bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical study of cavitation inception in the near field of an axisymmetric jet at high Reynolds number
Authors: Cerutti Stefano , Knio Omar M. , Katz Joseph
Published: July 2002

Cavitation inception in the near field of high Reynolds number axisymmetric jets is analyzed using a simplified computational model. The model combines a vorticity–stream-function finite-difference scheme for the simulation of the unsteady flow field with a simplified representation for microscopic bubbles that are injected at the jet inlet. The motion of the bubbles is tracked in a Lagrangian reference frame by integrating a semiempirical dynamical equation which accounts for pressure, drag, and lift forces. The likelihood of cavitation inception is estimated based on the distributions of pressure and microscopic bubbles. The computations are used to examine the role of jet slenderness ratio, Reynolds number, bubble size, and bubble injection location on the cavitation inception indices. The results indicate that, for all bubble sizes considered, the cavitation inception index increases as the jet slenderness ratio decreases. Larger bubbles entrain more rapidly into the cores of concentrated vortices than smaller bubbles, and the corresponding inception indices are generally higher than those of smaller bubbles. The inception indices for larger bubbles are insensitive to the injection location, while the inception indices of smaller bubbles tend to increase when they are injected inside the shear layer near the nozzle lip. Although it affects the bubble distributions, variation of the Reynolds number leads to insignificant changes in pressure minima and in the inception indices of larger bubbles, having noticeable effect only on the inception indices of smaller bubbles. Computed results are consistent with, and provide plausible explanations for, several trends observed in recent jet cavitation experiments.
Analytical study of bifurcation of a flow of a gas between coaxial circular cylinders with evaporation and condensation
Authors: Sone Yoshio , Doi Toshiyuki
Published: July 2002

Time-independent behavior of a gas between two coaxial circular cylinders made of the condensed phase of the gas, where the cylinders are rotating around their common axis and evaporation or condensation is taking place, is considered with special attention given to bifurcation of the flow. The problem is studied analytically for small values of the speeds of rotation of the cylinders and the Knudsen number on the basis of the Boltzmann equation, and the solution is obtained explicitly. The bifurcation of flow occurs even in a simple case where the gas is axially symmetric and uniform (or the flow field depends only on the radial coordinate). The comprehensive feature of the bifurcation of flow is clarified with the explicit forms of solutions and the bifurcation diagram.
Ferroelectric properties of alkoxy-derived CaBi2Ta2O9 thin films
Authors: Kato Kazumi , Suzuki Kazuyuki , Nishizawa Kaori , Miki Takeshi
Published: July 2002

CaBi 2 Ta 2 O 9 thin films were deposited on Pt-passivated quartz glass substrates. The 750 °C-annealed thin film was a single phase of layer-structured perovskite and showed random orientation. The thin film exhibited a P–E hysteresis loop. The remanent polarization (Pr) and coercive electric field (Ec) at 13 V were 6 μC/cm2 and 160 kV/cm, respectively. The polarization did not show fatigue after 2×1010 switching cycles.
The use of pressure controlled Fabry-Pérot interferometer with linear scanning of data for Brillouin-type experiments
Authors: Błachowicz Tomasz
Published: July 2002

The article presents results from work with Fabry-Pérot interferometers in Brillouin laser light scattering experiments, where optical signals of very low level intensity are observed. The information presented here can be useful in other types of optical experiments where scanning in the Fabry-Pérot interferometer spectral range has to be used. In such situations the shape of spectral lines as well as their relative distances can be detected. The key to the solution presented here is the use of a silicon-membrane pressure sensor coupled to a pressure chamber. It makes it possible to view spectral lines equally spaced after nonlinear flow of air from a chamber where the Fabry-Pérot interferometer is placed. Linear scanning in the spectral range equal to a frequency of about 150 GHz is possible. The method can be applied to Fabry-Pérot’s etalons, very frequently produced some years ago. Now it should find new fields of application, in a simple and cost effective way, in student laboratories as well as in other research institutions.
A medium-frequency interferometer for studying auroral radio emissions
Authors: Hughes J. M. , LaBelle J. , Trimpi M. L.
Published: July 2002

An interferometer for use between 2.5 and 3.0 MHz has recently been developed for the purpose of studying auroral radio emissions. The instrument consists of an array of 17 antennas, associated electronics for amplitude and phase measurements, and computer hardware for instrument control and data recording. In its standard operational mode, the instrument sweeps from 2.5 to 3.0 MHz in 1 kHz steps every 1.5 s. The intensity of the received signal and the phase at each antenna is measured for each 1-kHz-wide bin. These data can be used to produce spectrograms showing the intensity of the received signal versus frequency and time and for determining the direction of arrival for any signal in the instrument bandwidth.
High-resolution differential optical reflectivity measurements of freestanding liquid-crystal films
Authors: Pankratz S. , Johnson P. M. , Huang C. C.
Published: July 2002

A differential optical reflectivity measurement technique for freestanding films is presented. The system is highly sensitive to changes in the polarization state of the reflected light. The method is used to study the evolution of an optically anisotropic liquid crystalline structure with temperature. In addition we present a reflectivity method for determining the film thickness for films consisting of two to hundreds of layers.
Submicrometer resolution Yablonovite templates fabricated by x-ray lithography
Authors: Cuisin C. , Chelnokov A. , Lourtioz J.-M. , Decanini D. , Chen Y.
Published: July 2002

