bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of long-wavelength spin waves in exchange biased bilayers
Authors: Kim Joo-Von , Stamps R. L.
Published: July 2002

Long-wavelength spin waves are examined in an exchange biased ferromagnet/antiferromagnet bilayer. The frequencies of the excitations are calculated as a function of external field strength and orientation. It is suggested that the angular dependence of the frequencies can lead to linewidth enhancements, in agreement with recent experimental results. It is also shown that the study of the spin wave frequencies can yield the strengths of the antiferromagnet anisotropy and interlayer coupling.
Impact of exposure induced refractive index changes of photoresists on the photolithographic process
Authors: Erdmann Andreas , Henderson Clifford L. , Willson C. Grant
Published: July 2002

In many commercial and noncommercial photoresists the real and the imaginary parts of the refractive index are changed during exposure. Using a finite-difference beam-propagation algorithm, we analyze the impact of these nonlinear optical effects on the photolithographic process. Changes of the real part of the refractive index have a considerable impact on dose latitudes, sidewalls, swing curves, iso-dense bias and other process parameters. These effects become more dominant as the thickness of the resist layer increases.
Self-organized structures in thin liquid films on chemically heterogeneous substrates: Effect of antagonistic short and long range interactions
Authors: Zope Manoj , Kargupta Kajari , Sharma Ashutosh
Published: July 2002

Surface instability, dynamics, and morphology in spontaneous dewetting of a thin liquid film on a chemically heterogeneous substrate are studied based on nonlinear simulations for a system subjected to a long range van der Waals attraction and soft short-range repulsion. Characteristics of dewetting by a heterogeneity are clearly contrasted with the spinodal dewetting on a homogeneous surface. In the presence of a chemical heterogeneity, the instability is engendered by the gradient of intermolecular interactions that lead to a microscale wettability contrast. The time scale of instability can be substantially less than the spinodal time scale, especially for thinner films close to the critical thickness, and it varies inversely with the potential difference induced by the heterogeneity. Heterogeneity, on a very small length scale, can even destabilize a spinodally stable film. A local ordering of the structure (droplets and holes) around the heterogeneity produces “castle-moat,” “ripples,” and “flower” like morphologies, which are not predicted by the spinodal mechanism. A local enhancement of the long-range attraction on the patch leads to a longer ranged ordering of holes around the heterogeneous patch for relatively thicker films. In contrast, a decrease in the short-range repulsion on the heterogeneous patch encourages an even longer-ranged ordering of droplets for the thinner films.
Suppression of 90° spin switching in Co nanomagnets grown on the Cu(110)-{2×1}O surface
Authors: Tselepi M. , Bode P. J. , Xu Y. B. , Wastlbauer G. , Hope S. , Bland J. A. C.
Published: July 2002

We report on the effect of oxygen on the CO-induced 90° spin switching in the Co/Cu(110) system [Hope et al., Phys. Rev. B 57, 7454 (1998)]. The epitaxial fcc Co films were grown on the unsaturated Cu(110)-{2×1}O surface and their magnetic properties have been studied with in situ magneto-optic Kerr effect. The easy axis switch of the Co(110) films is suppressed when Co is grown on O exposed Cu surfaces. Scanning tunneling microscopy images of the film surface reveals the growth of elongated nanostructures preferentially oriented along the [001] direction, providing step-like edges for gas adsorption. The density of steps is similar to the density developing during growth on the clean Cu(110) surface but the step edges now run in the perpendicular direction. The suppression of the easy axis switch can be attributed to subtle changes of the number and adsorption probability of the available atop adsorption sites along the step edges for CO chemisorption. Our experiments highlight the remarkable chemical sensitivity and adsorption site dependence of the easy axis switch of the Co(110) nanomagnets.
InGaAs/InP quantum well intermixing studied by cross-sectional scanning tunneling microscopy
Authors: Chen Huajie , McKay H. A. , Feenstra R. M. , Aers G. C. , Poole P. J. , Williams R. L. , Charbonneau S. , Piva P. G. , Simpson T. W. , Mitchell I. V.
Published: July 2002

Cross-sectional scanning tunneling microscopy (STM) is used to study lattice matched InGaAs/InP quantum well (QW) intermixing induced by ion implantation and thermal annealing. Different strain development in QWs (determined by STM topography of elastic relaxation in cross sectionally cleaved samples) is found to be dependent upon the range of the implanted ions relative to the QWs. It is found that the quantum wells remain latticed matched to the barrier layers after intermixing when ions are implanted through the multiple quantum well (MQW) stack. A shallow implantation in which ions are implanted into the cap layer above the MQW stack leads to tensilely strained wells and compressively strained interfaces between wells and barriers. The strain development in the latter case is attributed to different degrees of interdiffusion on the group III and group V sublattices. Finite element elastic computations are used to extract the group V and group III interdiffusion length ratio, and results using different diffusion models are compared. A preferred group V interdiffusion in the case of shallow implantation is explained in terms of faster diffusing P related defects compared to In related defects. Images of as-grown QWs provide useful information about the growth technique related compositional fluctuations at the interfaces.
Neutron and x-ray evidence of charge melting in ferromagnetic layered colossal magnetoresistance manganites (invited)
Authors: Vasiliu-Doloc L. , Osborn R. , Rosenkranz S. , Mesot J. , Mitchell J. F. , Sinha S. K. , Seeck O. H. , Lynn J. W. , Islam Z.
Published: July 2002

