bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of Ta antidiffusion barriers on the thermal stability of tunnel junctions
Authors: Cardoso S. , Ferreira R. , Freitas P. P. , Wei P. , Soares J. C.
Published: July 2002

Previous work on spin-dependent tunnel junctions based on NiFe/CoFe/Al2O3/CoFe/MnIr reported structural thermal stability upon anneals up to 300 or 320 °C. Above 320 °C, the tunneling magnetoresistance (TMR) starts to decrease, and interdiffusion occurs at the MnIr/CoFe interface. MnIr diffuses into CoFe and reduces its magnetic moment. It was proposed that this mechanism could be partially responsible for the observed loss of TMR, due to the decrease in interface polarization. In this letter, tunnel junctions were prepared with a 2 Å-thin Ta antidiffusion barrier inserted at the CoFe/MnIr interface, to stop MnIr diffusion into the pinned electrode. Structural and magnetization measurements indicate that the CoFe and MnIr layers remain unchanged with anneals up to 440 °C, and that the antidiffusion barrier is working. However, the TMR degradation above 320 °C was not avoided, which suggests that improving the thermal stability of the junctions will probably require changes in the barrier itself.
Electrical properties of rapid thermal oxides on Si1−x−yGexCy films
Authors: Bera L. K. , Choi W. K. , Feng W. , Yang C. Y. , Mi J.
Published: July 2002

The electrical characteristics of rapid thermal oxides on Si1−x−yGexCy layers are reported. X-ray photoelectron spectroscopy results indicate segregation of Ge at the SiO2/Si1−x−yGexCy interface, a thin GeO2 layer at the oxide surface, and elemental Ge at the interface and in the oxide. The interface state density of the samples ranges from 3.0×1011 to 3.6×1012 eV−1 cm−2. All the samples show electron trapping behavior and the trap generation rate decreases with increasing C concentration. The charge-to-breakdown value and the oxide breakdown field are higher for Si0.887Ge0.113 than for Si1−x−yGexCy samples, and these values decrease with increasing C concentration.
Parallel implementation of the CI-vector evaluation in full CI/CAS-SCF
Authors: Klene Michael , Robb Michael A. , Frisch Michael J. , Celani Paolo
Published: July 2002

We present an implicitly parallel, direct reduced list method for integral-driven full CI (FCI) algorithms to be used in CAS-SCF. Our algorithm makes efficient use of modern supercomputer hardware supporting both shared memory and distributed memory architectures. The applicability and efficiency is demonstrated with a CAS-SCF(14,14) calculation on stilbene and a CAS-SCF(8,16) calculation on pentalene.
Laser vector measurement technique for the determination and compensation of volumetric position errors. Part II: Experimental verification
Authors: Janeczko John , Griffin Bob , Wang Charles
Published: July 2002

A Giddings and Lewis, model RAM 630 horizontal machine center was used to verify the laser vector measurement technique. The repeatability of the machine and the repeatability of the vector measurements were verified over a six month period. A compensation file was generated based on the laser vector measurement technique and the body diagonal measurements were used to check the volumetric accuracy of the machine. The results indicated that a gain of a factor of 3–4 in accuracy was achieved with the volumetric compensation than without compensation. The time required to compensate for the machine using the laser vector measurement technique is significantly less than that using conventional measurement procedures.
Vortical turbulence structure and transport mechanism in a homogeneous shear flow
Authors: Iida O. , Iwatsuki M. , Nagano Y.
Published: July 2002

Direct numerical simulations (DNS) are carried out to investigate the kinematics of vortical structures in a homogeneous shear flow, and their association with the momentum transfer is studied in detail. Longitudinal streamwise vortices are generated and conditionally averaged over all the computational region. The effects of the nonlinear term on their kinematics are investigated by comparing the DNS and Rapid Distortion Theory (i.e., RDT). As a result, some important similarities are found in the vortical structure between the homogeneous shear flow and near-wall turbulence. It is also found that the strain rate in the vortical structure, which is markedly affected by the nonlinear term, determines the transfer functions associated with the energy cascade of the turbulence.
Molecular dynamics evaluation of self-diffusion in Yukawa systems
Authors: Ohta H. , Hamaguchi S.
Published: July 2002

Self-diffusion coefficients of Yukawa systems in the fluid phase are obtained from molecular dynamics simulations in a wide range of the thermodynamical parameters. The Yukawa system is a collection of particles interacting through Yukawa (i.e., screened Coulomb) potentials, which may serve as a model for charged dust particles in a plasma or colloidal particles in electrolytes. The self-diffusion coefficients are found to follow a simple scaling law with respect to the system temperature, which is consistent with the universal scaling (i.e., temperature scaling independent of the ratio of interparticle distance to screening length) observed by Robbins et al. [J. Chem. Phys. 88, 3286 (1988)] if the fluid system is near solidification. Also discussed is the velocity autocorrelation function, which is in part used to determine the self-diffusion coefficients through the Green–Kubo formula.
Formulas for Cross Sections and Rate Coefficients for Partial Electron Impact Ionization of CH4 and H2
Authors: Dose V. , Pecher P. , Preuss R.
Published: July 2002

