bluuebunny/crossref_metadata_2025 huggingface dataset:
Entropy conservation as hTμ≈λ̄μ+dμ in neurobiological dynamical systems
Authors: Mandell Arnold J. , Selz Karen A.
Published: July 2002

That the topological entropy, hTμ, of a C1<r⩽2diffeomorphism, φ:M→M, of a surface, M, upon which invariant measure(s) μ are concentrated, varies as the product of its average leading Lyapunov characteristic exponent, λ̄μ, and the Hausdorff dimension of its support, dμ,was proven by Pesin [Russ. Math Surveys 32, 55–114 (1977)] for nonuniform partial hyperbolic systems and by Ledreppier and Young [Ergod. Theor. Dyn. Syst. 2, 109–123 (1982)], and Manning [Ergod. Theor. Dyn. Syst. 1, 451–459 (1981)] for uniformly hyperbolic (Axiom A) diffeomorphisms. When considered in conjunction with the post-Shannon information encoding theorems of Adler [Trans. Am. Math. Soc. 114, 309–319 (1965); Mem. Am. Math. Soc., No. 219 (1979)] and others, this suggests a way to differentiate equal entropy behaviors in systems with varying patterns of dynamical behaviors. Here we show this relation to be useful in the quantitative discrimination among the behaviors of abstract neuronal models and two real, finite time, partially and nonuniformly hyperbolic, brain-related dynamical systems. We observe a trade-off in finite time between two competing dynamical processes, jittery sticking (tending to increase dμ) and convective escaping (more prominently incrementing λ̄μ+). In finite time systems, these changes in combination can statistically conserve the dynamical entropy, hTμ, while altering the Levy characteristic exponent, α (describing the tail of the density distribution of observables, ρ(x)∼exp−γ|x|α,1⩽α⩽2), and the Mandelbrot-Hurst exponent 0<H*<1, such that H*>0.5 implicates sequential correlations and H*<0.5 sequential anticorrelation. When the relation hTμ=λ̄μ+dμ fails, the way it does so provides information about the system.
Parabolic resonances and instabilities
Authors: Rom-Kedar Vered
Published: July 2002

A parabolic resonance is formed when an integrable two-degrees-of-freedom (d.o.f.) Hamiltonian system possessing a circle of parabolic fixed points is perturbed. It is proved that its occurrence is generic for one parameter families (co-dimension one phenomenon) of near-integrable, two d.o.f. Hamiltonian systems. Numerical experiments indicate that the motion near a parabolic resonance exhibits a new type of chaotic behavior which includes instabilities in some directions and long trapping times in others. Moreover, in a degenerate case, near a flat parabolic resonance, large scale instabilities appear. A model arising from an atmospherical study is shown to exhibit flat parabolic resonance. This supplies a simple mechanism for the transport of particles with small (i.e. atmospherically relevant) initial velocities from the vicinity of the equator to high latitudes. A modification of the model which allows the development of atmospherical jets unfolds the degeneracy, yet traces of the flat instabilities are clearly observed.
Sharp diffraction peaks from chaotic structures
Authors: Hübler Alfred , Kuhl Ulrich , Wittmann Rolf , Nagata Takashi
Published: July 2002

Recently various models for spatially chaotic structures have been proposed. We study the diffraction patterns produced by plane chaotic waves incident on one-dimensional chaotic point scatterers. The spacing between the scatterers and the dynamics of the incident wave are given by a logistic map or standard map. We find a sharp diffraction peak when the incident dynamics is produced by the same map as the structure of the spatial configuration. The diffraction pattern is symmetric about the incident direction only if the map dynamics is invertible. Diffraction patterns with chaotic incident waves have a large signal-to-noise ratio and are well suited for pattern identification. We discuss possible applications to the scattering of microwaves from aperiodic structures.
Nonlinear prediction, filtering, and control of chemical systems from time series
Authors: Petrov Valery , Showalter Kenneth
Published: July 2002

