bluuebunny/crossref_metadata_2025 huggingface dataset:
A time-dependent quantum dynamics study of the H2+CH3→H+CH4 reaction
Authors: Wang Dunyou
Published: November 2002

We present a time-dependent wave-packet propagation calculation for the H2+CH3→H+CH4 reaction in six degrees of freedom and for zero total angular momentum. Initial state selected reaction probabilities for different initial rotational–vibrational states are presented in this study. Excitation of the H2 stretch enhances the reaction probability, whereas the excitation of the CH3 umbrella mode has the opposite effect. The cumulative reaction probability (CRP) is obtained by summing over initial-state-selected reaction probabilities. The energy-shift approximation to account for the contribution of degrees of freedom missing in the six-dimensional calculation is employed to obtain an approximate full-dimensional CRP. Thermal rate constant is compared with different experiment results.
Three-center versus four-center elimination of haloethene: Internal energies of HCl and HF on photolysis of CF2CHCl at 193 nm determined with time-resolved Fourier-transform spectroscopy
Authors: Wu Chia-Yan , Chung Chao-Yu , Lee Yu-Chang , Lee Yuan-Pern
Published: November 2002

Following photodissociation of 2-chloro-1,1-difluoroethene CF2CHCl) at 193 nm, vibration–rotationally resolved emission spectra of HCl(v⩽3) and HF(v⩽4) in spectral regions 2000–2900 and 3050–4410 cm−1, respectively, are detected with a step-scan time-resolved Fourier-transform spectrometer. All vibrational levels of HCl and HF show Boltzmann-type rotational distributions. HCl has an average rotational energy of 23±4 kJ mol−1 and a vibrational energy of 25±5 kJ mol−1, whereas HF has an average rotational energy of 20±4 kJ mol−1 and a vibrational energy of 48±6 kJ mol−1. The observed internal energy distribution indicates that HCl is produced via the three-center (α,α), but HF via the four-center (α,β) elimination. A modified separate statistical ensemble model predicts an internal energy distribution of HCl slightly greater than experimental observation. A modified impulse model taking into account geometries and displacement vectors of transition states during bond breaking predicts satisfactorily the rotational excitation of HF produced from four-center elimination. Ratios of rate coefficients (0.87:0.13) predicted for three-center or four-center elimination channels based on Rice–Ramsperger–Kassel–Marcus theory are consistent with a branching ratio of 0.88:0.12 determined based on observed populations of HCl and HF, respectively. We also compare these experimental and theoretical results with those of photolysis of vinyl halides (CH2CHX, X=F, Cl, or Br) at 193 nm.
The flow around a torsionally oscillating sphere
Authors: Hollerbach R. , Wiener R. J. , Sullivan I. S. , Donnelly R. J. , Barenghi C. F.
Published: November 2002

We investigate, experimentally and numerically, the flow around a torsionally oscillating sphere. The oscillation frequency is sufficiently high that the thickness of the Stokes boundary layer is small compared with the radius of the sphere. In addition to this boundary layer the flow then consists of a radial jet of periodically fluctuating speed emanating from the equator of the sphere. As the oscillation amplitude is increased, these fluctuations gradually become more pronounced, until the faster portions of the jet overtake the slower ones, causing them to curl back on themselves to form vortex pairs. The experimental results show that even after the appearance of the vortices the flow remains predominantly axisymmetric, and also equatorially symmetric, for a distance considerably greater than one sphere radius away. A two-dimensional numerical code is therefore used to elucidate the precise details of the flow, with excellent agreement on the range of amplitudes over which the vortices and other structures gradually emerge, and on the variation of that range with frequency. The turbulent breakdown of the vortices at higher amplitudes is also studied experimentally, and a connection with previous results is suggested.
Rapid synthesis of stable ZnO quantum dots
Authors: Mädler L. , Stark W. J. , Pratsinis S. E.
Published: November 2002

Zinc oxide nanocrystallites down to 1.5 nm in diameter were made by spray combustion of Zn/Si precursors. These crystallites exhibit a quantum size effect: blueshift of light absorption with decreasing crystallite size. X-ray diffraction, high-resolution transmission electron microscopy, and nitrogen adsorption showed that the addition of controlled amounts of silica prevented the growth and stabilized the ZnO crystals. The blue shift of the ultraviolet-vis absorption edge with decreasing ZnO crystal size closely followed a correlation between optical band gap and crystallite size from the literature. The band-gap energy of these ZnO quantum dots increased with silica content in the spray and particles consistently. The as-prepared quantum dots were stable and did not require any post processing.
Phase transformation and semiconductor-metal transition in thin films of VO2 deposited by low-pressure metalorganic chemical vapor deposition
Authors: Sahana M. B. , Subbanna G. N. , Shivashankar S. A.
Published: November 2002

