bluuebunny/crossref_metadata_2025 huggingface dataset:
Spin-glass and random-field effects in exchange-biased NiFe film on a NiO single-crystal substrate
Authors: Araújo A. E. P. de , Machado F. L. A. , Rodrigues A. R. , Azevedo A. , de Aguiar F. M. , de Almeida J. R. L. , Rezende S. M. , Egelhoff W. F.
Published: July 2002

Magnetization (M) data on a ferromagnetic/antiferromagnetic (FM/AF) sample made of a permalloy thin film sputtered on a thick NiO single-crystal substrate reveal remarkable irreversibilities. The field, temperature (T), and time dependencies of M were measured after the sample was cooled in zero-field or in the presence of a field, keeping T below the Néel temperature, with the field applied in the plane, either perpendicular or parallel to the anisotropy direction. While the behavior observed in the perpendicular configuration is characteristic of spin glasses, in the parallel configuration it exhibits features of random-field systems. The observed metastabilities and thermal history dependencies are shown to result from frustrations and randomness due to the interface roughness.
Dispersion relation and growth in a free-electron laser with planar wiggler and in-channel guiding
Authors: Hwang Un-Hak , Mehdian Hassan , Willett Joseph E. , Aktas Yildirim M.
Published: July 2002

A theory of a planar wiggler free-electron laser with ion-channel guiding is developed. An analysis of the quasi-steady-state electron trajectories is first obtained by solving the equations of motion for an electron in the ion-channel electrostatic field and the wiggler magnetostatic field. Next a sixth-degree polynomial dispersion equation for electromagnetic and space-charge waves in the wiggler is derived. Numerical solutions of the polynomial equation yield the complex wave number as a function of the frequency of the waves. These results are used to illustrate the dependence of growth rate–frequency curves on the ion-channel frequency, and the peak growth rate and corresponding wave frequency as functions of the ion-channel frequency.
Time-resolved Kerr measurements of magnetization switching in a crossed-wire ferromagnetic memory element
Authors: Zelakiewicz S. , Krichevsky A. , Johnson Mark , Freeman M. R.
Published: July 2002

Detailed knowledge of magnetization reversal dynamics in lithographically patterned, micrometer-scale thin film ferromagnetic elements is of basic interest, and is relevant to the development of integrated device applications such as nonvolatile random access memory. We report a time-resolved Kerr microscopy study of the magnetic response of a 1×7 μm2 FeCo element inductively coupled to two crossed write wires. Beginning from a remanent state, switching rates are measured for single pulses of various amplitudes, and for two pulses with fixed amplitude and variable overlap. While using pulse amplitudes near the switching threshold, we observe the magnetization plateauing below the saturation level which we interpret as probabilistic switching caused by thermal fluctuations.
n:m
phase synchronization with mutual coupling phase signals
Authors: Chen J. Y. , Wong K. W. , Shuai J. W.
Published: July 2002

We generalize the n:m phase synchronization between two chaotic oscillators by mutual coupling phase signals. To characterize this phenomenon, we use two coupled oscillators to demonstrate their phase synchronization with amplitudes practically noncorrelated. We take the 1:1 phase synchronization as an example to show the properties of mean frequencies, mean phase difference, and Lyapunov exponents at various values of coupling strength. The phase difference increases with 2π phase slips below the transition. The scaling rules of the slip near and away from the transition are studied. Furthermore, we demonstrate the transition to a variety of n:m phase synchronizations and analyze the corresponding coupling dynamics.
Emission from BaTiO3:Pr3+ controlled by ionic radius of added trivalent ion
Authors: Okamoto Shinji , Yamamoto Hajime
Published: July 2002

Emission enhancement from BaTiO3:Pr3+ by addition of trivalent ion under excitation above the fundamental absorption edge of BaTiO3 host crystals has been investigated. In this system, energy transfer occurs from BaTiO3 to Pr3+. Emission of Pr3+ is intensified by addition of small trivalent ions, e.g., Yb3+ or Sc3+, which substitutes for Ti4+ site, while it is hardly changed by addition of La3+ or Nd3+, which is larger in the ionic radius and substitutes for Ba2+ site. Moreover, smaller difference in ionic radius between the added trivalent ions and Ti4+ induces larger ratio of D21→H43 emission intensity to P03→H43 emission intensity. Addition of Ga3+ gives the largest ratio and the D21→H43 emission intensity among the trivalent ions. These observations demonstrate that one of the origins of the emission enhancement is charge compensation of Pr3+ at the Ba2+ site by the trivalent ion substituting for Ti4+ around the Pr3+, and the emission from BaTiO3:Pr3+ can be controlled by the ionic radius of the added trivalent ion.
Extrapolating the coupled-cluster sequence toward the full configuration-interaction limit
Authors: Goodson David Z.
Published: July 2002

Extrapolation methods that accelerate the convergence of coupled-cluster energy sequences toward the full configuration–interaction (FCI) limit are developed and demonstrated for a variety of atoms and small molecules for which FCI energies are available, and the results are compared with those from Møller–Plesset (MP) perturbation theory. For the coupled-cluster sequence SCF, CCSD, CCSD(T), a method based on a continued-fraction formalism is found to be particularly successful. It yields sufficient improvement over conventional CCSD(T) that the results become competitive with, and often better than, results from the MP4-qλ method (MP4 summed with quadratic approximants and λ transformation). The sequence SCF, CCSD, CCSDT can be extrapolated with a quadratic approximant but the results are not appreciably more accurate than those from the CCSD(T) continued fraction. Singularity analysis of the MP perturbation series provides a criterion for estimating the accuracy the CCSD(T) continued fraction.
Damping functions for the pairwise sum model of the atom–surface potential
Authors: Patil S. H. , Tang K. T. , Toennies J. P.
Published: July 2002

