bluuebunny/crossref_metadata_2025 huggingface dataset:
Two-layer Lanczos iteration approach to molecular spectroscopic calculation
Authors: Yu Hua-Gen
Published: October 2002

A two-layer Lanczos algorithm is suggested to calculate the rovibrational energy levels of polyatomic molecules in terms of a partitioned Hamiltonian. Such a Hamiltonian is formed in a set of orthogonal polyspherical coordinates. This algorithm solves the full dimensional eigenvalue problem in a reduced-dimensional (RD) way. By splitting the coordinates into radial and angular groups, one obtains two small RD Hamiltonians in each coordinate group. The eigenstates of each RD system are computed using either a standard or a guided spectral transform Lanczos method. These two subsystems are exactly coupled via a set of diabatic basis functions in the angular degrees of freedom without any dynamical approximation. The two-layer Lanczos algorithm is illustrated in detail using an example of the variational calculation of the vibrational energies of pentatomic molecules. An application to methane is given. Numerical results show that the two-layer Lanczos method is substantially more efficient, compared to the conventional Lanczos algorithm.
Ternary nucleation of inorganic acids, ammonia, and water
Authors: Napari I. , Kulmala M. , Vehkamäki H.
Published: October 2002

Homogeneous ternary nucleation rates of water, ammonia (NH3), and inorganic acids are calculated from classical nucleation theory at various combinations of temperature and ambient vapor concentrations. Mixtures with sulfuric acid (H2SO4), nitric acid (HNO3), hydrochloric acid (HCl), and methane sulfonic acid (CH3SO3H or MSA) are considered. The results are compared to assess the relative importance of different nucleation routes. Vapors with an acid and an alkaline (ammonia) component are shown to nucleate more readily than vapors having two acid species. Much lower concentrations of H2SO4 than HNO3 or HCl are required to reach the same nucleation rate at similar vapor concentrations of H2O and NH3. Nucleation rates in H2O–H2SO4–HNO3 and H2O–H2SO4–MSA vapors are close to the limit of H2O–H2SO4 nucleation, even at high concentrations of MSA and HNO3. The results suggest that H2O–H2SO4–NH3 nucleation is the foremost ternary nucleation route in the atmosphere.
Monte Carlo simulations of electrical double-layer formation in nanopores
Authors: Yang Kun-Lin , Yiacoumi Sotira , Tsouris Costas
Published: October 2002

The formation of the electrical double layer (EDL) in an aqueous solution in contact with the charged solid surfaces of a slit-type nanopore has been simulated by grand canonical Monte Carlo (GCMC) and canonical Monte Carlo (CMC) methods. In the GCMC simulations, a primitive EDL model in which water is considered as a continuum and the ions are considered as hard spheres is used. The results are found to be slightly different from those predicted by the Gouy–Chapman model at low electrolyte concentration and low surface charge density. The GCMC results were then used as an initial condition for the CMC simulations of a nonprimitive model in which the EDL is composed of molecular water (four-point transferable intermolecular potential, together with fluctuating charge model), cations (Na+), and anions (Cl−). The nonprimitive model provides a very different view of the EDL at the atomic level. For example, a single layer of water molecules, instead of counterions, is strongly adsorbed on negatively charged surfaces. This feature cannot be observed in the Gouy–Chapman or in the primitive model. Moreover, because the charged surfaces are covered by water molecules, the maximum counterion concentration in a nanopore occurs at the center, instead of on the surfaces, of the pore. This phenomenon is also supported by experimental data and theoretical calculations found in the literature.
Relationship between electronic and geometric structures of the O/Cu(001) system
Authors: Stolbov Sergey , Rahman Talat S.
Published: October 2002

The electronic structure of the (22×2)R45° O/Cu(001) system has been calculated using locally self-consistent, real space multiple scattering technique based on first principles. Oxygen atoms are found to perturb differentially the long-range Madelung potentials, and hence the local electronic subbands at neighboring Cu sites. As a result the hybridization of the oxygen electronic states with those of its neighbors leads to bonding of varying ionic and covalent mix. Comparison of results with those for the c(2×2) overlayer shows that the perturbation is much stronger and the Coulomb lattice energy much higher for it than for the (22×2)R45° phase. The main driving force for the 0.5 ML oxygen surface structure formation on Cu(001) is thus the long-range Coulomb interaction which also controls the charge transfer and chemical binding in the system.
Adsorption of hydrogen and deuterium atoms on the (0001) graphite surface
Authors: Zecho Thomas , Güttler Andreas , Sha Xianwei , Jackson Bret , Küppers Jürgen
Published: October 2002