Recent x-ray and neutron scattering studies have revealed static diffuse scattering due to polarons in the paramagnetic phase of the colossal magnetoresistive manganites La2−2xSr1+2xMn2O7, with x=0.40 and 0.44. We show that the polarons exhibit short-range incommensurate correlations that grow with decreasing temperature, but disappear abruptly at the combined ferromagnetic and metal–insulator transition in the x=0.40 system because of the sudden charge delocalization, while persisting at low temperature in the antiferromagnetic x=0.44 system. The “melting” of the polaron ordering as we cool through TC occurs with the collapse of the polaron scattering itself in the x=0.40 system. This short-range polaron order is characterized by an ordering wave vector q=(0.3,0,1) that is almost independent of x for x⩾0.38, and is consistent with a model of disordered stripes.
On the exact steepest descent method: A new method for the description of Stokes curves
Authors: Aoki Takashi , Kawai Takahiro , Takei Yoshitsugu
Published: July 2002

In order to determine the region where the Borel sum of a Wentzel–Kramers–Brillouin (WKB) solution ψ of an mth order (m⩾3) ordinary differential equation Pψ=0 with polynomial coefficients is well defined, we apply the steepest descent method to the Laplace integral of a WKB solution ψ̂ of P̂ψ̂=0, with P̂ denoting the Laplace transform of P. As a counterpart of the connection formula for ψ̂, we introduce a new notion of the exact steepest descent path, which is the union of bifurcated steepest descent paths. Both theoretical and computer-assisted experimental studies are given to show the importance of this notion.
Discrete approximations to integrals over unparametrized paths
Authors: Durhuus Bergfinnur , Jonsson Thordur
Published: July 2002

We discuss measures on spaces of unparametrized paths related to the Wiener measure. These measures arise naturally in the study of one-dimensional gravity coupled to scalar fields. Two kinds of discrete approximations are defined, the piecewise linear and the hypercubic approximations. The convergence of these approximations in the sense of weak convergence of measures is proven. We describe a family of sets of unparametrized paths that are analogous to cylinder sets of parametrized paths. Integrals over some of these sets are evaluated in terms of Dirichlet propagators in bounded regions.
Effect of moderate heating on the negative electron affinity and photoyield of air-exposed hydrogen-terminated chemical vapor deposited diamond
Authors: Piantanida G. , Breskin A. , Chechik R. , Katz O. , Laikhtman A. , Hoffman A. , Coluzza C.
Published: July 2002

The effect of moderate heating (200–300 °C) in vacuum on the photoemission from air-exposed hydrogen-terminated chemical vapor deposited diamond films was studied in the photon spectral range of 140–210 nm (8.9–5.9 eV). A three- to fivefold enhancement was observed, stable in high vacuum and in some high purity gases, but unstable in air. The surfaces were also examined by x-ray induced photoelectron spectroscopy and ultraviolet induced photoelectron spectroscopy before and after the heating process and upon exposure to air and to oxygen. The results provide good evidence that the strong dipole originating from H2O molecules absorbed on the diamond surface is responsible for the observed effect. A simple model is presented for quantitative estimation of the effect.
Single-electron tunneling in highly doped silicon nanowires in a dual-gate configuration
Authors: Tilke A. , Blick R. H. , Lorenz H. , Kotthaus J. P.
Published: July 2002

Lateral patterning of highly doped silicon-on-insulator films allows us to observe conductance oscillations due to single-electron charging effects. In our devices, silicon nanostructures are embedded into a metal–oxide–silicon configuration. The single-electron effects can be tuned both by an in-plane sidegate, as well as by a metallic topgate, a technology which is compatible with large-scale integration of single-electron devices with dimensions down to 10 nm. We compare the influence of different gating electrodes, important for ultralarge scale integration, on the electron islands.
Interdot interactions and band gap changes in CdSe nanocrystal arrays at elevated pressure
Authors: Kim Bosang S. , Islam Mohammad A. , Brus Louis E. , Herman Irving P.
Published: July 2002

Three-dimensional arrays of organically passivated CdSe nanocrystals were investigated under hydrostatic pressure using photoluminescence (PL) and absorption spectroscopies. Interdot separations were varied coarsely by varying the organic ligand on the nanocrystal and finely by applying hydrostatic pressure. The PL and absorption spectra of solutions and arrays of CdSe nanocrystals capped by either tri-n-octylphosphine oxide or tri-n-butylphosphine oxide are the same up to 60 kbar, which suggests that they exhibit no interdot coupling since the interdot separations in the solutions (∼50 nm) are much greater than those in the arrays (≲1 nm). While the variation with pressure is roughly that expected from the increase in band gap energy of bulk CdSe with pressure and the increase in confinement energies of electrons and holes with increased pressure, there is still a significant difference in the energy of the PL peak and the first exciton in absorption (the Stokes shift) for both these solutions and arrays that increases with pressure. This is attributed mostly to increased vibrational relaxation due to the movement of nuclei in the excited state. In contrast, there is a distinct difference between the pressure dependence of CdSe/pyridine dots in solution and arrays; the increase of the energy of the first exciton peak in absorption with pressure becomes markedly slower above about 30 kbar in CdSe/pyridine arrays, and is lower than that in the corresponding solution by ∼50 meV at 50 kbar and ∼70 meV at 60 kbar. Experiments with CdSe/shell/pyridine dots, with large electron and hole barriers, cast doubt on the mechanism of interdot electron and/or hole tunneling leading to a decrease in electron and/or hole confinement energy. Also, interdot tunneling of single carriers may be inhibited by the charge separation energy. The differences in the dielectric medium surrounding each dot in the solution and array explain their different absorption exciton energies at ambient pressure, but not the changes at elevated pressure. The observed loss of much of the pyridine ligands during array drying could be very significant, and contact between pyridine-capped dots at elevated pressure may be important.
The Painlevé analysis for N=2 super Korteweg–de Vries equations
Authors: Bourque S. , Mathieu P.
Published: July 2002