A pairwise sum model is used to derive the laterally averaged atom–surface potential using the Tang–Toennies potential for interactions between a free atom and the atoms in a solid. The atom–surface van der Waals attraction is shown to include all multipole terms with the proper damping. The analytical expressions for the damping functions have the same damping parameter as for the free atom–atom interactions.
Effect of film thickness on hydrogenated amorphous silicon grown with hydrogen diluted silane
Authors: Danesh P. , Pantchev B. , Grambole D. , Schmidt B.
Published: July 2002

Thin films of hydrogenated amorphous silicon (a-Si:H) prepared by plasma-enhanced chemical vapor deposition with 10% SiH4 in hydrogen have been studied concerning the effect of film thickness on the hydrogen concentration, interconnected void network and mechanical stress. The hydrogen concentration was determined by nuclear reaction analysis. The interconnected void network was studied by the method of ion exchange in glass substrate. The films were prepared at a substrate temperature in the range of 150–270 °C. The results show that at the substrate temperature of 150 °C the film starts to grow with an extensive void network, and its structural improvement with thickness is manifested by an increase of the film density. In contrast, at 270 °C the film starts to grow with a dense structure, and its improvement is manifested by an increase of the intrinsic compressive stress. The hydrogen concentration does not depend on the film thickness at any substrate temperature.
Supersonic jet and shock interactions
Authors: Foster J. M. , Wilde B. H. , Rosen P. A. , Perry T. S. , Fell M. , Edwards M. J. , Lasinski B. F. , Turner R. E. , Gittings M. L.
Published: July 2002

Supersonic fluid flow and the interaction of strong shock waves to produce jets of material are ubiquitous features of inertial confinement fusion (ICF), astrophysics, and other fields of high energy-density science. The availability of large laser systems provides an opportunity to investigate such hydrodynamic systems in the laboratory, and to test their modeling by radiation hydrocodes. We describe experiments to investigate the propagation of a structured shock front within a radiation-driven target assembly, the formation of a supersonic jet of material, and the subsequent interaction of this jet with an ambient medium in which a second, ablatively driven shock wave is propagating. The density distribution within the jet, the Kelvin–Helmholz roll-up at the tip of the jet, and the jet’s interaction with the counterpropagating shock are investigated by x-ray backlighting. The experiments were designed and modeled using radiation hydrocodes developed by Los Alamos National Laboratory, AWE, and Lawrence Livermore National Laboratory. The same hydrocodes are being used to model a large number of other ICF and high energy-density physics experiments. Excellent agreement between the different simulations and the experimental data is obtained, but only when the full geometry of the experiment, including both laser-heated hohlraum targets (driving the jet and counter-propagating shock), is included. The experiments were carried out at the University of Rochester’s Omega laser [J. M. Soures et al., Phys. Plasmas 3, 2108 (1996)].
First principles study of Pt adhesion and growth on SrO- and TiO2-terminated SrTiO3(100)
Authors: Asthagiri Aravind , Sholl David S.
Published: July 2002

We use density functional theory to investigate Pt growth on SrTiO3(100) surfaces. We have determined the favored monolayer arrangement for Pt on both terminations of SrTiO3(100). The work of separation as a function of the number of monolayers of Pt has been determined for Pt films of up to 5 monolayers. To examine which surface growth mode will dominate Pt film growth on this substrate, we have also examined isolated Pt clusters on each termination of SrTiO3(100). Our results indicate that the TiO2-terminated surface is more favorable for epitaxial growth of (100) oriented Pt films than the SrO-terminated surface.
Structure analysis of turbulent mixing patterns in inert and reactive turbulent liquid flows
Authors: Rys Franz S. , Rys Paul
Published: July 2002

The paper contains an extended summary of an invited plenary talk given at the Workshop on Active Chaos at the Los Alamos National Laboratory on 29–31 May 2001 by one of us (F.S.R.).
Measurement of the full shear-induced self-diffusion tensor of noncolloidal suspensions
Authors: Breedveld V. , van den Ende D. , Bosscher M. , Jongschaap R. J. J. , Mellema J.
Published: July 2002

The full diffusion tensor of shear-induced self-diffusion has been measured experimentally for the first time. In addition to the well-known components in the velocity gradient, Dyy, and vorticity direction, Dzz, the coefficients Dxx and Dxy have been determined for concentrated suspensions of noncolloidal hard spheres as a function of particle volume fraction. Owing to the shear-induced nature of the phenomenon, these four coefficients are the only nonzero elements of the diffusion tensor. The newly determined diffusion quantities have been obtained by extending our correlation based technique [J. Fluid Mech. 375, 297 (1998); Phys. Rev. E 63, 021403 (2001)] with a method to subtract convective displacements due to the shear flow. The diffusion in the velocity direction, Dxx, is almost an order of magnitude larger than the other components and the only nonzero off-diagonal component, Dxy, is negative and small compared to the diagonal components of the diffusion tensor. In principle the applied technique is also feasible for measuring other anisotropic diffusion mechanisms, e.g., Brownian diffusion in steady shear flow.
Computational studies on the infrared vibrational spectra, thermodynamic properties, detonation properties, and pyrolysis mechanism of octanitrocubane
Authors: Zhang Ji , Xiao Heming
Published: July 2002