Prediction, filtering and control of nonlinear systems is formulated in terms of corresponding nonlinear surfaces in the phase space of delayed system readings and control parameters. The construction of these surfaces from time series and their use is demonstrated with a simple chemical model in the chaotic regime.
Dynamics of transients in yttrium-iron-garnet
Authors: Mar D. J. , Pecora L. M. , Rachford F. J. , Carroll T. L.
Published: July 2002

Yttrium-iron-garnet (YIG) is an important technological material used in microwave devices. In this paper we use dual microwave (1–4 GHz) drives to study the dynamical bifurcation behavior of magnetostatic and spin-wave modes in YIG spheres and rectangular films. The samples are placed in a dc magnetic field and driven by cw and pulse-modulated microwave excitations at magnetostatic mode frequencies. A second microwave drive applied to the sample excites additional spin-wave modes that can interact with those arising from the original excitation and thereby affect the transmission characteristics at the primary frequency. We find a significant decrease in transmission of the primary when the secondary frequency is tuned to approximately half that of the primary drive. This decrease is observed both in the steady state behavior and in the initial overshoot transient associated with pulse modulation of the primary excitation. Results such as these are often treated by extending linear theory to include higher order interaction terms. Herein we present a simple dynamical model that reproduces results that qualitatively resemble the experimental data.
Control and synchronization of chaos in high dimensional systems: Review of some recent results
Authors: Ding Mingzhou , Ding E-Jiang , Ditto William L. , Gluckman Bruce , In Visarath , Peng Jian-Hua , Spano Mark L. , Yang Weiming
Published: July 2002

Controlling chaos and synchronization of chaos have evolved for a number of years as essentially two separate areas of research. Only recently it has been realized that both subjects share a common root in control theory. In addition, as limitations of low dimensional chaotic systems in modeling real world phenomena become increasingly apparent, investigations into the control and synchronization of high dimensional chaotic systems are beginning to attract more interest. We review some recent advances in control and synchronization of chaos in high dimensional systems. Efforts will be made to stress the common origins of the two subjects.
Motion in a stochastic layer described by symbolic dynamics
Authors: Misguich J. H. , Reuss J.-D. , Elskens Y. , Balescu R.
Published: July 2002

The motion in the stochastic layer surrounding an island can be studied by using the standard map: This problem is of direct relevance to the diffusion of magnetic field lines in a tokamak. In a previous work it was shown that this process can be adequately modelled by a continuous time random walk (CTRW) describing transitions of the running point between three basins representing, respectively, trapped motion around the island, and passing motion above or below the island. The sticking property of the island deeply modifies the nature of the transport process, leading to subdiffusive behavior. In the present work it is shown that the motion can be analyzed in terms of a symbolic dynamics which leads to the possibility of an automatic measurement of the data necessary for the construction of the CTRW. The logical features of the procedure are described, and the method is applied to an analysis of long time series, thus completing the results of the previous work.
Ordering of the Mandelbrot-like set of the exponential map
Authors: Romera M. , Pastor G. , Álvarez G. , Montoya F.
Published: July 2002

We graphically study the Mandelbrot-like set of the complex exponential family of maps Eλ(z)=λez, which we call the Baker–Rippon–Devaney (BRD) set. We observe that the period of every hyperbolic component can be deduced with the naked eye by using two simple rules.
Spectral decomposition of the tent map with varying height
Authors: Subbiah Suresh , Driebe Dean J.
Published: July 2002

The generalized spectral decomposition of the Frobenius–Perron operator of the tent map with varying height is determined at the band-splitting points. The decomposition includes both decay onto the attracting set and the approach to the asymptotically periodic state on the attractor. Explicit compact expressions for the polynomial eigenstates are obtained using algebraic techniques.
The control of high-dimensional chaos in time-delay systems to an arbitrary goal dynamics
Authors: Bünner M. J.
Published: July 2002

We present the control of high-dimensional chaos, with possibly a large number of positive Lyapunov exponents, of unknown time-delay systems to an arbitrary goal dynamics. We give an existence-and-uniqueness theorem for the control force. In the case of an unknown system, a formula to compute a model-based control force is derived. We give an example by demonstrating the control of the Mackey–Glass system toward a fixed point and a Rössler dynamics.
Scaling and interleaving of subsystem Lyapunov exponents for spatio-temporal systems
Authors: Carretero-González R. , O/rstavik S. , Huke J. , Broomhead D. S. , Stark J.
Published: July 2002