We report the fabrication and optical characterization of diamond-like photonic structures with 1.3 μm lattice constants. In analogy with the first Yablonovite obtained by a mechanical drilling of a dielectric material, the structures are fabricated in poly(methylmethacrylate) resist using three consecutive exposures to an x-ray beam through a triangular lattice of holes. Up to six crystal periods are obtained in a 6.2-μm-thick resist. The measured reflection and transmission spectra show well contrasted photonic gaps in agreement with numerical simulations. This demonstrates the good optical quality of the structures that can be used as porous templates for transferring the diamond-like pattern to high-refractive-index dielectrics or metals.
H
1
spin dynamics obtained from off-resonant decoupling in a C13-NMR signal
Authors: Levstein Patricia R. , Usaj Gonzalo , Pastawski Horacio M. , Raya Jésus , Hirschinger Jérôme
Published: July 2002

We extract the exact on-resonance position and local dynamical information of the H1 network from a dramatic variation of the peak amplitude of C13 signals as a function of the carrier frequency of the decoupling field for H1. Its Lorentzian dependence is explained within a simple theory.
On the theory of electron transfer reactions at semiconductor/liquid interfaces. II. A free electron model
Authors: Gao Yi Qin , Marcus R. A.
Published: July 2002

Electron transfer reactions at semiconductor/liquid interfaces are studied using the Fermi Golden rule and a free electron model for the semiconductor and the redox molecule. Bardeen’s method is adapted to calculate the coupling matrix element between the molecular and semiconductor electronic states where the effective electron mass in the semiconductor need not equal the actual electron mass. The calculated maximum electron transfer rate constants are compared with the experimental results as well as with the theoretical results obtained in Part I using tight-binding calculations. The results, which are analytic for an s-electron in the redox agent and reduced to a quadrature for pz- and dz2-electrons, add to the insight of the earlier calculations.
Optical solitons: Perspectives and applications
Authors: Ablowitz Mark J. , Biondini Gino , Ostrovsky Lev A.
Published: July 2002

This article serves as an introduction to the focus issue on optical solitons. After a short review of the history of solitons and the field of integrable systems, a brief overview of the development of nonlinear optics and optical solitons is provided. Next, the various contributions to this focus issue are presented, and a few separate remarks are devoted to optical communications, where solitons promise to play a decisive role in the next generation of commercial systems.
Direct-coupled-cavity bandpass filters using scrounged rectangular-to-circular waveguide transitions
Authors: Zelazny S. A. , Koch P. M.
Published: July 2002

We describe the design procedure for and realization of microwave bandpass filters that use two direct-coupled cylindrical cavities. Use of rectangular-to-circular waveguide transitions scrounged from commercial waveguide variable attenuators solved two problems: how to mate the filters to our microwave system that used rectangular waveguide and how to realize filters cheaply and quickly that could be used at different frequencies by changing only a small number of easily fabricated parts.
GaN light emitting diode as a photoreflectance pump source
Authors: Gaskill D. K. , Holm R. T. , Glembocki O. J.
Published: July 2002

A GaN light emitting diode is used as a photoreflectance pump source to acquire optical impedance spectroscopy data. Such a pump source has the advantage of having a controllable pump wave form (intensity, modulation depth, and shape) over a broad frequency range. Given the ready availability of light emitting diodes, many different wavelengths are potentially available.
Magnetoresistance of self-assembled lateral multilayers
Authors: Tober E. D. , Marks R. F. , Chambliss D. D. , Roche K. P. , Toney M. F. , Kellock A. J. , Farrow R. F. C.
Published: July 2002

The angular-dependent magnetoresistance and magnetization of epitaxial FeηAg1−η self-assembled lateral multilayers (SALMs) grown on Mo(110)/Al2O3(11-2 0) template layers has been examined for an optimum stoichiometry of η=0.38. The low-temperature anisotropic magnetoresistance (AMR) and low-field magnetoresistance (MR) are measured as a function of field angle for two nearly orthogonal current directions. The SALM structures are observed to display a significant AMR (roughly 1.6% maximum for the entire structure) and a pronounced field-dependent MR with a maximum ΔR/R of 0.88% at 2.7 K.
A simple approach for calibrating solid-state imaging systems
Authors: Leslie Fred W. , Cha Soyoung S. , Ramachandran Narayanan
Published: July 2002

Imaging by solid-state array sensors has become an indispensable tool in various modern scientific and engineering applications. In such instrumentation, a linear response of the sensor output to the incident light intensity is often very much desirable. The system linearity can be affected by both the sensing element and electronics. The linearity also needs to be accurately determined in order to assess and correct measurement errors. Some imaging systems, especially those of upper grade with high resolution, allow precise control of exposure time. If so, it is easy to check the linearity in response to the incident energy with changes of exposure time under a constant intensity of illuminating light. For imaging systems without precise time control, especially those of intermediate- or lower-grade, it is highly desirable to have some means for assessing linearity and applying corrections to improve accuracy for photometric uses. Here, we present two simple approaches, one for quickly checking linearity and the other for correcting the response, for those imaging systems without exact exposure-time control. The former allows a quick assessment of the linearity while the latter permits an accurate calibration with minimal hardware preparation. In initial testing, the calibration method works well with an intermediate-grade sensor interfaced to a frame grabber. The nonlinearity of the imaging system tested can be reduced from about 5.1% to much less than 1% in standard deviation.
Anomalous positive charge trapping in thin nitrided oxides under high-field impulse stressing
Authors: Lim P. S. , Chim W. K.
Published: July 2002