The molecular geometries, infrared vibrational spectra, and thermodynamic properties of octanitrocubane (ONC) are calculated using the density functional theory (DFT) method at the B3LYP/6-31G* level. The IR frequency scaling factor 0.9501 suitable for polynitrocubanes is obtained at the B3LYP/6-31G* level, and the calculated IR frequencies of ONC are scaled. The accurate heat of formation 726.47 kJ/mol of ONC in gas phase is obtained via designed isodesmic reaction in which the cubane cage skeleton has been kept. The sublimation enthalpy, density, and heat of formation for ONC crystal are also calculated, and they are 220.63 kJ/mol, 2.189 g/cm3, and 505.84 kJ/mol, respectively. In addition, the estimated detonation velocity and detonation pressure of ONC are 10.26 mm/ms and 520.86 kbar, respectively. Finally, the pyrolysis mechanism of ONC is studied using various theoretical methods, i.e., MP2, DFT, and selected MINDO/3 semiempirical MO, based on the unrestricted Hartree–Fock model. The calculated results show that the pyrolysis initiation reaction of ONC, i.e., rate-controlling step, is to form a diradical by the single C–C bond breaking in the cube. The second C–C bond breaking is easily followed to form a nitrocyclooctatetraene. The calculated activation energy for the pyrolysis initiation reaction of ONC, obtained from B3LYP/6-31G* method, is 155.30 kJ/mol, which this rather large activation energy indicates that ONC is a new type of energetic material with less sensitivity and better thermal stability, and has highly exploitable values.
Effects of nonuniformity in thin-film photovoltaics
Authors: Karpov V. G. , Compaan A. D. , Shvydka Diana
Published: July 2002

We discuss the physical origin and effects of micrononuniformities on thin-film photovoltaics. The key factors are the large device area and the presence of potential barriers in the grain boundaries (for polycrystalline films) and in device junctions. We model the nonuniformity effects in the terms of random microdiodes connected in parallel through a resistive electrode. The microdiodes of low open circuit voltages are shown to affect macroscopically large regions. They strongly reduce the device performance and induce its nonuniform degradation in several different modes. We support our predictions by experiments, which show that the device degradation is driven by the light-induced forward bias and is spatially nonuniform.
Stochastic webs and their applications
Authors: Zaslavsky George M.
Published: July 2002

The conditions for the appearance of a stochastic web in degenerate dynamic systems and typical physical problems that lead to such a web are analyzed. Examples of webs are considered, as well as their symmetry, width, and structural changes. A description is given of a change in the diffusion dynamics along the web channels as a function of the number of the degrees of freedom and the phenomenon of stochastic percolation is discussed.
Time-delayed map as a model for open fluid flow
Authors: Willeboordse Frederick H.
Published: July 2002

It is shown that a time-delayed map for just one (chaotic) element whose feedback is periodically interrupted can be exactly mapped to a coupled map lattice model for open fluid flow.
Complexity in spiral wave dynamicsa)
Authors: Nagy-Ungvarai Zsuzsanna , Ungvarai Janos , Müller Stefan C.
Published: July 2002

In closed systems of the Belousov–Zhabotinsky reaction a large number of dynamic states found in open systems is sampled as they evolve in time. During such slow aging processes of thin solution layers, prepared under appropriately chosen chemical conditions, an unexpectedly rich variety of spiral tip behavior was observed experimentally. Within a (concentration, time) parameter plane, the movement of free ends of waves was classified as follows: (a) in a stable domain–periodic rigid rotation with cores of small (200 μm) or very large (2 mm) diameter; quasiperiodic compound motion along a hypocycle, a straight loopy line or an epicycle; complex meandering composed of possibly more than two components; (b) rectilinear tip motion indicating the boundary of spiral wave stability; and (c) in an unstable domain—shrinking of open ends of wave fronts during propagation. The main properties of these parameters are compared with recently published computer calculations.
Exit times and transport for symplectic twist maps
Authors: Easton R. W. , Meiss J. D. , Carver S.
Published: July 2002

The exit time decomposition of a set yields a description of the transport through the set as well as a visualization of the invariant structures inside it. We construct several sets computationally easier to deal with than the construction of resonances, based on the ordering properties for orbits of twist maps. Furthermore these sets can be constructed for four- and higher-dimensional twist mappings. For the four-dimensional case—using the example of Froeshlé—we find ‘‘practically’’ invariant volumes surrounding elliptic fixed points. The boundaries of these regions are remarkably sharp; however, the regions are threaded by ‘‘tubes’’ of escaping orbits.
Bifurcations in a system described by a nonlinear differential equation with delay
Authors: Ueda Yoshisuke , Ohta Hirofumi , Stewart H. Bruce
Published: July 2002

