bluuebunny/crossref_metadata_2025 huggingface dataset:
Force-free field model of ball lightning
Authors: Tsui K. H.
Published: July 2002

Due to the nature that the force-free magnetic field, whose current carried by the conducting plasma is everywhere parallel to the magnetic field it generates, is the minimum energy configuration under the constraint of magnetic helicity conservation, ball lightning is considered as a self-organized phenomenon with a plasma fireball immersed in a spherical force-free magnetic field. Since this field does not exert force on the plasma, the plasma pressure, by itself, is in equilibrium with the surrounding environment, and the force-free magnetic field can take on any value without affecting the plasma. Due to this second feature, singular solutions of the magnetic field that are otherwise excluded are allowed, which enable a large amount of energy to be stored to sustain the ball lightning. The singularity is truncated only by the physical limit of current density that a plasma can carry. Scaling the customary soccer-size fireball to larger dimensions could account for day and night sightings of luminous objects in the sky.
Tangential x-ray imaging system for analysis of the small-scale modes in the T-10 tokamak
Authors: Savrukhin P. V. , Klimanov I. V.
Published: July 2002

A toroidally viewing x-ray imaging system is used in the T-10 tokamak for reconstruction of the small-scale plasma perturbations (high-m modes). The imaging system is based on the matrix array of the silicon surface-barrier diodes with a two-dimensional Soller collimator and an automatic positioning system. An additional hard x-ray detector and magnetic probe are placed inside the tokamak vessel for measurements of the nonthermal emission and fast magnetic field perturbations. Initial experiments in T-10 indicated improved sensitivity of the system for identification of the small-scale plasma perturbations prior the density limit disruptions.
Predicting free energy landscapes for complexes of double-stranded chain molecules
Authors: Zhang Wenbing , Chen Shi-Jie
Published: July 2002

We develop a statistical mechanical theory for the free energy landscapes for complexes of double-stranded chain molecules. The theory is based on the generalized polymer graph, a graphical representation for the conformations of the complexes. We compute the partition functions by “dividing and conquering” on the generalized polymer graph: we decompose a graph into simple subunits, calculate the partition function of each subunit exactly, and treat the interactions between subunits approximately, by calculating the localized interactions (of the nearest neighbor and the next-nearest neighbor monomers) at the interface of subunits. Our tests against the exact computer enumeration on the two-dimensional (2D) square lattice show that the theory is accurate. We apply the theory to the computation of the free energy landscapes of three representative systems: homopolymer–homopolymer, homopolymer–heteropolymer, and heteropolymer–heteropolymer complexes, using contact-based energy functions for the homopolymer–homopolymer and homopolymer–heteropolymer complexes, and stacking energies for the heteropolymer–heteropolymer complexes (to mimic RNA secondary structures). We find that the systems involving homopolymers show smooth free energy landscapes, and undergo noncooperative structural transitions during the melting process, and that the system of heteropolymers show rugged free energy landscapes, and the thermal denaturation involves intermediate states and cooperative structural transitions. We believe this approach maybe useful for computing the free energy landscapes and the thermodynamics of DNA or RNA interactions and RNA binding to a DNA or RNA target.
Validity of the Stoner–Wohlfarth model in hysteretic giant magnetoimpedance of annealed amorphous materials
Authors: Kim Cheol Gi , Yoon Seok Soo , Jang Kil Jae , Kim Chong-Oh
Published: July 2002

The hysteretic characteristics of giant magnetoimpedance (GMI) profiles have been measured in Co-based amorphous ribbon with various anisotropy angles θk, and they have been analyzed by using the Stoner–Wohlfarth model. A two-peak behavior with a dip near zero field is revealed in the measured GMI profile at 10 MHz, irrespective of θk. The negligible hysteresis of the field for the dip is in close agreement with that calculated assuming a magnetization jump from a metastable to a stable state. However, the hysteretic asymmetries for the increasing and decreasing fields in the samples with the angle range of 20°⩽θk<60° are well described by a divergence in the calculation without a magnetization jump. The asymmetry for the sample with θk⩾60° may also be due to the divergence. But the two peak of the measured profiles, which are due to the anisotropy distribution in the actual materials, are different from the calculated profiles, which have a single peak near zero field. This indicates that the Stoner–Wohlfarth model adequately describes a hysteretic asymmetry of GMI profiles for the anisotropy angle in the range of 20°⩽θk<60° at a frequency of 10 MHz in amorphous ribbons.
Carrier-type reversal in Pb–Ge–Se glasses: Photopyroelectric measurements of thermal conductivity and heat capacity
Authors: Philip J. , Rajesh R. , Menon C. Preethy
Published: July 2002

The thermal conductivity and heat capacity of Pb modified Ge–Se glasses, Pb20GexSe80−x (x=17–24) and PbyGe42−ySe58 (y=0–20), which exhibit carrier-type reversal from p type to n type at specific compositions, have been measured simultaneously by the photopyroelectric technique. The thermal conductivity as well as the heat capacity show anomalous variations at the compositions at which carrier-type reversals are reported to occur in these glasses. The results are explained in terms of the enhancement in electron concentration during p- to n-type changeover in these materials.
Coupled molecular dynamics/semiempirical simulation of organic solutes in polar liquids. I. Naphthalene in acetonitrile
Authors: Cichos F. , Brown R. , Bopp Ph. A.
Published: July 2002