The Painlevé analysis of a generic multiparameter N=2 extension of the Korteweg–de Vries (KdV) equation is presented. Unusual aspects of the analysis, pertaining to the presence of two fermionic fields, are emphasized. For the general class of models considered, we find that the only ones which manifestly pass the test are precisely the four known integrable supersymmetric KdV equations, including the SKdV1 case.
Relaxation spectra in poly(methylmethacrylate): Comparison of acoustic attenuation and light scattering data
Authors: Caliskan G. , Kisliuk A. , Novikov V. N. , Sokolov A. P.
Published: July 2002

Relaxation processes in polymethylmethacrylate were investigated using light scattering and mechanical relaxation techniques. The acoustic attenuation data cover the frequency range from 1 Hz to 17 GHz and the light scattering from 0.5 to 10 000 GHz. Analysis of the data in a broad temperature range (80–350 K) shows that the model of the asymmetric double-well potentials, traditionally used for description of the acoustic attenuation in glasses, cannot describe all the data consistently. It is shown that assuming an additional relaxation in GHz region with a constant loss spectrum provides a good agreement of the model with experimental data.
Quantum Monte Carlo studies of the structure and spectroscopy of NenOH (Ã 2Σ+, n=1–4) van der Waals complexes
Authors: Lee Hee-Seung , McCoy Anne B.
Published: July 2002

Adiabatic rigid-body diffusion Monte Carlo is used to study the structure and spectroscopy of complexes of OH(Ã 2Σ+) with several neon atoms. Although the potential energy surfaces for these systems have many low-lying minima, the ground state wave functions are localized in the global minimum. This trend is found to persist in the first few vibrationally excited states of Ne2OH/D. Low-lying vibrational states that are localized in the potential minimum that corresponds to the linear Ne–OH/D–Ne configuration of the complex are also found.
X-ray scattering evidence for the structural nature of fatigue in epitaxial Pb(Zr, Ti)O3 films
Authors: Thompson Carol , Munkholm A. , Streiffer S. K. , Stephenson G. B. , Ghosh K. , Eastman J. A. , Auciello O. , Bai G.-R. , Lee M. K. , Eom C. B.
Published: July 2002

We have probed the microscopic distribution of 180° domains as a function of switching history in 40 nm epitaxial films of Pb(Zr0.30Ti0.70)O3 by analyzing interference effects in the x-ray scattering profiles. These as-grown films exhibit voltage offsets (imprint) in the polarization hysteresis loops, coupled with a strongly preferred polarization direction in the virgin state. Our x-ray results are consistent with models attributing the loss of switchable polarization to the inhibition of the formation of oppositely polarized domains in a unipolar matrix. Using such model epitaxial films, we demonstrate that different microscopic ensembles of domains resulting from, for example, fatigue, may be resolved by this technique.
Asymmetric bias voltage dependence of the magnetoresistance of Co/Al2O3/Co magnetic tunnel junctions: Variation with the barrier oxidation time
Authors: Oepts W. , Gillies M. F. , Coehoorn R. , van de Veerdonk R. J. M. , de Jonge W. J. M.
Published: July 2002

Recently it has been observed that the magnetoresistance (MR) of plasma oxidized exchange biased Co/Al2O3/Co tunnel junctions can have a strongly asymmetric bias voltage (Vbias) dependence. In this article we report on the dependence of this phenomenon on barrier oxidation time tox. For junctions based on 1.5 nm Al, tox was varied from 20 to 120 s. For tox=20 s, for which the MR is approximately 20% at Vbias=0, and for tox⩾90 s symmetric MR(Vbias) curves are found, with the MR decreasing monotonically with |Vbias|. A strong asymmetric bias voltage dependence was observed for intermediate oxidation times, which correspond to essentially full oxidation of the Al layer, but almost no formation of stoichiometric CoO at the bottom electrode. Samples with tox=60 s show even an asymmetric double peak in MR(Vbias). Due to its strength, it has an important consequence for device applications: for a series of junctions with variable tox the maximum signal voltage (at a fixed current) is not necessarily obtained for junctions which have the largest MR ratio at Vbias=0.
Time-resolved photoluminescence study of InGaAs/GaAs quantum wells on (111)B GaAs substrates under magnetic fields
Authors: Tsai F. Y. , Lee C. P. , Voskoboynikov O. , Cheng H. H. , Shen Jinxi , Oka Yasuo
Published: July 2002

