bluuebunny/crossref_metadata_2025 huggingface dataset:
Tripolar vortices associated with toroidal ion temperature gradient modes in a magnetoplasma with sheared flows
Authors: Farid T. , Mirza Arshad M. , Shukla P. K. , Qamar A.
Published: July 2002

A system of nonlinear equations for low-frequency toroidal ion-temperature-gradient modes in a nonuniform magnetoplasma with sheared plasma flow is presented. For some specific profiles of the equilibrium density, the ion temperature and sheared plasma flows, the nonlinear equations admit a tripolar vortex. The numerical results show a negative potential vortex core between the lobs of a dipole-like structure which has a positive value of the potential.
Parametrically forced pattern formation
Authors: Armbruster Dieter , George Marguerite , Oprea Iuliana
Published: July 2002

Pattern formation in a nonlinear damped Mathieu-type partial differential equation defined on one space variable is analyzed. A bifurcation analysis of an averaged equation is performed and compared to full numerical simulations. Parametric resonance leads to periodically varying patterns whose spatial structure is determined by amplitude and detuning of the periodic forcing. At onset, patterns appear subcritically and attractor crowding is observed for large detuning. The evolution of patterns under the increase of the forcing amplitude is studied. It is found that spatially homogeneous and temporally periodic solutions occur for all detuning at a certain amplitude of the forcing. Although the system is dissipative, spatial solitons are found representing domain walls creating a phase jump of the solutions. Qualitative comparisons with experiments in vertically vibrating granular media are made.
A numerical study of dynamics of a temporally evolving swirling jet
Authors: Hu Guo-Hui , Sun De-Jun , Yin Xie-Yuan
Published: July 2002

Direct numerical simulation (DNS) of a swirling jet near the outlet of a nozzle with axisymmetric and non-axisymmetric disturbances is performed to investigate the dynamic characteristics of the flow. The early (linear) stage of the jet evolution agrees well with the predictions of linear stability theory. In the nonlinear stage, the axisymmetric DNS shows that the interaction between the primary vortex ring and the streamwise columnar vortex creates a secondary vortex structure with opposite azimuthal vorticity near the columnar vortex. Then a vortex pair consisting of the primary and secondary vortices forms and travels radially away from the symmetry axis, causing a rapid increase of the thickness of mixing layer. The non-axisymmetric DNS shows that the streamwise vortex layer developed in the early stage of evolution due to azimuthal instability breakdowns into small eddies under the joint stretch of the axial and azimuthal shear. The results reveal several mechanisms of mixing enhancement by swirl, i.e., the radial motion of vortex ring pairs, the rapid growth of streamwise vorticity, and the creation of three-dimensional small eddies. They are all favorable for fluid entrainment in swirling jets.
Enhanced intraband Stark effects in stacked InAs/GaAs self-assembled quantum dots
Authors: Sheng Weidong , Leburton Jean-Pierre
Published: July 2002

We present a theoretical study of the electronic properties and intersubband optical transitions in vertically aligned double InAs self-assembled quantum dots (QDs) which are subject to an electric field along their growth axis. The electron properties are calculated as a function of the applied electric field by using an eight-band strain-dependent k⋅p Hamiltonian. Transitions between ground s states and excited p states are found to be almost three times stronger than in single dot, with strong field anisotropy. The system also exhibits field tunable transitions between the bonding and antibonding s states, with polarization along the growth axis. Midinfrared photodetectors consisting of vertically coupled double-quantum-dot layers are expected to exhibit enhanced sensibility and voltage tunability, compared to devices using single-quantum-dot layer.
Lowering the dimension of polynomial vector fields in R2 and R3
Authors: Yanguas Patricia
Published: July 2002

We classify the normal forms associated to polynomial vector fields with dimensions two and three whose principal part is linear. Then we reduce by one the dimension of the associated differential systems. This is achieved by means of the extension of a symmetry of the unperturbed part to the whole system, up to a certain order of approximation. The corresponding transformation is formal in the sense that we have no estimation on its radius of convergence. We calculate the invariants, reduced phase spaces and differential systems associated to each reduction. This is the general setting for the study of the dynamics of the reduced system in order to obtain information about the original one.
Configurational temperature profile in confined fluids. II. Molecular fluids
Authors: Delhommelle Jerome , Evans Denis J.
Published: July 2002

In an earlier paper, we applied configurational expressions of the temperature to the calculation of temperature profiles within a confined atomic fluid. This paper focuses on the application of these expressions to confined molecular fluids using ethane and hexane as examples. We first give configurational expressions for the temperature for these constrained systems. The configurational temperature profiles so obtained are compared to the kinetic temperature calculated using the equipartition principle, in equilibrium systems. These expressions are then used in nonequilibrium molecular dynamics (NEMD) simulations of fluids undergoing planar Poiseuille flow. We show that these configurational expressions provide a direct and accurate determination of the temperature profile for these systems.
Tunnel junctions with AlN barriers and FeTaN electrodes
Authors: Wang Jianguo , Cardoso S. , Freitas P. P. , Wei P. , Barradas N. P. , Soares J. C.
Published: July 2002