Thin films of the semiconducting, monoclinic vanadium dioxide, VO2(M) have been prepared on ordinary glass by two methods: directly by low-pressure metalorganic chemical vapor deposition (MOCVD), and by argon-annealing films of the VO2(B) phase deposited by MOCVD. The composition and microstructure of the films have been examined by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Films made predominantly of either the B or the M phase, as deposited, can only be obtained over a narrow range of deposition temperatures. At the lower end of this temperature range, the as-deposited films are strongly oriented, although the substrate is glass. This can be understood from the drive to minimize surface energy. Films of the B phase have a platelet morphology, which leads to an unusual microstructure at the lower-deposition temperatures. Those grown at ∼370 °C convert to the metallic, rutile (R) phase when annealed at 550 °C, whereas those deposited at 420 °C transform to the R phase only at 580 °C. (When cooled to room temperature, the annealed films convert reversibly from the R phase to the M phase.) Electron microscopy shows that annealing leads to disintegration of the single crystalline VO2(B) platelets into small crystallites of VO2(R), although the platelet morphology is retained. When the annealing temperature is relatively low, these crystallites are nanometer sized. At a higher-annealing temperature, the transformation leads to well-connected and similarly oriented large grains of VO2(R), enveloped in the original platelet. The semiconductor-metal transition near 68 °C leads to a large jump in resistivity in all the VO2(M) films, nearly as large as in epitaxial films on single-crystal substrates. When the annealed films contain well-connected large grains, the transition is very sharp. Even when preferred orientation is present, the transition is not as sharp in as-deposited VO2(M), because the crystallites are not densely packed as in annealed VO2(B). However, the high degree of orientation in these films leads to a narrow temperature hysteresis.
Inhomogeneous structure of penetrable spheres with bounded interactions
Authors: Kim Soon-Chul , Suh Soong-Hyuck
Published: November 2002

The density functional theory (DFT) based on the bridge density functional and the fundamental-measure theory (FMT) have been used to investigate the structural properties of one- and two-component penetrable spheres in a spherical pore. The Monte Carlo simulations have been carried out to compare with the theoretical results. The result shows that at low temperature the FMT functional is better than the DFT based on the bridge density functional and compares well with the computer simulations. At high temperature the DFT based on the bridge density functional is better than the FMT functional. These results suggest the reliable accuracy of the modified Verlet closure for the penetrable spheres at high temperature. However, the accuracy of both the FMT functional and the DFT based on the bridge density functional deteriorates if the packing fraction is increased.
Calculations of the near-wall thermophoretic force in rarefied gas flow
Authors: Gallis M. A. , Rader D. J. , Torczynski J. R.
Published: November 2002

The thermophoretic force on a near-wall, spherical particle in a rarefied, monatomic gas flow is calculated numerically. The rarefied gas flow is calculated with the Direct Simulation Monte Carlo (DSMC) method, which provides the molecular velocity distribution. The force is calculated from the molecular velocity distribution using a force Green’s function. Calculations are performed over a Knudsen-number range from 0.0475 to 4.75 using Maxwell and hard-sphere collision models. Results are presented for the thermophoresis parameter, ξ, a dimensionless quantity proportional to the thermophoretic force. The spatial profiles of ξ show a clear progression from free-molecular conditions (ξ is constant throughout the domain) to near-continuum conditions (ξ is constant in the interior but increases in the Knudsen layers). For near-continuum conditions, the DSMC calculations and Chapman–Enskog theory are in excellent agreement in the interior, suggesting that their velocity distributions are similar in this region. For all conditions examined, ξ lies between the continuum and free-molecular limits, which differ by only 10%. Moreover, the near-wall ξ values differ from the interior values by less than 5% for a fully diffuse wall, in sharp contrast with most previous studies. An approximate theory for the wall effect is presented that agrees reasonably well with the calculations.
Deriving thermophysical properties of liquid Nb by containerless electrostatic levitation
Authors: Sung Y. S. , Takeya H. , Togano K.
Published: November 2002

Containerless solidification of Nb was carried out by the electrostatic levitation process in which a bulk sample was electrostatically levitated and melted by laser irradiation in high vacuum. A t(T) function in a time-temperature profile was analytically determined to describe the radiative cooling curves of Nb in the liquid state to estimate the ratio of constant pressure specific heat to hemispherical total emissivity, cP(T)/εT(T), and further to derive εT(T) and cP(T). The cooling behavior of liquid Nb was found to be well described by an inverse biquadratic polynomial of t(T). With the analytical t(T) function, the derivative dt/dT was easily obtained and the cP(T)/εT(T) ratio was readily estimated from the radiative cooling curve. The estimated cP(T)/εT(T) and the derived εT(T) and cP(T) agreed well with the reference data in the literature. This implies the rationale behind the analytical approach taken in this study was reasonable and can be applied to estimating cP(T)/εT(T) and further deriving cP(T) or εT(T) of materials in the liquid state.
Electro-optical response of reflected light from freely suspended smectic liquid-crystal films
Authors: Nakano Kazuyuki , Ozaki Masanori , Yoshino Katsumi
Published: November 2002

We have studied the electro-optical response of reflected light from the surface of freely suspended liquid-crystal films (FSFs) in SmA, SmC*, and SmCA* phases. Upon application of an electric field parallel to FSFs, the film vibrational spectrum generally contains a second-harmonic component of the applied frequency. In the SmA phase, only a second-harmonic component appeared in the fast Fourier transform (FFT) spectra of the reflected light. In the SmC* phase, however, many other components also appeared in the FFT spectra of the reflected light. These complicated characteristics observed in the ferroelectric phase might be attributed to the contribution of the transient scattering mode. On the other hand, in the SmCA* phase, only the second-harmonic component appeared, similar to the case of the SmA phase. In the electric field high enough to cause an antiferroelectric–ferroelectric phase transition, however, many other components appear in the spectrum, in the same manner as that in the SmC* phase.
Properties of amorphous Al–Yb alloy coating for scanning near-field optical microscopy tips
Authors: Liang K. F. , Grütter P.
Published: November 2002