In this paper we present coupled quantum mechanical/molecular dynamics simulations on the solvation of naphthalene in acetonitrile. A semiempirical treatment is introduced into classical molecular dynamics simulations to deal with the electronic polarizability of the solute. We study the structure of the solvation shell, the features of the electrostatic intermolecular interactions, the dynamics of solute and solvent molecules and the spectral shifts of the solute. For this particular system it is found that the influence of induced electrostatic interaction on the structure of the solvation shell is negligible. On the other hand, net induced dipolar contributions arise in the electrostatic interactions, in spite of the fact that the electric field is found to be inhomogeneous at the scale of the solute molecule. These induced dipolar interactions enhance the calculated absorption and emission shifts and contribute up to 40% to the electrostatic spectral shifts. They also modify or even dominate the fluctuations of the solute–solvent interaction and thus the solvation response. Both shifts and solvation dynamics are discussed in terms of dipole and quadrupole jumps.
Plasma buildup by short-pulse high-power microwaves
Authors: Bhattacharjee Sudeep , Amemiya Hiroshi , Yano Yasushige
Published: July 2002

The buildup of a plasma produced by short-pulse (0.05–1.2 μs), high-power (60–100 kW) microwaves is studied in a pressure range of 10 mTorr–10 Torr, by measurements of the temporal variation of the current and the optical intensity. The plasma is produced in a cylindrical tube and confined by a minimum-B field. The buildup of the electron current and the optical intensity are found to continue beyond the end of the pulse, for a few to tens of μs depending upon the pressure, and a minimum in their peak values and buildup times occur around 1 Torr. Increase in microwave pulse duration increases the buildup rate and peak current, whereas the pulse repetition frequency (10–500 Hz) has only a weak influence. The results are discussed from the growth of electron temperature during the pulse, and the following plasma evolution after the end of pulse. Collisional wave absorption, electron cyclotron heating, and diffusion are found to play important roles in plasma production and maintenance over the pressure range.
Electrical properties of fluorinated amorphous carbon films
Authors: Biswas N. , Harris H. R. , Wang X. , Celebi G. , Temkin H. , Gangopadhyay S.
Published: July 2002

We have studied the capacitance–voltage (C–V), conductance–voltage (G–V), and current–voltage characteristics of fluorinated amorphous carbon (a-C:Fx) films using metal/a-C:Fx/Si and metal/a-C:Fx/metal structures, respectively. Samples annealed in a vacuum were also studied. The C–V curves of the as-deposited sample are stretched about the voltage axis. Interface state density of 4.1×1011 cm−2 eV−1 at the midgap was calculated. Annealing the sample deposited on Si in a vacuum caused more frequency dispersion in the C–V and G–V curves, probably due to the diffusion of carbon into silicon. The bulk density of states for samples deposited on metal, measured by space-charge-limited current technique, decreased from 4×1018 eV−1 cm−3 for the as-deposited sample, to 7×1017 eV−1 cm−3 for the annealed sample.
Photodissociation of oriented HXeI molecules in the gas phase
Authors: Baumfalk Reinhard , Nahler N. Hendrik , Buck Udo
Published: July 2002

We report the first production of the molecule HXeI, which is bound by ionic forces, in the gas phase. The molecule is generated by the photodissociation of HI molecules on a large Xen cluster and is identified by detecting the asymmetric distribution of the H atom fragments of the oriented HXeI. The orientation is achieved in a combined pulsed laser and weak electrostatic field making use of the large anisotropy in the polarizability and the large dipole moment of this molecule.
Resonant and nonresonant hyper–Rayleigh scattering of charge-transfer chromophores
Authors: Wang C. H. , Woodford J. N. , Zhang C. , Dalton L. R.
Published: July 2002

The first molecular hyperpolarizabilities (β) of a series of charge-transfer nonlinear optical (NLO) chromophores are measured with the hyper-Rayleigh scattering (HRS) technique using two excitation wavelengths at 1064 and 1907 nm. The 1907 nm wavelength is the longest excitation wavelength used for the HRS experiment. For some of these chromophores, β values in excess of 1000×10−30 esu at 1907 nm are obtained, and due to two-photon enhancement, even greater β values are found with the 1064 nm excitation. Chromophores with such large hyperpolarizability are expected to have potential applications in practical electro-optical devices. The dispersion of β is analyzed using a two-vibronic-state model developed previously in our laboratory. The study shows that it is necessary to consider the vibronic structure of the chromophore in the excited state in order to account for the behavior of the first molecular hyperpolarizability of the charge-transfer NLO chromophores.
Lifetime of ferroelectric cathodes
Authors: Dunaevsky A. , Krasik Ya. E. , Felsteiner J. , Dorfman S. , Berner A. , Sternlieb A.
Published: July 2002

We study the lifetime and emission properties of ferroelectric cathodes made of perovskites with the ABO3 composition. To excite electron emission, we applied negative driving pulses with different amplitudes and durations to the rear electrode of the ferroelectric cathodes. The composition of the samples before and after the experiments was checked by energy dispersive spectroscopy and by Auger electron spectroscopy. The image and the structure of the emitting surface of the samples were studied by scanning electron microscopy. It was found that the lifetime of the tested cathodes depends strongly on their composition as well as on the parameters of the driving pulse. Namely, the driving pulse causes a substantial damage in lead–zirconium–titanate-type samples and a structural change in the surface of barium–titanate-type samples. In addition, fast deterioration was found in the emission properties of the tested cathodes along with increasing number of driving pulses. We discuss our results within the framework of the incomplete surface discharge model.
Hexagonal voids and the formation of micropipes during SiC sublimation growth
Authors: Kuhr Thomas A. , Sanchez Edward K. , Skowronski Marek , Vetter William M. , Dudley Michael
Published: July 2002

