramers is used for calculating the vibronic fine structure of the lowest-energy electronic transition in the sexithiophene crystal. Good semiquantitative agreement with experiment is achieved, lending credence to the applied model. The observed intensity pattern is interpreted in terms of the deviations from the adiabatic approximation in closely spaced electronic states. The intramolecular Herzberg–Teller corrections are of lesser importance, but are the probable cause of the minor discrepancies in the simulated spectra. The results suggest a modified interpretation of the experimental fluorescence spectra.
An ab initio direct-trajectory study of the photodissociation of ClOOCl
Authors: Kaledin Alexey L. , Morokuma Keiji
Published: July 2002

The photodissociation of chlorine peroxide, ClOOCl, is studied with classical trajectories where the energy and gradient are computed on the fly by means of the state-averaged (sa) complete active space self-consistent field (CASSCF) with the DZP(+) basis set. We show that six electronically excited states are involved in the process of decomposition, which proceeds via several competing pathways and at least three electronically unique fragment channels. The problem is treated in four-dimensional (4D) (C2 constraint) and five-dimensional (5D) (planar constraint) frameworks in order to model the mechanisms of synchronous and asynchronous or stepwise dissociation, respectively. A single trajectory with the initial conditions of a nonvibrating, nonrotating molecule is propagated on each excited state surface for an average time of 10 fs for the purposes of determining the early stages of bond breaking. We show that even in such a short propagation time the pathway competition can be more or less unambiguously understood. The results indicate that in the regime of a 308 nm photolysis, the major dissociation fragments are Cl atoms and O2 molecules, both in the ground state. The higher energy regime of a 248 nm photoexcitation yields additional fragments, e.g., ClO(X 2Π), O(3P) and ClOO(X 2A″,1 2A′). We have achieved an overall qualitative agreement with experiment that more than 70% of the available energy is transferred into the translational energy of the products for the case of the synchronous concerted dissociation. In all the cases, the rotational excitation of produced molecular oxygen is very high, while its vibration is in v=0. Implications of the results on the stratospheric ozone depletion cycle are also presented.
Quantum mechanical/quantum mechanical methods. I. A divide and conquer strategy for solving the Schrödinger equation for large molecular systems using a composite density functional–semiempirical Hamiltonian
Authors: Gogonea Valentin , Westerhoff Lance M. , Merz Kenneth M.
Published: July 2002

Herein we describe a new combined quantum mechanical/quantum mechanical (QM/QM) method for solving the Schrödinger equation for large molecular systems. The new method uses the divide and conquer (D&C) strategy to partition a large molecular system into subsystems and a composite density functional theory (DFT)–semiempirical (SEM) Hamiltonian to describe the molecular interactions. The DFT and SEM subsystems are coupled through the chemical potential and are equilibrated by exchanging electronic charge. Calculations performed with the DFT, SEM, and composite (DFT/SEM) methods on diatomic, triatomic, and polyatomic molecules show that as one moves away from the QM/QM boundary region the Mulliken charges converge to the values that would be obtained using the “pure” Hamiltonian. In other words, we find that the quality of each SEM and DFT wave function is largely conserved, which strongly suggests that this type of approach could be applied to study chemical reactivity much in the same way combined quantum mechanical/molecular mechanical (QM/MM) methods are presently utilized.
Correlation-based phase noise measurements
Authors: Rubiola Enrico , Giordano Vincent
Published: July 2002