Tunnel junctions with AlN (AlNxOy) barriers and CoFe and FeTaN electrodes were studied. The AlN barrier was formed by nitridizing a 10 Å thick Al layer using radio frequency N2 plasma. The nitrogen and oxygen content in the barriers was determined by Rutherford backscattering spectroscopy (RBS) on specially prepared barriers deposited on DLC coated substrates. RBS results indicate less than 10% O2 incorporation in the barrier. Top-pinned junctions formed by nitridizing a 10 Å thick Al layer (CoFe electrodes) show resistance×area products from 73 Ω μm2 to 8.5 kΩ μm2 for increasing nitridation times from 30 to 200 s, with corresponding tunneling magnetoresistance (TMR) values from 13% to 33%. Bottom-pinned junctions with FeTaN electrodes were also fabricated. Maximum TMR signal is 17% after anneal at 225 °C for 30 min. In both cases (CoFe or FeTaN electrodes), TMR degrades for anneals above 250 °C.
Nuclear reaction rates and energy in stellar plasmas: The effect of highly damped modes
Authors: Opher Merav , Silva Luis O. , Dauger Dean E. , Decyk Viktor K. , Dawson John M.
Published: July 2002

The effects of the highly damped modes in the energy and reaction rates in a plasma are discussed. These modes, with wave numbers k≫kD, even being only weakly excited, with less than kBT per mode, make a significant contribution to the energy and screening in a plasma. When the de Broglie wavelength is much less than the distance of closest approach of thermal electrons, a classical analysis of the plasma can be made. It is assumed, in the classical analysis, with ℏ→0, that the energy of the fluctuations ℏω≪kBT. Using the fluctuation-dissipation theorem, the spectra of fluctuations with ℏ≠0 is appreciably decreased. The decrease is mainly for the highly damped modes at high frequencies (∼0.5–3kBT). Reaction rates are enhanced in a plasma due to the screening of the reacting ions. This is taken into account by the Salpeter factor, which assumes slow motion for the ions. The implication of including the highly damped modes (with ℏ≠0) in the nuclear reaction rates in a plasma is discussed. Finally, the investigations presently done on these effects in particle simulations with the sheet model and the multiparticle quantum simulation code are described.
Photonic band gap properties of CdS-in-opal systems
Authors: Blanco A. , Míguez H. , Meseguer F. , López C. , López-Tejeira F. , Sánchez-Dehesa J.
Published: July 2002

Silica opals are used as templates where CdS is infiltrated with the aim to build inverse structures with enhanced photonic band gap properties. A control on the degree of infiltration, from 0% to 100%, is attained. The band gap at L is studied finding that the width decreases and then recovers as a function of CdS infilling (from bare opal to fully loaded structure). This is well accounted for by theory based on two different modes for the growth of CdS inside the opal pores. A shell mode seems to govern the growth at low infiltration (less than 10%). High quality opal templates, appropriate sintering, and a high and uniform infiltration are required to ensure further optical characterization of the inverse systems. Only heavily loaded structures are apt to be inverted. The gap in the fully loaded and the inverse opal are, respectively, two and three times broader than in the starting opal.
Strong visible photoluminescence from SiO2 nanotubes at room temperature
Authors: Chang H. J. , Chen Y. F. , Lin H. P. , Mou C. Y.
Published: July 2002

The optical studies of SiO2 mesoporous materials with hierarchical tubules-within-tubule structure have been investigated by photoluminescence and Fourier-transform infrared transmittance (FTIR). Our results suggest that the radiative intensity can be strongly enhanced by annealing the samples in N2 environment. From the FTIR spectra, we have pointed out that the origin responsible for the strong emission is Si–OH complexes located on nanotube surface. It has been observed that after turning off the pumping laser, the photoluminescence signal of SiO2 nanotubes can persist for several seconds, which is much longer than that of most materials performed under similar conditions. We have found that the decay of the photoluminescence signal is due to the quantum tunneling process. These are triplet and singlet states of Si–OH complexes that are responsible for the observed persistent photoluminescence.
Effects of π centers and symmetry on two-photon absorption cross sections of organic chromophores
Authors: Wang Chuan-Kui , Macak Peter , Luo Yi , Ågren Hans
Published: July 2002

We have theoretically examined a series of organic molecules that exhibit large two-photon absorption cross sections in the visible region and that have been synthesized in different laboratories. One- and two-photon absorption cross sections of the four lowest excited states of each molecule have been calculated at the same theoretical level using ab initio response theory. It is found that the molecular length and the one-photon absorption intensity are quite strongly correlated factors, but that a corresponding correlation for the two-photon absorption is much weaker or is missing. In contrast, a most crucial role for large two-photon absorption is played by the π center. For molecules with a given π center a symmetrical structure with strong donor groups can result in a maximum two-photon absorption cross section. Our theoretical findings are consistent with some recent experimental observations. The chromophore based on dithienothiophene as π center attached with symmetrical N,N-diphenylamine donors is found to have the largest two-photon cross section in the visible region among all known one-dimensional two-photon organic materials that have been reported in the literature.
On the magnetoelastic nature of the anisotropic domains in easy-plane crystals of iron-group dihalides
Authors: Kalita V. M. , Lozenko A. F.
Published: July 2002

A structure is proposed for the magnetoelastic antiferromagnetic domains in easy-plane antiferromagnets of the iron-group dihalides. The proposed structure is made up of a triad of regions with 120° rotation of the antiferromagnetic vector on passing between adjacent regions. This domain structure may be stabilized by defects which are isotropic in the easy plane and which preserve the orientational degeneracy of the directions of the antiferromagnetic vector in the easy plane.
High resolution nonlinear microscopy: A review of sources and methods for achieving optimal imaging
Authors: Squier Jeff , Müller Michiel
Published: July 2002