Hexagonal voids observed in sublimation grown SiC boules were examined using optical microscopy, atomic force microscopy (AFM), scanning electron microscopy, KOH etching, and synchrotron white-beam x-ray topography. Voids formed at imperfections in the attachment layer between the seed and crucible cap. They are platelet-like in shape with lateral sizes between 50 and 750 μm and thickness along the c axis between 5 and 25 μm. Growth steps were observed on the void facets closest to the seed and evaporation steps were observed on void facets closest to the growth surface, providing evidence for void movement during crystal growth. AFM images revealed that growth steps nucleate at a void sidewall, flow across the bottom of the void, and terminate in a trench-like depression. KOH etching of waters between the void and seed revealed dislocations lining up along the trace of the void path, often with higher densities corresponding to the location of the trench. X-ray topographs showed a random distribution of screw dislocations in the crystal volume above the void, and an absence of screw dislocations in the volume directly below the void. Hollow-core superscrew dislocations, called micropipes, were found at the corners of the void trace. Image forces associated with growth steps and void sidewalls are used to explain the formation of micropipes.
Diffusion photovoltage in poly(p -phenylenevinylene)
Authors: Duzhko V. , Dittrich Th. , Kamenev B. , Timoshenko V. Yu. , Brütting W.
Published: July 2002

Photovoltage phenomena in poly(p-phenylenevinylene) (PPV) are investigated under pulsed laser illumination. The photovoltage transients are strongly retarded in time depending on sample thickness, laser intensity, and bias illumination. It is shown that the photovoltage in PPV originates from separation of excess electrons and holes due to their concentration gradient and different diffusion coefficients (diffusion photovoltage). The diffusion coefficient of excess holes is found to be on the order of 1×10−6 cm2/s and it increases with increasing excitation intensity and intensity of bias illumination. The diffusion coefficient of excess electrons is about 1–2 orders of magnitude smaller than for excess holes.
Interfacing continuum and molecular dynamics: An application to lipid bilayers
Authors: Ayton Gary , Bardenhagen Scott G. , McMurtry Patrick , Sulsky Deborah , Voth Gregory A.
Published: July 2002

A new methodology is presented for interfacing atomistic molecular dynamics simulations with continuum dynamics, and the approach is then applied to a model lipid bilayer system. The technique relies on a closed feedback loop in which atomistic level simulations are coupled with continuum level modeling. This approach allows for the examination of the trans-temporal and trans-spatial phenomena that occur in many biological systems, and nonequilibrium molecular dynamics are used to calculate the relevant transport coefficients that are required at the continuum level. It is found that for the membrane system there is significant information transfer across disparate spatial and temporal regimes, resulting in significant nonlinear behavior.
Sintering of unequal grains
Authors: Colbeck S. C.
Published: July 2002

This theory shows that, as the ratio of the grain sizes increases, the boundary between the grains becomes more highly curved and the smaller grain becomes more deeply embedded in the larger one. At a ratio of only 1.57, the exposed portion of the small grain is too small to allow it to bond to a third, larger grain, and it cannot effectively connect to a chain of grains. For equal grains the theory reduces to the earlier theory but, as the size ratio increases, the stress gradient in the grain boundary decreases and the grain boundary becomes more tightly curved. These two factors decrease the rate of bonding, which decreases as either the ratio of the sizes, or their absolute sizes, increase.
Boron penetration in p-channel metal–oxide–semiconductor field-effect transistors enhanced by gate ion-implantation damage
Authors: Aoyama Takayuki , Suzuki Kunihiro , Tashiro Hiroko , Tada Yoko , Arimoto Hiroshi , Horiuchi Kei
Published: July 2002

We demonstrated that ion-implantation damage in gate SiO2-enhanced boron penetration in p-channel metal–oxide–semiconductor devices, easily enhances boron diffusion in SiO2 by ten times or more. This article describes the effectiveness of using high-temperature annealing above 1000 °C on damaged samples to suppress boron penetration in samples damaged by ion implantation. We also performed an electron spin resonance measurement to examine the damage recovery.
Effect of C and B sequential implantation on the B acceptors in 4H–SiC
Authors: Nakano Yoshitaka , Malhan Rajesh Kumar , Kachi Tetsu , Tadano Hiroshi
Published: July 2002

We have systematically investigated the effect of C and B sequential coimplantation on B-related acceptors and deep levels in 4H–SiC using thermal admittance spectroscopy. By increasing the concentration of coimplanted C, the density of deep levels decreased and was completely suppressed for a C and B ratio of 1:1. Moreover, the density and ionization energy of B acceptors increased and decreased, respectively, with increasing C concentration. However, we found that excess C content leads to the formation of a complex defect. Capacitance–voltage results also support the expected increase in the free hole concentration with increasing concentration of the coimplanted C atoms, which is followed by a decrease in the concentration under C-rich conditions. This is in reasonable agreement with the behavior of the B acceptors and deep defect levels. Therefore, the concentration of coimplanted C atoms is considered to be very sensitive to the formation of the B acceptor levels.
Thermal stability of the antiferromagnetically coupled Co/Cu/Co stack
Authors: Bal K. , van den Berg H. A. M. , Keen A. , Rasing Th.
Published: July 2002

