bluuebunny/crossref_metadata_2025 huggingface dataset:
A resolvable subfilter-scale model specific to large-eddy simulation of under-resolved turbulence
Authors: Zhou Yong , Brasseur James G. , Juneja Anurag
Published: July 2002

Large-eddy simulation (LES) of boundary-layer flows has serious deficiencies near the surface when a viscous sublayer either does not exist (rough walls) or is not practical to resolve (high Reynolds numbers). In previous work, we have shown that the near-surface errors arise from the poor performance of algebraic subfilter-scale (SFS) models at the first several grid levels, where integral scales are necessarily under-resolved and the turbulence is highly anisotropic. In under-resolved turbulence, eddy viscosity and similarity SFS models create a spurious feedback loop between predicted resolved-scale (RS) velocity and modeled SFS acceleration, and are unable to simultaneously capture SFS acceleration and RS–SFS energy flux. To break the spurious coupling in a dynamically meaningful manner, we introduce a new modeling strategy in which the grid-resolved subfilter velocity is estimated from a separate dynamical equation containing the essential inertial interactions between SFS and RS velocity. This resolved SFS (RSFS) velocity is then used as a surrogate for the complete SFS velocity in the SFS stress tensor. We test the RSFS model by comparing LES of highly under-resolved anisotropic buoyancy-generated homogeneous turbulence with a corresponding direct numerical simulation (DNS). The new model successfully suppresses the spurious feedback loop between RS velocity and SFS acceleration, and greatly improves model predictions of the anisotropic structure of SFS acceleration and resolved velocity fields. Unlike algebraic models, the RSFS model accurately captures SFS acceleration intensity and RS–SFS energy flux, even during the nonequilibrium transient, and properly partitions SFS acceleration between SFS stress divergence and SFS pressure force.
Squeezed potato orbits in a magnetic well
Authors: Shaing K. C.
Published: July 2002

It is shown that potato orbits in the near-axis region of a high beta tokamak are squeezed in a magnetic well. The squeezing factor is the same as that for the banana orbits derived in an earlier work [Phys. Plasmas 3, 2843 (1996)]. It depends on the energy of the particle. For high-energy particles, the size of the squeezed orbits is independent of their energy. This implies improved confinement for high-energy particles and for high beta tokamaks with advanced fuels.
A single-mode quantum transport in serial-structure geometric scatterers
Authors: Exner P. , Tater M. , Vaněk D.
Published: July 2002

We study transport in quantum systems consisting of a finite array of N identical single-channel scatterers. A general expression of the S matrix in terms of the individual-element data obtained recently for potential scattering is rederived in this wider context. It shows in particular how the band spectrum of the infinite periodic system arises in the limit N→∞. We illustrate the result on two kinds of examples. The first are serial graphs obtained by chaining loops or T-junctions. Another example concerns geometric scatterers where the individual element consists of a surface with a pair of leads; we show that apart from the resonances coming from the decoupled-surface eigenvalues, such scatterers exhibit the high-energy behavior typical for the δ′ interaction for the physically interesting couplings.
Near-wall layer of a positive dust–electron plasma in the presence of a nonequilibrium charging process
Authors: Benilov M. S. , Shukla P. K.
Published: July 2002

Accounting for a nonequilibrium charging process in a positive dust–electron plasma, a theory is developed for a quasi-neutral layer separating the near-wall space–charge sheath from the bulk plasma where the charging of the dust grains is quasi-equilibrium (i.e., the current of the electrons emitted by a dust grain is approximately equal to the current of the electrons collected by the grain). The problem is solvable when the velocity of the dust particles entering the layer from the quasi-equilibrium plasma satisfies an inequality which is mathematically, although not physically, similar to the Bohm criterion. The obtained results explain the reason for the absence of a continuous transition from the Bohm criterion in a dusty plasma with a variable dust particle charge to the conventional Bohm criterion for an electron–ion plasma with constant ion charge.
Interference of parallel cylindrical Langmuir probes
Authors: Sanmartı́n J. R. , Estes R. D.
Published: July 2002

Current to a cylindrical probe of arbitrary cross section is discussed. Previous results for circular cylinders at the high bias and moderate radius R of interest for electrodynamic bare tethers, for which space charge may be ignored over a large neighborhood of the probe, depend in separate ways on both R and perimeter p. These results are extended to a general convex cross section by introducing certain equivalent radius Req. For any concave cross section, results use a proper equivalent perimeter peq, in addition to Req. Finally, for the joint cross section of separate parallel probes, certain effective perimeter peff replaces peq. Rules to determine Req, peq, and peff are used to discuss collection interference among two or more parallel cylinders when brought from far away to contact.
The static dielectric constant of a colloidal suspension
Authors: Lu C.-Y. D.
Published: July 2002

We derive an expression for the static dielectric constant of the colloidal suspensions based on the electrokinetic equations. The analysis assumes that the ions have the same diffusivity, and that the double layer is much thinner than the radius of curvature of the particles. It is shown that the dielectric increment of the double-layer polarization mechanism is originated from the free energy stored in the salt concentration inhomogeneity. We also show that the dominant polarization charges in the theory are at the electrodes, rather than close to the particles.
Electron magnetohydrodynamic simulations of magnetic island coalescence
Authors: Dorelli John C. , Birn Joachim
Published: July 2002