Robust, accurate, and asymptotically exact cross section formulas for all direct and dissociative electron impact ionization channels of methane and hydrogen are presented. The parameter estimation employs Bayesian inference and allows for a consistent use of data from different experiments with possibly discordant calibrations. An efficient method for the calculation of rate coefficients and their temperature derivatives is outlined.
Coherent optical control of acoustic phonon oscillations in InGaN/GaN multiple quantum wells
Authors: Sun Chi-Kuang , Huang Yue-Kai , Liang Jian-Chin , Abare Amber , DenBaars Steven P.
Published: July 2002

We demonstrate coherent optical control of phonon oscillations using a femtosecond control pulse. The experiments were performed in InGaN/GaN multiple quantum wells. Coherent acoustic phonon oscillations was initiated with an UV femtosecond pulse. The subsequent manipulation, including magnitude and phase, of the coherent acoustic phonon oscillations was achieved using another UV femtosecond pulse by controlling the pulse time delay and intensity.
Beltrami fields in plasmas: High-confinement mode boundary layers and high beta equilibria
Authors: Yoshida Z. , Mahajan S. M. , Ohsaki S. , Iqbal M. , Shatashvili N.
Published: July 2002

A general solenoidal vector field, such as a magnetic field or an incompressible flow, can be decomposed into an orthogonal sum of Beltrami fields (eigenfunctions of the curl operator). Nonlinear dynamics of a plasma induces complex couplings among these Beltrami fields. In a single-fluid magnetohydrodynamic (MHD) plasma, however, the energy condensates into a single Beltrami magnetic field resulting in the self-organization of a force-free equilibrium, that is, the Taylor relaxed state. By relating the velocity and the magnetic fields, the Hall term in the two-fluid model leads to a singular perturbation that enables the formation of an equilibrium given by a pair of two different Beltrami fields. This new set of relaxed states, despite the simple mathematical structure, includes a variety of plasma states that could explain a host of interesting phenomena. The H-mode (high-confinement) boundary layer, where a diamagnetic structure is self-organized under the coupling of the magnetic field, flow, electric field, and pressure, is an example. The theory also predicts the possibility of producing high beta equilibrium.
Critical gradient formula for toroidal electron temperature gradient modes
Authors: Jenko F. , Dorland W. , Hammett G. W.
Published: July 2002

Under certain conditions, the electron heat transport induced by electron temperature gradient (ETG) streamers is sufficiently large and sensitive with respect to the normalized electron temperature gradient to represent a possible cause for electron temperature profile consistency (“stiffness”). Here, linear gyrokinetic simulations of toroidal ETG modes in tokamak core and edge plasmas are presented. An algebraic formula for the threshold of the linear instability is derived from the numerical solutions of the linear gyrokinetic equations which recovers previous analytical results in the appropriate limits.
Growth and superconducting properties of Bi2Sr2CaCu2O8+δ single-crystal whiskers using tellurium-doped precursors
Authors: Nagao Masanori , Sato Mitsunori , Maeda Hiroshi , Kim SangJae , Yamashita Tsutomu
Published: July 2002

Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) single-crystal whiskers were grown by annealing sintered Te-doped Bi–Sr–Ca–Cu–O precursors that had not passed through a melt-quenched glassy state, a process that is known well at this time. The optimum nominal composition of precursors is Ca-rich Bi2Sr2Ca2Cu2Te0.5Ox with a Te content of 0.5. The whiskers contain no Te element. The precursors are mainly composed of Bi2212 and (SrCa)3TeO6 (STO) phases. The STO or the element Te acts as a catalyst in enhancing whisker growth. The critical current (Ic) with a multibranched structure of the current–voltage characteristics due to the intrinsic Josephson junctions along the c axis is quite uniform, showing that the whiskers are homogeneous and of good quality.
Quantum size effect in magnetic tunnel junctions with ultrathin Fe(001) electrodes
Authors: Nagahama T. , Yuasa S. , Suzuki Y. , Tamura E.
Published: July 2002

The transport properties of magnetic tunnel junctions with single-crystalline ultrathin Fe(001) electrodes are studied. The tunnel spectra and the bias dependence of the differential magnetoresistance show quantum-well oscillations. This effect provides evidence of ballistic transport through the Al–O barrier and shows that magnetic tunnel junctions with quantum-well states can be a tool for studying spin-dependent transport mechanisms in magnetic tunnel junctions.
Two-surface wave decay
Authors: Macchi Andrea , Cornolti Fulvio , Pegoraro Francesco
Published: July 2002

The parametric excitation of pairs of electron surface waves (ESW) in the interaction of an ultrashort, intense laser pulse with an overdense plasma is discussed using an analytical model. The plasma has a simple step-like density profile. The ESWs can be excited either by the electric or by the magnetic part of the Lorentz force exerted by the laser and, correspondingly, have frequencies around ω/2 or ω, where ω is the laser frequency.
Exact numerical computation of a kinetic energy operator in curvilinear coordinates
Authors: Lauvergnat David , Nauts André
Published: July 2002