In the characterization of the phase noise of a component, it is common practice to measure the cross-spectrum density at the output of two phase detectors that simultaneously compare the component output signal to a common reference. This technique, which is based on correlation and averaging, allows the rejection of the phase detector noise. On the other hand, it is known that the interferometer exhibits lower noise floor and higher conversion gain than other phase detectors suitable to radio-frequency and microwave bands. Thus, we experimented on an improved instrument in which the phase noise of a component is measured by correlating and averaging the output of two interferometers. The measurement sensitivity, given in terms of noise floor, turns out to be limited by the temperature uniformity of the instrument, instead of the absolute temperature T. This feature makes the instrument suitable to investigate the spectrum Sφ(f) of phase fluctuations below kBT/Po, i.e., the thermal energy kBT referred to the carrier power Po. The described method is suitable to the implementation of instruments in a wide frequency range, from some 100 kHz to 40 GHz and beyond. In principle, this method can also be exploited for the measurement of amplitude noise. Theory and experimental proof are given.
Valve and nozzle design for injecting a shell-on-shell gas puff load into a
z
pinch
Authors: Song Y. , Coleman P. , Failor B. H. , Fisher A. , Ingermanson R. , Levine J. S. , Sze H. , Waisman E. , Commisso R. J. , Cochran T. , Davis J. , Moosman B. , Velikovich A. L. , Weber B. V. , Bell D. , Schneider R.
Published: July 2002

We have developed a dual-plenum gas valve coupled to a double shell nozzle for the generation of “shell-on-shell” gas loads in z-pinch plasma radiation source experiments. The gas density profiles of the nozzles have been characterized with laser interferometry. This valve/nozzle combination has been successfully fielded on the Double-EAGLE and Saturn pulsed-power generators. The design and characterization of the shell-on-shell valve/nozzle are presented in this article.
Experimental evidence of polarization effects on exchangeable cations trapped in zeolites
Authors: Giuntini J. C. , Maurin G. , Devautour S. , Henn F. , Zanchetta J. V.
Published: July 2002

The evolution of the activation energy of the conductivity depends on the nature of the exchanged cations, and differs in faujasites X and Y. This surprising phenomenon, reported in many works, is not yet satisfactorily explained. A qualitative explanation is proposed based on well-known results obtained in the study of the interactions between chemical species, by means of density functional theory.
Pressure in a micellized solution
Authors: de Moraes J. N. B. , Figueiredo W. , Henriques Vera B.
Published: July 2002

We investigate the effect of interaggregate interactions on the pressure of a micellar system in two dimensions. We consider a simple model that takes into account only hard-core interactions among aggregates. We derive an expression for the pressure of the micellar solution including the contribution of the second-virial correction. The distribution of the aggregates is determined from Monte Carlo simulations on a two-dimensional lattice, where a water molecule occupies a single site, and each amphiphile occupies three sites. The contribution of interaggregate interactions, that become apparent only above the critical micellar concentration, is a small fraction of the ideal gas pressure.
Anomalies in the temperature dependence of the microwave dielectric properties of Ba6−xSm8+2x/3Ti18O54
Authors: Belous Anatolii G. , Ovchar Oleg V. , Valant Matjaz , Suvorov Danilo
Published: July 2002

Measurements of the microwave dielectric properties of Ba6−xSm8+2x/3Ti18O54 solid solutions revealed the existence of a broad maximum in the temperature dependence of the permittivity as well as the dielectric loss. Submillimeter-wave frequency measurements confirmed the intrinsic origin of the observed anomaly, which could possibly be ascribed to phase transition with unknown nature in the Ba6−xSm8+2x/3Ti18O54 solid solution. The temperature of the anomaly, which is in the range of 90–120 °C, depends on composition of Ba6−xSm8+2x/3Ti18O54 and decreases with an increase in x.
Reactive scattering on multiple electronic surfaces: Collinear A+BC→AB+C reaction
Authors: Shin Changkyun , Shin Seokmin
Published: July 2002

Reaction dynamics on multiple electronic surfaces have been studied by performing quantum scattering calculations on a collinear A+BC→AB+C reaction. We propose model diabatic potentials with variable coupling potentials to examine the effect of the coupling between electronic states on the reaction. Time-independent quantum scattering theory based on diabatic representation provides a useful framework for treating reaction dynamics on coupled electronic states. Reaction probabilities calculated from the diabatic approach intrinsically include the effect of the presence of the excited state. We compared the results from the two-surface diabatic calculations with those using the single adiabatic surface alone. By varying the strength and the range of the diabatic coupling potential, we obtained a wide spectrum of reaction models. It is found that the reaction probabilities from the two-surface diabatic calculations showed noticeable differences from the single-surface adiabatic case. These results demonstrate that the effect of the excited states cannot be ignored even though the two electronic states are well-separated in energy.
Creation of strongly coupled plasmas using intense beams of 400 MeV/u uranium ions to be generated at the Gesellschaft für Schwerionenforschung (GSI) Darmstadt SIS-200
Authors: Tahir N. A. , Kozyreva A. , Shutov A. , Spiller P. , Hoffmann D. H. H.
Published: July 2002