Adsorption of H and D on HOPG surfaces was studied with thermal desorption (TDS), electronic (ELS), and high-resolution electron-energy-loss (HREELS) spectroscopies. After admission of H (D) from thermal (2000 K) atom sources to clean graphite surfaces TD spectra revealed recombinative molecular H2 (D2) desorption in a main peak around 445 K (490 K) and a minor peak at 560 K (580 K). After admission of higher fluences the main peak shifts to 460 K (500 K) and develops a shoulder at 500 K (540 K). The saturation coverages were calculated as 0.4±0.2 for H and D and initial sticking coefficients of 0.4±0.2 were obtained. Through leading edge analysis of the TD spectra desorption activation energies for H and D were determined as 0.6 and 0.95 eV, respectively. EL spectra suggest a 16% loss of the sp2 character of the surface carbon 2sp electrons upon D adsorption. HREEL spectra of H (D) graphite covered surfaces reveal in addition to two graphite-intrinsic optical phonon losses vibrational features at 1210 and 2650 cm−1 (and 640 and 1950 cm−1). These frequencies are in excellent agreement with those obtained from a recently published H (D)/graphite potential energy surface. A theoretical description of the desorption process through calculated H+H/graphite potential surfaces reveals the desorption mechanism and desorption activation energies which are in good agreement with the measured data.
Observation of mix in a compressible plasma in a convergent cylindrical geometry
Authors: Barnes Cris W. , Batha S. H. , Dunne A. M. , Magelssen G. R. , Rothman Steve , Day R. D. , Elliott N. E. , Haynes D. A. , Holmes R. L. , Scott J. M. , Tubbs D. L. , Youngs D. L. , Boehly T. R. , Jaanimagi P.
Published: October 2002

Laser beams that directly drive a cylindrical implosion are used to create a measurable region of mixed material in a compressible plasma state, for the first time in a convergent geometry. The turbulence driven by the Richtmyer–Meshkov instability by shock passage across a density discontinuity mixes marker material that is radiographically opaque. The width of the mix layer is compared between a system with large surface roughness and an initially smooth system. The experiment is described and results are compared to multi-dimensional numerical simulation, including three-dimensional turbulence calculations. The calculations adequately match the observations provided the measured initial conditions are used.
Monte Carlo simulation of electron transport in narrow gap heterostructures
Authors: Thobel Jean-Luc , Bonno Olivier , Dessenne François , Boutry Hervé
Published: October 2002

A Monte Carlo method is proposed for the study of in-plane electron transport in narrow gap heterostructures. Special attention is paid to the consequences of the strong nonparabolicity of the conduction band. The electron states are calculated within the framework of envelope function theory, which leads to a Schrödinger equation with an energy-dependent effective mass. This equation is solved in a numerically efficient way by including a standard eigenvalue solver in an iterative method. The mixing between conduction and valence band states is taken into account, at an approximate level, through a “Bloch overlap factor,” defined by analogy with the case of three-dimensional transport. This model was applied to a typical AlSb/InAs single well structure, and realistic results were obtained. The important role played by the Bloch overlap factor is demonstrated. When it is neglected, the mobility is strongly underestimated. A more sophisticated double well structure was also investigated. It is intended to reduce impact ionization, thanks to transfer toward the thinner well. This transfer is found to depend strongly on the potential profile.
Microcathodoluminescence and electron beam induced current observation of dislocations in freestanding thick n-GaN sample grown by hydride vapor phase epitaxy
Authors: Polyakov A. Y. , Govorkov A. V. , Smirnov N. B. , Fang Z-Q. , Look D. C. , Park S. S. , Han J. H.
Published: October 2002

Microcathodolumunescence (MCL) spectra measurements, MCL and electron beam induced current (EBIC) imaging of the freestanding n-GaN samples grown by hydride vapor phase epitaxy were made. Dark-spot defects in plan-view EBIC and MCL images and dark line defects in MCL images taken on the cleaved surface of the samples could be associated with dislocations. MCL spectra measurements in the vicinity of dislocations and in the matrix do not reveal specific luminescence bands that could be attributed to dislocations but rather suggest that dislocation regions have higher density of deep nonradiative traps.
Deep electron and hole traps in freestanding n-GaN grown by hydride vapor phase epitaxy
Authors: Polyakov A. Y. , Smirnov N. B. , Govorkov A. V. , Fang Z.-Q. , Look D. C. , Park S. S. , Han J. H.
Published: October 2002

Deep level electron and hole traps were studied by means of deep level transient spectroscopy with electrical and optical injection on a freestanding thick n-GaN sample with low dislocation density. It is shown that at both the upper and the lower surface of the sample there exists a thin, ∼0.5 μm layer of damaged material with lowered concentration of electrons and enhanced density of deep centers. Deep in the bulk of the film the densities of the majority of the electron and hole traps are shown to be very low, but measurably higher on the lower face (N face), which was originally closer to the Al2O3 substrate. The two faces are also shown to similarly differ in the density of deep hole traps whose concentration is deduced from low-temperature capacitance–voltage measurements in the dark and after illumination (such traps were previously associated with dislocation states). The concentration of persistent photocapacitance centers is shown to be very low on both sides and considerably lower than previously reported for other n-GaN samples with higher dislocation densities. Electron beam induced current measurements on both sides of the sample reveal the presence of dark spots whose density roughly correlates with the density of dislocations at the upper and lower faces. The reverse current at high voltages is shown to be considerably higher on the N face. The possible relationship of the observed phenomena to dislocations is discussed.
High resolution minority carrier transient spectroscopy of Si/SiGe/Si quantum wells
Authors: Gad M. A. , Evans-Freeman J. H.
Published: October 2002