The exciton dynamics in InxGa1−xAs/GaAs quantum wells grown on (111)B and (100) GaAs substrates were studied by time-resolved photoluminescence (PL) under magnetic fields in a Faraday configuration. We have found that the piezoelectric fields in (111)B samples affect the transient behavior of the PL spectra. Compared with the reference (100) sample, we have found that the strong piezoelectric fields, as well as the magnetic fields, cause a slower spin–flip process in (111)B strained quantum wells.
Role of water and oxygen in wet and dry oxidation of diamond
Authors: Larsson K. , Björkman H. , Hjort K.
Published: July 2002

The wetting properties of diamond surfaces have been theoretically and experimentally investigated in the temperature range of 200–700 °C. It was experimentally observed that the conversion from hydrophobic to hydrophilic surfaces will be more efficient with an increase in temperature when using a mixture of H2O and O2 in a wet-oxidation furnace, or only O2 in a dry-oxidation furnace. This conversion is especially efficient at temperatures higher than 400 °C. Molecular dynamic simulations of the wet- and dry-oxidation processes at 450 °C, together with a more thorough theoretical quantum mechanical study of various important adsorption processes at zero K, resulted in two favorable adsorption processes, dissociative adsorption of H2O and O2, respectively. Surface reactions including gaseous H2O were found to be responsible for the large hydrophilic properties of diamond surfaces at lower temperature, while the corresponding surface reactions including gaseous O2 species were most probably the dominant ones at higher temperatures. These results were strongly supported by the comparative experiments presented in this article.
Modeling of plasma dynamics at the air-water interface: Application to laser shock processing
Authors: Mazhukin V. I. , Nossov V. V. , Smurov I.
Published: July 2002

The gas-dynamic expansion stage of the plasma at the air-water interface is studied numerically for the setup corresponding to the laser shock processing of materials in the water-confined regime. The plasma is induced by a laser radiation of the intensity range 4–17 GW/cm2 at the 1.06 and 0.353 μm laser wavelength. A mathematical description of the plasma is performed in the frame of transient two-dimensional radiative gas dynamics, which incorporates the system of gas-dynamic equations and the radiation transfer equation. The studies performed indicate that the plasma evolution significantly depends on the laser wavelength. For the IR laser effect the expansion mechanism is the fast propagation of the ionization wave toward the laser source, and for the UV laser effect the laser supported detonation wave is formed. The plasma radiation contributes significantly to the redistribution of energy inside the plasma domain and, for the UV effect, forms the domain of preionization ahead of the shock wave. In both cases the plasma becomes opaque: for the IR effect it occurs over a very short period of time, 3–5 ns, while for the UV effect the process takes much longer. When the laser intensity is increased, the peak intensity and the duration of the transmitted pulse tend to reach a saturation level.
On the role of electrostatics in the heterolytic splitting of covalent bonds at defective oxide surfaces
Authors: D’Ercole Annalisa , Ferrari Anna Maria , Pisani Cesare
Published: July 2002

A model is proposed for the heterolytic dissociation of covalent bonds at the surface of perfect or defective oxide surfaces essentially based on the analysis of the electrostatic potential and its gradient at the bare surface. It permits us to obtain in a semiquantitative way the equilibrium geometry of the fragments and the reaction energy, and gives clues for the identification of the reaction path and for a rough estimate of the related activation energy. The predictive ability of the model has been verified by performing a number of calculations to simulate H2 dissociation at various defects at MgO and CaO: the isolated oxygen vacancy, the divacancy at the (100) face, the infinite edge and the divacancy at the edge. All calculations have been performed at an ab initio Hartree–Fock level of approximation, using the CRYSTAL program for the periodic structures and the EMBED program for the local defects. Generally satisfactory agreement is found between the model predictions and the results of actual calculations. The model could be useful for predicting with low cost computations if and how a local structure at a defective oxide surface is capable or not of heterolytically dissociating strong covalent bonds.
Spatiotemporal stochastic resonance and its consequences in neural model systems
Authors: Balázsi Gábor , Kish László B. , Moss Frank E.
Published: July 2002

The realization of spatiotemporal stochastic resonance is studied in a two-dimensional FitzHugh–Nagumo system, and in a one-dimensional system of integrate-and-fire neurons. We show that spatiotemporal stochastic resonance occurs in these neural model systems, independent of the method of modeling. Moreover, the ways of realization are analogous in the two model systems. The biological implications and open questions are discussed.
Noise-assisted traffic of spikes through neuronal junctions
Authors: Ruszczynski Peter S. , Kish Laszlo B. , Bezrukov Sergey M.
Published: July 2002

The presence of noise, i.e., random fluctuations, in the nervous system raises at least two different questions. First, is there a constructive role noise can play for signal transmission in a neuron channel? Second, what is the advantage of the power spectra observed for the neuron activity to be shaped like 1/fk? To address these questions a simple stochastic model for a junction in neural spike traffic channels is presented. Side channel traffic enters main channel traffic depending on the spike rate of the latter one. The main channel traffic itself is triggered by various noise processes such as Poissonian noise or the zero crossings of Gaussian 1/fk noise whereas the variation of the exponent k gives rise to a maximum of the overall traffic efficiency. It is shown that the colored noise is superior to the Poissonian and, in certain cases, to deterministic, periodically ordered traffic. Further, if this periodicity itself is modulated by Gaussian noise with different spectral exponents k, then such modulation can lead to noise-assisted traffic as well. The model presented can also be used to consider car traffic at a junction between a main and a side road and to show how randomness can enhance the traffic efficiency in a network.
Design optimized bistable twisted nematic liquid crystal display
Authors: Wang Bin , Bos Philip J.
Published: July 2002