The conformation and dynamical behavior of molecular systems is very often advantageously described in terms of physically well-adapted curvilinear coordinates. It is rather easy to show that the numerous analytical expressions of the kinetic energy operator of a molecular system described in terms of n curvilinear coordinates can all be transformed into the following more usable expression: T̂=∑ijf2ij(q)∂2/∂qi∂qj+∑if1i(q)∂/∂qi+ν(q), where f2ij(q), f1i(q), and ν(q) are functions of the curvilinear coordinates q=(…,qi,…). If the advantages of curvilinear coordinates are unquestionable, they do have a major drawback: the sometimes awful complexity of the analytical expression of the kinetic operator T̂ for molecular systems with more than five atoms. Therefore, we develop an algorithm for computing T̂ for a given value of the n curvilinear coordinates q. The calculation of the functions f2ij(q), f1i(q), and ν(q) only requires the knowledge of the Cartesian coordinates and their derivatives in terms of the n curvilinear coordinates. This coordinate transformation (curvilinear→Cartesian) is very easy to perform and is widely used in quantum chemistry codes resorting to a Z-matrix to define the curvilinear coordinates. Thus, the functions f2ij(q), f1i(q), and ν(q) can be evaluated numerically and exactly for a given value of q, which makes it possible to propagate wavepackets or to simulate the spectra of rather complex systems (constrained Hamiltonian). The accuracy of this numerical procedure is tested by comparing two calculations of the bending spectrum of HCN: the first one, performed by using the present numerical kinetic operator procedure, the second one, obtained in previous studies, by using an analytical kinetic operator. Finally, the ab initio computation of the internal rotation spectrum and wave functions of 2-methylpropanal by means of dimensionality reduction, is given as an original application.
Nondestructive defect delineation in SiC wafers based on an optical stress technique
Authors: Ma Xianyun , Parker Mathew , Sudarshan Tangali S.
Published: July 2002

The potential of using the optical stress technique to delineate the various defects in silicon carbide (SiC) wafers has been fully demonstrated. The observed defects include micropipes, dislocations, stress striations, grain boundary or dislocation walls, and regions of polytype nonuniformity. Revealed dislocation density is in the range 104–105 cm−2.
Using projections and correlations to approximate probability distributions
Authors: Karlen Dean
Published: July 2002

A method to approximate continuous multidimensional probability-density functions (PDFs) using their projections and correlations is described. The method is particularly useful for event classification when estimates of systematic uncertainties are required and for the application of an unbinned maximum-likelihood analysis when an analytic model is not available. A simple goodness-of-fit test of the approximation can be used, and simulated event samples that follow the approximate PDFs can be generated efficiently. The source code for a Fortran-77 implementation of this method is available. © 1998 American Institute of Physics.
Infrared reflectance evaluation of chemically vapor deposited β-SiC films grown on Si substrates
Authors: Holm R. T. , Klein P. H. , Nordquist P. E. R.
Published: July 2002

We use infrared reflectance from 400 to 4000 cm−1 to evaluate cubic SiC films grown by chemical vapor deposition on Si substrates. From different regions of the spectra we determine precise film thicknesses, estimate carrier concentrations in highly doped specimens, observe roughness at both film surfaces, and detect conducting regions at the interface. We show how the roughness and the interfacial conducting region introduce nonideality into the spectra. The method is nondestructive and the information can be obtained in less than 1 h after film growth.
Reinvestigation of the magnetic properties of MnSe2
Authors: Plumier R. , Sougi M.
Published: July 2002

Detailed neutron diffraction experiments performed on MnSe2 at various temperatures show the existence of a first-order transition at T*≂48.5 K. At this temperature, a transition takes place from the previously reported magnetic structure described in a (3a, a, a) cell with moments aligned along [100] to two independent sinusoidally modulated magnetic structures of comparable amplitudes and propagation vectors ‖kα‖=1/3a and ‖kβ‖=1/a aligned along [100]. The transition to paramagnetism takes place at a higher temperature T≂53 K. High-precision susceptibility measurements confirm the first-order character of the transition taking place at T*.
Photorefractive properties of strontium-barium niobate
Authors: Ewbank M. D. , Neurgaonkar R. R. , Cory W. K. , Feinberg Jack
Published: July 2002

We have grown and optically characterized strontium-barium niobate crystals, including both undoped and cerium-doped crystals having two different Sr/Ba ratios (61/39 and 75/25). By measuring the coupling of two optical beams in the crystals, we have determined the following photorefractive properties: the effective density, sign, and spectral response of the dominant charge carrier, the grating formation rate, dark conductivity, and carrier diffusion length. We find that electrons are the dominant photorefractive charge carriers in all of our samples; the typical density of photorefractive charges is ∼1×1016 cm−3 in the undoped samples. The grating formation rate increases with intensity, with a slope of ∼0.3 cm2/(W s) over an intensity range of ∼1–15 W/cm2 in undoped samples. Cerium doping improves both the charge density (increased by a factor of ∼3) and the response rate per unit intensity (∼5 times faster).
Theoretical intensity-dependent response of nonlinear periodic structures
Authors: Gohman Paul A. , Bambakidis Gust , Spry Robert J.
Published: July 2002