We have developed a high resolution technique for examining minority carrier emission from defect states in semiconductors called Laplace minority carrier transient spectroscopy (LMCTS). The experiment uses the same analytical approach to the capacitance transient as Laplace deep level transient spectroscopy (LDLTS), but minority carriers are injected into the depletion region by application of a suitable light pulse. The combination of LDLTS and LMCTS means that detailed emission properties of closely spaced majority and minority carrier traps across the whole band gap can be now characterized. The technique has been used to study minority carrier traps in gas source molecular beam epitaxy-grown Si/Si0.86Ge0.14 strained quantum wells. Initially the technique was evaluated by comparing LMCTS of a hole trap associated with the gold–hydrogen complex in n-type silicon with LDLTS of the same trap in p-type silicon. Both techniques confirm that this level consists of two states, as previously suggested in the literature. LMCTS was then applied to an n-type multiquantum well Si/SiGe layer. We have been able to measure directly the emission rate of holes out of SiGe quantum wells using LMCTS. The emission rate exhibited only slight temperature dependence, in strong contrast to that of holes which are thermally emitted from isolated point defects. We show that in the particular case of LMCTS, a temperature invariant emission rate out of quantum wells is to be expected, and this is consistent with theoretical predictions.
Field-dependent mobility from space-charge-limited current–voltage curves
Authors: Natali Dario , Sampietro Marco
Published: October 2002

In the framework of one-carrier space-charge-limited currents, exact equations are derived for the evaluation of arbitrary field-dependent mobility. A differential method, which simply needs the first and second derivatives of measured current–voltage (I–V) curves, is presented for a trap-free case. Extension to a mixed case of exponentially distributed traps and field-dependent mobility is also outlined. In addition, a tool based on a combination of capacitance–voltage and I–V measurements was developed to certify the applicability of this method by checking whether the device tested is actually in the space-charge-limited regime. The extraction of the mobility from measurements can be a valuable tool for use in the theory: theoretical predictions of mobility field dependence can be easily compared to experimental results, thus permitting improvement of the models and fostering the development of transport theories. This method can also be of particular relevance for organic semiconductors whose field-dependent mobility has recently attracted so much theoretical and experimental work.
Selective dissociation of HCl in Kr from vibrational overtones
Authors: Berghof V. , Schwentner N.
Published: October 2002

Vibrational levels v=1, 2, and 3 of HCl in Kr matrices are populated with tunable IR radiation and the excited molecules are dissociated by UV excitation to the repulsive A 1∏ state. Cl fragments are recorded by laser induced fluorescence of Kr2Cl and dissociation rates are determined from the increase in LIF with UV dose. The enlarged UV Franck–Condon range for overtones allows the study of cage exit of H fragments with small kinetic energy Ekin. A threshold at Ekin=1.4 eV and a steep rise indicate a predominant sudden exit. Monomers, different initial rotational states and transients in the relaxation cascade are preselected with overtone excitation and the feasibility of a discrimination between isotopes, aggregates, and local structures is illustrated.
Polarizabilities in the condensed phase and the local fields problem: A direct reaction field formulation
Authors: van Duijnen Piet Th. , de Vries Alex H. , Swart Marcel , Grozema Ferdinand
Published: October 2002

A consistent derivation is given for local field factors to be used for correcting measured or calculated static (hyper)polarizabilities in the condensed phases. We show how local fields should be used in the coupled perturbative Hartree–Fock or finite field methods for calculating these properties, specifically for the direct reaction field (DRF) approach, in which a quantum chemically treated “solute” is embedded in a classical “solvent” mainly containing discrete molecules. The derivation of the local fields is based on a strictly linear response of the classical parts and they are independent of any quantum mechanical method to be used. In applications to two water dimers in two basis sets it is shown that DRF matches fully quantum mechanical results quite well. For acetone in eleven different solvents we find that if the solvent is modeled by only a dielectric continuum (hyper)polarizabilities increase with respect to their vacuum values, while with the discrete model they decrease. We show that the use of the Lorentz field factor for extracting (hyper)polarizabilities from experimental susceptibilities may lead to serious errors.
Rod-pinch diode operation at 2 to 4 MV for high resolution pulsed radiography
Authors: Young F. C. , Commisso R. J. , Allen R. J. , Mosher D. , Swanekamp S. B. , Cooperstein G. , Bayol F. , Charre P. , Garrigues A. , Gonzales C. , Pompier F. , Vezinet R.
Published: October 2002

The rod-pinch diode is operated successfully at peak voltages of 2.4–4.4 MV for peak electrical currents of 55–135 kA delivered to the diode. At 4 MV, tungsten anode rods of 1 or 2 mm diam produce on-axis doses at 1 m of 16 rad(Si) or 20 rad(Si), respectively. The on-axis source diameter based on the full width at half-maximum (FWHM) of the line-spread function (LSF) is 0.9±0.1 mm for a 1 mm diam rod and 1.4±0.1 mm for a 2 mm diam rod, independent of voltage. The LANL source diameter, determined from the modulation transfer function of the LSF, is nearly twice the FWHM. The measured rod-pinch current is reproduced with a diode model that includes ions and accounts for anode and cathode plasma expansion.
Modeling of neoclassical tearing mode stability for generalized toroidal geometry
Authors: Rosenberg A. L. , Gates D. A. , Pletzer A. , Menard J. E. , Kruger S. E. , Hegna C. C. , Paoletti F. , Sabbagh S.
Published: October 2002