The heavy ion synchrotron, SIS-18 (that has an 18 Tm magnetic rigidity), at the Gesellschaft für Schwerionenforschung (GSI), Darmstadt is a unique facility worldwide that delivers intense beams of energetic heavy ions. The GSI has plans to extend its accelerator capabilities by building a new synchrotron (SIS-200) with a much higher magnetic rigidity of 200 Tm. According to the preliminary design considerations, the SIS-200 will generate a uranium beam that will consist of at least 1012 particles and that will be delivered in a 50 ns long pulse. This beam will be used to study various interesting problems, including fragmentation of the projectile ions while passing through solid matter and creation of high-density, strongly coupled plasmas. For the former type of studies, a particle energy of 1 GeV/u has been considered to be appropriate, while for the latter case, a lower value of 400 MeV/u has been found to be most suitable. In this paper we present, with the help of two-dimensional numerical simulations, the hydrodynamic and thermodynamic response of a solid lead cylindrical target that is irradiated with the future SIS-200 beam, which has a particle energy of 400 MeV/u. The beam focal spot is assumed to be circular and the power deposition profile is considered to be Gaussian along the radial direction. Calculations have also been done using a beam that has a ring-shaped (annular) focal spot that interacts with solid as well as hollow lead cylinders, respectively. In all the above cases it has been assumed that the cylinder length is shorter than the range of 400 MeV/u uranium ions in solid lead so that the Bragg peak does not lie inside the target and the energy deposition is almost uniform along the particle trajectory. These simulations show that it will be possible to create extended volumes of high-density, strongly coupled plasmas using the future SIS-200 beam.
Deposition of mass-selected ions in neon matrices: CS2+ and C6F6+
Authors: Lorenz M. , Bondybey V. E.
Published: July 2002

Infrared and visible absorption spectra and laser-induced fluorescence (LIF) and excitation spectra are obtained for several simple cations deposited from a mass-selected ion beam. In the present preliminary study we demonstrate successful and clean mass selection by presenting spectra of samples obtained by depositing the isotopic S1234C32S+ ion in natural isotopic abundance, and analyzing its spectrum. Spectra of C6F6+ deposited from a 20 eV ion beam exhibit quite different inhomogeneous line profiles, suggesting that even the relatively low kinetic energy results in considerable damage to the solid. Analysis of the spectra indicates that the Jahn–Teller-distorted vibrational structure in the doubly degenerate ground state of C6F6+ is strongly perturbed in the newly formed sites, which are presumably of lower symmetry. A 33–46 cm−1 splitting of the origin and other totally symmetric bands in emission is tentatively attributed to the spin–orbit splitting in the E1g2 ground state.
Destabilization of energetic particle modes by localized particle sources
Authors: Zonca Fulvio , Chen Liu
Published: July 2002

In the present paper, in order to delineate the essential physics, analytic dispersion relations for Energetic Particle Modes (EPM) excitation via precessional resonance in toroidal systems are derived in various limiting but still relevant cases. The analytic dispersion relations obtained here can be readily applied to EPM excitations by magnetically trapped energetic ions generated with ion cyclotron frequency heating. It is also demonstrated that “frequency chirping” is what the linear dispersion relation of EPM’s predicts when the local properties of either energetic particles or the thermal plasma are changed. A new novel result is that, for sufficiently strong spatial gradients, energetic particles can produce an effective potential well that radially (globally) traps the EPM. This property is a crucial and peculiar one of EPM’s, and it could have important implications to the self-consistent nonlinear dynamic evolution of the fast particle distribution.
Maser-beam instability of Bernstein waves
Authors: Ziebell L. F. , Yoon Peter H. , Wu C. S.
Published: July 2002