We have modeled the response of a nonlinear periodic structure by means of the Abelés 2×2 matrix method. Our structure differs from the usual rejection-band filter designs, in that we have chosen the filter elements to be index matched in the absence of radiation, providing a rejection band that both grows and shifts as a function of incident intensity. The intensity output function of the model not only directly demonstrates optical bistability, but also limiting, switching, self-pulsing, and chaos.
Intrinsic stress in diamond films prepared by microwave plasma CVD
Authors: Windischmann H. , Epps Glenn F. , Cong Yue , Collins R. W.
Published: July 2002

The stress in diamond films prepared by microwave plasma CVD was investigated as a function of methane concentration (0.2%–3.0%) and deposition temperature (600–900 °C). Tensile and compressive total (thermal and intrinsic) stress were observed, depending on the deposition conditions. The thermal stress is compressive and relatively constant (0.215–0.275 GPa) over the temperature range investigated. The intrinsic stress is tensile and its origin is interpreted in terms of the grain boundary relaxation model. Calculations indicate a value of 0.84 GPa, using the grain boundary model, which is in fair agreement with the measured value. For the methane series, the tensile intrinsic stress decreases with increasing the methane fraction. The increasing compressive stress is ascribed to increased impurity (hydrogen and nondiamond phase carbon) incorporation with increasing methane fraction. 15N nuclear reaction analysis shows a linear correlation between hydrogen in the film and methane in the supply gas while spectroscopic ellipsometry shows a direct correlation between optically absorbing nondiamond (sp2) carbon incorporation and methane. For the temperature series, the intrinsic tensile stress increases with deposition temperature. The increase is ascribed to decreasing sp2 C incorporation with temperature, as confirmed by spectroscopic ellipsometry measurements.
Magnetism and segregation in Fe-Cr-Mn
Authors: Schneider T. , Acet M. , Wassermann E. F. , Pepperhoff W.
Published: July 2002

Fe-Cr-Mn alloys with 10 at. % Mn have similar structural and magnetic properties as Fe-Cr alloys, but have considerably higher Néel temperatures for the same Fe/(Fe+Cr) content ratio. When segregated, the high Néel temperatures of Fe-Cr-Mn alloys make it possible to investigate the magnetic nature of the Cr- and Fe-rich components. Resistivity and magnetization measurements show distinct features that correspond to a superposition of the properties of the individual segregated components.
Reaction of Cu-Ti bilayer films in vacuum and hydrogen
Authors: Apblett C. , Muira D. , Sullivan M. , Ficalora P. J.
Published: July 2002

Thin (0.5-μm-thick) films of titanium and copper on titanium were studied in hydrogen and vacuum ambients at temperatures from 350 to 450 °C. The resistivity and stress in the films were measured as a function of time at temperature. Rutherford backscattering spectroscopy and x-ray analysis were used to identify the compounds formed due to interdiffusion. Resistivity increased significantly in vacuum due to the formation of TiCu and TiCu3, respectively. The stress generated during formation of TiCu was compressive, while stress generated during the formation of TiCu3 was tensile. Final stresses in the Ti-Cu films were tensile. The activation energy for the formation of the compounds was 1.7 eV for TiCu and 2.5 eV for TiCu3, determined by both stress and resistivity measurements. In hydrogen, no intermetallics formed, due to the diffusion of hydrogen through the copper to form TiH2 at the Cu-Ti interface. No resistivity changes were found, but the stress was compressive and remained so. The energy of formation of the hydride using stress for the kinetic parameter was 0.93 eV.
Effects of annealing InP(110) surfaces on Schottky barrier heights at Pd/InP(110) interfaces
Authors: Yamada Masao , Spindt Chris J. , Miyano Ken E. , Meissner Paul L. , Herrera-Gomez Alberto , Kendelewicz Tom , Spicer William E.
Published: July 2002

The InP(110) surface becomes phosphorus deficient with increasing annealing temperature up to 200 °C and recovers nearly the initial stoichiometry at about 350 °C. The Schottky barrier heights formed on these annealed surfaces using Pd show a good correlation with the phosphorus loss from the surface due to annealing. This band-bending study using photoemission spectroscopy suggests the following: (i) Annealing creates donor defects due to phosphorus loss from the surface with a level at about 0.6 eV above the valence-band maximum (VBM); (ii) Pd deposition on InP(110) creates some other defects at around 0.9 eV above the VBM as usual; (iii) the surface Fermi-level positions ranging from 0.62 to 0.9 eV above the VBM are determined by the balance of these interface defect-state densities.
Crystal structure and energy gap of CdTe thin films grown by radio frequency sputtering
Authors: Jiménez-Sandoval S. , Meléndez-Lira M. , Hernández-Calderón I.
Published: July 2002

We have investigated the influence of structural characteristics on the band gap of rf sputtered CdTe thin films grown at substrate temperatures in the 69–232 °C range. The results of scanning electron microscopy and x-ray diffraction studies indicated that the films are a polycrystalline mixture of cubic and hexagonal phases with preferential growth of columnar type parallel to the cubic [111] direction. The band gap of the films was obtained from photoreflectance spectroscopy experiments carried out at room temperature. It was found that the films had a band gap larger than that of CdTe single crystals. This result has been correlated with the existence of lattice strain, quantum size effects, and hexagonal phase regions. By using theoretical models it was possible to estimate the contribution to the band gap shift due to strain and quantum size effects obtaining results in good agreement with the experiment. The study of annealed samples indicated that the effects of thermal treatments were to promote the change of the hexagonal phase to cubic, increase grain size, and shift the band gap towards lower energies reducing its difference with respect to that of single crystals.
Hydrogen passivation of iron-related hole traps in silicon
Authors: Kouketsu Masami , Isomae Seiichi
Published: July 2002