The long-term thermal stability of the hard artificial antiferromagnetic subsystem found in giant magnetoresistance and tunnel magnetoresistance sensors has been investigated at 182, 210, 240, and 300 °C using magnetoresistive and vibrating sample magnetometry measurements. The observed changes in the magnetoresistance signal and the saturation field show two processes, where the slower one has an activation energy of 1.5±0.1 eV while the faster one occurs too fast to fit accurately. In order to probe the stability of the Co/Cu interface, nonlinear (magneto-optical magnetization-induced second-harmonic generation) measurements were done which reveal a dramatic enhancement of the second harmonic tensor components, also dominated by two time constants.
Cathode luminescence of samarium, europium, and gadolinium ion doped scintillator CdWO4 single crystals studied with x-ray photoelectron spectroscopy
Authors: Tanaka Katsumi
Published: July 2002

Cathode luminescence and x-ray excited luminescence are recorded on Sm-, Eu-, and Gd-doped CdWO4 single crystal surfaces. Luminescence spectra are convoluted to three species at 2.85, 2.54, and 2.22 eV. Yellow cathode luminescence (2.22 eV) is pronounced on a Sm-doped CdWO4 single crystal at room temperature whereas there is no difference in x-ray activated luminescence from pure CdWO4. The effects of impurities are elucidated with x-ray photoelectron spectroscopy. Significant stress is observed in the Sm-doped CdWO4 single crystal. Electron transfer easily occurs in a Gd-doped CdWO4 single crystal to reduce W and Cd ions. Band structures of Sm-, Eu-, and Gd-doped CdWO4 single crystals are elucidated and compared with pure CdWO4.
Origin of electric field enhancement in field emission from amorphous carbon thin films
Authors: Carey J. D. , Forrest R. D. , Silva S. R. P.
Published: July 2002

The observation of electron emission from amorphous carbon thin films at low applied electric fields is explained in terms of an enhancement of the field brought about by dielectric inhomogeneities within the film. These inhomogeneities originate from the differences between conductive, spatially localized sp2 C clusters surrounded by a more insulating sp3 matrix. By a more complete understanding of the concentration and distribution of the clusters, a generic model for field emission from amorphous carbon thin films can be developed. Extensions of this model to explain the emission properties of carbon nanotubes and carbon nanocomposite materials are also presented.
The equilibrium theory for Lennard-Jones dimer fluids
Authors: Lin C.-T. , Stell G.
Published: July 2002

We develop a new analytic theory for the thermodynamics of a fluid of dimers consisting of Lennard-Jones or Lennard-Jones-type atoms. The theory uses a hard-dimer reference system and an extension of the Weeks–Chandler–Andersen approach from a monatomic to a polyatomic fluid.
Electrical and optical properties of ZnS0.05Se0.95 nanocrystalline thin films
Authors: Ganguly A. , Mandal S. K. , Chaudhuri S. , Pal A. K.
Published: July 2002

ZnS 0.05 Se 0.95 nanocrystalline films were deposited onto fused silica and GaAs (100) substrates by the magnetron sputtering technique. The electrical conductivities of the films deposited onto fused silica substrates were measured in the temperature range of 180–300 K. It was observed that the low temperature conductivity could be explained by hopping of the charge carriers in the Coulomb gap while at elevated temperature Mott hopping is the predominant mode of conduction in these films. A distinct cross over from Efros–Shklovoskii to Mott hopping is observed for all the films. Optical spectra were found to be dominated by the combined effects of optical losses due to absorption and scattering. The absorption spectra indicated a blueshift, the extent of which depended on the crystallite size.
Branching ratio in the HD+OH reaction: A full-dimensional quantum dynamics study on a new ab initio potential energy surface
Authors: Zhang Dong H. , Yang Minghui , Lee Soo-Y.
Published: July 2002

A full-dimensional quantum dynamical study is reported for the title reaction on the YZCL2 potential energy surface. The influence of reagent rotation on the dynamics is examined in detail. It is found that the rotational excitation of the HD reagent substantially enhances the rate coefficient for forming the H2O+D product, and plays an important role in determining the branching ratio at low temperatures. The theoretical results are compared with available experimental data for an integral cross section, thermal rate coefficients, and their corresponding branching ratios. Excellent agreement between theory and experiment is revealed for available thermal rate coefficients and its branching ratio. The agreement between theory and experiment for the integral cross section is also satisfactory. The study clearly shows that the YZCL2 potential energy surface is capable of producing “quantitatively” accurate results for the title reaction.
Effects of an extremely thin buffer on heteroepitaxy with large lattice mismatch
Authors: Chen Yefan , Hong Soon-ku , Ko Hang-ju , Kirshner V. , Wenisch H. , Yao Takafumi , Inaba Katsuhiko , Segawa Yusaburo
Published: July 2002

We report on an experimental study of buffer mechanism in plasma-assisted molecular-beam epitaxy of ZnO on Al2O3(0001) with a MgO buffer. It lowers the surface energy and provides nucleation cores. As a result, lateral epitaxial growth of ZnO becomes favorable from the initial growth stage. The MgO buffer also affects the generation of dislocations in such a way that it reduces their density by introducing dislocation interactions. This study suggests that by employing an appropriate buffer to modify the initial nucleation environment, high quality heteroepitaxy is achievable even with large mismatch.
Effect of the inductive electric field on ion flow in tokamaks
Authors: Catto Peter J. , Hastie R. J. , Hutchinson I. H. , Helander P.
Published: July 2002