The role of electrons in driving magnetic island coalescence in the electron magnetohydrodynamic limit is investigated. In particular, the dependence of the coalescence time tc(f) (defined as the time required for a fixed fraction f of the initial island magnetic flux to reconnect) on the resistivity η for constant Lundquist number Sλ (defined in terms of the island wavelength λ) is studied. Two distinct island coalescence regimes are observed: (1) a “resistive diffusion regime” where tc(f)∝tw/η (tw≡(λ2/Ωcidi2), where Ωci is the ion cyclotron frequency, and di is the ion inertial length), and (2) an “electron driven regime” where tc(f)∝tw. Defining the “ion Lundquist number,” Si≡Sλdi/λ, the resistive diffusion regime is observed when Si≲50, while the electron driven regime is observed when Si≳50. In the electron driven regime, the coalescence time is insensitive to Si over the range 50≲Si≲250.
Micelle formation of randomly grafted copolymers in slightly selective solvents
Authors: Kreig Adam , Lefebvre Amy A. , Hahn Hyeok , Balsara Nitash P. , Qi Shuyan , Chakraborty Arup K. , Xenidou Maria , Hadjichristidis Nikos
Published: July 2002

Amphiphilic surfactants, molecules with chemical moieties that interact differently with the solvating medium, are important for technological applications and ubiquitous in biology. Understanding how to control surfactant properties is, therefore, of wide-ranging importance. Using a combination of light scattering experiments and field theory, we demonstrate that the behavior of polymeric surfactants can be controlled sensitively by manipulating molecular architecture. We find that branched polymeric amphiphiles can be much better surfactants than traditional linear analogs. This is indicated by micelle formation in solvents that are very slightly selective for the backbone of the branched molecule. Our experimental and theoretical findings also suggest that, for a given chemistry and architectural class, surfactant properties of polymeric amphiphiles are very sensitive to subtle changes in architectural features. Specifically, we find that choosing a particular branching density optimizes the propensity for micelle formation. The sensitivity of macromolecular surfactant properties to molecular architecture can perhaps be profitably exploited in applications wherein only certain chemical moieties are allowed. The physical origin of this sensitivity is the importance of conformational entropy penalties associated with the pertinent self-assembly process. This is in contrast to self-assembly of small molecule systems where conformational entropy is not of such significance.
The low-temperature reactive tunneling in condensed phase. II. Multidimensional transition model
Authors: Basilevsky M. V. , Davidovitch G. V.
Published: July 2002

The dynamical model of tunneling interlevel transitions is extended to describe the kinetics of H-atom transfer in real reactions proceeding in the condensed phase. The reaction mechanism includes an explicit consideration of three modes; the high-frequency reactive motion of the H atom, the low-frequency intermolecular promotion mode, and an intramolecular mode which changes significantly during the reaction and is associated with the hybridization geometry changes of the reaction center. The rate constant expression is derived in terms of the standard technique of the theory of radiationless transitions. Its high- and moderate-temperature analysis reproduces the results obtained earlier in the framework of the Golden Rule approach. Peculiarities of the present theory are revealed beyond this temperature range. A remarkable low-temperature threshold effect causes steep temperature trends in the reaction rate which are likely to account for some recent kinetic experiments.
Specular x-ray reflectivity study of ordering in self-assembled organic and hybrid organic–inorganic electro-optic multilayer films
Authors: Evmenenko G. , van der Boom M. E. , Kmetko J. , Dugan S. W. , Marks T. J. , Dutta P.
Published: July 2002

Specular x-ray reflectivity has been used to probe the microstructures of siloxane-based self-assembled electro-optic superlattices composed of high-hyperpolarizable organic chromophore arrays intercalated with Ga and In oxide sheets. The film thickness increases linearly as a function of the number of layers, underscoring the high structural regularity and efficiency of the synthetic approach. Relatively dense metal oxide structures are detected in these systems. The x-ray reflectivity data also indicate that the dependence of the relative surface roughness on the number of layers is nearly identical for self-assembled organic and organic–inorganic hybrid film structures.
Coexistence behavior of the CuPtB-type and the CuAu–I-type ordered structures in highly strained CdxZn1−xTe/GaAs heterostructures
Authors: Lee H. S. , Lee J. Y. , Kim T. W. , Lee D. U. , Choo D. C. , Park H. L.
Published: July 2002

Ordered structures in highly strained CdxZn1−xTe/GaAs heterostructures epitaxial layers grown on (001)GaAs substrates were investigated by using selected area electron diffraction pattern (SADP) and cross-sectional high-resolution transmission electron microscopy (HRTEM) measurements. The SADP results showed two sets of (1/2 1/2 1/2) superstructure reflections with symmetrical intensities along the [110] axis, and the corresponding HRTEM images indicated a doublet periodicity in the contrast of the {111} lattice planes. Two structures, one corresponding to the CuPtB-type ordering for each direction of the doublet periodicity on the {111} lattice planes along the [110] axis and the other corresponding to superstructure spots related to the CuAu–I type ordering were observed in the SADP. The doublet periodicity of 200 lattice fringes, associated with the CuAu–I-type ordered structure was also observed in the HRTEM image, and many antiphase boundaries were observed in ordered regions. The formation of the two ordered structures in the CdxZn1−xTe epilayers might originate from the minimization of the relaxation energy due to the high strain effect resulting from the large lattice mismatch between the CdxZn1−xTe epilayer and the GaAs substrate. These results provide important information on the microstructural properties for improving the efficiencies of CdxZn1−xTe-based optoelectronic devices operating in the blue-green spectral region.
The bridging conformations of double-end anchored polymer-surfactants destabilize a hydrogel of lipid membranes
Authors: Slack N. L. , Davidson P. , Chibbaro M. A. , Jeppesen C. , Eiselt P. , Warriner H. E. , Schmidt H.-W. , Pincus P. , Safinya C. R.
Published: July 2002