Computer simulations of a nonlinear differential equation with time delay have been carried out to determine the possible steady states over a wide range of parameter values. A variety of nonlinear phenomena, including chaotic attractors and multiple coexisting attractors, are observed. Precision of the solutions is verified by means of evaluating the computational error at each time step. A number of bifurcations are observed, and the involvement of unstable periodic orbits is confirmed. The phase space of the system is infinite dimensional, but nonetheless all the bifurcation phenomena observed, including the blue sky disappearance (boundary crisis) of a chaotic attractor, show geometric structures which are consistent with familiar low-dimensional center-manifold descriptions.
A minimal single-channel model for the regularity of beating in the sinoatrial node
Authors: Guevara Michael R. , Lewis Timothy J.
Published: July 2002

It has been suggested that the normal irregular beating of the heart is a manifestation of deterministically chaotic dynamics. Evidence proffered in support of this hypothesis includes a 1/f-like power spectrum, a small noninteger correlation dimension, and self-similarity of the time series. The major cause of the normal fluctuations in heart rate is the impingement of several neural and hormonal control systems upon the sinoatrial node, the natural pacemaker of the heart. However, intrinsic fluctuations of beat rate can be seen in the isolated node, devoid of all neural and hormonal inputs, and even in a single cell isolated from the node. The electrical activity in such a single cell is generated by ions flowing through discrete channels in the cell membrane.

We decided to test the hypothesis that the fluctuations in beat rate in a single cell might be due to the fluctuations in the activity of this population of single channels. We thus assemble a model consisting of 6000 channels and probe its dynamics. Each channel has one or more gates, all of which must be open to allow current to flow through the channel. Since these gates are thought to open and close in a random manner, we model each gate by a Markov process, assigning a pseudorandom number to each gate every time that it changes state from open to closed or vice versa. This number, in conjunction with the classical voltage-dependent Hodgkin–Huxley-like rate constants that control the speed with which a gate will open or close, then determines when that gate will next change state. We also employ a second method that is much more efficient computationally, in which one computes the lifetime of the ensemble of 6000 channels. We show that the Monte Carlo model has behavior consistent with the hypothesis that the irregular beating seen experimentally in single nodal cells is due to the (pseudo)random opening and closing of single channels. However, since the pseudorandom number generator used in the simulations is deterministic, one cannot state that the activity in the model is random (or stochastic). Thus, it would be premature to claim that the irregularity of beating in a single nodal cell is accounted for by the stochastic behavior of a population of a few thousand single channels lying in the membrane of the cell. Finally, we consider some implications of our work for the naturally occurring in situ fluctuations in heart rate (‘‘heart rate variability’’).
Variations on tremor parameters
Authors: Boose A. , Jentgens Ch. , Spieker S. , Dichgans J.
Published: July 2002

This paper describes our analysis procedure for long-term tremor EMG recordings, as well as three examples of applications. The description of the method focuses on how characteristics of the tremor (e.g. frequency, intensity, agonist–antagonist interaction) can be defined and calculated based on surface EMG data. The resulting quantitative characteristics are called ‘‘tremor parameters.’’ We discuss sinusoidally modulated, band-limited white noise as a model for pathological tremor-EMG, and show how the basic parameters can be extracted from this class of signals. The method is then applied to (1) estimate tremor severity in clinical studies, (2) quantify agonist–antagonist interaction, and (3) investigate the variations of the tremor parameters using simple methods from time-series analysis.
Generalized entropies of chaotic maps and flows: A unified approach
Authors: Badii R.
Published: July 2002

A thermodynamic study of nonlinear dynamical systems, based on the orbits’ return times to the elements of a generating partition, is proposed. Its grand canonical nature makes it suitable for application to both maps and flows, including autonomous ones. When specialized to the evaluation of the generalized entropies Kq, this technique reproduces a well-known formula for the metric entropy K1 and clarifies the relationship between a flow and the associated Poincaré maps, beyond the straightforward case of periodically forced nonautonomous systems. Numerical estimates of the topological and metric entropy are presented for the Lorenz and Rössler systems. The analysis has been carried out exclusively by embedding scalar time series, ignoring any further knowledge about the systems, in order to illustrate its usefulness for experimental signals as well. Approximations to the generating partitions have been constructed by locating the unstable periodic orbits of the systems up to order 9. The results agree with independent estimates obtained from suitable averages of the local expansion rates along the unstable manifolds.
The conjugate-pairing rule for non-Hamiltonian systems
Authors: Searles Debra J. , Evans Denis J. , Isbister Dennis J.
Published: July 2002

In systems that satisfy the Conjugate Pairing Rule (CPR), the spectrum of Lyapunov exponents is symmetric. The sum of each conjugate pair of exponents is identical. Since in dissipative systems the sum of all the exponents is the entropy production divided by Boltzmann’s constant, the calculation of transport coefficients from the Lyapunov exponents is greatly simplified in systems that satisfy CPR. Sufficient conditions for CPR are well known: the underlying adiabatic dynamics should be symplectic. However, the necessary conditions for CPR are not known. In this paper we report on the results of computer simulations which shed light on the necessary conditions for the CPR to hold. We provide, for the first time, convincing evidence that the standard molecular dynamics algorithm for calculating shear viscosity violates the CPR, even in the thermodynamic limit. In spite of this it appears that the sum of the maximal exponents is equal to the entropy production per degree of freedom. Thus it appears that the shear viscosity can still be calculated using the standard viscosity algorithm by summing the maximal pair of exponents.
Chaotic mixing of granular materials in two-dimensional tumbling mixers
Authors: Khakhar D. V. , McCarthy J. J. , Gilchrist J. F. , Ottino J. M.
Published: July 2002