The difference between the equilibrium and switching d/p ratio of a bistable twisted nematic (BTN) liquid crystal display is shown by computer simulation. It is clearly indicated that the optimized d/p ratio has to be determined by the switching rather than equilibrium d/p ratio. It is also shown that the switching d/p ratio is related not only to the pre-tilt angle, but also to liquid crystal viscosities, which indirectly indicates how temperature will affect the BTN device, and that there exists an optimized delay time for the device-driving waveform which will narrow the selection pulse and increase the matrix driving scanning speed. Further, the trade-off between the width of the waveform selection pulse and cell thickness tolerance is considered. By applying the determined optimized conditions and using the multidimensional alignment method, a long-term bistable BTN effect can be demonstrated.
Three-dimensional strain field calculations in multiple InN/AlN wurtzite quantum dots
Authors: Jogai B.
Published: July 2002

A detailed calculation of the three-dimensional elastic strain field and Gibbs free energy in and around InN/AlN wurtzite quantum dots is presented. The strain tensor is calculated by minimizing the Helmholtz free energy on a three-dimensional grid. The boundary conditions for a free surface are rigorously implemented to enable the strain field and Gibbs free energy at the surface to be modeled realistically. This has implications for the growth of additional layers of dots above a seed layer and can serve as an arbiter for determining possible nucleation sites. Results are presented for a single dot as well as coupled dots. The Gibbs free energy is seen to exhibit strong minima directly above a layer of seed dots, facilitating vertical ordering. Under certain conditions, satellite minima can also occur. Using the calculated strain field, the piezoelectric polarization field is also calculated. Because of the strong lattice mismatch, the strain field is quite large, particularly near the base and apex of the dots. This, in turn, leads to piezoelectric charges with magnitudes as high as 1014 cm−2 in regions of high strain.
A two-dimensional polymer growth model
Authors: Vogt Marc , Hernandez Rigoberto
Published: July 2002

A polymer growth Hamiltonian with an accompanying novel lattice has been constructed to model reaction dynamics of polydisperse polymer systems that have kinetics affected by an environment which is altered over time by the polymerization process itself. Monte Carlo dynamics are used to simulate the polymerization process with bond breaking/formation moves analogous to a correlated percolation model. Lattice events thus occur on the time scale of reactivity rather than the much shorter time scales of either monomer motion or conformational dynamics, though an effort is made to include the latter as well. The equilibrium behavior of the system has been well mapped out and is complex enough to model a variety of real polymer systems such as solid state polymerization, living polymers and thermosetting polymers. Detailed equilibrium structural information such as molecular weight distributions and extent of cross polymerization have been obtained. Pseudodynamic information such as the time-dependence in the polymer weight distribution and more detailed quantities may also be obtained with respect to Monte Carlo time scales.
Photoluminescence yield and decay time of proton irradiated and thermally annealed a-Si0.35C0.65:H alloys: A phenomenological model
Authors: Baeri P. , Malvezzi A. M. , Reitano R.
Published: July 2002

We have studied the yield and the time evolution of pulsed laser induced photoluminescence in proton irradiated and thermally annealed amorphous hydrogenated silicon carbon alloys prepared by plasma enhanced chemical vapor deposition. Three major fluorescence decay channels have been observed with decay rates independent from proton irradiation and thermal annealing. Lack of correlation between yield and average decay time suggests a very simple phenomenological model which allows evaluation of the nonradiative time constant which is found to be linearly correlated with the photoluminescence yield. Our model suggests that radiative recombination occurs via exciton decay while the nonradiative recombination is driven by the trapping of carriers in defects states.
Model for band-edge electroluminescence from metal–oxide–semiconductor silicon tunneling diodes
Authors: Chen Miin-Jang , Liang Eih-Zhe , Chang Shu-Wei , Lin Ching-Fuh
Published: July 2002

A detailed model is proposed to explain the electroluminescence spectrum from metal–oxide–silicon tunneling diodes. This model includes phonon-assisted processes and exciton involvement. According to this model, the main peak and the low-energy tail of the electroluminescence spectrum are attributed to the transverse optical phonon and the two-phonon assisted recombination, respectively. With very few fitting parameters, the model accurately predicts the measured electroluminescence spectra.
A many-body analysis of the effects of the matrix protons and their diffusional motion on electron spin resonance line shapes and electron spin echoes
Authors: Nevzorov Alexander A. , Freed Jack H.
Published: July 2002