Double-end-anchored poly-ethylene-glycol-surfactants (DEA-PEG-surfactants) induce the gelation of lyotropic lamellar Lα phases stabilized by undulation forces. The physical hydrogel (Lα,g) derives its viscoelasticity from the proliferation of defects at a mesoscopic level. The DEA-PEG-surfactants assume both looping and bridging conformations. The existence of novel bridging conformations is indicated by the coexistence of two lamellar phases and the limited swelling of the Lα and Lα,g phases. Modeling of the polymer decorated membranes demonstrates the existence of bridging and yields a rapidly decreasing density of bridging conformations with increasing interlayer spacing.
Response to “Comment on ‘High order finite difference algorithms for solving the Schrödinger equation in molecular dynamics’ ” [J. Chem. Phys. 115, 6794 (2001)]
Authors: Guantes Raul , Farantos Stavros C.
Published: July 2002

The comment of Mazziotti about the classification of the Lagrange distributed approximating functional method as a finite difference method is answered. Furthermore, the relations of high order finite difference approximation of the Laplacian of the Schrödinger equation to well known pseudospectral techniques such as the fast Fourier transform and discrete variable representations are clarified.
Fluctuations and asymmetry via local Lyapunov instability in the time-reversible doubly thermostated harmonic oscillator
Authors: Hoover Wm. G. , Posch H. A. , Hoover Carol G.
Published: July 2002

Forward and backward trajectories from time-symmetric equations of motion can have time-asymmetric stability properties, and exhibit time-asymmetric fluctuations. Away from equilibrium this symmetry breaking is the mechanical equivalent of the second law of thermodynamics. Strange attractor states obeying the second law are time-reversed versions of (unobservable) repeller states which violate that law. Here, we consider both the equilibrium and the nonequilibrium cases for a simple deterministically thermostated oscillator. At equilibrium the extended phase-space distribution is a smooth Gaussian function. Away from equilibrium the distribution is instead a fractal strange attractor. In both cases we illustrate local time-symmetry breaking. We also quantify the forward–backward fluctuation asymmetry for the thermostated oscillator.
Relaxation dynamics of hot carriers and phonons in semiconductors: Influence of the excitation conditions
Authors: Silva A. A. P. , Algarte A. C. S. , Vasconcellos A. R. , Luzzi R.
Published: July 2002

The so-called hot-phonon effect that accompanies the rapid relaxation processes in the photoinjected plasma in semiconductors is analyzed, resorting to a nonlinear quantum kinetic theory based on a nonequilibrium ensemble formalism. We concentrate the study on the process of generation and decay of the nonequilibrium longitudinal optical phonon population per mode. Particular attention is paid to the question of the influence of the conditions of excitation imposed on the system, which determine the occurrence of different regimes of relaxation by means of carrier–phonon interactions. Comparison of the relaxation dynamics in the case of several semiconductors with different polar strengths is done.
Bulk diffusion measurements to study the effectiveness of barrier layers: I. Mathematical treatment
Authors: Tian Lei , Thompson Michael O. , Dieckmann Rüdiger , Hui Chung-Yuen , Lin Yu-Yun
Published: July 2002

The exchange of a tracer material, e.g., a radioactive isotope, between two samples forming a diffusion couple depends on the rate of the diffusion of the tracer in each of the two samples and also on the rate of the transfer of the tracer across the interface between these samples which may or may not contain a barrier layer with a different chemical composition and/or structure. Diffusion couples with three different initial tracer distributions are considered. In order to extract from experimental data values for tracer diffusion coefficients and for the rate constant for the tracer transport across the interface, a detailed analysis of the required mathematics is given. This analysis is of interest as well for obtaining true values for bulk diffusivities and also to characterize quantitatively the resistance of interfaces (=barriers) to the exchange of certain species. Some examples of experimental results are presented and briefly discussed.
Heats of formation of CCl and CCl2 from ab initio quantum chemistry
Authors: Dixon David A. , Peterson Kirk A.
Published: July 2002

High level ab initio electronic structure theory has been used to calculate the heats of formation of CCl and CCl2. The calculations were done at the CCSD(T) level with new correlation-consistent basis sets up through augmented hextuple-ζ including tight d functions on the Cl and then extrapolated to the complete basis set limit. Additional corrections for core/valence correlation, relativistic effects both scalar and atomic spin–orbit, and zero-point energies have been included. The heat of formation at 0 K of CCl is 103.1±0.4 kcal/mol and that of CCl2 is 55.1±0.4 kcal/mol.
Deprotonation energy of HO2: Basis set limit energies
Authors: Francisco Joseph S.
Published: July 2002