We consider the mixing of similar, cohesionless granular materials in quasi-two-dimensional rotating containers by means of theory and experiment. A mathematical model is presented for the flow in containers of arbitrary shape but which are symmetric with respect to rotation by 180° and half-filled with solids. The flow comprises a thin cascading layer at the flat free surface, and a fixed bed which rotates as a solid body. The layer thickness and length change slowly with mixer rotation, but the layer geometry remains similar at all orientations. Flow visualization experiments using glass beads in an elliptical mixer show good agreement with model predictions. Studies of mixing are presented for circular, elliptical, and square containers. The flow in circular containers is steady, and computations involving advection alone (no particle diffusion generated by interparticle collisions) show poor mixing. In contrast, the flow in elliptical and square mixers is time periodic and results in chaotic advection and rapid mixing. Computational evidence for chaos in noncircular mixers is presented in terms of Poincaré sections and blob deformation. Poincaré sections show regions of regular and chaotic motion, and blobs deform into homoclinic tendrils with an exponential growth of the perimeter length with time. In contrast, in circular mixers, the motion is regular everywhere and the perimeter length increases linearly with time. Including particle diffusion obliterates the typical chaotic structures formed on mixing; predictions of the mixing model including diffusion are in good qualitative and quantitative (in terms of the intensity of segregation variation with time) agreement with experimental results for mixing of an initially circular blob in elliptical and square mixers. Scaling analysis and computations show that mixing in noncircular mixers is faster than that in circular mixers, and the difference in mixing times increases with mixer size.
Heteroclinic primary intersections and codimension one Melnikov method for volume-preserving maps
Authors: Lomelı́ Héctor E. , Meiss James D.
Published: July 2002

We study families of volume preserving diffeomorphisms in R3 that have a pair of hyperbolic fixed points with intersecting codimension one stable and unstable manifolds. Our goal is to elucidate the topology of the intersections and how it changes with the parameters of the system. We show that the “primary intersection” of the stable and unstable manifolds is generically a neat submanifold of a “fundamental domain.” We compute the intersections perturbatively using a codimension one Melnikov function. Numerical experiments show various bifurcations in the homotopy class of the primary intersections.
Hierarchical structures in the phase space and fractional kinetics: II. Immense delocalization in quantized systems
Authors: Iomin A. , Zaslavsky G. M.
Published: July 2002

Anomalous transport due to Lévy-type flights in quantum kicked systems is studied. These systems are kicked rotor and kicked Harper model. It is confirmed for a kicked rotor that there exist special “magic” values of a control parameter of chaos K=K*=6.908 745… for which an essential increasing of a localization length is obtained. Functional dependence of the localization length on both parameter of chaos and quasiclassical parameter h̃ is studied. We also observe immense delocalization of the order of 109 for a kicked Harper model when a control parameter K is taken to be K*=6.349 972. This “magic” value corresponds to special phase space topology in the classical limit, when a hierarchical self-similar set of sticky islands emerges. The origin of the effect is of the general nature and similar immense delocalization as well as increasing of localization length can be found in other systems.
Bifurcations accompanying monotonic instability of an equilibrium of a cosymmetric dynamical system
Authors: Kurakin L. G. , Yudovich V. I.
Published: July 2002

It is well known that equilibrium in a cosymmetric system in the general position is a member of a one-parameter family. In the present paper the Lyapunov–Schmidt method and the method of the central manifold are used to analyze bifurcations of such a family of equilibria as well as internal bifurcations: transitions of the type focus—node, node—saddle, and so on during motion along the family. A series of scenarios of branching of families of equilibria and the change in the structure of their arcs, consisting of equilibria of the same type, is described. Bifurcations of stable and unstable arcs, coalescence and decomposition of families of equilibria, bifurcation of the loss of smoothness by the family of equilibria, and branching of a small equilibrium cycle from a corner point of the family of equilibria are investigated in detail. The variability of the spectrum along a family gives rise to a variety of new phenomena that are not encountered in the classical case of an isolated equilibrium or in bifurcations of families of equilibria of a system with symmetry. They include protraction with respect to the branching parameter of the family of equilibria, Lyapunov instability of a family of equilibria with the attraction properties being preserved, and the appearance and disappearance of new stable and unstable arcs on the family of equilibria.
Fractal mirror images and chaotic scattering
Authors: Korsch H. J. , Wagner A.
Published: July 2002

Classical scattering from a cluster of N hard spheres, in particular for N=3 and 4, is discussed. In addition, the case of planar scattering from three disks is considered. The scattering is chaotic and shows a fractal structure, whose fractal dimension increases with the number of spheres as well as with the ratio radius/distance. The scattering system can be reinterpreted as a system of polished balls reflecting each other, which generates a fascinating nested structure of fractal mirror images.
Magnetic materials characterization using a fiber optic magnetometer
Authors: Lenz J. E. , Anderson C. D. , Strandjord L. K.
Published: July 2002