The present study constitutes a continuation and improvement of the preceding work by Yoon et al. [J. Geophys. Res. 104, 19801 (1999)]. In the present discussion, an instability of Bernstein waves excited by a beam of energetic electrons is investigated. Special attention is paid to the regime where the ratio of plasma frequency, ωpe, to electron gyrofrequency, Ωe, is sufficiently higher than unity. An approximate but fairly accurate scheme is introduced to deal with the situation dictated by the condition, ωpe2/Ωe2≫1. The present investigation is motivated by the research in solar radiophysics. However, in this article the emphasis is placed on basic properties of the instability rather than its application.
Thermodynamic equilibrium of hollow non-neutral plasmas
Authors: Kotelnikov I. , Pozzoli R. , Romé M.
Published: July 2002

New annular confinement configurations of one component plasmas, corresponding to global thermal equilibria in a cylindrical Penning–Malmberg trap with an axial conductor, are investigated both numerically and analytically. In the case of infinite length plasma, analytical solutions are calculated explicitly in the limit of small Debye length. In the case of finite length plasma, the self-consistent solution of the thermal equilibrium Poisson’s equation is obtained numerically, and the dependence of the density distribution on the different parameters of the system is illustrated.
Dependence of the critical thickness on Si doping of InGaAs on GaAs
Authors: Tanner B. K. , Parbrook P. J. , Whitehouse C. R. , Keir A. M. , Johnson A. D. , Jones J. , Wallis D. , Smith L. M. , Lunn B. , Hogg J. H. C.
Published: July 2002

The formation of misfit dislocations during the initial stages of relaxation of In0.04Ga0.96As epitaxial layers on (001) GaAs has been studied by in situ high-resolution double crystal x-ray topography during molecular beam epitaxy growth. Relaxation is initially anisotropic with the fast B(g) dislocations being nucleated before the slow A(g) set. On doping with Si up to a maximum concentration of 4×1018 atoms/cm3, an increase in critical thickness was observed for both dislocation sets. The data can be fitted to an extension of the Matthews–Blakeslee model that includes a lattice friction force varying linearly with the dopant concentration.
Lattice location of erbium in high-fluence implanted silicon–germanium: Backscattering/channeling study
Authors: Touboltsev V. S. , Räisänen J. , Johnson E. , Johansen A. , Sarholt L.
Published: July 2002

High-quality crystalline Si0.75Ge0.25 alloy crystals were implanted with 70 keV Er+ ions at 550 °C to a fluence of 1019 m−2. In situ Rutherford backscattering/channeling spectrometry with a 500 keV He2+ beam revealed Er atoms located on regular lattice sites of the host matrix. Angular scans taken around the 〈100〉, 〈110〉, and 〈111〉 crystallographic axes showed that a considerable fraction of Er atoms occupy tetrahedral interstitial sites.
Relative reactivity of arsenic and gallium dimers and backbonds during the adsorption of molecular oxygen on GaAs(100)(6×6)
Authors: Kruse Peter , McLean James G. , Kummel Andrew C.
Published: July 2002

The chemisorption sites of molecular oxygen on the mixed GaAs(100)(6×6) surface were imaged at room temperature using scanning tunneling microscopy (STM). This surface is terminated by both gallium dimers and arsenic dimers, neither of which react with oxygen. Instead, the As–Ga backbonds are shown to react with O2 with 100% chemical selectivity. The reason for this selectivity is found in the interaction of the highly electronegative oxygen atoms with the higher electron density at the arsenic atoms. One oxygen atom displaces the attacked arsenic atom while the other oxygen atom bonds to two nearby gallium atoms, resulting in the thermodynamically most stable reaction products: metallic arsenic clusters and gallium oxide.
Chemically selective adsorption of molecular oxygen on GaAs(100)c(2×8)
Authors: Kruse Peter , McLean James G. , Kummel Andrew C.
Published: July 2002