Pinholes due to grain boundaries are a major source of defects in coatings of scanning near-field optical probes. We found amorphous Al rich thin films to have superior properties when compared to conventional Al films: they are pinhole free and smoother than Al films. Al1−xYbx deposited by thermal coevaporation on glass and Si(100) substrates at room temperature was found to be amorphous in the range of 10<x<35 at. %. The optical and electrical properties of these films in the as-deposited and annealed state was studied. The morphologies of amorphous and crystallized films was investigated by atomic force microscopy and compared to pure Al films. For the optimal composition of Al88Yb12 the absence of grain boundaries in the amorphous phase leads to pinhole-free films that are also much smoother than pure Al films, even when the initially amorphous film is crystallized. Finally, we demonstrated deposition of Al–Yb coatings on scanning near-field optical microscopy fiber tips.
Origin of microwave noise from an n-channel metal–oxide–semiconductor field effect transistor
Authors: Pantisano Luigi , Cheung K. P.
Published: November 2002

The physics of noise is a complex subject. It is often difficult to clearly identify the physical origin of the observed noise. Electronic noise at microwave frequencies is technologically very important and has been extensively studied. While it is well known that many physical phenomena give rise to output current fluctuations (i.e., noise) in a metal–oxide–semiconductor field effect transistor (MOSFET), few physical phenomena have a time constant that can contribute in the microwave range. Current physical models of MOSFET microwave noise are all based on thermal agitation of electrons (thermal noise). However, what is the correct temperature (lattice or electron) to use in the noise calculation is an ongoing debate in the literature. All the modeling efforts have been using noise measured from pristine devices as a test for validity. In this work, we studied the n-MOSFET microwave noise as a function of electrical stress induced degradation. Our experiments thus introduced a new dimension in the noise behavior study. The results of our experiments cannot be explained by any of the current existing models. All existing models discounted flicker noise as being too small at microwave frequency. Our experimental results compel us to reexamine the validity of this common assumption. While we are not quite able to prove conclusively, our evidences are clearly leaning toward defect-induced fluctuation (flicker noise) as the origin of microwave noise in a n-MOSFET
Germanium diffusion in polysilicon emitters of SiGe heterojunction bipolar transistors fabricated by germanium implantation
Authors: Mitchell M. J. , Ashburn P. , Hemment P. L. F.
Published: November 2002

A study is made of germanium diffusion in polysilicon emitters of SiGe heterojunction bipolar transistors made by germanium implantation. Implanted Ge is found to diffuse from the single-crystal silicon substrate into deposited polysilicon emitter layers during rapid thermal anneal at 1045 °C. Measurements of germanium diffusivity in polycrystalline silicon are reported at temperatures between 800 and 900 °C and modeled by an Arrhenius relationship with a preexponential factor of Do=0.026±0.023 cm2/s and an activation energy of E=2.59±0.36 eV. The measured diffusivity in polycrystalline silicon is ≈104 times larger than that reported for single-crystal silicon. It is hypothesised that germanium diffusion in polysilicon occurs by diffusion along grain boundaries.
Morphology of KI Adsorbates on Si(111) and Si(100) Surfaces
Authors: Guo Hansheng , Souda R.
Published: November 2002

The growth of submonolayer KI on Si(111) and Si(100) surfaces at room temperature has been studied with ultrahigh vacuum scanning tunneling microscopy (STM). The STM results show that KI on these surfaces essentially follows island-growth with a reactive interface, and that the morphology of the KI adsorbates at submonolayer coverage critically depends on the atomic structure of the surfaces. On the Si(111) surface, KI initially (coverage below 0.4 monolayer) tends to coalesce into clusters of size smaller than the 7×7 unit-cell. With further deposition the clusters grow into well-defined islands. On the Si(100) surface, the KI initially shows dissociative adsorption; distinctive islands appear with coverages above 0.4 monolayer, preferentially clustering at the steps and growing into islands with less regularity of size and shape. This comparative study enables us to evaluate the effects of the adsorbate–substrate and adsorbate–adsorbate interactions on the adsorbing behavior and the morphological evolution of the KI/silicon systems.
Role of carbon in GaN
Authors: Seager C. H. , Wright A. F. , Yu J. , Götz W.
Published: November 2002

GaN samples, containing various concentrations of carbon and doped intentionally with silicon, have been grown heteroepitaxially on sapphire using metal–organic chemical-vapor deposition. These samples have been characterized by a variety of electrical and optical techniques, and the resulting experimental data are compared to density-functional-theory calculations of the formation energies and electronic states of substitutional and interstitial carbon in hexagonal GaN. We find that in samples where the silicon concentration exceeds that of carbon, carbon sits in the N substitutional site, acting as an acceptor and partially compensating the material. However, when carbon densities exceed those for Si, GaN becomes semi-insulating due to carbon occupation of both N and Ga substitutional lattice sites, and a new luminescence peak appears at ∼3 eV. Calculated formation energies of carbon in both sites are strong functions of both the Fermi level and growth stoichiometry. The former dependence gives rise to self-compensation when [C]>[Si] because the formation energy of the Ga substitutional configuration (the donor state) becomes equal to that of the N substitutional site, effectively pinning the Fermi level as it approaches midgap. Our results suggest that effective p-type doping of GaN can only be achieved under Ga-rich growth conditions.
Evolution of the electronic properties of small Nin− (n=1–100) clusters by photoelectron spectroscopy
Authors: Liu Shu-Rong , Zhai Hua-Jin , Wang Lai-Sheng
Published: November 2002