The computation of the entire Lyapunov spectrum for extended dynamical systems is a very time consuming task. If the system is in a chaotic spatio-temporal regime it is possible to approximately reconstruct the Lyapunov spectrum from the spectrum of a subsystem by a suitable rescaling in a very cost effective way. We compute the Lyapunov spectrum for the subsystem by truncating the original Jacobian without modifying the original dynamics and thus taking into account only a portion of the information of the entire system. In doing so we notice that the Lyapunov spectra for consecutive subsystem sizes are interleaved and we discuss the possible ways in which this may arise. We also present a new rescaling method, which gives a significantly better fit to the original Lyapunov spectrum. We evaluate the performance of our rescaling method by comparing it to the conventional rescaling (dividing by the relative subsystem volume) for one- and two-dimensional lattices in spatio-temporal chaotic regimes. Finally, we use the new rescaling to approximate quantities derived from the Lyapunov spectrum (largest Lyapunov exponent, Lyapunov dimension, and Kolmogorov–Sinai entropy), finding better convergence as the subsystem size is increased than with conventional rescaling.
The rotating bucket of sand: Experiment and theory
Authors: Baxter G. W. , Yeung C.
Published: July 2002

The surface shape of a bucket of sand rotating about its cylindrical axis is studied experimentally and theoretically. Focusing on fast time scales on which surface shape is determined by avalanches, we identify three regimes of behavior. At intermediate and high frequencies, the surface shape is always at its critical shape determined by the Coulomb yield condition. The low frequency behavior displays an unexpected subcritical region at the center of the bucket. To understand this central region, we adapt a continuum model of surface flow developed by Bouchaud et al. and Mehta et al. The model indicates that the subcritical region is due to a nonlinear instability mechanism.
Asynchronous updating of coupled maps leads to synchronization
Authors: Mehta Mitaxi , Sinha Sudeshna
Published: July 2002

We investigate the spatiotemporal dynamics of two dimensional coupled map lattices, in the strong coupling phase, evolving under updating rules incorporating varying degrees of asynchronicity. Interestingly, we observe that parallel updates never allow synchronization among the sites, while asynchroncity has the effect of opening up windows in parameter space where the synchronized dynamics gains stability. As asynchronicity increases, the parameter range supporting synchronization gets rapidly wider. Detailed numerics, including bifurcation diagrams and patterns formed en route to synchronization, are reported. We also attempt a mean-field analysis of the system in order to try and account for the stability of the spatiotemporal fixed point under asynchronous updates.
Quantum chaos for the radially vibrating spherical billiard
Authors: Liboff Richard L. , Porter Mason A.
Published: July 2002

The spherical quantum billiard with a time-varying radius, a(t), is considered. It is proved that only superposition states with components of common rotational symmetry give rise to chaos. Examples of both nonchaotic and chaotic states are described. In both cases, a Hamiltonian is derived in which a and P are canonical coordinate and momentum, respectively. For the chaotic case, working in Bloch variables (x,y,z), equations describing the motion are derived. A potential function is introduced which gives bounded motion of a(t). Poincaré maps of (a,P) at x=0 and the Bloch sphere projected onto the (x,y) plane at P=0 both reveal chaotic characteristics.
Quantum chaos of an ion trapped in a linear ion trap
Authors: Berman Gennady P. , James Daniel F. V. , Kamenev Dimitri I.
Published: July 2002