This article reviews the latest instrumentation used in high resolution nonlinear microscopy and techniques for the temporal and spatial calibration of this instrumentation. This includes an overview of currently available ultrashort laser sources, the dispersion characteristics of microscopes, methods for pulse measurement at high numerical aperture, dispersion compensation techniques, and finally a brief overview of a number of nonlinear imaging methods presently used in these systems.
A quantum mechanical and quasi-classical trajectory study of the Cl+H2 reaction and its isotopic variants: Dependence of the integral cross section on the collision energy and reagent rotation
Authors: Aoiz F. J. , Bañares L. , Castillo J. F. , Menéndez M. , Skouteris D. , Werner H.-J.
Published: July 2002

Quantum mechanical (QM) and quasi-classical trajectory (QCT) calculations have been performed for the Cl+H2, Cl+D2, Cl+HD→ HCl(DCl)+D(H) reactions in order to determine integral cross sections as a function of collision energy and for different reagent rotational quantum numbers using the recent ab initio BW2 potential energy surface (PES) by Bian and Werner [J. Chem. Phys. 112, 220 (2000)]. The results are compared with experimental data obtained by using the Doppler-selected time-of-flight technique. It has been found theoretically by both the QM and QCT methods that reagent rotation enhances reactivity in agreement with experiment. The QM results are found to be in quantitative agreement with the experimental excitation functions for the Cl+p-H2 and Cl+n-H2 reactions, whereas those obtained quasi-classically fail to reproduce the experimental data. These results are in strong contrast with those reported on the previous G3 PES, in which QM and QCT calculations predicted that reactivity decreases with reagent rotation. The intermolecular isotope effect, i.e., the ratio between the cross sections of the Cl+n-H2 and Cl+n-D2 reactions, Γinter(Cl+n-H2/Cl+n-D2), predicted by QM calculations on the BW2 surface is notably larger than that obtained experimentally.
Refractive index of CdTe: Spectral and temperature dependence
Authors: Hlı́dek P. , Bok J. , Franc J. , Grill R.
Published: July 2002

The refractive index of CdTe was determined from interference maxima in the transparency region from 10 to 295 K. Parameters in a four-oscillator model were fitted to experimental data and an empirical formula for the refractive index is given.
Thermal redistribution of photocarriers between bimodal quantum dots
Authors: Zhang Y. C. , Huang C. J. , Liu F. Q. , Xu B. , Wu J. , Chen Y. H. , Ding D. , Jiang W. H. , Ye X. L. , Wang Z. G.
Published: July 2002

We study the photoluminescence (PL) properties of InAs/GaAs self-assembled quantum dots (QDs) by varying excitation power and temperature. Excitation power-dependent PL shows that there exists bimodal size distribution in the QD ensemble. Thermal carrier redistribution between the two branches of dots is observed and investigated in terms of the temperature dependence of their relative PL intensity. Based on a model in which carrier transfer between dots is facilitated by the wetting layer, the experimental results are well explained.
Structure and vibrational dynamics of the strongly hydrogen-bonded model peptide: N-methyl acetamide
Authors: Kearley G. J. , Johnson M. R. , Plazanet M. , Suard E.
Published: July 2002

Density functional theory-based methods have been used to calculate the vibrations, in the harmonic approximation, of n-methyl acetamide in the solid state. Good agreement is obtained with previously published inelastic neutron scattering spectra. The starting point for the calculation is the crystal structure, which has to be measured at the same temperature as the vibrational spectra. Unit cell and atomic coordinates have been obtained using powder neutron diffraction on the methyl-deuterated material at 2 K. The controversial assignment of the N–H stretch mode at ∼1600 cm−1, made in the original analysis of the vibrational spectra, is not supported by the calculations presented here. Neither is evidence found for the proposed double-well potential for the proton in the hydrogen bond.
Charge-transfer mediated photochemistry in alkene–O2 complexes
Authors: DeBoer Gary , Preszler Prince Amy , Young Mark A.
Published: July 2002

The photochemistry of a series of alkene–O2 complexes was studied in a supersonic expansion using a resonance enhanced multiphoton ionization probe of the O(3Pj) photoproduct at 226 nm. The relative yield of oxygen atoms from each complex was correlated to the ionization potential of the alkene species and indicates that initial excitation of an intermolecular charge-transfer state mediates the subsequent excited state chemistry. The behavior is similar to that observed previously for the C6H6–I2 system: a reverse electron-transfer step yields electronically excited O2 which subsequently dissociates. The kinetic energy release of the O(3Pj) fragment was also measured using a time-of-flight analysis and found to be small with an isotropic spatial distribution. No evidence for photo-oxidation of the alkenes was observed in the mass spectra. A comparison is made to the charge-transfer absorption spectra observed in cryogenic oxygen matrices of similar alkene complexes. Ab initio models were used to identify the stable ground state geometry of the C2H4–O2 complex and complete active space self-consistent-field calculations were performed to identify the energy of the charge-transfer state for several alkene–O2 complexes.
The onset of superconductivity in a domain with a corner
Authors: Jadallah Hala T.
Published: July 2002