The effect of the inductive electric field of a tokamak on the parallel (and poloidal) ion flow in the banana regime is evaluated. It is demonstrated that the flow is in the direction of the parallel current and is surprisingly large—comparable to the usual banana regime ion temperature gradient drive.
Substrate effect on the magnetic microstructure of La0.7Sr0.3MnO3 thin films studied by magnetic force microscopy
Authors: Desfeux R. , Bailleul S. , Da Costa A. , Prellier W. , Haghiri-Gosnet A. M.
Published: July 2002

Colossal magnetoresistive La0.7Sr0.3MnO3 thin films have been grown under tensile strains on (100)-SrTiO3 substrates and compressive strains on (100)-LaAlO3 and (110)-NdGaO3 substrates by pulsed laser deposition. Using magnetic force microscopy (MFM), a “feather-like” magnetic pattern, characteristic of films with an in-plane magnetization, is observed for films deposited on both SrTiO3 and NdGaO3 while a “bubble” magnetic pattern, typical of films with an out-of-plane magnetization, is recorded for LaAlO3. We show that the shape of the magnetic pattern imaged by MFM is fully correlated to the easy direction of the magnetization in the film.
Determination of intrinsic barrier height in the Au/n-GaN contact system
Authors: Noh S. K. , Bhattacharya P.
Published: July 2002

We have measured the intrinsic Schottky barrier height of Au/n-GaN metal–semiconductor diodes by performing current–voltage measurement on a series of diodes with varying in the range 1017–1019 cm−3 in the GaN layer. The effective barrier height (ΦB) monotonically decreases with increasing doping level. Taking account of the image-charge lowering (ΔΦ), the intrinsic barrier height ΦB0=ΦB+ΔΦ, is almost constant at (0.934±0.015) V up to ∼5×1018 cm−3, which is close to the Schottky limit of 0.94 V.
Shear Alfvén-like waves in a weakly ionized self-gravitating nonuniform dusty magnetoplasma
Authors: Mamun A. A. , Shukla P. K.
Published: July 2002

The dispersion properties of obliquely propagating ultra-low-frequency electromagnetic shear Alfvén-like waves in a weakly ionized, self-gravitating, nonuniform dusty magnetoplasma are examined. It is found that the electromagnetic shear Alfvén-like waves having the wavelength of the order of 10 AU and the wave period of the order of 30 000 years are unstable due to the effect of the self-gravitational force acting on the dust grains. The present investigation may be of relevance to the regions (namely, “warm photodisassociation regions”) at the boundary between molecular clouds and H II regions surrounding hot early type stars.
On the current transport limitations in Bi-based high temperature superconducting tapes
Authors: Tönies S. , Weber H. W. , Guo Y. C. , Dou S. X. , Sawh R. , Weinstein R.
Published: July 2002

The transport critical current densities, Jc, of superconducting (Bi, Pb)2Sr2Ca2Cu3Ox/Ag tapes were measured before and after employing a special radiation technique, which leaves the grain boundary properties largely unaffected. We identify two regions separated by a temperature dependent crossover field Hgb−p. In the low field region, Jc is limited by the transport currents across the grain boundaries, which remain unchanged after irradiation. Above Hgb−p, Jc is limited by flux pinning. In this field region, the artificial defects optimize flux pinning and enhance Jc.
Scattering patterns of self-assembled gyroid cubic phases in amphiphilic systems
Authors: Garstecki Piotr , Hołyst Robert
Published: July 2002

We present scattering patterns (with surface contrast) for five triply periodic minimal surfaces of the Ia3̄d cubic symmetry. We obtain a very good agreement between the numerically obtained spectrum and experimental patterns for the simple gyroid G structure. We show the scattering patterns for four gyroid GX1, GX2, GX3, and GX5 structures of a complex topology. We show how the scattering patterns change with increasing complexity of the unit cell of the structure. The spectra of the complex structures can give wrong estimates about the cubic cell parameter and even wrong establishment of the space symmetry group. Thus the correct recognition of the structure present in the system requires the analysis of the intensities of the peaks and comparison with numerically obtained spectra.
Multireference Møller–Plesset method with a complete active space configuration interaction reference function
Authors: Choe Yoong-Kee , Nakao Yoshihide , Hirao Kimihiko
Published: July 2002

A multireference Møller–Plesset (MRMP) method with a complete active space configuration interaction (CASCI) reference function has been proposed as an accurate and computationally efficient method for treating the ground and excited states of molecules. The CASCI wave function was constructed using the self-consistent field orbitals and used as a reference function of the MRMP to incorporate the remaining dynamical correlation. The advantage in using the CASCI is that it does not require iterations, nor does it encounter convergence difficulties which may be found in complete active space self-consistent field (CASSCF) calculations. The scheme was applied to the potential curves of the ground and low-lying excited states of N2, the potential curve of the ground state of CO, and the vertical valence π–π* excited states of benzene. Excellent agreement between theory, experiment, and some benchmark calculations was obtained. MRMP with CASCI is comparable in accuracy with MRMP with CASSCF. The present scheme will open a possibility toward accurate treatment for the ground and excited states of small to large molecular systems.
Binary N-octanol–sulfur hexafluoride nucleation
Authors: Anisimov Michael P. , Hopke Philip K. , Shaimordanov Ildar N. , Shandakov Sergey D. , Magnusson Lars-Erik
Published: July 2002