We describe the transition to quantum chaos of an ion trapped in a linear ion trap and interacting with two laser fields. Under the conditions of adiabatic illumination of the upper level of the ion, and when the frequencies of the two laser beams are slightly different, the system is reduced to a quantum linear oscillator interacting with a monochromatic wave. The property of localization over the quantum resonance cells is proposed to exploit in order to facilitate the process of measurement of the probability distribution of an ion on the vibrational levels. In the regime of strong chaos the time-averaged values of the energy and dispersion of energy are computed and compared with the corresponding classical quantities for different values of the perturbation amplitude. In the exact resonance case, the classical analog of the system possesses an infinite inhomogeneous stochastic web. We analyze the quantum dynamics inside the inhomogeneous web. It is shown that the quantum system mimics on average the dynamics of the corresponding classical system. Formation of the quantum resonance cells is illustrated in the case of a finite detuning from the exact resonance, and under increasing of the wave amplitude. The parameters of the model and the initial conditions are close to the real physical situation which can be realized in the system of cold trapped ion perturbed by two lasers fields with close frequencies.
The Direct Simulation Monte Carlo Method
Authors: Alexander Francis J. , Garcia Alejandro L.
Published: July 2002

Numerical simulation of the hydrodynamics of gas flow and fluid flow is described using the Direct Simulation Monte Carlo method. (AIP) © 1997 American Institute of Physics.
Accurate electron probe determination of aluminum composition in (Al, Ga)As and correlation with the photoluminescence peak
Authors: Miller N. C. , Zemon S. , Werber G. P. , Powazinik W.
Published: July 2002

The composition of Alx Ga1−xAs has been measured with improved accuracy by electron probe microanalysis using an aluminum-copper alloy standard rather than an elemental aluminum standard. Details of the experimental procedure are given. A calibration curve, relating the photoluminescence peak energy (Ep) to aluminum concentration (x), has been obtained. A least-squares fit to the data gives Ep(eV)=1.42+1.45x−0.25x2 for 0<x≤0.45.
Study of the solidification front of Si films in lamp zone melting controlled by patterning the underlying SiO2
Authors: Dutartre D. , Haond M. , Bensahel D.
Published: July 2002

A periodic heat-sink structure has been tested in lamp recrystallization of thin polycrystalline silicon films deposited on oxidized wafers. The defects generally encountered in this type of recrystallization, grain and subgrain boundaries, are localized in specifically designed areas. This localization technique and especially the occurrence of unlocalized defects are interpreted taking into account the breakdown of the solidification front. Beside faceting effects, constitutional supercooling due to impurity segregation is found to play a major role in this type of recrystallization.
Pyroelectric detection with smectic liquid crystals
Authors: Glass A. M. , Patel J. S. , Goodby J. W. , Olson D. H. , Geary J. M.
Published: July 2002

The potential of ferroelectric liquid crystals for application as pyroelectric infrared detectors is examined. The liquid crystals studied belong to a family of biphenyl and phenylbenzoate esters. It is found that even within this first liquid-crystal family examined, the materials figure of merit compares well with ferroelectric solid materials commonly used for infrared detection and imaging. Because they permit very simple and flexible device design, liquid crystals are attractive candidates for these applications. However, for low-frequency applications, attention must be paid to the reduction of thermal loading by the cell walls.
Raman scattering of SiC: Application to the identification of heteroepitaxy of SiC polytypes
Authors: Okumura H. , Sakuma E. , Lee J. H. , Mukaida H. , Misawa S. , Endo K. , Yoshida S.
Published: July 2002

Heteroepitaxial growth of 3C- and 6H-silicon carbide (SiC) was investigated using Raman scattering. It was found that 3C-SiC(111) can be epitaxially grown on 6H-SiC(0001) by chemical vapor deposition, and that Raman spectra are useful for identification of heterostructure of thin layers.
Experimental investigations and analysis for high-quality Nb/Al-AlOx/ Nb Josephson junctions
Authors: Morohashi Shin’ichi , Hasuo Shinya
Published: July 2002