We study the variational problem related to the onset of superconductivity that identifies the transition from the normal state to the superconducting state of a sample in the presence of an applied magnetic field. Our concern is a thin sample whose 2-D cross-section has a corner. In particular, we focus on the quarter-plane. We show a first eigenfunction minimizing the associated Rayleigh quotient exists and decays away from the corner. We also give a rigorous upper bound for the eigenvalue which is related to the critical temperature at which superconductivity emerges.
Magnetostriction during field transformation of the domain structure of an easy-plane antiferromagnet in the case of a magnetoelastic mechanism for the multidomain state
Authors: Kalita V. M. , Lozenko A. F.
Published: July 2002

The features of the transformation of the multidomain state of an easy-plane antiferromagnet in an external magnetic field are investigated in the case of a magnetoelastic origin of the multidomain state. Experimental data are presented on the field dependence of the induced magnetostriction of the easy-plane two-sublattice antiferromagnet CoCl2 at different temperatures. In the discussion of the experiments and in the theoretical description we use the approximation of a continuous distribution of domains in the easy plane. It is shown that the orientational distribution of the domains upon the introduction of a magnetic field depends only on the magnitude and direction of the field and is independent of temperature. These features of the distribution of domains can be explained by a matching of their equilibrium magnetostriction with the elasticity of defects wherein the elasticity of the domains compensates the elasticity of the defects.
Domain formation in epitaxial Pb(Zr, Ti)O3 thin films
Authors: Lee K. S. , Choi J. H. , Lee J. Y. , Baik S.
Published: July 2002

Ferroelectric twin-domain structures in epitaxial Pb(Zr, Ti)O3 (PZT) thin films grown on various single-crystal substrates such as MgO(001), KTaO3(001), and SrTiO3(001) were investigated by two-dimensional reciprocal space mapping using synchrotron x-ray diffraction. Each system showed a characteristic domain structure. PbTiO3 thin films grown on MgO(001) showed highly c-axis oriented domain structures consisting of a periodic array of 90° twinlike domains. Perfectly c-axis oriented films were obtained on SrTiO3(001), while the films grown on KTaO3(001) showed a-domain dominant structures with a small amount of c domains embedded in matrix a domains. Contributions of net elastic strain stored in each heteroepitaxial layer and its relaxation to the final domain structures were evaluated considering thermodynamic equilibrium relief of coherency strain by misfit dislocation generation at the film growth temperature. A comparison between theoretical consideration and experimental results clearly demonstrates that the nature of effective misfit strain and its relaxation during film growth play a critical role in the formation of domain structures in epitaxial PZT thin films. Moreover, it was verified that the control of such critical strain factors by changing film composition could modify dominant domain structures in a drastic way. In addition, it was found that the crystalline quality of the films is closely correlated to the tilting nature of the domain structure in each system and coherency strain across the 90° domain boundary is accommodated mainly by the domain tilt of the minor domain.
Bifurcation in precessional switching
Authors: Acremann Y. , Back C. H. , Buess M. , Pescia D. , Pokrovsky V.
Published: July 2002

We explore the precessional motion of the magnetization vector in a model magnetic element. We find that the Landau–Lifshitz equation governing this motion allows trajectories of the magnetization vector to bifurcate. This yet unknown phenomenon is accompanied by a slowing down of the precessional motion and an abrupt shrinking of the size of the trajectory of the precessing magnetization. We discuss the implication of bifurcation for future devices using precessional switching and suggest how magnetic elements showing the classical phenomenon of bifurcation can be tuned to act as quantum bits.
Mass spectra and photoelectron spectroscopy of negatively charged benzene clusters, (benzene)n− (n=53–124)
Authors: Mitsui Masaaki , Nakajima Atsushi , Kaya Koji , Even Uzi
Published: July 2002

Negatively charged benzene clusters, (benzene)n−, were produced by injecting low- and high-energy electrons into an intense supersonic jet expansion. Threshold size of n=53 was observed by slow-electron attachment, while the smaller (benzene)n− with 2⩽n⩽52 were also observed through the fragmentation of larger (benzene)n− by high-energy electron attachment. Photoelectron spectroscopy for (benzene)n− with n=53–124 has revealed a bulklike electron solvated state in (benzene)n⩾53− through the vertical detachment energies (VDEs) versus n−1/3 relationship.
Photoluminescence performance of pulsed-laser deposited Al2O3 thin films with large erbium concentrations
Authors: Serna R. , Jiménez de Castro M. , Chaos J. A. , Suárez-Garcia A. , Afonso C. N. , Fernández M. , Vickridge I.
Published: July 2002

Erbium doped Al2O3 films with concentrations up to 6×1020 Er cm−3 have been prepared in a single step process by pulsed-laser deposition. Alternate ablation of Al2O3 and Er targets has been used to control the in-depth distribution and in-plane concentration of Er3+ ions independently. The characteristic Er3+ photoluminescence response at 1.53 μm has been studied as a function of the Er3+ distribution. It is found that lifetime values can be greatly increased by increasing the Er3+–Er3+ in-depth separation above 3 nm. This result can be related to a reduced Er3+–Er3+ energy migration process. The in-plane Er3+ concentration was increased by either increasing the number of pulses on the Er target or the laser energy density for ablation. By the latter method in-plane concentrations as high as 1.1×1014 Er cm−2 per layer (corresponding to 2×1020 Er cm−3) were achieved, while keeping lifetime values as high as 6 ms. This result is explained in terms of shallow Er3+ implantation during deposition.
Resonant and nonresonant transmission through waveguide bends in a planar photonic crystal
Authors: Olivier S. , Benisty H. , Rattier M. , Weisbuch C. , Qiu M. , Karlsson A. , Smith C. J. M. , Houdré R. , Oesterle U.
Published: July 2002