The deprotonation energy of HO2 radical has been accurately determined using ab initio coupled cluster methods. Basis sets up to aug-cc-pVQZ quality have been used, and an extrapolation for further basis set incompleteness has been applied. The best computed deprotonation energy for HO2 is 352.4±0.5 kcal mol−1.
Thermal energy consideration in micromagnetic simulation for laminated antiferromagnetically coupled recording media
Authors: Hee C. H. , Wang J. P. , Piramanayagam S. N. , Chong T. C.
Published: July 2002

Contribution of thermal energy, has been included in the micromagnetic simulation of laminated antiferromagnetically coupled (LAC) media. The antiferromagnetic coupling constant, J, required to obtain Mrt reduction in the existing simulation studies (T=0 K) of LAC media is much higher than the experimental values. In this letter, we describe some experimental results, which point out that the contribution of thermal energy in Mrt reduction is significant. Consequently, we find that the values of J and the reversal fields are comparable to the experimental results, when thermal energy (T=300 K) is included in the simulation.
Generalized anisotropic ellipsometry applied to an organic single crystal: Potassium acid phthalate
Authors: Sassella A. , Borghesi A. , Wagner Th. , Hilfiker J.
Published: July 2002

The results of generalized anisotropic ellipsometry on a biaxial organic single crystal, namely, potassium acid phthalate, are discussed and analyzed to obtain the optical functions of the crystal along the different crystal directions. The dispersion of the real refractive indices nx, ny, and nz in the spectral range from 300 to 1400 nm, as well as the values of the extinction coefficient kx,y at the absorption edge are determined and modeled.
Chemical vapor deposition of Si on chlorosilane-treated SiO2 surfaces. II. Selective deposition in the regions defined by electron-beam irradiation
Authors: Yasuda T. , Nishizawa M. , Yamasaki S.
Published: July 2002

This article reports a selective-area deposition process using chlorosilane-treated ultrathin SiO2 masks on which the patterns are directly defined by irradiation of a focused electron beam (EB). Three different chlorosilane gases (SiH2Cl2, SiHCl3, and SiCl4) were first adsorbed on the SiO2 surfaces, and the regions with reactive sites were defined by taking advantage of electron-stimulated desorption (ESD) of the surface adsorbates. For the SiHCl3- and SiH2Cl2-treated surfaces, the nucleation density was remarkably high in the EB-irradiated regions. Such nucleation enhancement was less significant for the SiCl4-treated surface. For the purpose of direct patterning and selective growth, we conclude that SiHCl3 is the optimum treatment gas because it effectively suppresses Si nucleation outside of the EB-irradiated patterns. The overall ESD sensitivity of the SiHCl3-adsorbed mask was 50 mC/cm2, and a significant nucleation enhancement was observed by irradiation of 7 mC/cm2. We also report an extension of this selective-area processing to window opening through the ultrathin SiO2 mask layer. The factors limiting the minimum feature size of the grown Si structures and the opened windows are discussed.
Amphiphile-rich phase in a model ternary solution on the honeycomb lattice
Authors: Huckaby Dale A. , Pȩkalski Andrzej , Buzatu Daniela , Buzatu Florin D.
Published: July 2002

A model three-component system is considered in which the bonds of a honeycomb lattice are covered by bifunctional molecules of types AA, BB, and AB. The AB type molecule represents an amphiphile. An accurate approximation to the boundary in temperature-composition space of an ordered amphiphile-rich phase is calculated by transforming an accurate closed-form approximation of the critical frontier of an antiferromagnetic phase in an equivalent Ising model. This Ising model has coupling constants and a magnetic field that are temperature dependent.
General trends in the barriers of catalytic reactions on transition metal surfaces
Authors: Liu Zhi-Pan , Hu P.
Published: July 2002

A catalyst preparation by design is one of the ultimate goals in chemistry. The first step towards this goal is to understand the origin of reaction barriers. In this study, we have investigated several catalytic reactions on some transition metal surfaces, using density functional theory. All the reaction barriers have been determined. By detailed analyses we obtain some insight into the reaction barrier. Each barrier is related to (i) the potential energy surface of reactants on the surface, (ii) the total chemisorption energy of reactants, and (iii) the metal d orbital occupancy and the reactant valency.
Laser melting of photoluminescent (Y0.92Eu0.08)2O3 films
Authors: Hirata G. A. , McKittrick J. , Trkula M. , Mourant J. , Sze R.
Published: July 2002

Fluorescent red-emitting (Y0.92Eu0.08)2O3 films were deposited on sapphire substrates by the metallorganic chemical vapor deposition technique. The films were weakly luminescent in the as-deposited condition. The as-deposited films were composed of nanocrystals embedded in columnar grains. A KrF laser with ultraviolet (λ=248 nm) pulses at a fluence level between 0.9 and 2.3 J/cm2 was applied to different regions of the film. Increasing the energy fluence density initially increased the photoluminescence intensity but decreased it at the highest level. Transmission and scanning electron microscopy verified that surface melting and ablation occurred at all fluence levels. Computational modeling of the laser melting and ablation process predicted that a significant fraction of the film is removed by ablation at the highest fluence levels, thereby decreasing the photoluminescence intensity of the films due to the significant amount of material removed.
Three-photon phenomena in the upconversion luminescence of erbium–ytterbium-codoped phosphate glass
Authors: Song Feng , Zhang Guangyin , Shang Meiru , Tan Hao , Yang Jia , Meng Fanzhen
Published: July 2002