We have made a detailed study of the fabrication process of Nb/Al-AlOx/Nb Josephson junctions with the aim of introducing guiding principles to obtain high-quality junctions. Junctions were fabricated by a novel process named the self-aligned contact process. We investigated the deposition conditions of Nb and Al films and optimized them. Oxidation conditions for obtaining target critical current were also investigated. Then composition of the tunneling barrier material and the coverage of the thin Al layer on the base electrode were analyzed by x-ray photoelectron spectroscopy, Auger electron spectroscopy, etc. We investigated aging and annealing effects on the junctions. We confirmed that the current-voltage (I-V) characteristics of the junctions did not change after storage for 450 days at room temperature or when subjected to temperatures of up to 150 °C for shorter periods. Applying the McMillan theory to our experimental results, we explained the characteristics of the junctions. As a result, we have demonstrated that the characteristics are determined by two proximity layers: The first is a double proximity layer composed of an elemental Al layer and Nb/Al alloy layer in the base electrode, and the second is a Nb layer with low transition temperature in the counterelectrode. We have demonstrated that the characteristics are drastically improved by reducing the Nb/Al alloy layer and the Nb layer with low transition temperature and have obtained high-quality junctions with little leakage current.
Stresses and radiation damage in Ar+ and Ti+ ion-implanted silicon
Authors: Madakson P. , Angilello J.
Published: July 2002

Comparative studies of stress and radiation damage in 〈111〉 silicon were carried out after ion implantation with 28-keV Ar+ and 30-keV Ti+ to doses ranging from 1012 to 1017 ions/cm2. The effect of annealing at 600 °C for 2 h was also investigated. Automatic x-ray diffraction measurement of strains in the specimens shows that in the as-implanted condition the stresses are compressive and they increase linearly with ion dose up to about 1016 ions/cm2, after which they relieve. This behavior was the same for both the ‘‘reactive’’ (Ti) and the ‘‘unreactive’’ (Ar) ion implantation. However, after annealing the stresses in the Ar+ implanted samples remain compressive and those in the samples implanted with Ti+ become tensile. The change from compressive to tensile stresses is associated with titanium-silicide formation. X-ray diffraction patterns show that the silicide consists of metastable and equilibrium phases. Sheet resistance measurements on the samples with a continuous TiSi2 layer show the resistivity to be as low as 14 μΩ cm. Rutherford backscattering channeling studies show radiation damage to increase linearly with ion dose and, in the case of Ar implantation, saturate at doses ≥5×1014 ions/cm2, due to the complete amorphization of the implanted region. A decrease in defect density with dose occurred at doses >5×1015 ions/cm2 for the Ti implantation. Annealing only reduced the damage observed, except for low doses (≤1014 ions/cm2), where complete recrystallization occurred.
Recent developments in gravity gradiometry from the Space-Shuttle-borne tethered satellite system
Authors: Lorenzini Enrico C. , Gullahorn Gordon E. , Fuligni Franco
Published: July 2002

This paper describes the current development of a noncryogenic gravity gradiometer for future use on board the tethered satellite system (TSS). The paper also proposes a way of testing a single-axis prototype of such an instrument that makes use of the free-fall technique in vacuum. This gravity gradiometer has a design sensitivity of the order of 10−2 EU (1 EU =Eötvös Unit=10−9 ms−2/m) in 10-s integration time. Because of the small size and light weight, a noncryogenic gravity gradiometer is a valid candidate to fit the limited accommodation capability of the TSS deployed from the shuttle and maintained, for several days, at an altitude of 120–130 km. This unique capability improves the resolution and accuracy achievable in reconstructing the earth gravity field, in particular at short wavelengths. A preliminary analysis of the dynamics of the TSS satellite indicates the potential of such satellite as a low-altitude research platform for gravity gradiometry.
Neutralization of phosphorus in polycrystalline silicon by hydrogenation
Authors: Sankara Narayanan E. M. , Annamalai S. , Sarma G. H. , Iyer Suman B. , Kumar Vikram
Published: July 2002

Experimental evidence that phosphorus in silicon is neutralized by hydrogenation is presented by measuring changes in sheet resistance and Hall mobility carrier in heavily phosphorus-doped polycrystalline films.
A self-consistent computer simulation of compound semiconductor metal-insulator-semiconductor C-V curves based on the disorder-induced gap-state model
Authors: He Li , Hasegawa Hideki , Sawada Takayuki , Ohno Hideo
Published: July 2002