The method for treating the evolution of the density matrix developed in the accompanying paper for many-spin systems is applied here for calculating magnetic resonance signals of a spin A interacting with a bath of N identical spins B. Spins B are assumed to have much smaller gyromagnetic ratios than the spin A (e.g., the former are nuclear spins, I and the latter is an electron spin, S). The experimentally observed quadratic dependence of the spin-echo envelope decay on concentration and time is explained from considering the dipolar coupling of spin A to all the B spins in the presence of B–B dipolar interactions. It is shown that the spin-echo envelope decay in the rigid limit is due to the interaction of the A spin with the coherent many-body states of the coupled spins B via the nuclear flip-flop terms I±I∓, which becomes a dissipative mechanism in the thermodynamic limit. This represents a more rigorous analysis than simplified models based on an incoherent version of “spin diffusion,” and it leads to good quantitative agreement with experiment. Moreover, this analysis represents a unified description of both the modulation and decay of the A-spin echoes. Spin echoes and line shapes for the A–BN systems are also calculated for finite motions which randomize the B spins. Even for very slow motions (modeled as translational diffusion) an effective mechanism for spin-echo envelope decay is generated, which readily overtakes the coherent mechanism in importance. The intensity distribution for the forbidden components in the A-spin line shape resulting from multiquantum transitions of the B spins caused by the pseudosecular interaction terms SzI±, is calculated. In the rigid limit it is found to behave like a Poisson distribution.
Cognition friendly interaction: A concept of partnership in human computer interaction
Authors: Das Balaram
Published: July 2002

This paper identifies yet another field of research, the discipline of human computer interaction, where the concept of self-similar fluctuations can play a vital role. A concept of interaction between computation and cognition is developed that is friendly toward the cognitive process. It is argued that friendly interactions must have a memory and be antipersistent. To cast this in a mathematical form, fluctuations in the interactions recorded over a period of time are studied, and it is shown that these fluctuations must necessarily be self-similar with the value of the self-similarity parameter confined to the interval (0, 1/2), for the interaction to be friendly. A statistical measure of complexity, of the interaction process, is also formulated as a function of the self-similarity parameter. Finally the question is raised as how to build a friendly software and a possible evolutionary process through which friendly softwares may emerge is indicated.
Analytical correction for temperature and diode characteristics for application in a wide dynamic range logarithmic electrometer
Authors: Acharya Y. B.
Published: July 2002

A study has been carried out on the I–V characteristics of junction diodes for low current applications. An analytical correction for temperature on the forward voltage is estimated using experimentally derived parameters. The parameters can all be easily measured. The analytical correction is found to be accurate within 4% for a current range of 10−11–10−4 A and for a temperature range of −30–70 °C. This study will be useful for logarithmic electrometers for low current measurement where space and power are at a premium.
Variational Sturmian approximation: A nonperturbative method of solving time-independent Schrödinger equation
Authors: Mostafazadeh Ali
Published: July 2002

A variationally improved Sturmian approximation for solving time-independent Schrödinger equation is developed. This approximation is used to obtain the energy levels of a quartic anharmonic oscillator, a quartic potential, and a Gaussian potential. The results are compared with those of the perturbation theory, the WKB approximation, and the accurate numerical values.
Finite-dimensional integrable systems related to the n-wave interaction equations
Authors: Shi Qi-Yan
Published: July 2002

Under a constraint between the potentials and the eigenfunctions, Lax pairs and adjoint Lax pairs of a soliton hierarchy associated with the n×n generalized Zakharov–Shabat eigenvalue problem are transformed into a spatial finite-dimensional Hamiltonian system and a hierarchy of temporal finite-dimensional Hamiltonian systems. The Lax representations, r-matrix structure and integrals of motion are explicitly presented. These integrals of motion are functionally independent and in involution in pairs, which shows that these systems, especially the whole hierarchy of temporal finite-dimensional Hamiltonian systems, are Liouville integrable.
High speed laser shadowgraphy for electromagnetically driven cylindrical implosions
Authors: Rodriguez G. , Roberts J. P. , Echave J. A. , Taylor A. J.
Published: July 2002

A laser shadowgraphy system for high-speed imaging of a convergent cylindrical shockwave generated by an electromagnetically driven solid density liner implosion in Lucite is described. The laser shadowgraphy system utilizes an advanced high-energy, long-pulse, frequency-doubled Nd:YAG laser for target illumination and a fast framing camera for multiple frame imaging of the shockwave as it radially converges and transits the Lucite. The time window resolution is 10 ns as determined by the fastest exposure time capable with the camera. Two on-axis symmetric implosions and two off-axis asymmetric implosion experiments were fielded at the Air Force Research Laboratory’s Shiva Star 4.2 MJ capacitor bank z-pinch facility. For each experimental shot, the shadowgraphy system captured several frames of shadowgraph images as the shockwave moved through the Lucite. Analysis of the shockwave shadowgraph image shapes is done by fitting each shadowgraph image to a generic elliptical fit function and plotting the resultant two-dimensional image fits for comparison. For the on-axis symmetric implosion shots, a radial trajectory plot is extracted and a radial shock velocity is calculated. The Lucite shock speed is seen to increase monotonically from an initial velocity of 7.9 mm/μs to a near final velocity of 13.4 mm/μs as convergence effects dominate the shock speed calculated at small radii.
Comparison of collisionless macroscopic models and application to the ion–electron instability
Authors: Ahedo E. , Lapuerta V.
Published: July 2002