The chemisorption sites of molecular oxygen on the technologically important As-rich GaAs(100)c(2×8) surface were imaged with scanning tunneling microscopy (STM). The oxygen atoms insert into the arsenic–gallium backbonds and, subsequently, replace the arsenic atoms in the dimer rows. The displaced arsenic atoms aggregate in clusters of increasing size forming metallic arsenic. The strongly electronegative oxygen atoms are initially attracted by the larger electron density at the arsenic atoms, but the reaction of the oxygen atoms with the gallium atoms is thermodynamically favored. This leads to a 100% chemical selectivity for oxygen insertion into the As–Ga backbonds and subsequent chemisorption of the oxygen atoms into the arsenic sites.
Ultraviolet/infrared-double resonance spectroscopy and ab initio calculations on the indole+ and indole(H2O)1+ cations
Authors: Unterberg C. , Jansen A. , Gerhards M.
Published: July 2002

In this paper we report on the application of infrared/photoinduced Rydberg ionization (IR/PIRI) and IR-photodissociation spectroscopy to investigate the CH, NH or OH stretching vibrations of indole+ and the indole(H2O)1+ cluster cation. All vibrational frequencies of indole+ and indole(H2O)1+ are compared with the values obtained from ab initio calculations. In the case of the indole+ cation the NH vibration is observed. This is the first observation of a NH vibration in a bare cation. For indole(H2O)1+ a hydrogen-bonded structure with a nearly linear hydrogen bond can be derived both from ab initio calculations and the IR-spectra. By applying the state selective IR/PIRI spectroscopy to indole(H2O)1+, no vibrational couplings between the intermolecular O–H⋯N stretching vibration and the intramolecular OH stretching modes of the water moiety are observed. In the IR-photodissociation spectra of indole(H2O)1+ the NH, OH, and CH stretching vibrations as well as overtones of bending modes are observed. In agreement with the prediction of ab initio calculations the frequency of the NH stretching vibration of the cation is shifted by 300 cm−1 to lower frequencies compared to the neutral cluster. By analyzing the fragmentation of indole(H2O)1+ as a function of excess energies within the ion it can be concluded that only one IR photon is absorbed if OH stretching vibrations of the ion are excited, whereas at least two photons can be absorbed if the NH stretching mode is excited. Furthermore, comparison of IR/PIRI and IR-photodissociation spectroscopy indicates that the frequency of the OH stretching vibration decreases very slightly with increasing excess energy within the ion.
Momentum space densities for the beryllium isoelectronic series
Authors: Gálvez F. J. , Buendía E. , Sarsa A.
Published: July 2002

One- and two-body densities in momentum space have been calculated for the atomic beryllium isoelectronic series starting from explicitly correlated multideterminant wave functions. The effects of electronic correlations have been systematically studied by comparing the correlated results with the corresponding Hartree–Fock ones. Some expectation values such as 〈δ(p⃗)〉, 〈pn〉, 〈δ(p⃗12)〉, 〈p12n〉, 〈δ(P⃗)〉, and 〈Pn〉, where p⃗, p⃗12, and P⃗ stand for the electron–nucleus, interelectronic, and two-electron center-of-mass momentum coordinates, respectively, and the angular correlation coefficient have been obtained. All the calculations have been carried out by using the Monte Carlo algorithm.
Two interacting electrons in a box: An exact diagonalization study
Authors: Alavi Ali
Published: July 2002

The behavior of two electrons confined to a three-dimensional box with infinite walls and interacting with a Coulomb potential is studied using an exact diagonalization technique. The use of symmetry operators enables the Hamiltonian to be block diagonalized. Apart from the total spin, the wavefunctions can be classified using three symmetry quantum numbers. The Coulomb integrals are shown to be amenable to efficient and accurate calculation. The energy of the lowest few eigenstates of both the singlet (S=0) and triplet (S=1) are calculated as a function of the box size (i.e., in effect rs) for a slightly tetragonally distorted box where the z-axis is longer than the x- and y-axes. The ground state is a singlet function with ggg symmetry at all densities. At small rs, the ground state has a maximum in electron density at the box center. Upon increasing rs, at rs≈8 a.u., the ground state density acquires a minimum in the box center. At this same rs, the first-excited state of the singlet manifold changes its symmetry from ggu to ugu, and the corresponding degeneracy is changed from one to two. The energy-rs curve shows a nonanalytic change in slope. Subsequent increasing of rs gives rise to increased localization of the charge density in the eight corners of the box, which can be identified as the “Wigner” crystal limit of the present model. The physical exchange-correlation hole is evaluated in the high and low density limits.
Vertically faceted lateral overgrowth of GaN on SiC with conducting buffer layers using pulsed metalorganic chemical vapor deposition
Authors: Qhalid Fareed R. S. , Yang J. W. , Zhang Jianping , Adivarahan Vinod , Chaturvedi Vinamra , Asif Khan M.
Published: July 2002