An anomalously high density of positive trapped charges was observed in thin (43 Å) nitrided gate oxides subjected to high-field impulse stressing. The hot-hole generation occurs via a regenerative feedback mechanism, with minimal charge relaxation due to the short duration of the impulse stress. This gives rise to an extremely high density of trapped holes that were not observed under direct current stressing conditions. The trapped holes can be easily annealed electrically at room temperature and the annihilation of positive trapped charges is accompanied by a higher number of interface states being created. A better understanding on thin oxide degradation under impulse stressing can help in the choice of a suitable programing/erasing pulse width/amplitude for use in endurance testing of nonvolatile memories.
Scaling properties of three-dimensional isotropic magnetohydrodynamic turbulence
Authors: Biskamp Dieter , Müller Wolf-Christian
Published: July 2002

A comprehensive picture of three-dimensional (3D) isotropic magnetohydrodynamic (MHD) turbulence is presented based on the first 5123-mode numerical simulations performed. Both temporal and spatial scaling properties are studied. For finite magnetic helicity H the energy decay is governed by the constancy of H and the decrease of the ratio of kinetic and magnetic energy Γ=EK/EM. A simple model consistent with a series of simulation runs predicts the asymptotic decay laws E∼t−1/2, EK∼t−1. For nonhelical MHD turbulence, H≃0, the energy decays faster, E∼t−1. The energy spectrum follows a k−5/3 law, clearly steeper than k−3/2 previously found in 2D MHD turbulence. The scaling exponents of the structure functions are consistent with a modified She–Leveque model ζpMHD=p/9+1−(1/3)p/3, which corresponds to a basic Kolmogorov scaling and sheet-like dissipative structures. The difference between the 3D and the 2D behavior can be related to the eddy dynamics in 3D and 2D hydrodynamic turbulence.
Comment on “Numerical study of electrical transport in homogeneous Schottky diodes” [J. Appl. Phys. 85, 1935 (1999)]
Authors: Tung R. T.
Published: July 2002

In a recent article [J. Appl. Phys. 85, 1935 (1999)], Osvald simulated forward and reverse current–voltage and capacitance–voltage characteristics of inhomogeneous Schottky barrier (SB) diodes and concluded that the currents flowing in interacting and noninteracting inhomogeneous SBs were largely identical. This Comment points out the inappropriateness of some of the conditions chosen for these simulations which likely has rendered that conclusion untenable.
Electro-optic effect of periodically poled optical superlattice LiNbO3 and its applications
Authors: Lu Yan-Qing , Wan Zhi-Liang , Wang Quan , Xi Yuan-Xin , Ming Nai-Ben
Published: July 2002

The electro-optic effect of periodically poled optical superlattice LiNbO3 (PPLN) was studied. Because of the periodic electro-optic (EO) coefficient, the reciprocal vector of the periodic structure can be used to compensate for the phase mismatch between the ordinary and extraordinary waves, which is similar to the nonlinear optical frequency conversion process. If the quasi-phase-matching condition is satisfied, polarization of a light propagated in PPLN can rotate linearly with the applied electric field, which shows that PPLN may be used as a precise spectral filter or an EO switch.
Unified analytic model of direct and Fowler–Nordheim tunnel currents through ultrathin gate oxides
Authors: Khairurrijal  , Mizubayashi W. , Miyazaki S. , Hirose M.
Published: July 2002

A theoretical model to predict the gate tunnel current in metal–oxide–semiconductor structures has been developed by employing the nonparabolic E-k dispersion for describing the tunneling electron momentum. The tunnel electron effective mass mox and the Fermi energy in the gate have been used to fit the calculated tunnel current to the measured one. It is shown that in the direct tunneling regime the tunnel electron effective mass mox apparently increases with decreasing oxide thickness presumably due to the reduction of Si–O–Si bond angle in the compressively strained layer near the SiO2/Si interface, while in the Fowler–Nordheim tunneling regime mox remains constant at 0.50 m0.
Out-of-plane losses of two-dimensional photonic crystals waveguides: Electromagnetic analysis
Authors: Lalanne Ph. , Benisty H.
Published: July 2002

A method for the electromagnetic analysis of photonic crystal waveguides is described. It is tested against experimental transmission data obtained for AlGaAs slab-waveguide structures perforated by a two-dimensional hexagonal lattice. A good quantitative agreement is obtained for the band edge locations, for the ripples in the transmission windows, and more importantly, for the out-of-plane losses induced by the finite hole depth. The ultimate performance of the structure for deep etched holes is predicted.
Impact of short-range repulsive interactions between nuclei on the evolution of a phase transformation
Authors: Tong W. S. , Rickman J. M. , Barmak K.
Published: July 2002