We have investigated the passivation of iron-related hole traps in p-type silicon by deep level transient spectroscopy (DLTS) and recombination lifetime measurements. After hydrogen ion implantation (ranging in dose from 1.0×1014 to 1.0×1016 cm−2), all DLTS peaks related to iron impurities disappeared. This indicates that implanted hydrogen passivates the Fe-B pair as well as other iron-related hole traps that are not passivated by hydrogen plasma treatment. On the other hand, two kinds of hole traps were produced at Ev+0.23 eV and Ev+0.38 eV by the hydrogen ion implantation. The recombination lifetime increases from 3 to 18 μs (which is about 45% of the lifetime in uncontaminated samples) with the implantation dose. The maximum value of the recombination lifetime was observed at a dose of 1.0×1015 cm−2. We attribute the decrease in recombination lifetime in more heavily implanted samples (1.0×1016 cm−2) to hole traps created by the ion implantation.
Stability of the grain configurations of thin films—A model for agglomeration
Authors: Rha J. J. , Park J. K.
Published: July 2002

We have calculated the energy of three distinct grain configurations, namely, completely connected, partially connected, and unconnected configurations, evolving during a spheroidization of polycrystalline thin film by extending a geometrical model due to Miller et al. to the case of spheroidization at both the surface and film-substrate interface. “Stability” diagram defining a stable region of each grain configuration has been established in terms of the ratio of grain size to film thickness versus equilibrium wetting or dihedral angles at various interface energy conditions. The occurrence of spheroidization at the film-substrate interface significantly enlarges the stable region of unconnected grain configuration thereby greatly facilitating the occurrence of agglomeration. Complete separation of grain boundary is increasingly difficult with a reduction of equilibrium wetting angle. The condition for the occurrence of agglomeration differs depending on the equilibrium wetting or dihedral angles. The agglomeration occurs, at low equilibrium angles, via partially connected configuration containing stable holes centered at grain boundary vertices, whereas it occurs directly via completely connected configuration at large equilibrium angles except for the case having small surface and/or film-substrate interface energy. The initiation condition of agglomeration is defined by the equilibrium boundary condition between the partially connected and unconnected configurations for the former case, whereas it can, for the latter case, largely deviate from the equilibrium boundary condition between the completely connected and unconnected configurations because of the presence of a finite energy barrier to overcome to reach the unconnected grain configuration.
Temperature dependence of the photoluminescence of all-porous-silicon optical microcavities
Authors: Cazzanelli M. , Vinegoni C. , Pavesi L.
Published: July 2002

Photoluminescence measurements in all-porous-silicon optical microcavities (PSM) are reported over a wide temperature range. Both continuous wave and time resolved measurements have been performed. The microcavity is formed by an all porous silicon Fabry–Perot filter made by two distributed Bragg reflectors separated by a λ-thick PS cavity layer. The luminescence properties of PSM are changed with respect to those of PS: a temperature independent narrowing in the emission line shape, a different temperature dependence of the emission intensity, and a fractional shortening of the luminescence decay time over the 50–300 K temperature interval are achieved. The PSM luminescence properties are explained by the spatial redistribution of the spontaneous emission, by an effective refractive index probed by the photon mode confined in the cavity layer and by the coupling between the singlet exciton state and the photon mode confined in the cavity layer. The saturation of the absorption of the distributed Bragg reflector is also addressed.
Larger magnetocaloric effect in two-layered La1.6Ca1.4Mn2O7 polycrystal
Authors: Zhou Tie-Jun , Yu Z. , Zhong W. , Xu X. N. , Zhang H. H. , Du Y. W.
Published: July 2002

The magnetocaloric effect in La1.6Ca1.4Mn2O7 polycrystal with the layered Sr3Ti2O7-type perovskite structure has been studied. It is found that the magnetic entropy change of La1.6Ca1.4Mn2O7 is comparable to that of Gd, but smaller and more broadened than that of La0.8Ca0.2MnO3.
Temperature dependence of the E2 and A1(LO) phonons in GaN and AlN
Authors: Link A. , Bitzer K. , Limmer W. , Sauer R. , Kirchner C. , Schwegler V. , Kamp M. , Ebling D. G. , Benz K. W.
Published: July 2002

The frequencies and dampings of the zone-center optical phonons E2 and A1(LO) in wurtzite-type GaN and AlN layers have been measured by Raman spectroscopy in the temperature range from 85 to 760 K. The GaN layer was grown by metalorganic vapor phase epitaxy and the AlN layer by molecular beam epitaxy both on sapphire substrate. The experimentally obtained frequencies and dampings are modeled by a theory taking into account the thermal expansion of the lattice, a symmetric decay of the optical phonons into two and three phonons of lower energy, and the strain in the layers induced by the different thermal expansion coefficients of layer and substrate. The results were used to determine the local temperature of a GaN pn diode in dependence on the applied voltage.
High resolution infrared spectrum of methyleneimine, CH2NH,in the 3 μm region
Authors: Halonen Lauri , Duxbury Geoffrey
Published: July 2002