In a first part, different macroscopic models of linear Landau damping are compared using a concise one-dimensional (1-D) collisionless formulation. The three-moment model of Chang and Callen (CC) [Phys. Fluids B 4, 1167 (1992)] with two closure relations (complex in the Fourier space) for the viscous stress and the heat conduction is found to be equivalent to the two-moment model of Stubbe–Sukhorukov (SS) [Phys. Plasmas 6, 2976 (1999)], which uses a single (complex) closure relation for the pressure. The comparison of the respective closure relations favors clearly the SS pressure law, which associates an anomalous resistivity to the Landau damping. In a second part, a macroscopic interpretation, with the SS model, of the ion–electron instability shows its resistive character for low and intermediate drift velocities, and the transition to the reactive Buneman limit. The pressure law for the electrons is found to verify a simple law, whereas approximate laws are discussed for the ion pressure. These laws are used to close a macroscopic model for stability analyses of nonhomogeneous plasma structures, where SS and CC models are not applicable easily.
Optical properties of CeO2 films prepared from colloidal suspension
Authors: Petrovsky V. , Gorman B. P. , Anderson H. U. , Petrovsky T.
Published: July 2002

Optical absorption of nanocrystalline thin films can be influenced by the presence of both porosity and grain size effects. If both are present simultaneously, their effects are difficult to separate. In this study it is shown that the combination of uv-vis transmittance and reflectance measurements on porous CeO2 films provides enough data to make this separation. The CeO2 films were prepared by deposition of nanosized (∼5 nm) particles from a water colloidal suspension onto sapphire and subjecting these films to sintering temperatures sufficiently high to provide a series of films with a typical thickness of 0.6 μm with a wide range of grain sizes and porosity. X-ray diffraction, scanning electron microscopy, ellipsometry, and profilometry were used to characterize the films and to compare the observed grain sizes and porosity with that obtained from optical measurements. All of the techniques used gave results on porosity and grain size which were in good agreement, from 15% to 50% and 5 to 65 nm, respectively. For these porous films it was found that the changes in absorption which are normally explained by quantum confinement effects due to the small crystallite size can be attributed primarily to changes in porosity rather than in grain size.
Evolution of hydrogen and helium co-implanted single-crystal silicon during annealing
Authors: Duo Xinzhong , Liu Weili , Zhang Miao , Wang Lianwei , Lin Chenglu , Okuyama M. , Noda M. , Cheung Wing-Yiu , Wong S. P. , Chu Paul K. , Hu Peigang , Wang S. X. , Wang L. M.
Published: July 2002

H + was implanted into single-crystal silicon with a dose of 1×1016/cm2 and an energy of 30 KeV, and then He+ was implanted into the same sample with the same dose and an energy of 33 KeV. Both of the implantations were performed at room temperature. Subsequently, the samples were annealed in a temperature range from 200 to 450 °C for 1 h. Cross-sectional transmission electron microscopy, Rutherford backscattering spectrometry/channeling, elastic recoil detection, and high resolution x-ray diffraction were employed to characterize the strain, defects, and the distribution of H and He in the samples. The results showed that co-implantation of H and He decreases the total implantation dose, with which the surface could exfoliate during annealing. During annealing, the distribution of hydrogen did not change, but helium moved deeper and its distribution became sharper. At the same time, the maximum of the strain in the samples decreased a lot and also moved deeper. Furthermore, the defects introduced by ion implantation and annealing were characterized by slow positron annihilation spectroscopy, and two positron trap peaks were found. After annealing, the maximum of these two peaks decreased at the same time and their positions moved towards the surface. No bubbles or voids but cracks and platelets were observed by cross-sectional transmission electron microscopy. Finally, the relationship between the total implantation dose and the fraction of hydrogen in total implantation dose was calculated.
Sputtered silver oxide layers for surface-enhanced Raman spectroscopy
Authors: Büchel D. , Mihalcea C. , Fukaya T. , Atoda N. , Tominaga J. , Kikukawa T. , Fuji H.
Published: July 2002

We present results of reactively sputtered silver oxide thin films as a substrate material for surface-enhanced Raman spectroscopy (SERS). Herein, we show that deposited layers develop an increasingly strong SERS activity upon photoactivation at 488 nm. A benzoic acid/2-propanol solution was used to demonstrate that the bonding of molecules to SERS active sites at the surface can be followed by investigating temporal changes of the corresponding Raman intensities. Furthermore, the laser-induced structural changes in the silver oxide layers lead to a fluctuating SERS activity at high laser intensities which also affects the spectral features of amorphous carbon impurities.
Monte Carlo based analysis of intermodulation distortion behavior in GaN–AlxGa1−xN high electron mobility transistors for microwave applications
Authors: Li T. , Joshi R. P. , del Rosario R. D. , Fazi C.
Published: July 2002

Monte Carlo based calculations of the large-signal nonlinear response characteristics of GaN–AlxGa1−xN high electron mobility transistors with particular emphasis on intermodulation distortion (IMD) have been performed. The nonlinear electrical transport is treated on first principles, all scattering mechanisms included, and both memory and distributed effects built into the model. The results demonstrate an optimal operating point for low IMD at reasonably large output power due to a minima in the IMD curve. Dependence of the nonlinear characteristics on the barrier mole fraction x is also demonstrated and analyzed. Finally, high-temperature predictions of the IMD have been made by carrying out the simulations at 600 K. An increase in dynamic range with temperature is predicted, due to a relative suppression of interface roughness scattering.
Highly sensitive recognition element based on birefringent porous silicon layers
Authors: Gross E. , Kovalev D. , Künzner N. , Timoshenko V. Yu. , Diener J. , Koch F.
Published: July 2002