Frequency upconversion into green and red luminescence in erbium–ytterbium-codoped phosphate glass pumped with a 970 nm diode laser is presented. The red upconversion emission results from a two-photon excitation process. The green emission results from a two- or three-photon excitation process with the two-photon process dominating at low pumping power. Based on measurement of the excitation and emission spectra, a model of the three-photon excitation process has been developed. This model provides a good basis for explaining the effect of the three-photon process on the saturation of 1.54 μm laser power of Er, Yb-codoped phosphate glass laser.
Carrier lifetime measurement on electroluminescent metal–oxide–silicon tunneling diodes
Authors: Chen Miin-Jang , Lin Ching-Fuh , Lee M. H. , Chang S. T. , Liu C. W.
Published: July 2002

The temporal response of the electroluminescence at the Si band gap energy from a metal–oxide–silicon (MOS) tunneling diode is used to characterize the minority carrier lifetime near the Si/SiO2 interface. The temporal responses reveal that the Shockley–Read–Hall (SRH) recombination lifetimes are 18 and 25.8 μs for the rising and falling edges, respectively, and that the ratio for SRH, radiative, and Auger recombinations is 1:0.196:0.096 at injection current density of 39 A/cm2. The investigation shows that the electroluminescence of the MOS tunneling diode can be significantly increased by reducing the number of the nonradiative recombination centers.
Microspectroscopy and imaging using a delay line detector in time-of-flight photoemission microscopy
Authors: Oelsner A. , Schmidt O. , Schicketanz M. , Klais M. , Schönhense G. , Mergel V. , Jagutzki O. , Schmidt-Böcking H.
Published: July 2002

A method for microspectroscopy and energy-selective imaging using a special photoemission electron microscope (PEEM) is presented. A modified commercial PEEM was combined with a delay line device as x, y, t detector serving as the basic arrangement for spectromicroscopy. One can measure the time of flight of the electrons passing a drift section in order to analyze the energy distribution of photoelectrons in PEEM. The time of flight is referenced to the time structure of the synchrotron radiation from an electron storage ring. At electron kinetic energies of less than 20 eV within the drift region a spatial resolution of about 100 nm has been obtained. Fast counting electronics (instead of a camera) delivers an image for real-time monitoring on an oscilloscope screen or for image acquisition by a computer. A time resolution of about 500 ps has been obtained with the potential of further improvement. The spatial resolution of the delay line detector is about 50 μm in the image plane corresponding to 1000 pixels in the image diagonal. Direct photoemission from the W-4f core level of a W(110) single-crystal sample was observed at several photon energies. The W-4f fine-structure splitting of 2.3 eV could be well resolved at a pass energy around 40 eV through the drift region.
Characterization of holographic polymer dispersed liquid crystal transmission gratings
Authors: Jazbinšek Mojca , Drevenšek Olenik Irena , Zgonik Marko , Fontecchio Adam K. , Crawford Gregory P.
Published: July 2002

We investigate physical properties of polymer dispersed liquid crystal transmission gratings which influence the light diffraction efficiency behavior. Scanning electron microscopy is used to determine the orientation and shape of nanometer-sized liquid crystal droplets in a polymer host. Dependence of diffraction efficiency on light polarization suggests that the orientation of a liquid crystal within the ellipsoidal droplets is axial. The distribution of droplet orientations appears to vary for different grating spacings. The switching properties of the gratings are investigated and analyzed with a sinusoidal dielectric tensor modulation model. The analysis also takes into account orientational distribution of average directors and spatial distribution of the droplets. Temperature dependence of diffraction efficiency is tentatively explained by a structural transition within the droplets and the presence of impurities in droplets.
Strong unidirectional anisotropy in mechanically alloyed spinel ferrites
Authors: Shi Y. , Ding J.
Published: July 2002

Cluster glass and relatively high coercivity at low temperatures were found in disordered ultrafine nickel ferrite powders. High-energy mechanical milling of spinel NiFe2O4 led to formation of a wüstitelike structure. Our investigation suggested that ferrimagnetic clusters formed in an antiferromagnetic matrix. The strong ferri/antiferromagnetic exchange coupling resulted in a strong unidirectional anisotropy and a coercivity of over 10 kOe at 4.2 K.
O
2
⋅
N
2
photochemistry in the present and Precambrian atmosphere
Authors: Zipf Edward C. , Prasad Sheo S.
Published: July 2002

The absorption of radiation [λ200–260 nm] by the O2⋅N2 collision complex produces NOX with an average yield of (0.03±0.008) odd-nitrogen per absorbed photon. This process is also a new source of isotope enriched odd-nitrogen in the atmospheres.
Advection of passive particles in the Kolmogorov flow
Authors: Beyer P. , Benkadda S.
Published: July 2002

A statistical analysis of the advection of passive particles in a flow governed by driven two-dimensional Navier–Stokes equations (Kolmogorov flow) is presented. Different regimes are studied, all corresponding to a chaotic behavior of the flow. The diffusion is found to be strongly asymmetric with a very weak transport perpendicular to the forcing direction. The trajectories of the particles are characterized by the presence of traps and flights. The trapping time distributions show algebraic decrease, and strong anomalous diffusion is observed in transient phases. Different regimes lead to different types of diffusion, i.e., no universal behavior of diffusion is observed, and both time and space properties are needed to define anomalous transport.
Impact of forming gas annealing on the fatigue characteristics of ferroelectric SrBi2Ta2O9 thin films
Authors: Wang Dong-Sheng , Yu Tao , Hu An , Wu Di , Li Ai-Dong , Liu Zhi-Guo , Ming Nai-Ben
Published: July 2002