The infrared spectrum of the short-lived methyleneimine, CH2NH, was obtained in the gas phase with a resolution of about 0.01 cm−1 from 2800 to 3200 cm−1, using the Kitt Peak National Observatory Solar Fourier Transform Spectrometer. The ν2 band and the high order Coriolis coupled ν3 and 2ν5 bands have been analyzed rotationally. Several hundred transitions have been fitted and accurate band origins and rotational constants have been determined for each of the bands.
Polarizabilities for light scattering from chiral particles
Authors: Singham Shermila B. , Patterson Chris W. , Salzman Gary C.
Published: July 2002

The coupled dipole method is used to calculate the circular intensity differential scattering (CIDS) from chiral particles. The particles are described by a collection of spherical or ellipsoidal dipoles. In the case of ellipsoidal dipoles, it is shown that triaxial polarizabilities have to be used to quantitatively describe the scattering matrix of the particle. For certain collections of ellipsoidal dipoles, the dipolar interactions may be neglected in calculating CIDS from the structure. The coupled dipole approximation also provides a convenient method for calculating CIDS from a one dimensional helical crystal.
The dissociative recombination rate coefficients of H+3, HN+2, and HCO+
Authors: Amano T.
Published: July 2002

The dissociative recombination rate coefficients for H+3, HN+2, and HCO+ are determined at 110, 210, and 273 K by monitoring the decay of the infrared absorption signals as a function of time. The rate coefficients are 1.8, 7.0, and 3.1 in units of 10−7 cm3 s−1 for H+3, HN+2, and HCO+, respectively, at 273 K. These values agree very well with those obtained using the stationary afterglow or the merged beam techniques, but the values for H+3 disagree with that obtained by Smith and co-workers (≤2×10−8 cm3 s−1) using the flowing afterglow/Langmuir probe method. The rate coefficients for H+3 and HCO+ disagree with theory which has predicted very slow dissociative recombinations in the lower vibrational states. The temperature dependences obtained here, although the temperature range is rather limited, are consistent with those obtained previously using the stationary afterglow (for H+3 and HCO+) and the merged beam (for HN+2) techniques. The measurements are extended to several vibration–rotation levels and no significant rotation dependence of the rate coefficients is observed. It has also been found that the ions investigated here can be equally abundant at ice temperature as at liquid nitrogen temperature.
A 7Li nuclear magnetic resonance study of LiCF3SO3 complexed in poly(propylene-glycol)
Authors: Chung S. H. , Jeffrey K. R. , Stevens J. R.
Published: July 2002

7Li nuclear magnetic resonance (NMR) linewidths and spin–lattice relaxation times for poly(propylene-glycol) complexed with a range of concentrations of LiCF3SO3 are reported over the temperature region from 205 to 405 K. Calculations suggest that the spin–lattice relaxation mechanism is caused by the interaction between the 7Li (I=3/2) quadrupole moment and fluctuations in the surrounding electric field gradients, whereas the line shapes are influenced by both the dipolar and quadrupolar interactions. The motional parameters reported indicate that ion–polymer or ion–ion interactions are important in determining the Li+ cation mobilities. This is reflected in the lengthening of the correlation time with increase in Li+ ion concentration which suggests a decreased mobility for the cations resulting from a transient coordination of the cation to the polymer matrix or ion aggregation. Also, the activation energies in this study (∼0.24 eV) are in agreement with values obtained from recent pulsed field gradient studies suggesting that the NMR techniques employed in this study are approriate methods for probing the dynamics of ion transport on a macroscopic scale in these materials.
Spectroscopy and photodissociation of Rydberg states of N2O
Authors: Szarka Michael G. , Wallace Stephen C.
Published: July 2002

The optical spectrum of N2O in the region from 80 000 to 87 000 cm−1 has been investigated by two- and three-photon resonant multiphoton ionization mass spectrometry. State symmetry identification has been performed by the use of linearly and circularly polarized light in both two- and three-photon absorption, conclusively confirming the assignment of the spectrum as that of the 3pσ 1Π Rydberg state of N2O. The new spectra and polarization experiments also permit observation and analysis of previously unreported vibronic coupling effects in this state. Mass spectral data reveal significant variation in the degrees of fragmentation into NO+ and N+2 as a function of vibrational and rotational excitation in the intermediate state. An apparent progression of a previously unreported second electronic band is also observed and for which a possible assignment is discussed.
Velocity-gauge formalism in the theory of vibrational circular dichroism and infrared absorption
Authors: Nafie Laurence A.
Published: July 2002