The impact of short-range repulsive interactions between nuclei on the evolution of a phase transformation is studied in order to describe quantitatively the kinetics of the transformation and to characterize systematically the resulting microstructures. For these purposes both computer simulation and analytical methods are employed in order to investigate an idealized, two-dimensional model of nucleation and growth wherein nuclei contained in a background phase (e.g., liquid) interact via a “hard-core” repulsion, a prototype for systems with a variety of possible interactions. Several quantities, including the dynamic area and perimeter fractions as well as the distribution of coalesced grain areas, are calculated. It is found, for example, that the temporal evolution of the system differs markedly from that in which the nuclei are spatially uncorrelated, particularly as the core diameter becomes large relative to the characteristic internuclear separation for noninteracting nuclei, and an approximate, analytic description of this behavior is obtained. Finally, the morphologies of the grains constituting the transformed microstructures are linked with the range of the internuclear interaction.
Luminescence properties of nanocrystalline Y2O3:Eu3+ in different host materials
Authors: Schmechel R. , Kennedy M. , von Seggern H. , Winkler H. , Kolbe M. , Fischer R. A. , Xaomao Li , Benker A. , Winterer M. , Hahn H.
Published: July 2002

In this study, the optical properties of nanocrystalline europium doped yttria, Y2O3:Eu3+ were investigated in dependence on different caging hosts such as porous MCM-41, porous silica, and porous alumina with pore sizes ranging between 2.7 to 80 nm. These results were compared to nanopowders measured in air and aqueous solution whose particle sizes were 5 nm and 8 nm, respectively. All these results were compared to a commercial lamp phosphor powder with a grain size of about 5 μm. The structural properties of the samples were determined by x-ray diffraction and transmission electron microscopy. Investigated optical properties are the photoluminescence emission spectra, the excitation spectra, the lifetimes, and the quantum efficiencies. A heavy dependence of the charge transfer process on the surrounding will be reported and discussed.
Bending instability in electrospinning of nanofibers
Authors: Yarin A. L. , Koombhongse S. , Reneker D. H.
Published: July 2002

A localized approximation was developed to calculate the bending electric force acting on an electrified polymer jet, which is a key element of the electrospinning process for manufacturing of nanofibers. Using this force, a far reaching analogy between the electrically driven bending instability and the aerodynamically driven instability was established. Continuous, quasi-one-dimensional, partial differential equations were derived and used to predict the growth rate of small electrically driven bending perturbations of a liquid column. A discretized form of these equations, that accounts for solvent evaporation and polymer solidification, was used to calculate the jet paths during the course of nonlinear bending instability leading to formation of large loops and resulting in nanofibers. The results of the calculations are compared to the experimental data acquired in the present work. Agreement of theory and experiment is discussed.
Comment on “The viscoelastic response of Brownian suspensions” [J. Chem. Phys. 111, 8708 (1999)]
Authors: Felderhof B. U.
Published: July 2002

It is pointed out that the pair cluster integral expression for the viscoelastic response of a Brownian suspension includes a factor g(σ), the radial distribution function at contact. It is suggested that in the comparison with the theory of Verberg et al. for the zero frequency response it should be taken into account that the pair dynamics of a hard sphere suspension without hydrodynamic interactions is approximately a factor g(σ) faster than in a suspension with hydrodynamic interactions.
Adsorption of atomic oxygen on GaAs(001)-(2×4) and the resulting surface structures
Authors: Yi S. I. , Kruse P. , Hale M. , Kummel A. C.
Published: July 2002

The naturally occurring oxide of GaAs has a high density of defects that pin the Fermi level at the GaAs surface. The principle electronic defect causing the Fermi level pinning is widely believed to be an arsenic antisite double donor. We have used scanning tunneling microscopy to show that the arsenic antisite defects are formed during the initial period of oxidation of GaAs(001) by atomic oxygen. Atomic oxygen displaces a single arsenic atom in the top layer leaving behind an undimerized arsenic atom with a half-filled dangling bond. The displaced arsenic atoms bond to remaining arsenic dimers to form arsenic antisites. Cluster calculations confirm the assignment of the oxygen binding sites and the role of undimerized single arsenic atoms as acceptor sites.
Yb
3+
distribution in LiNbO3:(MgO) studied by cooperative luminescence
Authors: Montoya E. , Bausá L. E. , Schaudel B. , Goldner P.
Published: July 2002

This work presents a study of the distribution of Yb3+ ions in LiNbO3 and LiNbO3:MgO using cooperative luminescence as a probe. The cooperative rate is measured as a function of Yb3+ concentration in the samples. After comparing the experimental results with simple possibilities for rare earth distribution, a model for the distribution of Yb3+ ions is proposed, in which a fraction of the dopant ions forms pairs with an Yb3+ ion placed at Li+ site and the other one at Nb5+ site, while the rest of the ions are randomly placed at Li+ sites. Moreover, the codoping with MgO enhances the cooperative emission and this is discussed in terms of a more efficient redistribution of Yb3+ ions to Nb5+ sites.
The effects of hot-wire atomic hydrogen on amorphous silicon
Authors: Brockhoff A. M. , van der Weg W. F. , Habraken F. H. P. M.
Published: July 2002