The metalorganic decomposition-derived SrBi2Ta2O9 (SBT) ferroelectric thin films were annealed in forming gas (5%H2+95%N2), and their fatigue characteristics were investigated. Although the hysteresis loops of these films had an unacceptable degradation under such a H2-containing reducing atmosphere, no obvious polarization fatigue with electric field cycling could be observed. However, an increase of P*r was observed over the initial period of the fatigue test. It could be viewed as a competition between the increase of P*r due to the leakage current and the decrease of P*r due to switching polarization. H2 played an important role in the increasing in the leakage current of SBT thin films, forming more weak pinning centers of domain walls, and degradation in the fatigue characteristics.
Impurity contamination control by an ion-cyclotron wave generated ponderomotive force in dusty plasmas
Authors: Amroun M. , Annou R.
Published: July 2002

A dust contamination control procedure in plasmas is proposed. The ponderomotive force generated by an ion-cyclotron wave is used to separate dust grains from electrons and ions. The relevance of the process to semiconductors technology is pointed out.
Spontaneously forming nanostructures: Quantum dot molecules and rings
Authors: Zhang Yangting , Floyd Margaret , Drucker Jeff , Kellogg G. L.
Published: July 2002

Annealing molecular beam epitaxy grown Ge/Si(100) islands spontaneously forms quantum dot molecules and rings. Quantum dot molecules (QDM) are close-packed arrays of several SiGe alloy hut clusters arranged with rectangular or circular symmetry. Rings are 3D structures with a depressed center. Both of these structures are related to island ensemble coarsening. Si interdiffusion into the perimeter of nominally pure Ge dome clusters forms a Si-rich annulus surrounding a Ge rich core. This lower melting point core diffuses away during coarsening of the island ensemble leaving behind the QDM. Large ring structures are formed by rapid coarsening of large, dislocated clusters during high temperature anneals.
Electric measurements with constant current: A practical method for characterizing dielectric films
Authors: Giacometti José A. , Wisniewski Célio , Ribeiro Paulo Antonio , Moura Walterley Araújo
Published: July 2002

This article assesses the use of the constant current (CC) method for characterizing dielectric films. The method is based on charging the sample with a constant current (current stress) and measuring the corresponding voltage rise under the closed circuit condition. Our article shows that the CC method is an alternative to the constant voltage stressing method to study the electric properties of nonpolar, ferroelectric, and polar polymers. The method was tested by determining the dielectric constant of polytetrafluoroethylene, and investigating the electric conduction in poly(ethylene terephthalate). For the ferroelectric polymer poly(vinylidene fluoride), it is shown that hysteresis loops and the dependence of the ferroelectric polarization on the electric field can be obtained.
Some navigation rules for D-brane monodromy
Authors: Aspinwall Paul S.
Published: July 2002

We explore some aspects of monodromies of D-branes in the Kähler moduli space of Calabi–Yau compactifications. Here a D-brane is viewed as an object of the derived category of coherent sheaves. We compute all the interesting monodromies in some nontrivial examples and link our work to recent results and conjectures concerning helices and mutations. We note some particular properties of the 0-brane.
Mechanism of the reduction of dislocation density in epilayers grown on compliant substrates
Authors: Pei C. W. , Turk B. , Wang W. I. , Kuan T. S.
Published: July 2002

A simplified model of the mechanism of dislocation reduction in epilayers grown on compliant substrates by molecular-beam epitaxy has been developed based on the dislocation theory and detailed experiments. Theoretical results calculated with this model indicate that up to 100-fold defect reduction can be achieved by using a silicon-on-insulator compliant substrate for the thick epilayer growth as compared to that of using a conventional Si substrate. The advantage of growing thick epilayers on compliant substrates can be predicted quantitatively. The mechanism of a nearly dislocation-free SiGe alloy, as well as GaAs epilayers grown on silicon-on-insulator compliant substrates, is explained and the dislocation density calculated with this model is in good agreement with our experimental results.
Carbon acceptor doping efficiency in GaAs grown by metalorganic chemical vapor deposition
Authors: Mimila-Arroyo J. , Lusson A. , Chevallier J. , Barbé M. , Theys B. , Jomard F. , Bland S. W.
Published: July 2002

Carbon doping efficiency in GaAs grown by metalorganic chemical vapor deposition using intrinsic and extrinsic doping sources is studied. Independent of the carbon source, carbon hydrogen complexes are systematically present and depending on the growth conditions, carbon dimers can be present and form complexes with hydrogen as well. Carbon–hydrogen related complexes and dimers reduce the hole concentration decreasing the doping efficiency. Additionally, the carbon dimer introduces a deep level, decreases the hole mobility and hydrogen bonds stronger to it than to isolated carbon. Depending on the growth conditions it is possible to reach 100% doping efficiency with high hole mobility.
Coherence collapse and redshifting in vertical-cavity surface-emitting lasers exposed to strong optical feedback
Authors: Quay C. H. L. , Maxwell I. Z. , Hudgings J. A.
Published: July 2002