Compound semiconductor metal-insulator-semiconductor (MIS) capacitance-voltage (C-V) curves are simulated on a computer, assuming the presence of a disorder-induced gap-state (DIGS) continuum near the interface in which bonding and antibonding states are distributed both in space and in energy around a particular charge neutrality point, EHO . A program based on Shockley–Read–Hall statistics is developed and applied to simulate a set of experimental MIS C-V curves measured on Al2 O3 /native oxide/InP MIS capacitors. By assuming a particular type of energy and spatial distribution of DIGS continuum and a suitable single set of dynamic parameters, the simulation can completely and self-consistently reproduce the experimental MIS C-V curves with complex hysteresis behavior, which varies with the bias amplitude and swing speed. The excellent agreement between theory and experiment supports the DIGS model. The danger involved in simply applying Terman’s method to determine an Nss distribution is pointed out. A quick procedure for reasonably accurate Nss measurement is suggested.
Thermal conductivity of synthetic diamond films
Authors: Morelli D. T. , Beetz C. P. , Perry T. A.
Published: July 2002

We have measured the thermal conductivity of two diamond films grown by a chemical vapor deposition process. At room temperature the thermal conductivity is of the order of 10 W cm−1 K−1, i.e., as high as naturally occurring (type Ia) single-crystal diamonds. This value exceeds the thermal conductivity of copper at room temperature by a factor of 2. Since these films consist of microcrystallites of diamonds of dimensions on the order of 2 μm, boundary scattering of phonons is expected to be large. The expected effect of boundary scattering on the lattice thermal conductivity is calculated, and is in qualitative agreement with the observed results above about 30 K. However, important differences between the measured conductivity and that expected for boundary scattering are observed below this temperature. It is proposed that a small amount of disorder present in the lattice (identified through Raman studies) can account for this unusual behavior.
A method to calculate the laser heating of layered structures
Authors: Anderson Roger J.
Published: July 2002

This work describes a method to calculate the heating that occurs when a Gaussian laser beam is scanned across the surface of a flat object which may be coated with one or more layers of differing materials. The laser beam is characterized by its power, radius, and scan velocity. Each layer of the object is characterized by its thickness, density, heat capacity, and thermal conductivity. Absorption of laser light is assumed to vary linearly across any one layer, and this absorption is characterized by two dimensionless constants. Temperature rise versus time is calculated as a function of position within the multilayer stack. Use of the technique is illustrated by considering the case of an optically opaque organic dye/polymeric binder coated onto a polymeric substrate. Such a construction may be used in optical data storage applications.
Spatially and time-resolved detection of gallium atoms formed in the laser photochemical vapor deposition process of trimethylgallium by laser-induced fluorescence: Decomposition in the adsorbed state
Authors: Suzuki Hideyuki , Mori Koji , Kawasaki Masahiro , Sato Hiroyasu
Published: July 2002

Spatially and time-resolved detection of gallium atoms formed in the excimer-laser photochemical vapor deposition (CVD) process of trimethylgallium at 248 nm was made by the laser-induced fluorescence technique. The result shows the importance of chemically adsorbed species in the photo-CVD process. The presence of a thermally assisted effect in the photochemical decomposition was indicated.
Structural, optical, and spin properties of hydrogenated amorphous silicon-germanium alloys
Authors: Stutzmann M. , Street R. A. , Tsai C. C. , Boyce J. B. , Ready S. E.
Published: July 2002

We report on a detailed study of structural and electronic properties of hydrogenated amorphous silicon-germanium alloys deposited by rf glow discharge from SiH4 and GeH4 in a diode reactor. The chemical composition of the alloys is related to the deposition conditions, with special emphasis on preferential incorporation of Ge into the solid phase and on the role of inert dilutant gases. Hydrogen bonding in the alloys is investigated with nuclear magnetic resonance and vibrational (Raman and infrared) spectroscopy. The optical properties of a-SiGe:H samples deposited under optimal conditions are analyzed with the help of subgap absorption measurements and band-tail luminescence for the entire range of alloy composi-tions. A large part of the article describes an investigation of the electron-spin-resonance response of undoped alloys. The spin density associated with dangling bond defects localized on Si and Ge atoms has been measured as a function of alloy composition for optimized material. In addition, the dependence of the two defect densities on the detailed deposition conditions (rf power, substrate temperature, and dilution) has been determined in a systematic way for alloys deposited from a plasma with a fixed SiH4/GeH4ratio. The results of this study, especially the preferential creation of Ge dangling bonds, are discussed in the context of our structural data. Furthermore, spin resonance is employed to investigate the light-induced degradation (Staebler–Wronski effect) of a-SiGe:H. Finally, the changes of the spin-resonance spectra of a-Si0.7 Ge0.3 :H upon substitutional doping with phosphorus and boron have been obtained experimentally, and are used to construct a model for the electronic density of states in this material.
ac impedance measurement of environmental water in adhesive interfaces
Authors: Takahashi K. M. , Sullivan T. M.
Published: July 2002