Recently, the accuracy of vapor nucleation rate measurements has increased substantially. However, when experimental conditions become close to the values to the critical parameters of investigated systems, significant discrepancies were found between the experimental results and theoretical predictions. Some studies have found agreement between results and theory only within a narrow range of nucleation conditions. Therefore, it is appropriate to investigate the nucleation of additional systems to obtain new information to guide nucleation theory development. Alcohols are the most studied class of chemical compounds, but currently there are no experimental data on n-octanol vapor nucleation. The present study is devoted to the investigations of the homogeneous nucleation of the n-octanol in sulfur hexafluoride. Comparison of the critical pressures for binary mixture with the Laplace pressure calculated in the droplet approximation using the bulk liquid surface tension shows that the carrier gas has a more active role in the nucleation than assumed by classical theory. A very high Laplace pressure in the critical embryo is calculated in the droplet approximation. When compared with the highest possible critical pressure for the binary system under investigation, the problems associated with calculating the critical embryo excess energy using the bulk liquid surface tension are revealed. Disagreement of the experimental results with predictions of classical nucleation theory, Dillmann–Meier theory, or any other theory reveals the same basic problems. These results suggest that it is appropriate to consider the general problem of vapor–liquid nucleation in a carrier gas including the estimation of the size and temperature dependence of the critical embryo excess energy as a binary nucleation problem instead of that of a single component. Theories for single-component vapor nucleation should be compared with the truly single-component vapor nucleation experimental results.
Crossed molecular beam studies of the O(1D)+NH3 reaction
Authors: Shu Jinian , Lin Jim J. , Wang Chia C. , Lee Yuan T. , Yang Xueming
Published: July 2002

The O(1D)+NH3 reaction has been investigated using the improved universal crossed molecular beam technique. Two different reaction channels, OH+NH2 and NHOH/NH2O+H, have been observed. OH formation is the dominant process, while the H formation process is minor. Different product angular distributions are also observed for these two product channels, indicating that the two reaction channels proceed with significantly different dynamics.
Nonequilibrium initial conditions of a Brownian oscillator system observed by two-dimensional spectroscopy
Authors: Suzuki Yoko , Tanimura Yoshitaka
Published: July 2002

We study effects of a nonequilibrium initial condition of a Brownian oscillator system upon two-, three-, and four-time correlation functions of an oscillator coordinate as a subject of multidimensional spectroscopy. A nonequilibrium initial condition is set by a displacement of a Gaussian wave packet in an oscillator potential. Such situation may be found in a vibrational motion of molecules after a sudden bond breaking between a fragmental molecule and a targeting vibrational system or a movement of wave packet in an electronic excited state potential surface created by a laser pump pulse. Multitime correlation functions of oscillator coordinates for a nonequilibrium initial condition are calculated analytically with the use of generating functional from a path integral approach. Two-, three-, and four-time correlation functions of oscillator coordinates correspond to the third-, fifth-, and seventh-order Raman signals or the first-, second-, and third-order infrared signals. We plotted these correlation functions as a signal in multidimensional spectroscopy. The profile of the signal depends on the initial position and momentum of the wave packet in the fifth- and seventh-order Raman or the second and third order infrared measurement, which makes it possible to measure the dynamics of the wave packet directly in the phase space by optical means.
Continuous phase transition in polydisperse hard-sphere mixture
Authors: Blaak Ronald , Cuesta José A.
Published: July 2002

In a previous paper [Zhang et al., J. Chem. Phys. 110, 5318 (1999)] we introduced a model for polydisperse hard-sphere mixtures that is able to adjust its particle size distribution. Here we give the explanation of the questions that arose in the previous description and present a consistent theory of the phase transition in this system, based on the Percus–Yevick equation of state. The transition is continuous, and like Bose–Einstein condensation, a macroscopic aggregate is formed due to the microscopic interactions. A treatment based on the equation of state by Boublı́k, Mansoori, Carnahan, Starling, and Leland leads to the same conclusion with slightly more accurate predictions.
Stabilization of the Mercier modes in a tokamak by toroidal plasma rotation
Authors: Wahlberg C. , Bondeson A.
Published: July 2002

The stability of localized modes (Mercier modes) in a tokamak with a toroidally rotating plasma is analyzed within the framework of compressible, ideal magnetohydrodynamics. For equilibria with large aspect ratio, poloidal beta value of order unity, and isothermal magnetic surfaces, it is found that sonic, toroidal rotation provides a strongly stabilizing effect for the Mercier modes, similar to the stabilization recently found for the internal kink mode in a rotating plasma [Wahlberg and Bondeson, Phys. Plasmas 7, 923 (2000)]. A finite oscillation frequency (Brunt–Väisälä frequency), of the order of the sound frequency, is shown to be associated with each magnetic surface. If Γ>1, where Γ is the exponent in the equation of state, the rotation transforms the Mercier instabilities to stable oscillations at the local Brunt–Väisälä frequency associated with the magnetic surface where the mode is located. If the plasma satisfies an isothermal equation of state (Γ=1), however, the stability of the Mercier modes becomes sensitive to the profile of the toroidal flow. In this case, the rotation is found to be stabilizing if the kinetic energy density of the rotation is an increasing function of the minor radius. In the opposite case, the rotation is destabilizing unless the pressure profile is much more peaked than the kinetic energy density profile.
Full-band-structure calculation of Shockley–Read–Hall recombination rates in InAs
Authors: Krishnamurthy Srinivasan , Berding M. A.
Published: July 2002