The response of a commercial-grade, single-mode vertical-cavity surface-emitting laser to very strong optical feedback is investigated. With increasing feedback magnitude, the lasing spectrum broadens dramatically, eventually resulting in coherence collapse, and the lasing peak shifts to longer wavelengths. These effects became less severe with the onset of higher-order modes. A theoretical model is provided to explain the redshifting of the lasing spectrum with feedback.
Energy distributions of electrons in a low-current self-sustained nitrogen discharge
Authors: Vrhovac S. B. , Stojanović V. D. , Jelenković B. M. , Petrović Z. Lj.
Published: July 2002

Electron energy distribution functions (EEDF) in N2 Townsend discharges for E/N between 1 and 30 kTd (1 Td=10−21 V m2) were measured using a multigridded energy analyzer behind a small (0.1 mm) aperture in the graphite anode. Experimental results are further analyzed by applying Monte Carlo simulations in order to describe nonequilibrium transport in steady state Townsend discharges and to determine the origin of the observed EEDF features. Boundary effects at electrodes are described by allowing exact representation of absorption, reflection, and secondary electron production at the anode. It was found that it is necessary to include electron reflection and secondary electron production in order to model the low energy part of the observed EEDF.
Barrier roughness effects in resonant interband tunnel diodes
Authors: Magno R. , Bracker A. S. , Bennett B. R. , Nosho B. Z. , Whitman L. J.
Published: July 2002

Peak current densities of InAs/AlSb/GaSb/AlSb/InAs resonant interband tunneling diodes (RITD) grown by molecular beam epitaxy have been measured as a function of the growth temperature. The growth procedures were designed to produce nominally identical AlSb tunneling barriers. The variations observed in the peak current for positive bias are consistent with the barrier on the substrate side of the RITD becoming effectively thicker for diodes grown at high temperatures. Plan-view in situ scanning tunneling microscopy (STM) measurements indicate that smoother AlSb barriers are grown at high temperature. The growth temperature dependence of the peak current density and STM results are consistent, because tunneling is highly dependent on barrier thickness. While the high and low temperature growths were designed to have the same barrier thickness, the large current flowing through the thin areas of a rough barrier result in an effectively thinner barrier compared to the smooth one.
Response to “Comment on ‘A fallacious argument in the finite time thermodynamics concept of endoreversibility’ ” [J. Appl. Phys. 90, 6557 (1998)]
Authors: Sekulic Dusan P.
Published: July 2002

In this reply, the author reiterates that his argument that there is a logical inconsistency in introducing the concept of endoreversibility into a finite-time thermodynamics model of power plants characterized with a presence of heat exchanger thermal couplings is valid. As long as the couplings involve finite temperature differences between working fluids across the suggested endoreversible boundary, the inherent irreversibilities of thermal couplings cannot selectively be eliminated due to the entropy flow across that boundary and entropy generation within a heat exchanger. Hence the concept of endoreversibility in such a case does violate a proper definition either of the system (e.g., power plant) or the component (a heat exchanger). A heat exchanger as a system component, if included in the endoreversible compartment, would operate reversibly, which is impossible for a finite size heat exchanger; if excluded from it the endoreversible part of the system becomes open, which constitutes a violation of the initial set of assumptions; and if it is split between the endoreversible part and the surroundings, each part carries its respective irreversibility. Hence the endoreversible compartment would become irreversible.
Photochromic response of bismuth germanium oxide doped with chromium
Authors: McCullough J. S. , Harmon Bauer A. L. , Hunt C. A. , Martin J. J.
Published: July 2002

Doping the photorefractive material bismuth germanium oxide Bi12GeO20(BGO) with chromium significantly affects its optical properties. An absorption spectrum taken at 300 K shows that the introduction of chromium produces a strong unresolved band which overlaps the absorption cutoff of BGO and a series of smaller overlapping bands between 650 and 1100 nm. Cooling BGO:Cr to 10 K sharpens the small bands and resolves the strong band into a peak near 510 nm. This peak is responsible for the reddish-brown color of BGO:Cr. Excitation at room temperature with visible light increases the bands in the 650–1100 nm range. The production of this additional room temperature photochromic absorption is most efficient with excitation near 490 nm but extends throughout the visible. Chromium occupies the tetrahedral germanium site in BGO; and the observed spectra are consistent with a Cr4+ tetrahedral state. During crystal growth some of the chromium gives up an electron to the antisite bismuth native defect and becomes Cr5+. Exposing the sample to light returns the electron to the chromium and increases the Cr4+ absorption. The photoinduced absorption is thermally stable for temperatures up to about 425 K. This stability suggests that doping BGO with Cr should lead to room temperature persistent photorefractive gratings. © 2001 American Institute of Physics.
Persistent refractive index gratings in bismuth germanium oxide doped with chromium
Authors: McCullough J. S. , Bauer A. L. Harmon , Hunt C. A. , Martin J. J.
Published: July 2002

The kinetics of the production and dark-decay of index gratings in chromium-doped bismuth germanium oxide Bi12GeO20 (BGO) and undoped BGO were investigated as a function of temperature from room temperature to about 500 K. During the write process in undoped BGO the gratings show a fast leading edge peak, which drops quickly to a steady saturated value. The leading-edge peak does not change significantly at the higher temperatures. Grating production was much slower in the BGO:Cr samples and the gratings grew monotonically throughout the 0.5 s write interval. At room temperature the gratings in BGO:Cr are highly persistent. A grating written at room temperature retains 70% of its initial strength after 24 h when it was read only once an hour. Optical erasure plays a major role in the decay of such gratings; reading the grating every 6 min reduced its strength to 48% of its initial value after 24 h. The grating written at room temperature and monitored as the sample was heated decayed within the same 10 K temperature range as the photochromic absorption bands. As expected, undoped BGO shows a rapid dark decay at room temperature that becomes faster at higher temperatures.
A family of integrable nonlinear equations of hyperbolic type
Authors: Tongas A. , Tsoubelis D. , Xenitidis P.
Published: July 2002