An in situ method based on interfacial electrical impendance measurements is described for following the time-dependent intrusion of water in an adhesive interface that is exposed to a humid environment. Water ingress is measured by its effect on bulk and interfacial conductivity, while regions of liquid water are identified by the presence of aqueous double-layer capacitances. Preliminary measurements are made using aluminum triple-track electrodes to study the environmental stability of adhesion between oxidized silicon (SiO2, 4% P) substrates and three coating materials (RTV silicone, amine-cured epoxy, and mercaptan-cured novolac epoxy) at 80 °C and 94% relative humidity. Moisture-induced changes in conductivity of all coatings are measured, and the presence of condensed water in the mercaptan-cured epoxy interface is clearly identified. Based on static potential field calculations, a modified experimental procedure is outlined that can be used to quantitatively isolate interfacial conduction processes from ac conduction in bulk substrate and coating phases. By indicating the state and quantity of interfacial moisture, capacitance and conductance measurements will allow interface displacement processes to be monitored.
Magnetic properties of new ternary R6Ga3Fe11 compounds
Authors: Li Hong-Shuo , Hu Bo-Ping , Cadogan J. M. , Coey J. M. D. , Gavigan J. P.
Published: July 2002

Three new rare earth–iron ternary compounds with the La6Ga3Co11 structure (I4/mcm), Pr6Ga3Fe11, Nd6Ga3Fe11, and Sm6Ga3Fe11, are found to be ferromagnetic with Curie temperatures of 320, 397, and 462 K, respectively. Large anisotropies have been observed from the magnetization curves and the anisotropy field is larger than 7 T at room temperature. First-order magnetization processes were observed for all samples throughout the temperature range of 4.2–300 K. Point charge calculations give A20=1070 K a−20 at the 16l site and −283 K a−20 at the 8f site.
ac impedance measurements of moisture in interfaces between epoxy and oxidized silicon
Authors: Takahashi Ken M.
Published: July 2002

I n situ interfacial impedance measurements are used to study the effects of moisture at 80 °C on the interface between oxidized silicon and a diglycidyl ether of bisphenol A epoxy cured with diethylene triamine. Bulk impedance measurements follow Randles behavior; conduction in the bulk epoxy is ionic, and is diffusion controlled at low frequencies. An additional conduction process between interfacial electrodes is observed even though a water layer does not condense in the interface. Through comparative use of linear network models, it is found that the ‘‘interfacial’’ conduction path represents distributed bulk conduction in the epoxy with displacement current leakage into the conductive silicon substrate through an interfacial capacitance. Both electrochemical and bulk coating properties jump at humidities near 70%, indicating greatly increased ionic mobility, coating permittivity, and interfacial capacitances. The permittivity and ionic mobility behavior suggest the formation of large water-swollen demains or highly elongated water clusters near the property jump threshold. Because humidity effects are resolved into bulk and interfacial components, interfacial impedance measurements appear to have great utility for the in situ study of transport and electrochemical properties of interfaces and coatings during environmental exposure.
Effects of interface layer sequencing on the transport properties of InAs/AlSb quantum wells: Evidence for antisite donors at the InAs/AlSb interface
Authors: Tuttle Gary , Kroemer Herbert , English John H.
Published: July 2002