We report a calculational procedure to obtain the rate of electron–hole recombination, mediated by the Shockley–Read–Hall (SRH) mechanism. Our method uses a combination of first-principles calculations and accurate empirical band structures. First, we use ab initio calculations to identify the point defects, their densities and energy levels in the gap. Then we parametrize the tight-binding interaction between defect and the host atoms in a Green’s function approach to obtain the defect levels as identified by the first-principles calculations. Finally, the resulting tight-binding Hamiltonian is used to obtain the dipole matrix element between the conduction and valence band states, mediated through the defect levels in the gap, in second-order perturbation theory. The states are integrated over the entire Brillioun zone, subject to energy and momentum conservation, to obtain the limiting lifetimes of the carriers. This method is applied to study the minority carrier lifetimes in n-doped InAs. The calculated effective lifetimes that include Auger and SRH recombinations as functions of temperature agree reasonably well with experiment. Our calculation of lifetimes in 3.5×1016 and 2.0×1016 cm−3 n-doped InAs indicate that SRH is dominant at low temperatures and that the lifetimes vary between 10−8 and 10−7 s.
An investigation into the local segmental dynamics of polyethylene: An isothermal/isobaric molecular dynamics study
Authors: Hotston S. D. , Adolf D. B. , Karatasos K.
Published: July 2002

Molecular dynamics simulations of unentangled linear polyethylene melts have been performed for systems composed of 10 chains of 100 united atoms over a pressure range of 1 to 5000 bar and a temperature range of 375 to 475 K. Transition rates, activation volumes, and activation energies are in good agreement with values from similar simulations quoted in literature for systems well above Tg. Second-neighbor torsional angle coupling is observed to increase with increasing pressure and decreasing temperature. The lifetime of this coupling between conformational events is presented for the first time. Geometric autocorrelation functions are analyzed in terms of their distribution of relaxation times and reveal a process on the time scale of a few picoseconds and another on the time scale of a few nanoseconds. An intermediate process develops between these two time scales at high pressure and low temperature.
Photopumped red-emitting InP/In0.5Al0.3Ga0.2P self-assembled quantum dot heterostructure lasers grown by metalorganic chemical vapor deposition
Authors: Ryou J. H. , Dupuis R. D. , Walter G. , Kellogg D. A. , Holonyak N. , Mathes D. T. , Hull R. , Reddy C. V. , Narayanamurti V.
Published: July 2002

We report the 300 K operation of optically pumped red-emitting lasers fabricated from InP self-assembled quantum dots embedded in In0.5Al0.3Ga0.2P layers on GaAs (100) substrates grown by metalorganic chemical vapor deposition. Quantum dots grown at 650 °C on In0.5Al0.3Ga0.2P layers have a high density on the order of 1010 cm−2 and the dominant size of individual quantum dots ranges from ∼5 to ∼10 nm for 7.5 monolayer “equivalent growth.” These InP/In0.5Al0.3Ga0.2P quantum dot heterostructures are characterized by atomic force microscopy, high-resolution transmission electron microscopy, and photoluminescence. Laser structures are prepared from wafers having two vertically stacked InP quantum dot active layers within a 100-nm-thick In0.5Al0.3Ga0.2P waveguide and upper and lower 600 nm InAlP cladding layers. We observe lasing at λ∼680 nm at room temperature in optically pumped samples.
Methods for surface roughness elimination from thermal-wave frequency scans in thermally inhomogeneous solids
Authors: Nicolaides Lena , Mandelis Andreas
Published: July 2002

Two approaches for eliminating surface roughness in the thermal-wave frequency response of inhomogeneous solids are developed. The first approach is based on the theoretical formulation of roughness as an effective homogeneous overlayer and is adequate for eliminating low roughness levels from experimental data. The second approach models roughness as random spatial white noise resulting in a linear superposition of logarithmic-Gaussian distributions representing roughness scales in the spatial frequency spectrum and in the modulation frequency domain. Two scales of roughness on the surface of hardened AISI 8620 steel with the same hardness depth profiles are found and the experimental data are reconstructed to retrieve similar inhomogeneous thermal diffusivity depth profiles.
What is the true electronic ground state of the disilaethynyl radical (SiSiH): 2B1 or A12?
Authors: Pak Chaeho , Wesolowski Steven S. , Rienstra-Kiracofe Jonathan C. , Yamaguchi Yukio , Schaefer Henry F.
Published: July 2002

The two lowest-lying (H-bridged, cyclic) electronic states (2B1 and A12) of the disilaethynyl (SiSiH) radical have been investigated using ab initio electronic structure theory. Theoretical methods through the full coupled cluster with all triple excitations (CCSDT) have been used, and basis sets as large as Dunning’s correlation consistent pentuple set adopted. While the SCF, MP2, CISD, and CCSD levels of theory predict the B12 state to be lower in energy, the CCSD(T) and CCSDT methods show that the A12 state is the true electronic ground state. With our most reliable method, the energy difference is predicted to be Te(2B1)=0.60 kcal/mol (0.026 eV,210 cm−1) and T0(2B1)=0.37 kcal/mol (0.016 eV,128 cm−1). This theoretical finding confirms the experimental assignment by Xu et al. [J. Chem. Phys. 108, 7645 (1998)] in 1998 that the ground state of SiSiH is the A12 state and it is 0.020±0.005 eV lower in energy than the B12 state.
High-resolution energy-selected study of the reaction NH3+→NH2++H: Accurate thermochemistry for the NH2/NH2+ and NH3/NH3+ systems
Authors: Song Y. , Qian X.-M. , Lau K.-C. , Ng C. Y. , Liu Jianbo , Chen Wenwu
Published: July 2002