Amorphous silicon is strongly affected by exposure to hot-wire atomic hydrogen. In this study we show the various effects of atomic hydrogen on thin hydrogenated amorphous silicon films. The as-deposited material is of high quality. It appears possible to increase the hydrogen concentration by a factor of 2. At least 80% of the additional hydrogen atoms are bonded to silicon. Prolonged hydrogen loading causes total removal of the amorphous silicon layer by etching. An underlying crystalline silicon substrate is etched inhomogeneously. We link these various effects, describe a probable mechanism, and suggest processes taking place during hot-wire deposition. Atomic hydrogen appears to play a major role during film growth.
Strong in-plane birefringence of spatially nanostructured silicon
Authors: Kovalev D. , Polisski G. , Diener J. , Heckler H. , Künzner N. , Timoshenko V. Yu. , Koch F.
Published: July 2002

We report on a strong intrinsic optical anisotropy of Si induced by its dielectric patterning. As a result, an in-plane birefringence for nanostructured (110) Si surfaces is found to be 104 times stronger than that observed in bulk silicon crystals. We found the value of birefringence to be strongly dependent on the dielectric surrounding of the silicon nanoparticles assembling these layers. Beyond numerous potential implications for realization of optical devices and sensors, this gives a favorable route for studying the physics of condensation phenomena in a mesoscopic geometrical scale.
Electronic parameters of the two-dimensional electron gas in modulation-doped single quantum wells due to an embedded deep step layer
Authors: Kim T. W. , Lee D. U. , Choo D. C. , Jung M. , Yoo K. H. , Song M. S. , Yeo T. , Comanescu G. , McCombe B. D. , Kim M. D.
Published: July 2002

The Shubnikov–de Haas (S–dH) measurements at 1.5 K clearly demonstrated the existence of a two-dimensional electron gas (2DEG) in the modulation-doped Al0.25Ga0.75As/InyGa1−yAs/GaAs single and step quantum wells, and the fast Fourier transformation results for the S–dH data clearly indicated the electron occupation of one subband in the asymmetric single and step quantum wells. While the electron carrier density of the 2DEG in the step quantum well was larger than that in the single quantum well due to the larger conduction-band discontinuities, the mobility of the 2DEG in the step quantum well was smaller than that in the single quantum well because of the interface scattering resulting from the embedded step well. The electron effective mass in the step quantum well was smaller than that in the single quantum well, which was consistent with a smaller mass of the embedded deep step layer. The electronic subband energy, the energy wave function, and the Fermi energy in the InyGa1−yAs step quantum wells were calculated by using a self-consistent method taking into account exchange-correlation effects together with strain and nonparabolicity effects.
Analysis of longitudinal mode wave guiding in vertical-cavity surface-emitting lasers with long monolithic cavity
Authors: Mahmoud S. W. Z. , Unold H. J. , Schmid W. , Jäger R. , Michalzik R. , Ebeling K. J.
Published: July 2002

Lasing mode switching between two longitudinal modes is observed in transverse single mode vertical-cavity surface-emitting lasers with an extended cavity. Near- and far-field analyses carried out on devices with 2, 4, and 8 μm cavity spacers show an inherent relation between the transverse mode diameter and the calculated oxide- or thermally induced index guiding for the oscillating modes. Depending on the designed alignment of the optical longitudinal standing wave patterns relative to the oxide layer, mode switching can either be promoted or suppressed. Record-high single mode output powers up to 5.5 mW obtained from devices with 8 μm spacers and 8μm active diameter indicate the potential of the given device concept for low-divergence fundamental mode emission as required for many applications.
Influence of electrode contact on luminescence from alumina ceramic surface under ac electric field in vacuum
Authors: Zhang G. J. , Yan Z. , Liu Y. S. , Yasuoka K. , Ishii S.
Published: July 2002

The luminescence from a planar metal–alumina–metal structure was investigated under ac voltage application in vacuum. Two kinds of electrode contacts, i.e., with/without sputtered gold film, showed quite different optical phenomena. For the nonsputtered alumina samples, no luminescence was detected until extremely irregular light pulses resulted from partial discharges under higher voltage. While for the sputtered samples, from far lower applied voltage, there was faint but stable light emission due to electroluminescence (EL) observed. Based on the band theory of solids, we proposed a model to explain the interesting phenomena qualitatively. Taking account of the sputtering process, a marked reduction of the potential barrier between electrodes and alumina surface would result. Thus, charge carriers can be easily injected from the electrodes into the surface layer of alumina, and EL will emit due to the radiative recombination of electrons and holes. It is considered that EL has a significant influence on the developing process of surface flashover.
Theoretical modeling of crossed-field electron vacuum devices
Authors: Kaup D. J.
Published: July 2002

The modeling of crossed-field electron vacuum devices, such as magnetrons and crossed-field amplifiers with a dc background mode and an rf pump mode is discussed. The dominant interaction in these devices is the wave-particle interaction (diocotron). This interaction drives the classical Brillouin flow nonlinearly unstable through a Rayleigh-type instability in a shear flow. This linear instability triggers the nonlinear instability, which is a second-order nonlinear diffusion process. This diffusion process is driven by the density gradient at the edge of the sheath, which causes the electron density to evolve into a new density profile, one which will be in equilibrium with the nonlinear diffusion process. The general physics of the various processes contained in this model is discussed, including a possible explanation for the ultra-low noise phenomena.
Growth of strained-layer GaAs/Ge superlattices by magnetron sputtering: Optical and structural characterization
Authors: Rosendo E. , Rodrı́guez A. G. , Navarro-Contreras H. , Vidal M. A. , Asomoza R. , Kudriavtsev Y.
Published: July 2002