A new system of integrable nonlinear equations of hyperbolic type, obtained by a two-dimensional reduction of the anti-self-dual Yang–Mills equations, is presented. It represents a generalization of the Ernst–Weyl equation of General Relativity related to colliding neutrino and gravitational waves, as well as of the fourth order equation of Schwarzian type related to the KdV hierarchies, which was introduced by Nijhoff, Hone, and Joshi recently. An auto-Bäcklund transformation of the new system is constructed, leading to a superposition principle remarkably similar to the one connecting four solutions of the KdV equation. At the level of the Ernst–Weyl equation, this Bäcklund transformation and the associated superposition principle yield directly a generalization of the single and double Harrison transformations of the Ernst equation, respectively. The very method of construction also allows for revealing, in an essentially algorithmic fashion, other integrability features of the main subsystems, such as their reduction to the Painlevé transcendents.
Kinetics of Te doping in disodering GaInP grown by organometallic vapor phase epitaxy
Authors: Jun S. W. , Stringfellow G. B. , Howard A. D. , Fetzer C. M. , Shurtleff J. K.
Published: July 2002

Te-doped GaInP epitaxial layers were grown by organometallic vapor phase epitaxy in an effort to clarify the Te disordering mechanism. CuPt ordered GaInP is produced under normal growth conditions. The addition of Te has been reported to induce disorder. One suggested mechanism for disordering GaInP is the increased step velocity caused by the addition of Te. To test this hypothesis, the effects of growth rate and growth temperature on the disordering effect of Te were studied. The Te/III ratio in the vapor and the partial pressure of the P precursor, tertiarybutylphosphine, were kept constant. The behavior of Te incorporation is found to be unusual. The decrease with increasing temperature is consistent with Te acting as a volatile impurity. However, the Te incorporation is also found to be inversely proportional to the growth rate, a characteristic of nonvolatile dopants. A suggested solution to this apparent contradiction is that the Te, which accumulates at step edges, is not able to keep pace with the steps when they move at the higher velocities. As the growth rate was decreased, with a corresponding decrease in measured step velocity, the degree of order was observed to increase, in support of this kinetic model. GaInP layers grown at higher temperatures were observed to become much less ordered. Analysis of these data indicates that the effect is due mainly to the effect of temperature on step velocity. The direct correlation between the step velocity and the degree of order, as these two growth parameters were varied, confirms that Te disorders GaInP for kinetic reasons.
PbZr
0.5
Ti
0.5
O
3
/La
0.5
Sr
0.5
CoO
3
heterostructures prepared by chemical solution routes on silicon with no fatigue polarization
Authors: Wang G. S. , Meng X. J. , Sun J. L. , Lai Z. Q. , Yu J. , Guo S. L. , Cheng J. G. , Tang J. , Chu J. H.
Published: July 2002

PbZr 0.5 Ti 0.5 O 3 /La 0.5 Sr 0.5 CoO 3 (PZT/LSCO) heterostructures have been grown directly on (100)Si by chemical solution routes. Field-emission scanning electron microscopy and x-ray diffraction analysis show that PZT and LSCO thin films are polycrystalline and entirely perovskite phase. PZT thin films grown on LSCO thin films showed no preferential orientation and excellent ferroelectricity. The remnant polarization Pr is about 22.8 μC/cm2 and the coercive field Ec is about 95 kV/cm at an applied electric field of 400 kV/cm. The ferroelectric capacitor has been fabricated and showed no polarization fatigue after 3×109 fatigue cycles. The Pt/PZT/LSCO capacitor is suitable for nonvolatile random access memory application.
Theoretical ab initio study of the water trimer anion: Ground and excited state
Authors: Chen Hsing-Yin , Sheu Wen-Shyan
Published: July 2002

The ground and excited state of the water trimer anion, (H2O)3−, are studied via high-level ab initio calculations. A systematic search for the stable (H2O)3− configuration is carried out, based on the configuration formed by the stable (H2O)2− plus one additional water molecule. Four isomers, including three surface structures and one interior structure, are located on the (H2O)3− potential energy surface. While the linear chainlike structure is determined to be the major species detected in supersonic expansion experiments, consistent with previous theoretical studies, the other three isomers may also exist in molecular beams as evidenced by the matches of their vertical detachment energies with high-energy shoulders observed in photoelectron spectra. A significant geometric distortion of water molecules directly interacting with the excess electron indicates that the Franck–Condon effects are the major cause for the experimental observation of the excitation of water vibrational modes upon the photodetachment of (H2O)3−. This is in contrast to the vibronic effects for the case of (H2O)2−. In addition, the excited states of (H2O)3− are also investigated. The results show the existence of electronically bound excited states in surface structures of (H2O)3−. These states possess considerable p character, and therefore, can be regarded as a precursor of three p-like excited states of bulk hydrated electrons.