Neoclassical tearing modes (NTMs) can lead to disruption and loss of confinement. Previous analysis of these modes used large aspect ratio, low β (plasma pressure/magnetic pressure) approximations to determine the effect of NTMs on tokamak plasmas. A more accurate tool is needed to predict the onset of these instabilities. As a follow-up to recent theoretical work, a code has been written which computes the tearing mode island growth rate for arbitrary tokamak geometry. It calls PEST-3 [A. Pletzer et al., J. Comput. Phys. 115, 530 (1994)] to compute Δ′, the resistive magnetohydrodynamic (MHD) matching parameter. The code also calls the FLUXGRID routines in NIMROD [A. H. Glasser et al., Plasma Phys. Controlled Fusion 41, A747 (1999)] for Dnc, DI, and DR [C. C. Hegna, Phys. Plasmas 6, 3980 (1999); A. H. Glasser et al., Phys. Fluids 18, 875 (1975)], which are the bootstrap current driven term and the ideal and resistive interchange mode criterion, respectively. In addition to these components, the NIMROD routines calculate αs–H, a new correction to the Pfirsch–Schlüter term. Finite parallel transport effects were added and a National Spherical Torus Experiment (NSTX) [M. Ono et al., Nucl. Fusion 40, 557 (2000)] equilibrium was analyzed. Another program takes the output of PEST-3 and allows the user to specify the rational surface, island width, and amount of detail near the perturbed surface to visualize the total helical flux. The results of this work will determine the stability of NTMs in a spherical torus (ST) [Y.-K. M. Peng et al., Nucl. Fusion 26, 769 (1986)] plasma with greater accuracy than previously achieved.
Heteroepitaxial growth of β-LiGaO2 thin films on ZnO
Authors: Ohkubo I. , Hirose C. , Tamura K. , Nishii J. , Saito H. , Koinuma H. , Ahemt P. , Chikyow T. , Ishii T. , Miyazawa S. , Segawa Y. , Fukumura T. , Kawasaki M.
Published: October 2002

High-quality interface between an insulator and ZnO as a wide-band-gap semiconductor should realize devices based on field-effect carrier modulation or superlattices having large band offset over 1 eV. We demonstrate that LiGaO2 could be a possible candidate for this purpose. Heteroepitaxy of LiGaO2 is demonstrated on ZnO films, giving atomically sharp interface and fairly good exciton-related optical properties in the ZnO under layer. Although slight distortion of a basal-plane hexagon with a lattice mismatch of about 3% in LiGaO2 gives multidomain epitaxial structure with relaxed lattices, a possible solution is proposed to realize coherent heterointerface.
Transition from amorphous boron carbide to hexagonal boron carbon nitride thin films induced by nitrogen ion assistance
Authors: Gago R. , Jiménez I. , Agulló-Rueda F. , Albella J. M , Czigány Zs. , Hultman L.
Published: October 2002

Boron carbon nitride films (BCN) were grown by B4C evaporation under concurrent N2 ion beam assistance. The films were characterized by x-ray absorption near-edge spectroscopy, infrared and Raman spectroscopies, and high-resolution transmission electron microscopy. The bonding structure and film composition correlate with the momentum transfer per incoming atom during deposition. As the momentum transfer is increased, the film structure evolves from an amorphous boron carbide network towards a hexagonal ternary compound (h–BCN) with standing basal planes. The growth of h–BCN takes place for momentum transfer in the window between 80 and 250 (eV×amu)1/2. The characteristic vibrational features of the h–BCN compounds have also been studied. Finally, the solubility limit of carbon in the hexagonal BN structure, under the working conditions of this article, is found to be ∼15 at. %.
Heat capacity of the n-InSe single crystal layered semiconductor
Authors: Dmitriev A. I. , Lashkarev G. V. , Baida A. A. , Kovalyuk Z. D. , Szewczyk A. , Piotrowski K. , Gutowska M.
Published: October 2002

Measurements of heat capacity and conductivity of InSe single crystal have been carried out over the temperature range 5–300 K. Heat capacity Cp, entropy ΔS, and enthalpy ΔH as well as Debye temperatures have been calculated. The singularities observed on the temperature dependencies of heat capacity and electric conductivity can be attributed to a phase transition to the charge density wave state. This transition perturbs considerably the electron and phonon spectra of the studied layered crystal.
Multiple “time step” Monte Carlo
Authors: Hetényi Balázs , Bernacki Katarzyna , Berne B. J.
Published: October 2002

We propose a sampling scheme to reduce the CPU time for Monte Carlo simulations of atomic systems. Our method is based on the separation of the potential energy into parts that are expected to vary at different rates as a function of coordinates. We perform n moves that are accepted or rejected according to the rapidly varying part of the potential, and the resulting configuration is accepted or rejected according to the slowly varying part. We test our method on a Lennard-Jones system. We show that use of our method leads to significant savings in CPU time. We also show that for moderate system sizes the scaling of CPU time with system size can be improved (for n=40 the scaling is predominantly linear up to 1000 particles).
Experimental and theoretical infrared spectra of Co2CO
Authors: Tremblay Benoı̂t , Manceron Laurent , Gutsev Gennady L. , Andrews Lester , Partridge Harry
Published: October 2002

Matrix infrared spectra of Co2CO were obtained from cobalt vapors condensed with different isotopically substituted mixtures of carbon monoxide diluted in argon at 10 K. New bands at 1953.3, 488.7, 357.9, and 270.9 cm−1 are assigned to the νC–O, νM–CO, νM–C–O, and νM–M fundamentals, respectively. Computations performed using density functional theory with the generalized gradient approximation (DFT-GGA) for the exchange-correlation potential predict the lowest energy state of Co2CO to be A″5 with the angle Co–Co–C of 116°. Our DFT-GGA vibrational frequencies of this state agree with the experimental frequencies extremely well—within 15 cm−1. Our estimate for the Co2–CO bond strength is 1.60 eV. Computed vibrational frequencies for Co2CO+ and Co2CO− are included for comparison.
Relaxation dynamics of the electronically excited vanadium Met–Car cluster
Authors: Leskiw B. D. , Knappenberger K. L. , Castleman A. W.
Published: October 2002