The first use of velocity-gauge factors in the theory of vibrational circular dichroism (VCD) and infrared absorption intensities is described. The approach involves the exact incorporation of all or part of the dependence of the electronic wave function on an electron-velocity perturbation, such as the vector potential or the velocities of the nuclei, into the atomic orbital basis functions as a gauge transformation. Any remaining dependence of the wave function on these perturbations is carried by the basis-function coefficient derivatives which are determined to first order by coupled Hartree–Fock perturbation theory. The magnetic field perturbation formulations of VCD in the common origin and distributed origin gauges are identified within the new formalism, providing a new direct derivation of the distributed origin gauge theory. The formalism also yields new a priori VCD intensity expressions, derived using nuclear velocity-gauge factors, in the complete adiabatic approximation. Several distinct a priori computational approaches to VCD intensities can now be identified—the vibronic coupling theory implemented with a direct sum over states (VC/SOS), the field adiabatic theory implemented with magnetic field perturbation (FA/MFP), and the complete adiabatic theory implemented with nuclear velocity perturbation (CA/NVP). In addition, basic expressions are presented for an energy gradient theory of VCD that employs both magnetic-field and nuclear-velocity perturbations (EG/MFNVP). It is shown that the CA/NVP theory of VCD possesses a higher Born–Oppenheimer content than the VC/SOS or FA/MFP theories and provides an improved basis for reducing a priori VCD theory to various models of VCD intensity.
The exactness of the extended Koopmans’ theorem: A numerical study
Authors: Sundholm Dage , Olsen Jeppe
Published: July 2002

The exactness of the extended Koopmans’ theorem has been studied by comparing multiconfiguration Hartree–Fock calculations on Be(1S) with configuration interaction calculations on Be+(2S) performed in the same basis. The lowest ionization potentials calculated as the energy difference between Be and Be+ and those obtained from the extended Koopmans’ theorem approach each other with increased size of the orbital basis. The difference between the ionization potentials obtained in an 11s basis is 0.034 meV (1.2×10−6 a.u.), and in a 12s basis the difference is 0.019 meV (0.7×10−6 a.u.) which shows that the s-limits for the two ionization potentials are equal. Similarly, the sp-, and spd-, and the spdf-limits of the two ionization potentials approach each other in the limit of complete basis of the given irreducible representations, indicating that the extended Koopmans’ theorem is exact for the lowest ionization potential.
Comment on ‘‘The exactness of the extended Koopmans’ theorem: A numerical study’’ [J. Chem. Phys. 98, 3999 (1993)]
Authors: Morrison Robert C.
Published: July 2002

Recent systematic, numerical calculations by Sundholm and Olsen [J. Chem. Phys. 98, 3999 (1993)] support the proof that the lowest ionization potential can in principle be obtained exactly using the extended Koopmans’ theorem. Because of the nature of their calculations it cannot be concluded that only the lowest ionization potential could be obtained exactly using the extended Koopmans’ theorem.
Efficient implementation of the Hiller–Sucher–Feinberg identity for the accurate determination of the electron density
Authors: Challacombe Matt , Cioslowski Jerzy
Published: July 2002

A new, highly optimized implementation of the Hiller–Sucher–Feinberg (HSF) identity is presented. The HSF identity, when applied to molecular wave functions calculated with Gaussian-type basis functions, not only improves the overall accuracy of the electron density by more than an order of magnitude, but also yields approximate cusps at nuclei. The three classes of molecular integrals, ℒ, 𝒰, and 𝒱, which are encountered in the calculation of the HSF density, are derived in compact form. Efficient algorithms for the accurate evaluation of these integrals are detailed, including a novel approach to the necessary numerical quadratures and the thresholding of two-electron 𝒱 integrals. Hartree–Fock (HF) electron densities calculated with both the conventional definition and from the HSF identity are compared to their respective HF limits for a variety of diatomic molecules and basis sets. The average error in the calculated HSF electron densities at non-hydrogen nuclei equals 0.17%, which constitutes a marked improvement over an error of 5.77% in the conventional densities.
Thermodynamics of strongly-coupled Yukawa systems near the one-component-plasma limit. II. Molecular dynamics simulations
Authors: Farouki R. T. , Hamaguchi S.
Published: July 2002

Molecular dynamics simulations are employed to study the equilibrium thermodynamics of strongly-coupled systems of particles interacting through the Yukawa potential. Such systems serve, under the Debye–Hückel approximation, as a model for the physical behavior of plasma or colloidal suspensions of charged particulates. The thermodynamics may be characterized in terms of two dimensionless parameters—the ratio κ of the mean interparticle distance to the Debye length, and an approximate measure Γ of the interparticle potential energy in units of the thermal kinetic energy. Employing an accurate representation of infinite periodic boundary conditions, we focus on the regime of weak Debye screening (κ ≲ 1) and strong coupling (Γ≫1). Excess internal energies measured at many points (κ,Γ) are fitted to simple functional forms for the fluid and solid phases, representing extrapolations of the classical one-component plasma (OCP) limit, κ=0. Quantitative expressions for the Helmholtz free energy and the ‘‘equation of state’’ of the Yukawa system— giving the pressure p in terms of κ and Γ—are thus derived, and the freezing/melting curve in the (κ,Γ) plane is traced as the intersection of the fluid and solid free-energy surfaces.
Structure of powder deuteroammonia between 2 and 180 K revisited: A refinement of the neutron diffraction pattern taking into account molecular reorientations: analysis of the diffuse intensity
Authors: Leclercq Françoise , Damay Pierre , Foukani Mohammed
Published: July 2002