Employing the newly developed high-resolution pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence (PFI-PEPICO) technique, we have examined the dissociation of energy-selected NH3+ to form NH2++H near its threshold. The breakdown curves for NH2+ and NH3+ thus obtained yield a value of 15.765±0.001 eV for the 0 K dissociation threshold or appearance energy (AE) for NH2+ from NH3. This value, together with the known ionization energy (IE=10.1864±0.0001 eV) and 0 K bond dissociation energy (D0=4.6017±0.0025 eV) for NH3, allows the determination of the D0(NH2+–H) and IE(NH2), which are 5.5786±0.0010 and 11.1633±0.0025 eV, respectively. Using the known 0 K heats of formation (ΔHf0°) for NH3 and H and the AE(NH2+), we obtain the ΔHf0°(NH2+)=302.60±0.08 kcal/mol. The PFI-PE spectrum for NH3 exhibits a step-like feature at the 0 K AE(NH2+), indicating that the dissociation of excited NH3 in high-n (n⩾100) Rydberg states at energies slightly above the dissociation threshold occurs on a time scale ⩽10−7 s. This step confirms the AE(NH2+) value derived from the PFI-PEPICO measurements. Highly accurate energetic data with well-founded error limits, such as those obtained in the present and other studies using the PFI techniques, are expected to play an important role for the development of the next generation of ab initio quantum computation procedures. This experiment has stimulated a state-of-the-art ab initio quantum chemical calculation [Dixon et al., J. Chem. Phys. 115, 2576 (2001)]. The comparison between theoretical predictions and the best experimental results for the NH2/NH2+ and NH3/NH3+ systems indicates that the accuracy of the computational scheme used is ⩽0.4 kcal/mol.
Parallel integration and characterization of nanoscaled epitaxial lattices by concurrent molecular layer epitaxy and diffractometry
Authors: Ohnishi T. , Komiyama D. , Koida T. , Ohashi S. , Stauter C. , Koinuma H. , Ohtomo A. , Lippmaa M. , Nakagawa N. , Kawasaki M. , Kikuchi T. , Omote K.
Published: July 2002

A material highway for building up crystal lattices and heterojunctions from molecular layers has been developed based on a concept of combinatorial lattice integration. The atomic-scale precision of automated multilane paving of multilayered thin films is in situ monitored by concurrent reflection high-energy electron diffraction. The designed nanolayered structures are rapidly verified by a concurrent x-ray diffractometer which has been developed for the purpose of this technology. This scheme corresponds to the concurrent two-dimensional Merrifield synthesis to form a variety of sequence-controlled layer structures in parallel and should be widely applicable for systematic fabrication and property screening of nanostructured materials and devices.
Ionization energy studies for ozone and OClO monomers and dimers
Authors: Probst M. , Hermansson K. , Urban J. , Mach P. , Muigg D. , Denifl G. , Fiegele T. , Mason N. J. , Stamatovic A. , Märk T. D.
Published: July 2002

Electron impact ionization cross sections measured close to threshold are reported for both the monomers and dimers of ozone and OClO using a new high resolution electron impact apparatus. The present appearance energies AE(O3+/O3)=12.70±0.02 eV, AE (OClO+/OClO)=10.55±0.02 and AE(ClO+/OClO=13.37±0.03 eV derived from the measured ionization cross sections are in excellent agreement with the vertical threshold values determined for these ions by high resolution PES and PIMS photoionization studies. The corresponding appearance energies determined for the dimer ions, 10.10±0.3 eV for (O3)2+ and 9.87±0.2 eV for (OClO)2+, are both red shifted with respect to the monomer case. The bond energy (0.70–0.3+0.5) eV of (OClO)2+ estimated from these data is similar to that of other dimer ions, whereas the bond energy of (O3–O3+) with (2.55−0.4+0.6) eV is rather large suggesting an unusual structure for the cationic ozone dimer ion. Based on quantum chemical calculations on various levels we are led to the conclusion that the ion produced by ionization of the ozone dimer is no longer a conventional dimer ion where the two monomer units are still present (as is the case for the OClO system), but rather an ion of form O2⋯O4+ or a twisted ring structure of (O6)+.
A generalized Ornstein–Zernike integral equation study of atomic impurities in quantum fluids
Authors: Shinoda Keiko , Miura Shinichi , Okazaki Susumu
Published: July 2002

In this paper, solvation structure and thermodynamic properties of rare gas and alkali impurities in liquid helium-4 have been studied theoretically. A generalized Ornstein–Zernike integral equation for pure quantum fluids [J. Chem. Phys. 114, 7497 (2001)] was extended to the quantum solutions at infinite dilution. Self-correlation function of the solute atom which appears in the integral equation was determined self-consistently with the solvent density fluctuation. Numerical calculations have been performed for the helium-4 solutions at 4 K, with Boltzmann statistics being assumed. It was found that all the rare gas species investigated in this study have negative partial molar volumes, owing to the well-defined solvation structure around the impurities. In contrast to this, the alkali atoms have large positive partial molar volumes, primarily coming from the excluded volume contribution. Further, while the rare gas atoms have negative excess chemical potentials, the alkali atoms have large positive values. The former may be dominated by the negative interaction energy between the impurity and surrounding solvent atoms, and the latter by the work done by the volume of the solute to exclude the solvent atoms.