The relaxation dynamics of the vanadium Met–Car cluster, V8C12, excited electronically using femtosecond laser pulses of various wavelengths, is reported. Particular attention is focused on time-resolved measurements in the vicinity of 2 eV where experimental evidence of an electronic state is acquired.
Acceleration and orbits of charged particles beneath a monolayer plasma crystal
Authors: Schweigert V. A. , Schweigert I. V. , Nosenko V. , Goree J.
Published: October 2002

Experiments and simulations are reported for a monolayer plasma crystal that is disturbed by an extra particle moving in a plane below the monolayer. Numerical simulations and experiments are performed to find an explanation for the motion of the extra particle. In contrast to earlier simulations where an extra particle did not move spontaneously as in the experiment, here an ion wakefield downstream of the monolayer of particles is included. This resulted in the spontaneous motion of an extra particle as in the experiment, so that it is concluded that the wakefield produces this motion. In both the experiment and the simulation a trend is observed where the orbit of an extra particle becomes more crooked and less energetic when the gas damping is stronger. The simulation reveals that the energy of the extra particle exhibits distinctive transitions between three regimes.
Forward coupling phenomena between artificial left-handed transmission lines
Authors: Liu Lei , Caloz Christophe , Chang Chin-Chang , Itoh Tatsuo
Published: October 2002

Coupling phenomena between artificial left-handed (LH) transmission lines are explored on the basis of a transmission line approach of LH materials. A LH forward coupler in the form of an ideal lumped-element ladder network is presented and is shown to exhibit strong forward coupling that increases with decreasing of frequency. A quasi-lumped element microstrip implementation of a LH coupler is proposed. It is demonstrated that this coupler requires a shorter coupling length in comparison with the conventional right-handed coupled-line forward couplers, and hence makes the device much more compact.
Nucleation and growth kinetics of preferred C54 TiSi2 orientations: time-resolved x-ray diffraction measurements
Authors: Özcan A. S. , Ludwig K. F. , Lavoie C. , Cabral C. , Harper J. M. E. , Bradley R. M.
Published: October 2002

The transformation from the C49 to the C54 phase in TiSi2 thin films has been studied for more than a decade because of its importance to the semiconductor industry. In previous work we identified three dominant C54 orientations in TiSi2 thin films on Si(001) substrates using x-ray pole figure analysis. In the current study the evolution of the three prominent C54 texture components was further examined using synchrotron based time-resolved in situ x-ray diffraction. The kinetics results show significant differences in the nucleation and growth kinetics of the individual orientations. Simulations suggest that this could be explained by an inhomogeneous nucleation of different texture components in the thin film, but not by simple differences in nucleation and/or growth rates. Classical Johnson–Mehl–Avrami–Kolmogorov (JMAK) analysis of nucleation and growth was applied to model the C49–C54 phase transformation kinetics for each C54 orientation. These results showed a systematic decrease of the Avrami exponent n from 3 to 2 with increasing annealing temperature. Within a JMAK approach this could be explained by the presence of a limited number of C54 nucleation sites coupled with a faster increase in nucleation rate than in growth velocity with increasing anneal temperature.
Temperature dependence of the Seebeck coefficient and the potential barrier scattering of n-type PbTe films prepared on heated glass substrates by rf sputtering
Authors: Kishimoto Kengo , Tsukamoto Masayoshi , Koyanagi Tsuyoshi
Published: October 2002

Thermoelectric properties of polycrystalline degenerate n-type PbTe films have been investigated in order to understand potential barrier scattering. The Seebeck coefficients of the PbTe films obtained in this study were larger than those of bulk samples having the same carrier concentrations in the temperature range from 100 to 450 K. Some of their power factors were larger than those of bulk samples having the same carrier concentrations in the temperature range from 200 to 450 K, while their electrical conductivities were smaller than those of bulk samples. From a comparison of these results with those previously reported, we concluded that potential barrier scattering occurred at grain boundaries in our films, resulting in the above favorable changes in thermoelectric properties. By analyzing their properties using the energy filtering model, we estimated the height of grain-boundary potential barriers, which probably influenced the increases in the Seebeck coefficient. We also examined the origin of the potential barriers accordingly. Consequently, we consider that the origin of the potential barriers was mainly due to point defects, probably Te vacancies, and that the barrier height may be controlled, for example, by the preparation conditions.
Formation of Mo5Si3 phase in Mo/Si multilayers
Authors: Srivastava A. K. , Tripathi Pragya , Nayak M. , Lodha G. S. , Nandedkar R. V.
Published: October 2002

Mo/Si multilayers deposited using electron beam evaporation in ultrahigh vacuum were used to study the formation of silicides at high temperatures, in situ in an analytical transmission electron microscope. Studies were carried out on cross-sectional as well as on end-on samples. We observed the origin of the formation of Mo5Si3 phase above 400 °C and nearly complete transformation at 750 °C in both the samples. Formation of Mo5Si3 phase was confirmed by selected area diffraction and high-resolution electron microscopy and by compositional analysis using energy dispersive x-ray analysis.
Electrostatic interactions in computer simulations of a three-dimensional system periodic in two directions: Ewald-type summation
Authors: Bródka A. , Grzybowski A.
Published: October 2002