The standard Rietveld profile analysis is a powerful tool for refining the structure of powder crystals. Nevertheless, in case of molecular crystals, where rigid groups undergo reorientations of large amplitude in well-defined directions, thermal ellipsoids, even with anisotropic terms, may not be sufficient to account for the motion. That is the case for deuteroammonia for which strong librations of the ND3 groups are observed even at 2 K. In this paper we present a refinement of the structure of deuteroammonia using a model allowing the ND3 group to perform reorientations about the easy C3 axis of the cubic lattice; the adjustment of diffraction patterns measured at 2, 78, and 180 K shows the temperature dependence of the librational amplitude. The geometry of the ammonia molecule in the crystal is found to be the same as in the gas phase [rN–D=1.008(4) Å] and is in excellent agreement with that determined by the analysis of the intramolecular structure factor for large momentum transfers; furthermore the molecular parameters are directly obtained from the fit, without need of rigid body corrections of bond lengths. This model assumes correlated motion of the D atoms which is confirmed by a semiquantitative analysis of the Q dependence of diffuse scattering observed under the Bragg peaks.
Electron intracule densities with correct electron coalescence cusps from Hiller–Sucher–Feinberg-type identities
Authors: Cioslowski Jerzy , Stefanov Boris B. , Tan Agnes , Umrigar C. J.
Published: July 2002

Identities for the electron intracule density I(R) in atoms and molecules are derived within the Hiller–Sucher–Feinberg (HSF) formalism. It is proven that, when applied to arbitrary (exact or approximate) electronic wave functions, these identities produce intracule densities that satisfy a modified condition for the electron coalescence cusp. A corollary of this proof provides a new, simplified derivation of the cusp condition for the exact I(R). An expression for the Hartree–Fock approximation to the HSF electron intracule density that contains only two- and three-electron terms is obtained and its properties are analyzed. A simple scaling of the three-electron contributions in this expression assures integrability of the approximate I(R) and improves its overall accuracy. Numerical tests carried out for the H−, He, Li+, Be2+, Li, and Be systems demonstrate that the application of the scaled HSF-type identity to Hartree–Fock wave functions affords dramatic improvements in the short-range behavior of the electron intracule density.
Number of clusters formed in nucleation-growth processes
Authors: Slezov V. V. , Schmelzer J. , Tkatch Ya. Y.
Published: July 2002

The number of clusters, N, formed in nucleation-growth processes in first-order phase transformations, and their average size, 〈R〉, at the end of the stage of independent growth of the supercritical clusters are determined analytically. It is shown that the number of clusters, N, depends only on the thermodynamic parameters like initial supersaturation, specific interfacial energy etc. but not on the values of kinetic parameters like the diffusion coefficient. In addition, estimates are given for the duration of the different stages of the nucleation-growth process. In this way, a complete quantitative description of the basic characteristics of first-order phase transformations (e.g., segregation in solid or liquid solutions, vapor condensation, etc.) is established. The results are in excellent agreement both with experimental results as well as with computer calculations modelling nucleation and growth based on the numerical solution of the set of kinetic equations describing this process in the framework of classical nucleation theory.
Collision-induced double transition effects in the 3ν3 CO2 band wing region
Authors: Filippov N. N. , Bouanich J.-P. , Boulet C. , Tonkov M. V. , Le Doucen R. , Thibault F.
Published: July 2002

IR absorption beyond the head of the 0003–0000 (3ν3) band of CO2 near 7000 cm−1 has been analyzed. This absorption is found to consist of two comparable intensity contributions, namely, the allowed band wing and a collision-induced absorption (CIA) band. The band wing profile has been described by using a non-Markovian theory and the rotational perturbation densities for CO2–CO2 collisions, which was previously calculated from the intensity distribution in the high-frequency wing of the 0001–0000 CO2 band. The CIA component has a typical shape of CO2 CIA bands with the maximum at the double transition (0001+0002)–(0000+0000) frequency. The integrated binary coefficient of this CIA band was estimated to be B2=(1.0±0.6)×10−5 cm−2 Amagat−2. The CIA spectral moment theory has been used for the intensity calculation, which takes into account for the first time the collision-induced vibrational force field in CO2 pairs. By comparing the calculated and measured intensity for the double transition, the polarizability anisotropy matrix element for the 2ν3 band has been estimated as β20=0.26±0.08 a.u., value in reasonable agreement with an independent estimation from previous results of polarizability matrix elements of CO2.
An analytical derivation of the locally enhanced sampling approximation
Authors: Zheng Wei-Mou , Zheng Qiang
Published: July 2002

The locally enhanced sampling (LES) approximation used in molecular modeling is analytically derived for a simple quadratic energy function. Starting with multiple copies of full protein-ligand complexes the thermodynamic probability distribution is reduced to that of a complex with a single protein and multiple ligands. The reduction of the protein phase space and the Hamiltonian obtained from the reduction suggest a new LES formalism, whose definition and interpretation are closely related to, but different from the traditional ones. One characteristic of the new formalism is that it is free from the inherent temperature disparity of the traditional LES. The error involved in the LES approximation is identified as the truncation of an induced inter-ligand interaction in the reduced Hamiltonian. This truncated term contains important information on the correlation between the protein intra-molecular and the protein-ligand inter-molecular interactions. These results may provide insight and guidance for interpreting and improving the LES simulations.