Anisotropically nanostructured silicon layers exhibit a strong in-plane birefringence. Their optical anisotropy parameters are found to be extremely sensitive to the presence of dielectric substances inside of the pores. Polarization-resolved transmittance measurements provide an extremely sensitive tool to analyze the adsorption of various atoms and molecules in negligible quantities. A variation of the transmitted linearly polarized light intensity up to two orders of magnitude combined with a fast optical response in the range of seconds make these layers a good candidate for sensor applications.
Resonant spectroscopy of electric-field-induced superlattices
Authors: Vitusevich S. A. , Förster A. , Lüth H. , Belyaev A. E. , Danylyuk S. V. , Konakova R. V. , Sheka D. I.
Published: July 2002

A system of interdigital gates is used to create a periodic potential profile in a multilayer heterostructure. The electrostatic problem for the spatial distribution of the potential is solved and experimentally examined by measurements of current–voltage characteristics of resonant-tunnelling diodes embedded in the depletion region of the Schottky contact. It is shown that the position of the resonant peak voltage is sensitive to the spatial potential distribution and that with appropriate parameters of the heterostructure the sensitivity of the gates can be considerably enhanced.
Molecule frame photoelectron angular distributions from oriented methyl chloride and methyl fluoride molecules
Authors: Hikosaka Yasumasa , Eland John H. D. , Watson Tim M. , Powis Ivan
Published: July 2002

Molecule-frame photoelectron angular distributions are reported for the Ã band photoionization of CH3Cl and CH3F molecules whose spatial orientation is effectively fixed using an electron–ion recoil vector correlation technique. Measurements are made at various photon wavelengths with the polarization set both perpendicular and parallel to the molecular axis. Subsidiary measurements on the lab-frame distributions of photoelectrons and photofragment ions are also presented. An extensive comparison is made with the results of a multiple scattering calculation of the photoionization dynamics and a convincing description of the data is obtained. Scattering influences arising in the photon–electron and electron–ion core interactions can be distinguished as the polarization geometry and identity of the halogen atoms is varied.
Polymorphous Si thin films from radio frequency plasmas of SiH4 diluted in Ar: A study by transmission electron microscopy and Raman spectroscopy
Authors: Viera G. , Huet S. , Bertran E. , Boufendi L.
Published: July 2002

In this study, we present a detailed structural characterization by means of transmission electron microscopy and Raman spectroscopy of polymorphous silicon (pm-Si:H) thin films deposited using radio-frequency dust-forming plasmas of SiH4 diluted in Ar. Square-wave modulation of the plasma and gas temperature was varied to obtain films with different nanostructures. Transmission electron microscopy and electron diffraction have shown the presence of Si crystallites of around 2 nm in the pm-Si:H films, which are related to the nanoparticles formed in the plasma gas phase coming from their different growth stages, named particle nucleation and coagulation. Raman scattering has proved the role of the film nanostructure in the crystallization process induced “in situ” by laser heating.
Potential energy curves and dipole transition moments for electronic states of ArHe and HeNe
Authors: Petsalakis Ioannis D. , Theodorakopoulos Giannoula , Liebermann Heinz-Peter , Buenker Robert J.
Published: July 2002

Relativistic core-potential calculations have been carried out on the ground and excited Ω states of ArHe and HeNe correlating with the atomic limits Ar*(3p54s, 3P,1P) and Ne*(2p53s, 3P,1P), respectively, and with ground-state He atoms. The potential energy curves of the excited states of ArHe are repulsive. The potential energy curves of the 0−(I), 1(I) and 0+(II)2pσ→3s states of HeNe show a local Rydberg minimum at 2.8 bohr and a barrier at 4.0 bohr, while the other excited state potentials of HeNe are repulsive. For both ArHe and HeNe, large dipole transition moments are calculated connecting the ground state with states dissociating to atomic limits from which radiative transitions to the atomic ground state are allowed. Small but nonzero dipole transition moments are obtained for short internuclear distances for the transitions from the 1(I) state, which correlates with the metastable P23 state of Ar (in ArHe) or Ne (in HeNe). The radiative lifetime of the 1(I) state of HeNe (estimated from the vertical transition at 2.8 bohr) is 18 μs.
Approach of the critical point of gas–liquid transitions in an electrostatically stabilized colloidal suspension
Authors: Cousin F. , Dubois E. , Cabuil V.
Published: July 2002

We report on the study of the critical point of gas–liquid transitions in an electrostatically stabilized colloidal suspension. Suspensions are constituted of spherical magnetic nanoparticles dispersed in water and stabilized by electrostatic repulsions. The coexistence line is constructed on the Π-V diagram (Π: osmotic pressure, Φ: volume fraction) by the determination of transitions’ threshold by optical microscopy on a large range of volume fractions. This coexistence line presents a maximum that corresponds to a “critical area” where density fluctuations are seen at the microscopic scale. The measurement of these density fluctuations when reaching transitions by small angle neutron scattering (SANS) shows that the transition is of the second order in the critical area and of the first order out of the critical area. SANS measurements also allow to conclude that the interparticle potential is attractive at long range near the coexistence lines of the phase diagram. Long-range attractions are due to dipolar magnetic interactions between particles. Such a potential authorizes gas–liquid transitions and thus the existence of a liquid colloidal phase.