We show analytically that the two-dimensional Ewald summation of the Coulomb and dipole–dipole interaction energy may be approximated by the conventional three-dimensional Ewald method. However, an empty space must be introduced in the simulation box, and |M|2/3 must be replaced with Mz2, where M is the total dipole moment of the system and Mz is its component along the nonperiodicity direction.
Formation of quasistationary vortex and transient hole patterns through vortex merger
Authors: Ganesh R. , Lee J. K.
Published: October 2002

Collection of point-like intense vortices arranged symmetrically outside of a uniform circular vortex patch, both enclosed in a free-slip circular boundary, are numerically time evolved for up to 10–15 patch turnover times. These patterns are found to merge with the patch by successively inducing nonlinear dispersive modes (V-states) on the surface of the patch, draw off fingers of vorticity (filamentation), trap the irrotational regions as the fingers symmetrize under the shear flow of the patch and point-like vortices (wave breaking) followed by the vortex–hole capture. While the hole patterns are observed to break up over several turnover periods the vortex patterns appear to evolve into quasistationary patterns for some cases of an initial number of point-like vortices Npv. The bounded V-states, filamentation, and vortex (hole) pattern formation are discussed in some detail and their possible connection to recently observed vortex “crystals” is pointed out.
On the consistency of the collisionless sheath model
Authors: Godyak Valery , Sternberg Natalia
Published: October 2002

It is shown that the collisionless sheath model found in textbooks is internally inconsistent. It generates results that contradict the main assumption of the sheath model, namely the negligible ionization. The source of the inconsistency is in the improper boundary condition at the plasma–sheath interface and in the inconsistent derivation of the sheath equations.
Transport and structural properties of binary skutterudite CoSb3 thin films grown by dc magnetron sputtering technique
Authors: Savchuk V. , Boulouz A. , Chakraborty S. , Schumann J. , Vinzelberg H.
Published: October 2002

The results of the studies of structural, electrical, and thermoelectric properties of binary skutterudite CoSb3 thin films are presented. The n-type films were prepared without intentional doping on oxidized Si(100) and ceramic Al2O3 substrates using magnetron dc sputtering technique. The electron probe microanalysis has shown that the composition of the films is strongly affected by the preparation conditions. The x-ray measurements indicate the films to be amorphous if they are deposited on substrates kept at room temperature. On the basis of the differential thermal and x-ray analyses data, the temperature of the transition from the amorphous to crystalline state was established to be (153±1)°C. The films are polycrystalline with a preferential orientation of the growth along the (310) direction of the CoSb3 lattice if grown on substrates kept at 200 °C. The electrical resistivity ρ(T) and the Seebeck coefficient S(T) were measured from room temperature up to about 700 K. The value of ρ(T) decreases with increasing temperature. The maximum absolute value of the Seebeck coefficient is found to be −250 μV/K at around 550 K for both substrates. This is the highest value of the Seebeck coefficient for CoSb3 films deposited by dc magnetron sputtering. The predominant mechanisms of the scattering in CoSb3 are discussed on the basis of the temperature dependence of the Hall mobility. The carrier concentration n(T) is found to be around 1.2×1019 cm−3 at 300 K and is almost constant over the temperature range of (250–400) K. The room temperature measurements of the thermal conductivity carried out by the 3ω method have shown a value of about 3 W/(m K), which is significantly lower than that reported for bulk material.
Localized mixed acoustic modes in superlattices with structural defects
Authors: Wang Xue-Hua , Chen Ke-Qiu , Gu Ben-Yuan
Published: October 2002

We investigate the property of the localized mixed acoustic modes lying in the interior minigap in structural defect superlattices grown away from the cubic axis of bulk materials. It is found that the localized mixed modes always appear in pairs inside each interior minigap for certain structural parameters. Furthermore, there are degeneracy points between two branches of localized mixed modes. These characteristics are markedly different from localized folded acoustic phonon modes in minigaps that occur at the center and the edge of the Brillouin zone. We also find that the interior minigap and localized mixed modes strongly depend on the growth direction of the superlattices.
Excited and ionic states of formamide: An excited-state photoelectron spectroscopy and ab initio study
Authors: ter Steege D. H. A. , Lagrost C. , Buma W. J. , Leigh D. A. , Zerbetto F.
Published: October 2002

High-resolution excited-state photoelectron spectroscopy has been applied to unravel the spectroscopic and dynamic properties of the excited states of formamide populated by two- and three-photon excitation. In combination with ab initio calculations, this approach has led to various reassignments of previously observed states, and to the observation of new states. One of the aspects that particularly emerges from the present study is the important role of vibronic coupling, which leads to states of heavily mixed character. Projection on the ionic manifold—as is done in our studies—is, however, able to determine the various contributions of the wave function. Our studies have enabled us as well to resolve an apparent disagreement concerning the values of the ionization energies of the ground and first excited state of the radical cation. We find here adiabatic values of 10.233±0.008 and 10.725±0.020 eV, respectively. A final issue our studies shed light on concerns the vibrational properties of the ground state of the radical cation.
Dynamical control of the chaotic state of the current-driven ion acoustic instability in a laboratory plasma using delayed feedback
Authors: Fukuyama T. , Shirahama H. , Kawai Y.
Published: October 2002