Photoelectron spectra of size-selected and relatively cold Nin− (n=1–100) clusters were obtained at three detachment photon energies; 355, 266, and 193 nm. The evolution of the electronic structure of Nin− clusters from molecular to bulklike behavior was systematically investigated. Well-resolved threshold peaks were observed for small Nin− clusters (n⩽9), beyond which a single broadband was observed due to the high electronic density of states at large cluster sizes. This spectral change coincides with a dramatic decrease of the magnetic moment in this size range. In addition, narrow and well-resolved spectral features were observed around n=13 and 55, consistent with high symmetry icosahedral structures proposed for these clusters. A sharp threshold peak was observed in the spectra of Ni19− and Ni23−, also evident of more symmetric cluster structures. The spectra of Ni3− measured at various photon energies suggested the existence of two isomers. The electron affinities of the Nin clusters were observed to follow the prediction of a metallic droplet model at large cluster sizes above n=10 and extrapolate to the bulk work function of Ni at infinite size.
Origin of the contrasting effects of oxygen reduction on the transport properties of La1−ΔMnO3+δ and La1−xMxMnO3+δ (M:Ca,Sr) thin films
Authors: Nakamura Keikichi , Ogawa Keiichi
Published: November 2002

The effects of oxygen reduction treatments on the transport properties of La1−ΔMnO3+δ and La1−xMxMnO3+δ thin films (M: Sr,Ca) have been investigated to understand the origin of the contrasting behaviors observed for the vacancy and Ca, Sr doped lanthanum manganites. It is found that the vacancy doped (La deficient) films with different doping levels (La deficient levels) showed a continuous change in the transport properties with increasing reduction of oxygen. The films reduced under PO2 of 1 Pa at 1173 K showed almost insulating behavior independent of the doping level, whereas the Sr doped films showed well-known Sr doping level dependent properties. The continuous change in the transport properties for the vacancy doped (La deficient) thin films suggests that the Mn4+ concentration of the films decreases continuously with the reduction of oxygen. This behavior is in contrast with that found in the Sr doped films in which the Mn4+ concentration approaches the value characteristics of the Sr doping level under the same reduction conditions. The origin of the contrasting effects of oxygen reduction is interpreted on the basis of the proposed defect model that vacancies generated by the La deficiency Δ are partially replaced with Mn ions to give both La and Mn site vacancies, which decrease continuously with decreasing excess oxygen δ.
A stable, single-chip, two-dimensional gradiometric device based on two superconducting quantum interference devices
Authors: Yokosawa Koichi , Suzuki Daisuke , Tsukada Keiji , Tsukamoto Akira
Published: November 2002

A planar gradiometric device for estimating current vectors in specimens, was fabricated as two orthogonal magnetic field gradiometers integrated on a 14.5-mm-square substrate. The gradiometers consist of superconducting quantum interference devices (SQUIDs) and pickup coils made from a low-critical-temperature superconductor. The two gradiometers (one for the x direction and one for the y direction) have exactly the same structure, and their gradiometric centers coincide at the center of the substrate. The gradiometric balance of each gradiometer is 1/2000–1/3700, the cross-talk ratio between the two gradiometers is less than 1% below a few hundred hertz, and their intrinsic noise is 2–3 pT/(m√Hz). The gradiometric configurations of both the pickup coils and the SQUIDs provide good gradiometric balance. The parallel-type gradiometric SQUIDs produced a simple yet effective structure, although the superconducting loops of such SQUIDs have conventionally been thought to lead to adverse effects in the presence of a changing magnetic field. Magnetic fields up to 1 mT, i.e., 20 times the geomagnetic field, applied to a zero-field cooled SQUID reduced the critical current, but did not affect the intrinsic noise. This result shows that the configuration of the SQUIDs kept the circulating current in the superconducting loop away from the Josephson junctions.
Shear layer oscillation along a perforated surface: A self-excited large-scale instability
Authors: Celik E. , Rockwell D.
Published: November 2002

Self-sustained oscillations of a free-shear flow, i.e., a jet, mixing layer, or wake, can arise from its impingement upon a downstream boundary; these oscillations are well documented. The present experiments demonstrate the existence of analogous oscillations for the case of a shear layer along a perforated plate. The spectrum of the pressure fluctuation at impingement shows sharply defined peaks. Moreover, the predominant frequency of oscillation exhibits a variation with either impingement length or inflow velocity that is remarkably similar to the case of the corresponding free-shear layer in absence of the perforated plate. Quantitative imaging yields patterns of large-scale vorticity concentrations and streamline topology, which have similar features for the shear layer along the perforated plate and the free-shear layer in absence of the plate. All of these observations indicate existence of a feedback mechanism for the shear layer along the perforated plate of finite length.
Origins of field enhancement in electron field emission from ion beam synthesized SiC layers
Authors: Tsang W. M. , Wong S. P. , Lindner J. K. N.
Published: November 2002

SiC layers were synthesized by high-dose carbon implantation into silicon. Their electron field emission properties were studied and correlated with results from atomic force microscopy (AFM) and conducting AFM measurements. It is clearly demonstrated that there are two types of field enhancement mechanisms responsible for the improvement of the electron field emission properties of these ion beam synthesized SiC layers. In the as-implanted samples, the local field enhancement effect is attributed to electrical inhomogeneity due to the existence of small conducting graphitic clusters embedded in the layer. On the other hand, in the annealed samples, the dominant field enhancement mechanism is attributed to a surface morphology effect due to the formation of small protrusion structures at the surface.
Intervalence band absorption loss coefficients of the active layer for InGaAs/InGaAsP multiple quantum well laser diodes
Authors: Kakimoto Syouichi
Published: November 2002