A “pulsed metalorganic chemical vapor deposition” technique has been developed for lateral overgrowth of GaN thin films on SiC with conducting buffer layers for vertically conducting devices. Growth was carried out at temperatures as low as 950 °C keeping a constant gallium flux while pulsing NH3. We demonstrate that, by varying the NH3 pulse duration, growth morphology can be gradually changed from triangular to rectangular for the lateral overgrowth. Even at a V/III ratio as low as 550, high quality smooth layers with (11̄00) vertical facets were successfully grown with a lateral to vertical growth rate ratio as high as 4:1. Atomic force microscopic measurements show the root-mean-square roughness of the laterally overgrown layers to be 7.0 Å. Scanning thermal microscopy was used to measure a thermal conductivity of 1.7 and 1.5 W/cm K, respectively, for the laterally overgrown film and the GaN deposition in the window region.
Catalytic mechanism of divertor plasma recombination provided by hydrocarbon impurities
Authors: Janev R. K. , Kato T. , Wang J. G.
Published: July 2002

It is shown that the hydrocarbon molecules CxHy, when present in a divertor plasma, provide a powerful catalytic mechanism for volume plasma recombination at temperatures below 6–8 eV. The mechanism is based on the proton charge exchange with CxHy and the subsequent recombination of CxHy+ with plasma electrons, while its catalytic property derives from the simultaneous creation of new hydrocarbon molecules Cx′Hy′ in each charge exchange–dissociative recombination cycle. The plasma recombining efficiency of CH4, C2H6, and C3H8 molecules is for a factor of about 10, 30, and 40 higher than the efficiency of the similar ion conversion–dissociative recombination mechanism based on the molecule hydrogen. A 2.5% of C3H8 in the divertor molecular gas has the same effect on divertor plasma recombination as the entire content of molecular hydrogen in the divertor.
Dynamics of a Z-pinch x-ray source for heating inertial-confinement-fusion relevant hohlraums to 120–160 eV
Authors: Sanford T. W. L. , Olson R. E. , Mock R. C. , Chandler G. A. , Leeper R. J. , Nash T. J. , Ruggles L. E. , Simpson W. W. , Struve K. W. , Peterson D. L. , Bowers R. L. , Matuska W.
Published: July 2002

A Z-pinch radiation source has been developed that generates 60±20 kJ of x rays with a peak power of 13±4 TW through a 4-mm-diam axial aperture on the Z facility. The source has heated National Ignition Facility-scale (6-mm-diam by 7-mm-high) hohlraums to 122±6 eV and reduced-scale (4-mm-diam by 4-mm-high) hohlraums to 155±8 eV—providing environments suitable for indirect-drive inertial confinement fusion studies. Eulerian-RMHC (radiation-magnetohydrodynamics code) simulations that take into account the development of the Rayleigh–Taylor instability in the r–z plane provide integrated calculations of the implosion, x-ray generation, and hohlraum heating, as well as estimates of wall motion and plasma fill within the hohlraums. Lagrangian-RMHC simulations suggest that the addition of a 6 mg/cm3 CH2 fill in the reduced-scale hohlraum decreases hohlraum inner-wall velocity by ∼40% with only a 3%–5% decrease in peak temperature, in agreement with measurements.
Magnetic domains and twin structure of the La0.7Sr0.3MnO3 single crystal
Authors: Khapikov A. , Uspenskaya L. , Bdikin I. , Mukovskii Ya. , Karabashev S. , Shulyaev D. , Arsenov A.
Published: July 2002