The fiber optic magnetometer is a relatively new sensor concept which shows promise for high sensitivity measurements. It employs glass fibers which are configured in a Mach–Zehnder interferometer where the fringe pattern output is extremely sensitive to path length changes. The sensing leg is sensitized to magnetic fields by electroplating the fiber with magnetostrictive Ni-Fe-Co alloys. The magnetic M properties of the metal-coated fiber are measured using an M-H looper and its magnetostriction λ using the fiber magnetometer setup. Data for Ni-Fe alloys are presented which compare the measurements of M and λ versus tension. From this data physical relationships between λ and M for the metal-coated fibers are identified.
Compression studies of a nickel-based superalloy, MAR-M200, and of Ni3Al
Authors: Mauer F. A. , Munro R. G. , Piermarini G. J. , Block S. , Dandekar D. P.
Published: July 2002

The lattice parameter of a cubic nickel-based alloy, MAR-M200, has been determined as a function of pressure for 0<p<14 GPa at room temperature. A similar study was made for Ni3Al in the range 0<p<11 GPa at room temperature. In both cases, the diamond anvil pressure cell was used in conjunction with the energy dispersive method of x-ray diffraction. The data were analyzed in the context of model equations of state and in comparison with other results from ultrasonic studies.
Evaluation of ionic mobility and transference number in a polymeric solid electrolyte by isothermal transient ionic current method
Authors: Watanabe M. , Rikukawa M. , Sanui K. , Ogata N.
Published: July 2002

We proposed a new method for evaluating mobilities of both Li+ and SCN− ions in a polymeric solid electrolyte formed by poly (ethylene succinate) and LiSCN. The method is based on the isothermal transient ionic current (ITIC) technique by using an ion-blocking platinum electrode and an ion-reversible lithium electrode. The differences of the ITIC profiles, depending on the kind of the electrodes, provided both mobilities of Li+ and SCN− ions, and their assignment. The mobilities of Li+ and SCN− ions at 90 °C were 1.3–4.2×10−6 cm2 V−1 s−1 and 5.3–10.7×10−8 cm2 V−1 s−1, respectively. The transference number of Li+ ions was 0.92–0.99.
Deposition and photoconductivity of hydrogenated amorphous silicon films by the pyrolysis of disilane
Authors: Chu T. L. , Chu Shirley S. , Ang S. T. , Lo D. H. , Duong A. , Hwang C. G.
Published: July 2002

The thermal decomposition of disilane (Si2H6) in a hydrogen or helium flow has been used for the deposition of hydrogenated amorphous-silicon (a-Si:H) films on the surface of Corning 7059 glass substrates at 450–500 °C. The reaction product consists of monosilane and trisilane in addition to the unreacted disilane and ethylsilane (the major impurity in commercial disilane). The concentration of Si2H6 in the reaction mixture has been found to strongly affect the deposition rate and the photoconductivity of a-Si:H films. At a given Si2H6 concentration, the deposition rate of a-Si:H films increases exponentially with temperature. At a given substrate temperature, the AM1 conductivity of a-Si:H films increases with increasing Si2H6 concentration and approaches 10−5 Ω cm−1 at Si2H6 concentrations higher than about 4%, and the conductivity ratio in better films is about 105. The conductivities of CVD a-Si:H films have been found to show negligible change under illumination over a period of several days. The optical band gap of a-Si:H films has been determined to be 1.65–1.68 eV. The doping of a-Si:H films with arsenic and boron has been carried out, and their activation energies determined.
Spark breakdown of liquid hydrocarbons. I. Fast current and voltage measurements of the spark breakdown in liquid n-hexane
Authors: Fuhr Jitka , Schmidt Werner F. , Sato Shuji
Published: July 2002

A fast coaxial discharge line for the study of the electric breakdown of liquid hydrocarbons is described. It allows the simultaneous measurement of discharge current (up to 5 kA) and voltage (up to 50 kV) with nanosecond time resolution. The spark gap is of tip-plane geometry. A detailed frequency analysis of the discharge and measurement circuits has been performed. Examples for the spark breakdown of n-hexane under a variety of conditions are presented.
Twin nucleation in layers of CdTe on {lll} CdTe grown by metalorganic vapor-phase epitaxy
Authors: Hails J. E. , Russell G. J. , Brinkman A. W. , Woods J.
Published: July 2002

Homoepitaxial layers of CdTe ranging from 0.5 to 5 μm in thickness have been grown on twin free, single-crystal {111}A oriented CdTe substrates by metalorganic vapor phase epitaxy over the temperature range 360–440 °C. The as-grown surfaces were invariably very smooth resulting in reflection high energy electron diffraction (RHEED) patterns characterized by streaks of diffracted intensity lying perpendicular to the shadow edge of the sample. After a light chemical etch, the 〈110〉 zone axis RHEED patterns exhibited an arrangement of double spots which corresponds to a twinned fcc structure. Furthermore, this twinned structure was observed in all layers grown on substrates of this specific orientation.
Liquid-phase epitaxial growth of very thick In1−xGaxAs layers with uniform composition by source-current-controlled method
Authors: Nakajima Kazuo
Published: July 2002

A source-current-controlled method for the continuous supply of solute elements into solutions during growth was developed. In this method, a continuous electric current is passed through a GaAs source material of the solute elements during growth and the source material is dissolved into the growth solution due to Peltier and Joule heating at the interface between the source material and the solution. This method was applied to the liquid-phase epitaxial growth of In1−xGaxAs on InP. The temperature variation of the solution was directly measured by immersing a thermocouple in the solution and a temperature gradient of more than 6 °C could easily be obtained in the solution between the GaAs source material and the InP substrate. The amount of dissolved GaAs was measured as a function of the current density. Using these results, an optimum growth condition for the continuous supply of solute elements was determined, and an 80-μm-thick In0.54Ga0.46As layer with uniform composition (x=0.458±0.002) was obtained.
Energy band-gap bowing parameter in an AlxGa1−x N alloy
Authors: Koide Y. , Itoh H. , Khan M. R. H. , Hiramatu K. , Sawaki N. , Akasaki I.
Published: July 2002