We have calculated the intervalence band absorption loss coefficients of the active layer for InGaAs/InGaAsP multiple quantum well laser diodes and InGaAsP bulk laser diodes. The calculated intervalence band absorption loss coefficients of the active layer for the multiple quantum well laser diodes are smaller than those for the bulk laser diodes at the same lasing wavelength, which coincides with the experimental data reported.
Elastic-strain mechanisms for the influence of temperature, magnetic field, and pressure on the resistive and magnetic properties of magnetic semiconductors
Authors: Polyakov P. I. , Kucherenko S. S.
Published: November 2002

An analysis is made of the results of studies of the variation of the resistivity of a bulk polycrystalline sample of La0.56Ca0.24Mn1.2O3 under the influence of temperature (T), pressure (P), and magnetic field (H) and a study of the variation of the magnetostriction in single-crystal LaMnO3 as a function of T and H. It is found that the peaks of the magnetoresistive, baroresistive, and baromagnetoresistive effects occur at the same temperature, which is independent of magnetic field and pressure and corresponds to the temperature Tms of the metal–semiconductor phase transition in the absence of magnetic field and pressure. “Cooling” and “heating” effects of pressure and magnetic field are detected, and an equivalency of the influence of T, P, and H on the resistivity of the polycrystalline sample and of T and H on the magnetostriction of the single-crystal LaMnO3 is observed. The linearity of the shifts of the Tms(P) and Tms(H) peaks in the resistive properties of La0.56Ca0.24Mn1.2O3 is demonstrated and also the linearity of Hg(T) in the magnetic properties for the example of the changes in the hysteresis of the magnetostriction in the LaMnO3 single crystal. The role of the regularities that obtain for an elastic-strain mechanism for the influence of T, P, and H on the magnetic and resistive properties and phase states is revealed and explained. The sign-varying nature of the influence of T, P, and H is established, and its role in the variation of the resistive and magnetic properties is found (magnetic phase transitions). The relationship of the structural, elastic, resistive, and magnetic properties in magnetic semiconductors is established.
Effects of chaotic local crystal fields in pseudobinary rare-earth intermetallides
Authors: Ermolenko A. S.
Published: November 2002

Works on the influence of local chaotic crystal fields on the magnetic properties of pseudobinary rare-earth intermetallides are briefly reviewed. Attention is focused primarily on compounds of the type RNi5−xMx, M=Cu, Al. Experimentally observed effects of local crystal fields, such as, the appearance of ferromagnetism in the system Pr(Ni, Cu)5 and a decrease of the spontaneous magnetic moment and an increase in the anisotropy energy in the basal plane and coercive force in the intermediate pseudobinary alloys R(Ni, Cu)5 and R(Ni, Al)5 as compared with the limiting binary compounds, are discussed.
Magnetic field induced phase transition in KEr(MoO4)2. Vibronic model
Authors: Loginov A. A.
Published: November 2002

A vibronic model with Kramers magnetic ions is investigated in a parameter region that does not admit phase transitions on changes of the temperature T in the absence of magnetic field H. The H–T (θ=0) and θ–H (T=const) phase diagrams (the angle θ specifies the orientation of the field relative to the symmetry axis of the crystal) are constructed in the mean field approximation. A comparison is made with the experimentally obtained phase diagrams of the KEr(MoO4)2 crystal.
Solitons in elastic plates
Authors: Kovalev A. S. , Sokolova E. S. , Mayer A. P. , Éckl’ C.
Published: November 2002

The nonlinear dynamics of elastic shear waves is investigated taking into account the interaction of the shear component of the displacements with low-amplitude sagittal components. Nonlinear evolutionary equations are derived for the shear-displacement field. These equations contain additional nonlinear dispersion terms due to the interaction with displacements in the sagittal plane. The soliton solutions of the equations obtained are studied and the possibility of the existence of exotic solitons—compactons and peakons—is discussed.
Splashing impact of a spray onto a liquid film
Authors: Sivakumar D. , Tropea C.
Published: November 2002

The impact of single droplets within a spray onto a thin liquid film produced by this spray has been studied using direct visualization and the phase Doppler technique. The temporal evolution of the crown radius and its height has been compared with results corresponding to the impact of an isolated drop onto an undisturbed liquid film. Significant differences in crown formation indicate to what extent interactions between neighboring drop impacts can affect the resulting secondary spray. The results of this study have significance for future modeling of spray impact.
Experimental and theoretical study of third-order harmonic generation in carbon nanotubes
Authors: Stanciu C. , Ehlich R. , Petrov V. , Steinkellner O. , Herrmann J. , Hertel I. V. , Slepyan G. Ya. , Khrutchinski A. A. , Maksimenko S. A. , Rotermund F. , Campbell E. E. B. , Rohmund F.
Published: November 2002

Third-harmonic generation from solid samples of carbon nanotubes has been studied experimentally, using ultrashort pulses generated by a Cr:Forsterite laser, at a wavelength of 1250 nm. The results show an unusual nonperturbative behavior of the third-harmonic yield, for relatively low input laser fields, of ∼1010 W/cm2. This strong nonlinearity of the laser interaction with carbon nanoubes is also confirmed theoretically, in a full quantum-mechanical theory for harmonics generation from a single-walled carbon nanotube.
Computational design of a material for high-efficiency spin-polarized electron source
Authors: Janotti A. , Wei Su-Huai
Published: November 2002