Using a magneto-optical indicator film imaging technique and an x-ray topography, we have observed a strong correlation between magnetic and twin structure of a single crystal of La0.7Sr0.3MnO3 (LSMO). The correlation between magnetic and twin domains can be understood in terms of a rhombohedral deformation of the cubic cell of LSMO which is accompanied by an occurrence of a magnetic anisotropy. Our data and analysis suggest that twin domains play a fundamental role in a low-field magnetization behavior and in magnetotransport properties of LSMO.
Collision-induced dissociation dynamics in O2++Ne(Ar) collisions: The role of electronic excitation
Authors: Dressler Rainer A. , Chiu Yu-hui , Levandier Dale J. , Ng C. Y.
Published: July 2002

Integral collision-induced dissociation (CID) cross sections at center-of-mass (CM) energies ranging from threshold to 22 eV and product ion recoil velocity distributions are presented for O2++Ne(Ar) collisions. The O2+ ions are produced in an electron impact ion source and thus have ∼0.5 eV of vibrational energy. The O2++Ne CID cross sections are observed at energies below the 0 K thermodynamic threshold of 6.66 eV, implying the possibility of significant vibrational effects. A second CID threshold is apparent at 9±1 eV. This is close to the charge-transfer (CT) threshold energy. No CT products are observed. The O2++Ar CID cross section rises rapidly above a relative translational energy onset of 7.5±0.2 eV and reaches a maximum value of ∼0.8 Å2 at 14 eV (CM). A CT onset of 6.0±0.2 eV is observed which is close to the thermodynamic threshold for CID. The energy dependence of the CT cross section is very similar to the CID cross section except for a small step at energies where CID becomes efficient. The CID recoil velocity distributions near threshold are centered at the CM velocity and shift to forward preference at higher energies. The O2++Ar CT recoil velocities are forward scattered near threshold and become centered with respect to the CM velocity at energies where competition with CID is apparent.
Inhomogeneous mixture system: A density functional formalism based on the universality of the free energy density functional
Authors: Zhou Shiqi
Published: July 2002

A density functional theory is proposed for an inhomogeneous mixture system, which is based on the universality of the free energy density functional. The present formalism requires as input only the second-order direct correlation function and bridge function of the corresponding uniform fluid mixture. Applicability of the density functional theory was verified by applying the formalism to study on the structure of binary hard-sphere fluid mixture near a hard-planar wall; the predictions of the theory compare well with the corresponding computer simulation results.
Theoretical study of the photodetachment spectroscopy of the IHBr and IDBr anions
Authors: Kaledin Alexey , Skokov Sergei , Bowman Joel M. , Morokuma Keiji
Published: July 2002

The coupled-cluster method with a large basis set with quasi-relativistic effective core potentials on the halogens was utilized to investigate the ground electronic state X 1Σ+ of the IHBr anion. A semi-global, three-dimensional potential energy surface was obtained via spline interpolation of ab initio data. Variational rovibrational calculations were carried out with this potential energy surface, and assignment of fundamentals and low-lying overtones, as well as combination bands, is presented for IH(D)Br anions. Photodetachment spectra for IH(D)Br−+hν→[IH(D)Br]+e− were computed using three-dimensional time-independent real and complex L2 methods and a recently developed real L2 wave packet propagation technique; all methods employed a previous London–Eyring–Polanyi–Sato empirical potential for the neutral system. Well resolved fine structure is found in photodetachment spectra of IHBr−. This structure is assigned to bending excitation based on analysis of the wave functions and the neutral potential energy surface near the Franck–Condon region. This fine structure is largely eliminated in the IDBr− spectrum. The photodetachment spectrum for the first excited asymmetric stretch of IHBr− shows a new feature that is associated with sampling of the transition state region of the neutral surface. Spin-orbit corrected multi-reference configuration interaction calculations indicate that the first excited electronic state is only about 0.06 eV above the ground electronic state of the neutral IHBr system in the Franck–Condon region. The model empirical potential used in the calculations of photodetachment spectra is shown to have roughly the average behavior of these two ab initio potentials.