Strained layer superlattices of GaAs/Ge/GaAs and Ge/GaAs/Ge have been grown by magnetron sputtering of different intercalated layer thickness. The samples exhibited good crystallographic quality, pseudomorphic growth on the substrate, as well as superlattice characteristics. Layer periodicity, concentration profile and the thicknesses of the resultant films were examined by high-resolution x-ray diffraction, secondary ion mass spectroscopy, infrared (IR) optical transmission measurements, and Raman spectroscopy. The heterostructures exhibited IR attenuation peaks in transmission between 0.5 and 1.0 eV, whose energy position was characterized as a function of the thickness of the intercalated thinner layers. The combined results of these techniques reveal that the intended GaAs layers are in fact composed of (GaAs)1−x(Ge2)x alloys with a few percent Ge content. Experimental and theoretical results have been modeled with the transmittance model, which assumes that light hits the surface normally and takes the alternating layer thicknesses as variable parameters. Both, experimental and theoretical results agree to within 3%.
Magnetic anisotropy of thin film La0.7Ca0.3MnO3 on untwinned paramagnetic NdGaO3 (001)
Authors: Mathur N. D. , Jo M.-H. , Evetts J. E. , Blamire M. G.
Published: July 2002

We describe in detail a method by which to establish the magnetic anisotropy of thin ferromagnetic films on strongly paramagnetic substrates that are slightly anisotropic. The film that we consider is composed of the much studied manganite La0.7Ca0.3MnO3 and the substrate is NdGaO3, a good lattice match. Below a Curie temperature Tc of 260 K it was found, using a vibrating sample magnetometer, that 72±3 nm La0.7Ca0.3MnO3 films grown epitaxially by pulsed laser deposition on untwinned orthorhombic NdGaO3 (001) substrates exhibit uniaxial anisotropy with K=(3.6±0.1)×105 erg cm−3. The easy direction is along [110] of the pseudocubic unit cell, i.e., diagonal to the O–Mn–O bond directions and parallel to the side of the actual unit cell which is orthorhombic. We attribute an 11±4% loss of the low temperature moment to the proximity of the paramagnetic substrate rather than to stress. It is argued that stress is minimal such that the observed anisotropy must be magnetocrystalline. Both the reduction in moment and the anisotropy must be taken into account when designing thin film experiments.
Studies of emission spectra in helium plasmas at atmospheric pressure and local thermodynamical equilibrium
Authors: Margot Joëlle
Published: July 2002

A collisional-radiative model coupled with the electron Boltzmann equation is used to perform a parametric study of the excitation spectrum in microwave-induced helium plasmas at atmospheric pressure. It is shown that the populations of the helium excited levels are much higher than predicted by the Saha equation. The excitation spectrum does not follow a Boltzmann distribution and is typically characterized by two excitation temperatures independent of the electron temperature. At sufficiently large electron density, the levels very close to the continuum limit come in partial Saha equilibrium. When nitrogen impurities are introduced into helium, the populations of the helium excited levels strongly decrease and drastically change the behavior of the plasma with respect to Saha equilibrium.
Size reassignments of the S1–S vibronic spectra of benzene clusters
Authors: Iimori Toshifumi , Ohshima Yasuhiro
Published: July 2002

The vibronic band systems of (benzene)n clusters in the S1–S0 region are revisited by mass-selective resonant two-photon ionization and ultraviolet–ultraviolet hole burning spectroscopies. A detailed examination of the spectra of isotopomers composed of C6H6 and C6D6 reveals that there is substantial fragmentation following photoionization for the trimer and the tetramer. Transitions which have been assigned to the trimer are most probably due to the tetramer. Instead, reassigned to the trimer is the band system which has been believed to be of an isomeric form of the dimer.
Behavior of laser-induced ultrasonic waves radiated from a wet surface. Part II. Experimental work
Authors: Wooh Shi-Chang , Zhou Quanlin
Published: July 2002

The theoretical model presented in Part I of this article is experimentally verified using a Nd:YAG pulsed laser as a source to illuminate a wet surface. The longitudinal and shear directivities are determined from the displacements measured experimentally using a laser interferometer or a piezopolymer transducer. The experiments are carried out for circular sources of various beam diameters at two frequencies, in which the results are in good agreement with the theoretical predictions. The differences between this model and the conventional model are discussed by comparing the directivities and observing the influences of source beam size, frequency, and the source intensity. It is confirmed that the elliptical loading model predicts the behavior more accurately than the constant loading model, especially for larger laser beams operating in a high frequency regime.
Charge centers induced in thermal SiO2 films by high electric field stress at 80 K
Authors: Fujieda Shinji
Published: July 2002

Charge centers induced in wet and dry SiO2 films by high electric field stress [Fowler–Nordheim (FN) stress] at 80 K were characterized. The gate current of wet oxides was found to increase steeply during the FN-stress application at 80 K. In contrast, the gate current of dry oxides increased little. The thermally stimulated current of the oxides stressed by a high electric field and that of oxides irradiated with vacuum-ultraviolet light were measured. The measurements revealed one negative charge center (tr-1) and three positive charge centers (tr-2–tr-4) in the temperature range of 80–350 K. Their activation energies were estimated to be 0.26 eV (tr-1), 0.50 eV (tr-2), 0.60 eV (tr-3), and 0.86 eV (tr-4). The wet oxides were found to have a higher density of charge centers than the dry oxides. In particular, positive charge center tr-3 was characteristic of the wet oxides. The steep increase in the gate current of the wet oxides during the application of FN stress is attributed to tr-3. This center appears to be related to H+.
An improved approach to the problem of nonlinear electrostatic structures in plasma columns
Authors: Ramachandran H.
Published: July 2002