The search for the highest quality and efficient spin-polarized electron source (SPES) is of fundamental importance in the investigation of spin-dependent phenomena in atomic, condensed-matter, nuclear, and particle physics. We propose a material, AgGaSe2 in the CuAu phase, as a high-quality SPES. We show that AgGaSe2 in the CuAu phase has an “ideal” band structure that allows the generation of electron beams with 100% polarization: a direct band gap with large and positive spin-orbit splitting and crystal-field splitting. Moreover, we predict that strain-free AgGaSe2 in CuAu phase can be obtained by epitaxial growth on ZnSe substrate.
Formation of the photoluminescence Cu center on in-diffusion and out-diffusion of Cu in dilute Cu-contaminated silicon crystals
Authors: Nakamura Minoru
Published: November 2002

Changes of photoluminescence (PL) intensity of the 1.014 eV Cu center (CuPL) with in- and out-diffusion of Cu in dilute Cu-contaminated silicon crystals (∼1013 atom/cm3) were observed. The intensity of CuPL increased with in-diffusion temperature of Cu to 700 °C and then decreased with increasing temperature above 800 °C for a short in-diffusion time. The formation barrier for CuPL (0.65 eV) obtained from the in-diffusion process of Cu below 600 °C was much smaller than the known effective formation enthalpy of an interstitial solution of Cu in silicon from Cu metal plated on silicon (1.5 eV), i.e., CuPL was formed more easily for a dilute Cu-contaminated sample than for a thickly Cu-plated sample. The decrease of the PL intensity of CuPL above 800 °C is attributed to the formation of another, more stable species than CuPL. A part of the in-diffused Cu out-diffused at room temperature after removing the surface oxide of the crystal for p-type crystals. Little change of CuPL intensity occurred on out-diffusion of Cu, indicating that CuPL and the out-diffusing species differed from each other. The changes of CuPL intensity on in- and out-diffusion processes of Cu were reasonably explained by assuming participation of several stable Cu species (at least three) in the silicon crystal.
Variation in material parameters for hexagonal barium ferrite films on (111) magnesium oxide as a function of oxygen pressure
Authors: Yoon S. D. , Vittoria C. , Oliver S. A.
Published: November 2002

This work examines the material characteristics and magnetic properties of hexagonal M-type barium ferrite (BaFe12O19) films deposited by pulsed laser ablation deposition onto (111) magnesium oxide substrates at different oxygen growth pressures. X-ray diffraction data showed the lattice constant of the film grown at 300 mTorr c=23.23 Å increased to c=23.30 Å for the film grown at 20 mTorr. The saturation magnetization ranged from 4.03 to 4.33 kG and the magnetic uniaxial anisotropy field ranged from 16.73 to 19.49 kOe as the oxygen pressure was varied from 10 to 300 mTorr. The narrowest ferrimagnetic resonance (FMR) linewidths (ΔH) for as-produced films were obtained for the 10 mTorr films, where ΔH=0.39 kOe. Films grown with oxygen pressure at 200 mTorr or higher showed two distinct resonance modes, which exhibited ΔH1⩾0.90 kOe and ΔH2⩾1.10 kOe at 59 GHz for a film deposited at 925 °C. The FMR linewidths for this film decreased to ΔH1=0.52 kOe and ΔH2=0.45 kOe after annealing the film at 1000 °C for 1 h. These results indicate that thick films grown at higher oxygen pressures have magnetic parameters closer to bulk values, but also show larger magnetic losses than films grown at lower oxygen pressures.
Diffusion mechanism of cation-exchange process for fabrication of HgBa2CaCu2O6+δ superconducting films
Authors: Xie Y. Y. , Wu J. Z. , Aytug T. , Christen D. K. , Cardona A. H.
Published: November 2002

In a cation-exchange process, epitaxial Hg-based high-temperature superconducting (HTS) films are obtained from epitaxial Tl-based HTS precursor films by diffusing Tl cations out of, and Hg cations into the crystal lattice. In order to understand the cation diffusion mechanism, we have carried out a systematic study on conversion of Tl2Ba2CaCu2O8 precursor films to HgBa2CaCu2O6 films. Such a conversion was found to be composed of two steps. In the first step, Tl2Ba2CaCu2O8 collapses structurally into the (Tl,Hg)Ba2CaCu2O6 lattice, while in the second step, Tl cations diffuse out while Hg cations diffuse in, resulting in pure HgBa2CaCu2O6 phase. A time constant τ1 of about 45–60 min was found for the first stage, while the second stage τ2 is beyond 2τ1. This provides potential to optimize the cation exchange processing parameters for Hg-based HTS films and devices.
Infrared-to-violet upconversion from Yb3+ and Er3+ codoped amorphous fluoride film prepared by pulsed laser deposition
Authors: Qin Guanshi , Qin Weiping , Huang Shihua , Wu Changfeng , Zhao Dan , Chen Baojiu , Lu Shaozhe , Shulin E.
Published: November 2002

Yb 3+ – Er 3+ codoped amorphous Zn0.3Al0.25Pb0.3Li0.09Yb0.1Er0.01F2.37 film has been prepared by pulsed-laser deposition. Violet, green, red, and ultraviolet upconversion emissions were observed under infrared excitation at 988 nm. In comparison with the upconversion of the target, the violet and ultraviolet emissions are enhanced greatly. The enhancement is attributed to the diminishing of the cross relaxation: G11/24→4F9/2 (Er3+): F7/22→2F5/2 (Yb3+) induced by the decrement of the structure parameter Ω2.
Adhesion force of polymeric three-dimensional microstructures fabricated by microstereolithography
Authors: Wu Dongmin , Fang Nicholas , Sun Cheng , Zhang Xiang
Published: November 2002