We have measured the near-infrared transmission spectra of 60° bends defined in two-dimensional photonic crystal waveguides consisting of three missing rows. Two limit cases are studied: a basic nonresonant bend and a bend built around a resonant lozenge cavity, which is found to exhibit peaked transmission. Finite-difference-time-domain simulations show very good agreement with the data allowing general design issues for efficient bends to be discussed.
Superconducting bulk magnets: Very high trapped fields and cracking
Authors: Gruss S. , Fuchs G. , Krabbes G. , Verges P. , Stöver G. , Müller K.-H. , Fink J. , Schultz L.
Published: July 2002

Improved trapped fields are reported for bulk melt-textured YBa2Cu3O7−δ (YBCO) material in the temperature range between 20 and 50 K. Trapped fields up to 12.2 T were obtained at 22 K on the surface of single YBCO disks (with Ag and Zn additions). In YBCO minimagnets, maximum trapped fields of 16 T (at 24 K) and of 11,2 T (at 47 K) were achieved using (Zn+Ag) and Zn additions, respectively. In all cases, the YBCO disks were encapsulated in steel tubes in order to reinforce the material against the large tensile stress acting during the magnetizing process and to avoid cracking. We observed cracking not only during the magnetizing process, but also as a consequence of flux jumps due to thermomagnetic instabilities in the temperature range betweeen 20 and 30 K.
Modification of nonrelativistic Gaussian basis sets for relativistic calculations
Authors: Tatewaki Hiroshi , Mochizuki Yuji , Koga Toshikatsu , Karwowski Jacek
Published: July 2002

A simple method is proposed in which basis sets of Gaussian-type functions (GTFs), suitable for relativistic Dirac–Fock–Roothaan (DFR) calculations, are derived from their nonrelativistic analogs. The relativistic basis set is obtained through augmenting the nonrelativistic one by several GTFs determined from relativistic calculations for hydrogen-like atoms. The usefulness and reliability of the method is illustrated by DFR calculations of the ground-state energies of lanthanide and actinide atoms.
Characterization of anharmonicities on complex potential energy surfaces: Perturbation theory and simulation
Authors: Calvo F. , Doye J. P. K. , Wales D. J.
Published: July 2002

We have systematically investigated the effect of anharmonicity on the equilibrium properties of systems with a complex potential energy surface. Anharmonicities are modeled by the temperature dependence of the harmonic frequencies {νi} near a stationary point of the PES. The low-temperature behavior is described by a simple thermal expansion ν(i)(β)=ν0(i)[1−α1(i)/β+α2(i)/2β2+⋯], where the coefficients {αj(i)} are obtained from perturbation theory. Using a simple diagrammatic representation, we give the complete expressions for the first two coefficients α1 and α2 in terms of derivatives of the potential. This approach is illustrated for the example of a bulk Lennard-Jones system of 32 particles, in both the solid and the liquid states. We also determine the anharmonic frequencies from reversible-scaling Monte Carlo simulations, which appear particularly well suited to this problem. As an example, we have studied a model biopolymer that exhibits significant first and second order anharmonicities. To show the importance of treating anharmonicities properly, we have calculated the caloric curve (heat capacity) of the quantum Ne13 cluster in both the classical and quantum regimes. For this calculation we have used a superposition approximation and exact anharmonic classical corrections to second order in perturbation theory. When every vibrational mode of each inherent structure is treated separately, we find good agreement between our results and previous quantum Monte Carlo calculations.
Unsteady heat transfer from a sphere in a uniform cross-flow
Authors: Balachandar S. , Ha M. Y.
Published: July 2002

Three different types of unsteady heat transfer problems are considered: the first considers the free thermal evolution of a spherical particle subjected to a uniform isothermal ambient flow, where the particle temperature evolves toward the ambient flow temperature; the second considers the unsteady heat transfer problem in response to a sudden jump in the particle temperature; and the third considers oscillatory heat transfer due to oscillatory particle temperature in an isothermal ambient flow. In each case a range of Reynolds or Peclet number is considered. The first set of simulations show that unsteady heat transfer from a spherical particle under free thermal evolution can be described by an effective Nusselt number, at least over the range of parameters considered in this study. The effective Nusselt number under unsteady free thermal evolution deviates from the steady-state counterpart and the difference depends on the heat capacity ratio between the surrounding fluid to the particle. The existence of an effective Nusselt number indicates a faster decay of the thermal history kernel, which is confirmed with response to a step change in particle temperature in the second set of simulations. The final set of simulations considers the behavior of the thermal history kernel in the frequency space. We observe the low frequency response, or correspondingly the long time behavior, of the thermal history kernel at finite Peclet number to deviate significantly from the classical one over square root decay obtained for zero Peclet number limit.
Dynamics of one- and two-dimensional kinks in bistable reaction–diffusion equations with quasidiscrete sources of reaction
Authors: Rotstein Horacio G. , Zhabotinsky Anatol M. , Epstein Irving R.
Published: July 2002