An improved method of solving for the structure of nonlinear electrostatic structures in plasma columns is presented. The fully nonlinear solution of the Trivelpiece–Gould (GT) mode has been obtained both for the case where the perpendicular mode structure is held constant and for the case where it is a function of the nonlinear amplitude. Unlike conventional approaches, the perpendicular structure of the mode is permitted to vary with amplitude. Temperature effects and radial profile effects are included in this treatment. The results are compared with those obtained if the perpendicular eigenmode is held constant and with the literature.
Photoionized gaseous nebulae and magnetized stellar winds: The evolution and shaping of H II regions and planetary nebulae
Authors: Franco José , Garcı́a-Segura Guillermo , Kurtz Stan E. , López José A.
Published: July 2002

The early evolution of hydrogen+ (H II) regions is controlled by the properties of the star-forming cloud cores. The observed density distributions in some young H II regions indicate that the power-law stratifications can be steeper than r−2. Ionization fronts can overrun these gradients and the ionized outflows are strongly accelerated along these steep density distributions. Thus, photoionized regions can either reach pressure equilibrium inside the inner parts of the high-pressure cores [with sizes and densities similar to those observed in ultra compact (UC) H II regions], or create bright H II regions with extended emission. The density inhomogeneities engulfed within the ionization fronts create corrugations in the front, which in turn drive instabilities in the ionization-shock (I-S) front. These instabilities grow on short time scales and lead to the fragmentation of the dense shells generated by the shock fronts. Thus, new clumps are continuously created from the fragmented shell, and the resulting finger-like structures can explain the existence of elephant trunks and cometary-like globules in most H II regions. In the case of planetary nebulae (PNe), wind asymmetries and magnetic fields from rotating stars, along with precession of the rotation axis, can create the wide range of observed PNe morphologies and collimated outflows (jets). Magnetic collimation and jet formation in PNe become very efficient after the flow has passed through the reverse shock of the PN.
Computer simulation of surface and adatom properties of Lennard-Jones solids: A comparison between face-centered-cubic and hexagonal-close-packed structures
Authors: Somasi Sweta , Khomami Bamin , Lovett Ronald
Published: July 2002

We introduce a new molecular dynamics simulation path to easily calculate solid–vapor surface free energies. The method is illustrated with explicit calculations of the surface free energies of a face-centered-cubic (fcc) crystal (the [110], [111], and [100] surfaces) and a hexagonal-close-packed (hcp) crystal (the [111] surface) of Lennard-Jones atoms. We verify that, because of the reduced symmetry at interfaces, simulation of the surface structure and free energy requires a large cutoff distance for the range of the pair potential. To estimate when a growing crystal resolves the fcc/hcp structural ambiguity, we observe the binding free energy and dynamics of clusters of adatoms on [111] surfaces of fcc and hcp crystals. A structural distinction only appears when clusters become large enough that their slow translational motion allows a structural relaxation of the crystal’s surface. From the observed distribution over cluster structures we deduce thermodynamic parameters that can be used to model the equilibrium between fcc-like clusters and hcp-like clusters on [111] surfaces and the rate of transformation between these.
Effect of the density of collision cascades on implantation damage in GaN
Authors: Kucheyev S. O. , Williams J. S. , Titov A. I. , Li G. , Jagadish C.
Published: July 2002

Damage accumulation in wurtzite GaN films bombarded with 0.5 MeV Bi1 and 1 MeV Bi2 ions (the so-called molecular effect) is studied by Rutherford backscattering/channeling spectrometry. Results show that an increase in the density of collision cascades dramatically enhances the level of implantation-produced lattice disorder in GaN. This effect is attributed to (i) an increase in the defect clustering efficiency with increasing density of ion-beam-generated point defects and/or (ii) to collective nonlinear energy spike processes. Such a strong influence of the density of collision cascades is important to take into account for a correct estimation of implantation-produced lattice disorder in GaN.
Analytic magnetohydrodynamic equilibria of a magnetically confined plasma with sheared flows
Authors: Simintzis Ch. , Throumoulopoulos G. N. , Pantis G. , Tasso H.
Published: July 2002

Analytic solutions of the magnetohydrodynamic equilibrium equations for a symmetric magnetically confined plasma with sheared incompressible flows associated with electric fields similar to those observed in the transition from the low- to the high-confinement mode in tokamaks are constructed in cylindrical and toroidal geometries. In particular, an exact toroidal solution is obtained which for vanishing flows reduces to the Solovév equilibrium which has been extensively employed in tokamak confinement studies. Owing to the flow, several toroidal configurations having either one or two stagnation points are possible in addition to the usual ones with a single magnetic axis. For flows pertaining to tokamak operational regime the extremum of the electric field becomes larger as flow and its shear increase, the location of the extremum being, however, nearly independent of these variations. In addition, the flow affects the safety factor profile and the shape of the magnetic surfaces and results in an increase of the magnetic shear and a decrease of the toroidal beta. The impact of plasma elongation on the above-mentioned confinement figures of merit is also evaluated.
Current crowding and optical saturation effects in GaInN/GaN light-emitting diodes grown on insulating substrates
Authors: Guo X. , Schubert E. F.
Published: July 2002