The adhesion between microstructures represents a great challenge in reliability of polymeric three-dimensional structures fabricated by microstereolithography (μSL). During the evaporative releasing, the capillary force of the solvent causes the deformation and adhesion of the fabricated beams. We present a method to determine the adhesion force of polymeric microstructures fabricated by μSL. The test structures with parallel beams were fabricated and released from the liquid resin via evaporation. By measuring the relationship between the adhesion length and the geometry of the beams, the adhesion force between two 1,6-hexanediol diacrylate (HDDA) polymeric parallel beams is determined as γ=72±5 mN/m. This simple method and the determined adhesion force provide a key in designing reliable polymeric microelectromechanical systems in preventing the stiction problem.
Temperature dependence of ferroelectric and dielectric properties of PbZr0.5Ti0.5O3 thin film based capacitors
Authors: Meng X. J. , Sun J. L. , Wang X. G. , Lin T. , Ma J. H. , Guo S. L. , Chu J. H.
Published: November 2002

The temperature dependence of the ferroelectric and dielectric properties of PbZr0.5Ti0.5O3 thin films deposited on LaNiO3-coated SrTiO3 substrate was investigated. The results showed that both the saturation polarization and remanent polarization increased with decreasing temperature from 300 to ∼50 K, and decreased as the temperature continued to decrease below 40 K. The capacitance of the PbZr0.5Ti0.5O3 ferroelectric thin film capacitor as a function of small ac field and temperature was measured, and the data were processed using Rayleigh law. It was demonstrated that both the reversible and irreversible contributions to the dielectric constant decreased with decreasing temperature; however, they showed an increase when the temperature dropped below 50 K. The anomalous behavior of the temperature dependence may be attributed to a phase transition in the PbZr0.5Ti0.5O3 thin film in the vicinity of 50 K.
Relationships between macroscopic polarization hysteresis and local piezoresponse of fatigued Pb(Zr,Ti)O3 films within a Landau theory-based lattice model
Authors: Ricinschi Dan , Okuyama Masanori
Published: November 2002

Characterization of the fatigued state in Pb(Zr,Ti)O3 (PZT) films has been carried out by nanoscale piezoelectric displacements measured simultaneously with the macroscopic polarization hysteresis. For explaining the fatigue peculiarities of PZT films prepared by sol-gel and rf sputtering, a Landau theory-type model based on formation of degraded regions in a ferroelectric lattice has been used. The relevance of local piezoelectric loops for the macroscopic polarization hysteresis has been assessed and the possibility of recovering the latter as a weighted convolution of the former has been shown to depend on the film quality.
High remanent polarization in Sr1−xCaxBi2Ta2O9 ferroelectric thin films
Authors: Das Rasmi R. , Bhattacharya P. , Pérez W. , Katiyar Ram S.
Published: November 2002

SrBi 2 Ta 2 O 9 (SBT) thin films with different concentrations of Ca at the Sr-site were grown on platinized silicon substrates. A systematic shift of diffraction lines towards higher diffraction angle confirmed the decrease in lattice parameter and attributed to the smaller ionic radii of Ca than for Sr. The grain size of the films was found to be increased upon Ca incorporation in SBT. The dielectric constant of the SBT films was systematically decreased with increasing Ca contents and it was attributed to lower dielectric permittivity of CaBi2Ta2O9 system. The ferroelectric properties were gradually increased up to 25% Ca at the Sr site, above which the ferroelectric properties started degrading. The maximum value of remanent polarization (∼23.8 μC/cm2) of the films was obtained at 20% Ca substituted SBT. The systematic increase in coercive field was attributed to the higher electronegativity of Ca. The films showed minimal fatigue degradation (<23%) after 1010 switching cycles. Substitution of Ca (up to 25%) in SBT thin films did not show any pronounced influence on leakage current characteristics.
Robustness to optical feedback of oxide-confined versus proton-implanted vertical-cavity surface-emitting lasers
Authors: Judge P. A. , Quay C. H. L. , Hudgings J. A.
Published: November 2002

We show that the coupling efficiency of optical feedback into a single-mode oxide-confined vertical-cavity surface-emitting laser (VCSEL) is half that of a similar proton-implanted VCSEL, so the oxide-confined devices are less sensitive to feedback-induced variations in threshold current and lasing wavelength. However, strong optical feedback causes coherence collapse in both types of lasers and can also result in satellite modes and coherence length discontinuities in oxide-confined VCSELs. While polarized feedback tends to suppress polarization-switching in oxide-confined devices, the opposite effect is observed in proton-implanted VCSELs.
Boltzmann theory of engineered anisotropic magnetoresistance in (Ga,Mn)As
Authors: Jungwirth T. , Abolfath M. , Sinova Jairo , Kučera J. , MacDonald A. H.
Published: November 2002

We report on a theoretical study of dc transport coefficients in (Ga,Mn)As diluted magnetic semiconductor ferromagnets that accounts for quasiparticle scattering from ionized Mn2+ acceptors with a local moment S=5/2 and from nonmagnetic compensating defects. In metallic samples Boltzmann transport theory with golden rule scattering rates accounts for the principle trends of the measured difference between resistances for magnetizations parallel and perpendicular to the current. We predict that the sign and magnitude of the anisotropic magnetoresistance can be changed by strain engineering or by altering chemical composition.
Using waviness to reduce thermal warpage in printed circuit boards
Authors: Grenestedt Joachim L. , Hutapea Parsaoran
Published: November 2002