Optical measurements are performed near the fundamental absorption edge for single-crystal AlxGa1−x N epitaxial layers in the composition range of 0≤x≤0.4. The dependence of the energy band gap on composition is found to deviate downwards from linearity, the bowing parameter being b=1.0±0.3 eV. The origin of the large bowing is discussed in terms of the pseudopotential of Al and Ga based on the pseudopotential of the Heine–Abarenkov type. With increasing x the absorption edges broaden, which is attributed to the increase of the compositional nonuniformity.
Numerical simulation of streamers in SF6
Authors: Dhali Shirshak K. , Pal Anup K.
Published: July 2002

The results of computer simulations of streamer development and propagation are reported in SF6. The two-dimensional algorithm makes use of flux-corrected transport techniques for the solution of transport equation of charged species under the action of the space-charge electric field. This field is obtained from the solution of Poisson’s equation assuming cylindrical symmetry. The role of attachment, space-charge field, secondary electrons, and the shape and size of the initiating charge have been determined. The results presented are for a uniform applied field-to-gas density ratios (E/N) of 355–435 Td for background neutral ionization densities of 104–106 cm−3. The results in SF6, which is an attaching gas, are compared with a nonattaching gas like nitrogen.
Electron-spin resonance and fluorescence investigation of LaMgAl11O19:Ti3+, a potential tunable laser material
Authors: Gourier D. , Colle L. , Lejus A. M. , Vivien D. , Moncorge R.
Published: July 2002

Single crystals of LaMgAl11O19:Ti (LMA:Ti) have been grown from the melt by the Verneuil (flame fusion) process. As-grown crystals contain titanium in both 3+ and 4+ oxidation states. Ti4+ ions can be reduced into the 3+ state by annealing at 1200 °C under hydrogen atmosphere. The crystals exhibit a broadband fluorescence extending from ∼600 to 1200 nm and peaking around 775 nm. The fluorescence lifetime is found to be ∼6 μs at 10 K against ∼4 μs at 300 K; it does not depend significantly on the emission wavelength, this indicating that fluorescence originates mainly from one type of Ti3+ active ions. Electron-spin-resonance (ESR) investigations reveal that Ti3+ ions are distributed among the three octahedral sites of the magnetoplumbite structure of the host, with a marked predominance for the regular octahedral site (2a), which contains about 90% of the titanium detected by ESR. Crystal field splitting of the t2g level has been estimated for the three sites and it is proposed that Ti3+ in the (12k) site is adjacent to a cationic defect. This new material is of potential interest for near infrared tunable laser application.
Anisotropic etching of polycrystalline silicon with a hot Cl2 molecular beam
Authors: Suzuki Keizo , Hiraoka Susumu , Nishimatsu Shigeru
Published: July 2002

A hot Cl2 molecular (Cl*2) beam was successfully applied to achieve highly anisotropic, highly selective, and almost damage-free etching of polycrystalline Si. The anisotropy, the ratio of etch rates in vertical and horizontal directions, was larger than 25. The selectivity, the ratio of polycrystalline Si and SiO2 etch rates, was larger than 1000. The Cl*2 beam was produced by free jet expansion of a Cl2 gas heated in a graphite furnace. The furnace temperature was 830 °C. The substrate temperature was 180 °C. The average total energy (0.38 eV) of a Cl*2 molecule impinging on a substrate surface is much lower than the critical energy (approximately 10 eV) to displace the atoms of the etched material and to cause surface damage. This is the essential reason why this highly selective and almost damage-free etching has been achieved. The highly anisotropic etching mechanism is explained by a model taking into account the directional incidence of Cl*2 molecules to the surface, and the deactivation process of the Cl*2 molecules on a cold surface.
Production of H− ions with addition of cesium or xenon to a hydrogen discharge in a small multicusp ion source
Authors: Walther S. R. , Leung K. N. , Kunkel W. B.
Published: July 2002

The effect on H− ion production by adding cesium or xenon to a hydrogen discharge, in a small magnetically filtered multicusp ion source, has been investigated. Addition of cesium vapor to the hydrogen discharge resulted in a factor of 16 increase in H− output relative to the uncesiated discharge for the same operating parameters. Likewise, the addition of xenon gas to a hydrogen discharge resulted in a factor of 2.7 increase in H− output over pure hydrogen operation at optimum H2 source pressure, while maintaining the same arc parameters. Operation of the source with the plasma electrode at optimum bias voltage was essential to obtain this result.
High-frequency bias susceptibilities of ultrasoft CoB amorphous thin films
Authors: Jeong I. S. , Valanju A. P. , Walser R. M. , Herring J. H.
Published: July 2002