Current crowding in mesa-structure GaInN/GaN light emitting diodes (LEDs) grown on insulating substrates is analyzed. A model developed reveals an exponential decrease of the current density with distance from the mesa edge. Devices with stripe-shaped mesa geometry display current crowding and a saturation of the optical output power at high injection currents. It is shown that the optical power saturation depends on the device geometry. It is also shown that saturation is less pronounced in LEDs employing a ring-shaped mesa geometry, which reduces current crowding, as compared to the conventional square-shaped mesa geometry.
Testing strong-segregation theory against self-consistent-field theory for block copolymer melts
Authors: Matsen M. W.
Published: July 2002

We introduce a highly efficient self-consistent-field theory (SCFT) method for examining the cylindrical and spherical block copolymer morphologies using a standard unit cell approximation (UCA). The method is used to calculate the classical diblock copolymer phase boundaries deep into the strong-segregation regime, where they can be compared with recent improvements to strong-segregation theory (SST). The comparison suggests a significant discrepancy between the two theories indicating that our understanding of strongly stretched polymer brushes is still incomplete.
An analysis of the magnetic hyperfine interactions in the microwave spectrum of NO–HF: Evidence of electron transfer
Authors: Dennis C. R. , Whitham C. J. , Howard B. J.
Published: July 2002

One hundred and fifty-six magetic hyperfine transitions belonging to four rotational transitions of the NO–HF complex have been observed using a Fourier transform microwave spectrometer. The magnetic hyperfine structure shows interactions due to the nitrogen, hydrogen, and fluorine nuclei. Analysis of both the fine and hyperfine structure gives a bent geometry with an a axis–NO angle of about 44° and indicates a strong stabilization of the in plane NO π* orbital with respect to the out of plane orbital. The Fermi contact interactions at the H and F nuclei have the opposite sign and comparison with ab initio calculations suggests assigning the larger positive interaction to the F nucleus. The results reveal details of the distortions of the electronic structure of the partners in the complex caused by the hydrogen bond.
The triad in the region of the lowest-frequency parallel fundamental band (v5=1←0) of CH3SiH3: Fermi-type interactions and giant torsional splittings
Authors: Schroderus J. , Moazzen-Ahmadi N. , Ozier I.
Published: July 2002

The lowest frequency parallel fundamental band ν5 of CH3SiH3 near 700 cm−1 has been measured at a resolution of 0.004 cm−1 with Fourier transform spectroscopy to investigate vibration–torsion–rotation interactions in symmetric tops. The torsional splittings in the spectrum are increased from ∼0.005 cm−1 to ∼1 cm−1 by Fermi-type vibration–torsion interactions between the torsional stack (v6=0,1,2,…) in the ground vibrational state and the corresponding stack for v5=1. Resonant interactions were observed between the states (v5=1,v6=0) and (v5=0,v6=5) for the rotational series with (k=±1,σ=∓1), where σ labels the torsional sublevels. In this resonance, the two unperturbed states are near opposite limits for torsional motion: (v5=0,v6=5) involves nearly free rotation, while (v5=1,v6=0) involves small amplitude torsional oscillation. For the (k=±1,σ=∓1) rotational series, perturbation-allowed transitions in the high overtone (v6=5←0) were observed. Over 750 frequencies measured here have been analyzed together with more than 2500 measurements involved in the recent analysis of the lowest-lying degenerate fundamental band ν12 given by Moazzen-Ahmadi et al. [J. Mol. Spectrosc. 175, 54 (1996)]. A fit to within experimental error was achieved using 41 parameters, an increase of only 4 when the new band is added. The analysis shows that the inclusion of the Fermi-type interactions leads to a considerable simplification of the Hamiltonian for the ground vibrational state. For example, both the second and third terms (V0,6,V0,9) in the Fourier expansion of the hindering potential as well as the torsional flexing term (F0,m) vanish in the ground state. The changes in the leading terms in the torsional Hamiltonian have been quantitatively explained by a contact transformation. The large perturbations produced by the interaction matrix elements off-diagonal by 5 units in v6 have serious implications for vibrational relaxation in molecules undergoing internal rotation.
Thermoelectric properties of spark plasma sintered Ca2.75Gd0.25Co4O9 ceramics
Authors: Matsubara Ichiro , Funahashi Ryoji , Takeuchi Tomonari , Sodeoka Satoshi
Published: July 2002

Spark plasma sintering (SPS) was applied for preparing dense Ca2.75Gd0.25Co4O9 ceramics, and their thermoelectric properties were investigated from room temperature to 700 °C in air. The SPS process was effective in obtaining dense Ca2.75Gd0.25Co4O9 ceramics, typically 98% of the theoretical x-ray density, with lower resistivity, without any degradation of the thermoelectric power. The power factor and figure of merit of the spark plasma sintered samples attain 4.8×10−4 W m−1 K−2 and ZT=0.23 at 700 °C, respectively.