Thermal deformations in printed circuit boards (PCB) were reduced by using wavy rather than straight electric lines or traces. The traces had either in-plane or out-of-plane waviness. Thermal deformations of a few different models were calculated using the finite element (FE) method. Periodic unit cell FE models were developed, and subjected to temperature changes. In-plane trace waviness was seen to reduce the thermal warpage of the PCB by ∼40%. Out-of-plane waviness also reduced the warpage, but to a lesser extent.
Indium supply from triisopropylindium onto a GaAs(001) surface at room temperature
Authors: Kuramochi H. , Cui J. , Ohtake A. , Ozeki M. , Kishida M. , Uchida H. , Akinaga H.
Published: November 2002

The adsorbates derived from triisopropylindium (TIPIn) on a GaAs(001)-2×4 surface were studied by scanning tunneling microscopy and spectroscopy. TIPIn was adsorbed on a GaAs(001)-2×4 surface dissociatively at room temperature, the characteristic spectrum was detected from each adsorbate of different size. The differences among spectra are discussed qualitatively from energy calculation and comparative experiments, and the change of the electronic states according to size fluctuation is explained by the change of constituents of adsorbates. The smallest adsorbate assigned as In atoms(s) by comparing the electronic states with deposited indium. The mechanism of InAs quantum dots fabrication by the supersonic molecular-beam method is also confirmed at atomic level.
An ab initio investigation on aromaticities in semiconductor systems
Authors: Li Si-Dian , Yu Hong-Lang , Wu Hai-Shun , Jin Zhi-Hao
Published: November 2002

Second-order Moller–Plesset perturbation theory investigations indicate that the highly strained three- and four-membered semiconductor ring systems A3R3+, A2BR3+, GeSiCR3+ A3R3Ha, and A4R42+ (A,B=C,Si,Ge; R=H, SiH3; Ha=F, Cl, Br) exhibit aromaticities featured with the delocalized π orbitals perpendicular to the ring planes.
Acoustic anomalies and central peak in SrBi2Ta2O9 single crystals studied by micro-Brillouin scattering
Authors: Ko Jae-Hyeon , Hushur Anwar , Kojima Seiji , Sih Bryan C. , Ye Zuo-Guang
Published: November 2002

Acoustic properties of layered perovskite SrBi2Ta2O9 (SBT) single crystals were investigated by micro-Brillouin scattering. Two elastic stiffness coefficients, c33 and c44, defined within the pseudotetragonal crystallographic axes, and the corresponding sound velocities were determined in the temperature range between 290 and 773 K. The longitudinal sound velocity propagating along the pseudotetragonal c axis was found to be very small, which can be related to the weak interlayer bonding of SBT. The corresponding c33 shows marked softening on approaching the ferroelectric phase transition temperature Tc around 600 K from both high- and low-temperature sides. The hypersonic attenuation increases markedly around Tc, which seems to reflect not only the bilinear coupling between the order parameter and the strain in the ferroelectric phase, but also the electrostrictive coupling in the paraelectric phase. Furthermore, a quasielastic central component was observed near Tc, which is believed to be partly responsible for the change of the low-frequency dielectric permittivity at Tc, besides the contributions from the soft mode and the low-frequency dielectric dispersion.
Visible emission from N-rich turbostratic boron nitride thin films doped with Eu, Tb, and Tm
Authors: Liu Q. L. , Xu F. F. , Tanaka T.
Published: November 2002

Red, green, and blue light emissions have been obtained at 80 and 300 K by cathodoluminescence (CL) from N-rich turbostratic BN thin films doped with Eu, Tb, and Tm, respectively. The films were grown by rf magnetron sputtering in an atmosphere of Ar and NH3 gas mixture using EuB6, TbB6, and TmB12 targets, respectively. Infrared absorption measurements, transmission electron microscopy, and electron probe microanalysis show that the matrix of the films is N-rich turbostratic BN. The sharp characteristic emission lines corresponding to Eu3+, Tb3+, and Tm3+ intra-4fn shell transitions are resolved in the CL spectral range from 350 to 800 nm at 80 and 300 K.
Amorphous structures of Cu, Ag, and Au nanoclusters from first principles calculations
Authors: Oviedo J. , Palmer R. E.
Published: November 2002

We have carried out first-principles density functional calculations for clusters of the coinage metals containing thirteen atoms (M13, where M=Cu, Ag, or Au). We find that for this geometric “magic number” the low energy isomers are actually disordered, forming almost a continuous distribution as a function of energy.
Genetic Spot Optimization for Peak Power Estimation in Large VLSI Circuits
Authors: Hsiao Michael S.
Published: November 2002

Estimating peak power involves optimization of the circuit′s switching function. The switching of a given gate is not only dependent on the output capacitance of the node, but also heavily dependent on the gate delays in the circuit, since multiple switching events can result from uneven circuit delay paths in the circuit. Genetic spot expansion and optimization are proposed in this paper to estimate tight peak power bounds for large sequential circuits. The optimization spot shifts and expands dynamically based on the maximum power potential (MPP) of the nodes under optimization. Four genetic spot optimization heuristics are studied for sequential circuits. Experimental results showed an average of 70.7% tighter peak power bounds for large sequential benchmark circuits was achieved in short execution times.