Controlling chaos caused by the current-driven ion acoustic instability is attempted using delayed feedback, i.e., the time-delay auto synchronization (TDAS) method introduced by Pyragas [Phys. Lett. A 170, 421 (1992)] with flexibility. When the control parameter of the system Vm is increased, the current-driven ion acoustic instability is excited, and then, the system demonstrates chaotic oscillation in a certain range of the parameter. When the electronic circuit to control chaos based on the TDAS method is applied to the chaotic regime, the chaotic orbit changes to a periodic one maintaining the instability, that is, the chaotic state caused by the current-driven ion acoustic instability is well controlled using the TDAS method. Furthermore, it is shown that controlling chaos can be achieved when the delay time τ is chosen to be close to a fundamental frequency of the unstable periodic orbit embedding in the chaotic system.
Electrical and electro-optical investigations of liquid crystal cells containing TiO2–V2O5 thin films prepared by sol–gel synthesis
Authors: Bruno V. , Cazzanelli E. , Scaramuzza N. , Strangi G. , Ceccato R. , Carturan G.
Published: October 2002

This work is aimed at a deeper understanding of the polarity-sensitive electro-optical response observed in a liquid crystal (LC) cell with asymmetric insertion of thin films of TiO2–V2O5 having a Ti/V atomic ratio of 1/1, prepared by sol–gel synthesis on a transparent indium tin oxide substrate. After preliminary structural characterization of the films, the electro-optical response of the liquid crystal cells containing a TiO2–V2O5 layer has been analyzed. The voltage thresholds of the Freedericksz transition are increased or decreased for anodic or the cathodic polarization, respectively, of the TiO2–V2O5 film. In such a way a polarity-sensitive electro-optical response is generated that has the same frequency as the field applied. Impedance and cyclic voltammetry measurements were performed on liquid crystal cells having TiO2–V2O5 films inserted as the electrode, for as-deposited films as well as for films annealed at 400 °C. The LC cell containing thermally annealed Ti/V 1/1 film showed a rectified square wave response instead of the usual impulsive one, quadratic versus electric field. On the contrary, the LC cell containing as-deposited Ti/V 1/1 film exhibits more complex electro-optical behavior with a weak asymmetric response. All the measurements suggest that charge redistribution of the ions, always present in the films, occurs during the intercalation–deintercalation processes induced by the voltage applied, and this is responsible for changes of the effective electric field that act on the liquid crystal layer.
Nanoscale modifications of chalcogenide glasses using scanning tunneling microscopes
Authors: Ohto Masataka , Tanaka Keiji
Published: October 2002

Nanoscale modifications in Cu–As–Se glasses (⩽30 Cu at. %) produced by a scanning tunneling microscope have been studied comparatively with those in As2Te3 and Ag35As26Se39 glasses and in Ag and Cu3AsSe4 crystals. When subjected to the tip voltages greater than ±3 V, all the samples, except Cu–As–Se glasses, produce hillocks or holes. In the Cu–As–Se glasses, nanometer hills accompanying peripheral grooves appear upon applications of negative tip voltage of about −5 V for 1 s. The size increases if the voltage is applied with light illumination, but it does not depend upon temperature (20–100°C) at which the voltage is applied. The size also increases in the glasses with higher Cu contents. This deformation can be accounted for as a Taylor cone produced under electronically induced fluidity.
Formation processes of CdTe quantum dots on ZnTe substrates studied by reflection high-energy electron diffraction and photoluminescence
Authors: Godo K. , Chang J. H. , Makino H. , Takai T. , Hanada T. , Yao T. , Sasao T. , Goto T.
Published: October 2002

Formation processes of self-organized CdTe quantum dots (QDs) grown on ZnTe (001) substrates by molecular beam epitaxy were studied. In situ reflection high-energy electron diffraction (RHEED) was used to study the initial growth processes and strain relaxation behaviors of CdTe QDs. The growth process of CdTe layer at the initial stage can be divided into three stages: (1) stable two-dimensional (2D) growth, (2) metastable 2D growth just before the QD formation, and (3) QD formation. It was found that the critical thickness for the 2D–3D transition is about 5.5 monolayers (ML) at the growth rate of 0.12 ML/s. The results of photoluminescence (PL) and μ-PL agree with the RHEED observations. In addition, by investigating the dependence of PL peak energy on the growth temperature, we found that interdiffusion between the cations (Cd and Zn) is activated at a higher growth temperature.
Infrared spectra of N2O–hydrogen complexes
Authors: Tang Jian , McKellar A. R. W.
Published: October 2002

The infrared spectra of five distinct species of N2O–hydrogen van der Waals complexes (N2O–paraH2, –orthoH2, –orthoD2, –paraD2, and –HD) were studied in the region of the ν1 fundamental band of N2O (≈2224 cm−1) by tunable diode laser spectroscopy in a pulsed supersonic jet expansion. The spectra corresponded to those of T-shaped asymmetric rotors with mainly a-type transitions (ΔKa=0), plus weaker b-type transitions (ΔKa=±1) in the case of N2O–D2. They were analyzed using a conventional Hamiltonian with quadratic centrifugal distortion parameters. The fitted rotational parameters were consistent with structures having intermolecular separations between 3.25–3.4 Å and angles of about 80° between the N2O axis and the hydrogen center-of-mass. These intermolecular distances are about 0.3 Å shorter than those observed previously for the related system, OCS–hydrogen. The band origins were blueshifted relative to the free N2O molecule by from +0.25 to +0.79 cm−1. These vibrational shifts showed regularities among the different spin and isotopic forms of hydrogen that were analogous to those in OCS–hydrogen.
Ion flux uniformity in inductively coupled plasma sources
Authors: Denysenko Igor , Dudin Stanislav , Zykov Aleksander , Azarenkov Nikolay , Yu M. Y.
Published: October 2002