We study the evolution of fronts in a bistable reaction–diffusion system when the nonlinear reaction term is spatially inhomogeneous. This equation has been used to model wave propagation in various biological systems. Extending previous works on homogeneous reaction terms, we derive asymptotically an equation governing the front motion, which is strongly nonlinear and, for the two-dimensional case, generalizes the classical mean curvature flow equation. We study the motion of one- and two-dimensional fronts, finding that the inhomogeneity acts as a “potential function” for the motion of the front; i.e., there is wave propagation failure and the steady state solution depends on the structure of the function describing the inhomogeneity.
Significant improvement of the trajectory surface hopping method by the Zhu–Nakamura theory
Authors: Zhu Chaoyuan , Kamisaka Hideyuki , Nakamura Hiroki
Published: July 2002

By taking the three-dimensional D++H2 reaction system, the trajectory surface hopping method based on the Zhu–Nakamura theory is demonstrated to work much better than the old one and to be very promising to treat high-dimensional electronically nonadiabatic processes. The difference between the new and old survives even at high initial vibrational states and high energies.
Complexity of regular invertible p-adic motions
Authors: Pettigrew J. , Roberts J. A. G. , Vivaldi F.
Published: July 2002

We consider issues of computational complexity that arise in the study of quasi-periodic motions (Siegel discs) over the p-adic integers, where p is a prime number. These systems generate regular invertible dynamics over the integers modulo pk, for all k, and the main questions concern the computation of periods and orbit structure. For a specific family of polynomial maps, we identify conditions under which the cycle structure is determined solely by the number of Siegel discs and two integer parameters for each disc. We conjecture the minimal parametrization needed to achieve—for every odd prime p—a two-disc tessellation with maximal cycle length. We discuss the relevance of Cebotarev’s density theorem to the probabilistic description of these dynamical systems.
Microstructural characterization of rf sputtered polycrystalline silicon germanium films
Authors: Choi W. K. , Teh L. K. , Bera L. K. , Chim W. K. , Wee A. T. S , Jie Y. X.
Published: July 2002

Polycrystalline silicon germanium (Si1−xGex) films deposited by the rf sputtering technique were characterized using a combination of transmission electron microscopy (TEM), atomic force microscopy (AFM), x-ray diffraction (XRD), and Raman spectroscopy techniques. The TEM results showed small grains (10–20 nm) with microtwins, and AFM showed islands of 100–200 nm in the films. The XRD results show that our films consist of Si1−xGex alloy with no cluster of Ge or a Ge-rich material embedded in a Si matrix. The smaller grains in our films could be a result of an abundance of nucleating sites or impurities in the films. Raman spectroscopy results indicate that our films were strain free.
Intervalenceband and plasmon optical absorption in heavily doped GaAs:C
Authors: Songprakob W. , Zallen R. , Tsu D. V. , Liu W. K.
Published: July 2002

By using direct numerical-solution techniques for the reflectance (R) and transmittance (T) equations of a multilayer structure, we have analyzed infrared R and T measurements on heavily doped p-type GaAs:C films grown by molecular beam epitaxy. The optical properties, for films with hole concentrations up to 1.4×1020 cm−3, were determined for photon energies from 0.07 to 0.6 eV, in which region plasmon (intraband) and intervalenceband contributions are in competition. Our results for the optical absorption coefficient resolve two separate peaks located (at high doping) at about 0.1 and 0.2 eV. By carrying out calculations of the intervalenceband (IVB) absorption processes for our dopings, we identify the peak near 0.2 eV with light-hole to heavy-hole IVB transitions, and we attribute the lower-energy peak to hole-plasmon excitations. Our experimental absorption spectra are very well described by a model combining the IVB contribution to the dielectric function with a plasmon contribution. The hole-plasmon parameters we obtain for highly doped p-GaAs yield an infrared mobility which (unlike the too-small IVB-entangled infrared mobility implied by the use of the usual effective-plasmon model) is in substantial agreement with the dc mobility.
Electrostatic forces between sharp tips and metallic and dielectric samples
Authors: Gómez-Moñivas S. , Froufe-Pérez L. S. , Caamaño A. J. , Sáenz J. J.
Published: July 2002

A detailed analysis of electrostatic interactions between a dc-biased tip and a metallic or insulating sample is presented. By using a simple method to calculate capacitances and forces, tip shape effects on the force versus tip-sample distance curves are dicussed in detail. For metallic samples the force law, except for a constant background, only depends on the tip radius of curvature. In contrast, for dielectric samples the forces depend on the overall geometry of the tip. Interestingly, we found that the contact (adhesion) force does not depend on the tip size and is bound by a simple expression which only depends on the applied bias and the sample dielectric constant.
Stability of HF-etched Si(100) surfaces in oxygen ambient
Authors: Zhang X. , Garfunkel E. , Chabal Y. J. , Christman S. B. , Chaban E. E.
Published: July 2002