We report the results of the first high-resolution measurements of the transverse susceptibilities of ultrasoft (Hc<0.2 Oe, Hk≊0.8 Oe) rotating field annealed amorphous CoB thin films over an extended frequency range of 2–200 MHz. At zero bias, their hard axis susceptibility exhibited a classic relaxation-type response with maximum loss in the 50–70-MHz range. We also systematically measured the dc bias field dependence of the transverse susceptibilities χt(0) and χt(π/2). These data were compared with typical, soft (Hc≊0.6 Oe, Hk≊1.8 Oe) thermally annealed films with the same composition. The transverse susceptibilities χt(0) and χt(π/2) of the soft, thermally annealed samples were irreversible with field. The ultrasoft magnetic thin film, however, exhibited a rotatable anisotropy and completely reversible and continuous χt. These data suggest that the quasistatic and small signal dynamical responses of the magnetization distributions in these ultrasoft films were substantially influenced by the internal dipolar fields associated with the magnetization ripple.
Interface capacitance in metal-semiconductor junctions
Authors: Wu Xu , Yang Edward S.
Published: July 2002

A new theory of the interface capacitance at metal-semiconductor junctions is presented. The diode capacitance is attributed to the modulation of the effective Schottky barrier height by interface charge. Relations between the measured capacitance and the physical properties of the interface states are formulated by using Shockley–Read statistics and taking into account the electron relaxation-time dispersion. This theory is applied to NiSi2-nSi diodes with both epitaxial and nonepitaxial interfaces. Spectra of density distribution, as well as other interface parameters, are obtained, indicating that interface states in these diodes are most probably defect related.
Electrothermal measurements: A calorimetric method to examine power dissipation in photovoltaic devices
Authors: Büchner Bettine , Flaisher Harvey , Wolf Martin , Cahen David
Published: July 2002

Power loss mechanisms in illuminated and nonilluminated photovoltaic cells are investigated by an ac calorimetric method. In this electrothermal (ET) method, periodic temperature changes, caused by periodic electrical excitation, are sensed as a function of external parameters, e.g., applied voltage. ET signals on crystalline silicon and thin-film CuInSe2/CdS solar cells are measured as a function of applied voltage. The results are compared to those obtained from an energy balance model. Inherent to the model is the occurrence of injected carrier cooling. The ability of ET measurements to separate different power loss mechanisms is discussed and compared to that of conventional photothermal ones (e.g., photoacoustic).
Critical layer thickness for misfit dislocation stability in multilayer structures
Authors: Hirth J. P. , Feng Xiaoxin
Published: July 2002

The critical thickness for which the injection of misfit dislocations into a strained layer becomes thermodynamically favorable is treated in a Volterra dislocation model. Both a single interior layer and a multilayer (superlattice) case are considered. The results agree with earlier simple dipole predictions in the limit of large misfit dislocation spacing, but deviate at smaller spacings. With a particular core parameter, the results also agree with atomistic calculations using a parabolic potential for a constrained glide equilibrium case.
Model of plasma source ion implantation in planar, cylindrical, and spherical geometries
Authors: Scheuer J. T. , Shamim M. , Conrad J. R.
Published: July 2002

A model has been developed that describes the propagation of the transient sheath during a pulse of high negative voltage applied to a conductor immersed in a plasma such as that present in plasma source ion implantation. This model assumes that the transient sheath obeys the Child–Langmuir law for space-charge-limited emission at each instant during the propagation of the sheath. Expressions are obtained for the sheath-edge position as a function of time. The model predicts the final sheath extent and average ion current to the target during each pulse for planar, cylindrical, and spherical geometries.
Photoluminescence characterization of the surface layer of chemically etched CdTe
Authors: García-García J. , González-Hernández J. , Mendoza-Alvarez J. G. , Cruz Elías López , Contreras-Puente Gerardo
Published: July 2002

The effects of several reducing and oxidizing etches on CdTe surfaces have been characterized by photoluminescence. For excitation, several lines from three different types of gas lasers, emitting at 325 nm (He-Cd laser), 488 nm (argon-ion laser), and 632.8 nm (He-Ne laser) were used. The corresponding light penetration depth varied from approximately 25 to 200 nm. The analysis of the photoluminescence as a function of the depth not only allows the characterization of the type of defects created by the etching but also their location from the treated surfaces. Proper etching solutions produce surfaces with a crystalline quality comparable to that of a cleaved surface and the photoluminescence spectra do not depend on the energy of the excitation.
Properties of very uniform InxGa1−xAs single crystals grown by liquid-phase electroepitaxy
Authors: Bryskiewicz T. , Edelman P. , Wasilewski Z. , Coulas D. , Noad J.
Published: July 2002

InxGa1−xAs (0<x<0.09) crystals with excellent compositional and luminescence efficiency uniformity were grown on (100) GaAs substrates by liquid-phase electroepitaxy. Compositional variations in these ternary alloys, ΔX, parallel and perpendicular to the growth direction, did not exceed 0.001 over a 1 in. diameter. These ternary bulk crystals are superior to comparable melt-grown materials.
Lattice instability at excited states of the self-trapped excitons in MgF2
Authors: Tanimura Katsumi , Itoh Noriaki
Published: July 2002

We have investigated lattice relaxation in MgF2 following electronic excitation of the self-trapped excitons, employing a cascade excitation technique in which self-trapped excitons generated by an electron pulse are excited further by a 308-nm laser pulse. It is found that the F centers are generated by excitation of self-trapped excitons at 5 K, although they are not produced by an electron pulse alone. The nature of the excited states of the self-trapped excitons from which the F centers are evolved and the absence of F-center generation from electron-hole pairs are discussed.