The spatial distribution of the ion flux at the substrate or work surface in inductively coupled plasma source is studied experimentally and theoretically. The theoretical study is based on a two-dimensional fluid model and the well-known global model. The spatial profiles of the electron density and temperature, as well as the ion flux density, are obtained. The theoretical results are compared with the experimental data obtained using a plate probe. It is found that at high pressures the temperature dependence of the electron-neutral collision rate can be important. For argon, maximum uniformity of the ion flux at the work surface occurs for p∼20 mTorr, with the induction coils located on the cylindrical sidewall of the discharge chamber.
Three-dimensional Hall magnetic reconnection
Authors: Huba J. D. , Rudakov L. I.
Published: October 2002

New numerical results of three-dimensional magnetic reconnection in the Hall limit (L<c/ωpi where c/ωpi is the ion inertial length) are presented. The reconnection process is initiated with a magnetic field perturbation localized along the current channel in a reversed field plasma configuration. The perturbation induces a magnetic wave structure that propagates opposite to the current, and leads to the asymmetric thinning of the plasma layer, strong plasma flows in the direction of the current, and rapid magnetic reconnection. The propagating wave structure is a Hall phenomenon associated with magnetic field curvature. The reconnection rate is independent of a (weak) guide field and the boundary conditions (i.e., periodic or outflowing).
Effects of multiple resistive shells and transient electromagnetic torque on the dynamics of mode locking in reversed field pinch plasmas
Authors: Guo S. C. , Chu M. S.
Published: October 2002

The effects of multiple resistive shells and transient electromagnetic torque on the dynamics of mode locking in the reversed field pinch (RFP) plasmas are studied. Most RFP machines are equipped with one or more metal shells outside of the vacuum vessel. These shells have finite resistivities. The eddy currents induced in each of the shells contribute to the braking electromagnetic (EM) torque which slows down the plasma rotation. In this work we study the electromagnetic torque acting on the plasma (tearing) modes produced by a system of resistive shells. These shells may consist of several nested thin shells or several thin shells enclosed within a thick shell. The dynamics of the plasma mode is investigated by balancing the EM torque from the resistive shells with the plasma viscous torque. Both the steady state theory and the time-dependent theory are developed. The steady state theory is shown to provide an accurate account of the resultant EM torque if (dω/dt)ω−2≪1 and the time scale of interest is much longer than the response (L/R) time of the shell. Otherwise, the transient theory should be adopted. As applications, the steady state theory is used to evaluate the changes of the EM torque response from the resistive shells in two variants of two RFP machines: (1) modification from Reversed Field Experiment (RFX) [Gnesotto et al., Fusion Eng. Des. 25, 335 (1995)] to the modified RFX: both of them are equipped with one thin shell plus one thick shell; (2) modification from Extrap T2 to Extrap T2R [Brunsell et al., Plasma Phys. Controlled Fusion 43, 1457 (2001)]: both of them are equipped with two thin shells. The transient theory has been applied numerically to study the time evolution of the EM torque during the unlocking of a locked tearing mode in the modified RFX.
Quantum correction to the linear response for a magnetized electron gas
Authors: Melrose D. B. , Weise J. I.
Published: October 2002

It is shown how the fully relativistic quantum expression for the response of an arbitrary magnetized electron (plus positron) gas reproduces its nonquantum counterpart. In the relativistic quantum case the dispersion is due to both gyromagnetic absorption and one-photon pair creation. Although one-photon pair creation has no classical counterpart, somewhat surprisingly it needs to be retained to reproduce the nonquantum limit correctly. For unpolarized electrons it is shown that the first quantum correction (order ℏ) to the nonquantum limit for the dispersion vanishes when one sums over all excited states, as for an unmagnetized electron gas. However, in the magnetized case there is a contribution of order ℏ from the ground state, which is the only state with a specific spin.
How to patch active plasma and collisionless sheath: A practical guide
Authors: Kaganovich Igor D.
Published: October 2002

Most plasmas have a very thin sheath compared with the plasma dimension. This necessitates separate calculations of the plasma and sheath. The Bohm criterion provides the boundary condition for calculation of plasma profiles. To calculate sheath properties a value of the electric field at the plasma–sheath interface has to be specified in addition to the Bohm criterion. The value of the boundary electric field and a robust procedure to approximately patch plasma and collisionless sheath with a very good accuracy are reported.
Self-consistent model for the radial current generation during fishbone activity
Authors: Lutsenko V. V. , Marchenko V. S.
Published: October 2002

Line broadened quasilinear burst model, originally developed for the bump-on-tail instability [H. L. Berk et al., Nucl. Fusion 35, 1661 (1995)], is extended to the problem of sheared flow generation by the fishbone burst. It is supposed that the radial current of the resonant fast ions can be sufficient to create the transport barrier.