In situ multiple internal reflection infrared absorption spectroscopy of H-passivated silicon surfaces in controlled oxygen environments reveals that direct oxygen incorporation into the surface Si–Si bonds occurs without surface hydrogen removal, in the temperature range of 550–590 K for 1–20 mTorr O2 pressures. The kinetics of the O2 insertion process display overall effective activation energies of 1.6 to 1.7 eV and prefactors controlled primarily by Si–H steric hindrance for O2 to access Si–Si backbonds.
Boron carbide/n-silicon carbide heterojunction diodes
Authors: Adenwalla S. , Welsch P. , Harken A. , Brand J. I. , Sezer A. , Robertson B. W.
Published: July 2002

The fabrication, initial structural characterization, and diode measurements are reported for a boron carbide/silicon carbide heterojunction diode. Current–voltage curves are obtained for operation at temperatures from 24 to 351 °C. Plasma-enhanced chemical-vapor deposition (PECVD) -deposited undoped boron carbide material is highly crystalline and consists of a variety of polytypes of boron carbide (BC) with crystal sizes as large as 110 nm. Crystal phases are similar to those for PECVD BC on Si but only partially match known boron and boron-rich BC phases.
Quantum coherent transport versus diode-like effect in semiconductor-free metal/insulator structure
Authors: Tiusan C. , Chshiev M. , Iovan A. , da Costa V. , Stoeffler D. , Dimopoulos T. , Ounadjela K.
Published: July 2002

Quantum coherent transport in double barrier tunnel junctions has been exploited for building micrometric-size semiconductor-free diodes. At room temperature, we observe strongly asymmetric current–voltage characteristics with an asymmetry ratio increasing with the bias voltage, reaching a maximum of 20 at 1 V. Our experimental data can be perfectly explained using a theoretical model involving resonance-assisted tunneling. The coherent/resonant tunneling regime is achieved using metallic 3 nm diameter monodisperse Cu clusters, sandwiched between two Al2O3 barriers.
Symplectic areas, quantization, and dynamics in electromagnetic fields
Authors: Karasev M. V. , Osborn T. A.
Published: July 2002

A gauge invariant quantization in a closed integral form is developed over a linear phase space endowed with an inhomogeneous Faraday electromagnetic tensor. An analog of the Groenewold product formula (corresponding to Weyl ordering) is obtained via a membrane magnetic area, and extended to the product of N symbols. The problem of ordering in quantization is related to different configurations of membranes: A choice of configuration determines a phase factor that fixes the ordering and controls a symplectic groupoid structure on the secondary phase space. A gauge invariant solution of the quantum evolution problem for a charged particle in an electromagnetic field is represented in an exact continual form and in the semiclassical approximation via the area of dynamical membranes.
Local lattice strain distribution around a transistor channel in metal–oxide–semiconductor devices
Authors: Toda Akio , Ikarashi Nobuyuki , Ono Haruhiko , Ito Shinya , Toda Takeshi , Imai Kiyotaka
Published: July 2002

The local lattice strain around the channel in metal–oxide–semiconductor (MOS) field-effect transistors of 0.1 μm gate length was measured by using convergent-beam electron diffraction. It was found that the normal strain along the gate-length direction is compressive beneath the gate and is larger for devices having smaller diffusion sizes in the gate length direction L′. The drive current Ion decreased for an n-channel MOS and increased for a p-channel MOS as L′ decreased. These results are consistent with those of a previous study. However, our results also revealed that the strain distribution around the channel region was strongly affected not only by the stress from the shallow trench isolation but also by the device structures around the gate.
Theoretical studies of ligand-free cadmium selenide and related semiconductor clusters
Authors: Deglmann Peter , Ahlrichs Reinhart , Tsereteli Kakha
Published: July 2002

The results of density functional theory potential surface investigations are presented for small ZnS, ZnSe, CdS, and CdSe clusters, up to the heptamer. Clusters of CdSe are investigated further by geometry optimizations of larger crystal structure sections with up to 198 atoms that correspond to the wurtzite modification. In addition to structures and energies of these clusters, we present electron affinities, ionization potentials as well as electronic excitation spectra, obtained from time dependent density functional theory.
Surface model and exchange-correlation functional effects on the description of Pd/α-Al2O3(0001)
Authors: Gomes J. R. B. , Illas F. , Hernández N. Cruz , Sanz J. F. , Wander A. , Harrison N. M.
Published: July 2002

The interaction of Pd with the Al-terminated α-Al2O3(0001) surface has been investigated using an embedded cluster model and periodic-supercell approaches. Furthermore, several treatments of electronic exchange and correlation within density functional (DF) theory have been employed including generalized gradient approximation (GGA) and hybrid exchange functionals. In the periodic calculations the influence of pseudopotentials and basis sets have also been investigated by comparing GGA results obtained using all electron basis set and pseudopotential plane-wave approaches. For a given choice of the exchange-correlation functional and for a fixed substrate, the cluster and slab models predict nearly the same structural parameters and adsorption energies. All structural models reproduce the general trend for the interaction of Pd with the α-Al2O3(0001) surface, which is that there is a slight preference for adsorption above surface sites sitting directly above oxygen atoms either from the second or fifth layer. However, significantly larger differences exist when comparing different DF methods within a given surface model. The cluster and periodic slab models predict a large adsorbate-induced relaxation with a similar description of the metal-oxide interface provided a minimum number of surface layers is included in the optimization procedure.
Effect of thermal expansion mismatch on the dielectric peak temperature of thin film relaxors
Authors: Catalan G. , Corbett M. H. , Bowman R. M. , Gregg J. M.
Published: July 2002

Strongly oriented thin film capacitors of Pb(Mg1/3Nb2/3)O3 were made by pulsed laser deposition and found to show a decreased temperature of the dielectric maximum (Tm) relative to bulk. The in-plane strain was measured and found to be compressive, a feature that is normally associated with increased Curie temperatures in conventional ferroelectrics. Other features, such as a more pronounced frequency dependence of Tm in thin films than in bulk, were also noted. An attempt to rationalize these differences by consideration of thermal expansion mismatch between substrate and film is presented. The shifts in Tm are modeled using a semiempirical Landau–Ginzburg–Devonshire model, in which the first coefficient of the thermodynamic potential has been modified to have a quadratic, rather than linear, temperature dependence. The predictions of the model are compared with experimental results from this work and other reports on relaxor thin films extracted from the literature.
Surface-emitting spin-polarized In0.4Ga0.6As/GaAs quantum-dot light-emitting diode
Authors: Ghosh S. , Bhattacharya P.
Published: July 2002

We report the properties of a spin-polarized In0.4Ga0.6As/GaAs quantum-dot surface-light-emitting diode with a Ga0.974Mn0.026As spin injector layer. Spin-polarized holes from this ferromagnetic layer recombine with electrons in the quantum dots to produce circularly polarized light output. The peak optical polarization efficiency at 5.1 K is 18% and the spin injection efficiency is estimated to be ∼36%. The temperature dependence of spin injection is almost identical to the temperature dependence of magnetization in the (Ga, Mn)As layer.
New implementation of the trajectory surface hopping method with use of the Zhu–Nakamura theory. II. Application to the charge transfer processes in the 3D DH2+ system
Authors: Zhu Chaoyuan , Kamisaka Hideyuki , Nakamura Hiroki
Published: July 2002

The newly implemented trajectory surface hopping (TSH) method for the collinear system with use of the Zhu–Nakamura semiclassical theory of nonadiabatic transition [C. Zhu, K. Nobusada, and H. Nakamura, J. Chem. Phys. 115, 3031 (2001)] is extended to treat 3D nonadiabatic reactions. Since the avoided crossing seam becomes a two-dimensional surface in the 3D system, the nonadiabatic transition region and the possibility of classically forbidden hops are enlarged very much in comparison with those in the collinear case. As a result, the contribution of the classically forbidden hops is quite a bit enhanced in the 3D system. Conservation of total angular momentum J is taken into account by slightly rotating the direction of momentum during the hop in the classically forbidden case. The method is tested by applying to the charge transfer processes in the 3D DH2+ system for J=0. Numerical results clearly demonstrate that the new TSH method works very well at all energies and for all initial vibrational states considered compared to the old TSH method based on the Landau–Zener formula. The significant discrepancy between the two TSH methods survives even at high collision energy and high vibrational states in contrast to the collinear case, indicating the importance of the classically forbidden hops in 3D systems. The new TSH method is considered to be a very promising method to deal with high dimensional nonadiabatic dynamics. It should also be noted that the new TSH method does not require any knowledge of nonadiabatic coupling and is based only on adiabatic potentials.
Tantalum ions produced by 1064 nm pulsed laser irradiation
Authors: Torrisi L. , Gammino S. , Andò L. , Làska L.
Published: July 2002

A Q-switched Nd:YAG (yttrium aluminum garnet) laser (1064 nm wavelength) with a 9 ns pulse width, 1–900 mJ pulse energy, and 0.5 mm2 target spot, is employed to irradiate tantalum targets in vacuum. The irradiation produces a strong etching of the metal and forms a plasma in front of the target. The plasma contains neutrals and ions with a high charge state and a wide energy distribution. Time-of-flight measurements are presented for the ionic production. A cylindrical electrostatic ion analyzer permits to measure the yield and the charge state of the emitted ions and to extrapolate the ion energy distribution as a function of the laser fluence in the range 10–100 J/cm2. The measurements indicate that at high laser fluence the tantalum charge state may reach 8+ and the maximum ion energy about 6 keV. The ion energy distribution is presented as a function of the charge state. It follows approximately a “shifted Maxwellian distribution.” A better theoretical approach has been further developed considering the Coulomb interactions occurring inside the plasma, which produce ion acceleration at velocities comparable with thermal ones, according to the so-called “shifted Maxwellian-Coulombian distribution.”
Symmetry breaking, bifurcations, quasiperiodicity, and chaos due to electric fields in a coupled cell model
Authors: Hasal P. , Merkin J. H.
Published: July 2002

A model for the asymmetric coupling of two oscillatory cells is considered. The coupling between the cells is both through diffusional exchange (symmetric) and through the electromigration of ionic reactant species from one cell to the other (asymmetric) in applied electric fields. The kinetics in each cell are the same and based on the Gray–Scott scheme. Without the electric field, only simple, stable dynamics are seen. The effect of the asymmetry (applying electric fields) is to create a wide variety of stable dynamics, multistability, multiperiodic oscillations, quasiperiodicity and chaos being observed, this complexity in response being more prevalent at weaker coupling rates and at weaker field strengths. The results are obtained using a standard dynamical systems continuation program, though asymptotic results are obtained for strong coupling rates and strong electric fields. These are seen to agree well with the numerically determined values in the appropriate parameter regimes.