Data are presented on the role of the InAs/AlSb interface in determining the electron transport in AlSb/InAs/AlSb quantum wells grown by molecular-beam epitaxy. Because both anion and cation change across an InAs/AlSb interface, it is possible to grow such wells with two different types of interfaces, one with an InSb-like bond configuration, the other AlAs-like. Electron mobility and concentration were found to depend very strongly on the manner in which the quantum well’s interfaces were grown, indicating that high mobilities are seen only if the bottom interface is InSb-like. An As-on-Al sites antisite defect model is postulated for bottom AlAs-like interfaces. Such antisites were used in subsequent samples as donors in modulation-doped high-mobility InAs/AlSb quantum wells.
Effect of annealing on the ac leakage components of the ZnO varistor. I. Resistive current
Authors: Gupta T. K. , Straub W. D.
Published: July 2002

The ZnO varistor is known to require a postsintering heat treatment (annealing) to impart stability to an otherwise unstable device, when subjected to an applied voltage stress. A large body of experimental data in the literature indicate that the most desirable temperature of annealing is between 600 and 700 °C at which the varistor becomes highly stable. This paper is aimed at understanding the significance of this annealing temperature from the analysis of the activation energies, estimated from the Arrhenius plots of the resistive current versus temperature in the prebreakdown region. It is found that the activation energy in the annealing temperature range of 600–700 °C is significantly higher than at any other annealing temperature between 500 and 800 °C. This conclusion is borne out by the statistical evaluation of the experimental data. When this result is combined with the equally notable observations in the literature that the interface state density [J. Mater. Sci. 20, 3487 (1985)], the grain-boundary trap density [J. Appl. Phys. 63, 5375 (1988)], and the density of the negatively charged zinc vacancy at the interface [J. Appl. Phys. 66, 6132 (1989)] are all reduced, also at the same annealing temperature range, it becomes clear that the annealing temperature of 600–700 °C is very unique to varistor stability. The significance of these observations is that the annealing and accompanying stability should be recognized as a very complex process that brings fundamental changes in the grain-boundary junction characteristic of the polycrystalline ceramic.
On the effective thermal conductivity of coated short-fiber composites
Authors: Benveniste Y. , Miloh T.
Published: July 2002

Effective-medium theories are formulated which predict the effective thermal conductivity of coated short-fiber composites. Composite aggregates that contain coated inclusions which may be aligned or randomly oriented are considered. A basic result which simplifies considerably the analysis of such systems in heat conduction problems is first established: It is shown that under certain situations a coated ellipsoidal inclusion can be replaced by an equivalent homogeneous but anisotropic one. For the composite with aligned short fibers, a microgeometry is constructed which possesses an exact solution for the effective conductivity. Self-consistent and differential schemes are formulated for the composite with randomly oriented coated short fibers.
The growth of high-quality AlGaAs by metalorganic molecular-beam epitaxy
Authors: Hersee S. D. , Martin P. A. , Chin A. , Ballingall J. M.
Published: July 2002

The electrical and optical properties of AlGaAs grown by metalorganic molecular-beam epitaxy using triethylaluminum, tri-isobutylaluminum, and trimethylamine-alane are compared. It is found that tri-isobutylaluminum yields the lowest residual carbon incorporation in the layers (Na − Nd = 4 × 1015 cm−3) and the highest electron and hole mobilities. Photoluminescence spectra for the higher-quality AlGaAs, grown using TiBAl, show excitonic luminescence. However, this luminescence appears to be defect related.
Electron-paramagnetic-resonance study of silver halide photochromic glasses: Darkening mechanism
Authors: Caurant D. , Gourier D. , Prassas M.
Published: July 2002

The mechanism of darkening of standard silver halide photochromic glass is studied by electron paramagnetic resonance (EPR) of photogenerated divalent copper ions. A time resolution of the EPR spectra was obtained by recording the evolution with the irradiation time of the EPR intensity at various magnetic-field strengths. It is shown that only one copper species is photogenerated during darkening. This species is a CuII-silver vacancy complex oriented along a [110] direction. The electron ground state has a predominantly dz2 character. The activation energy of the formation of this complex, E=0.06±0.01 eV, is controlled by the migration of interstitial silver ions. It is also shown that the silver vacancy of the complex is generated during the hole trapping by the monovalent copper ion.
The effect of ionizing and displacive radiation on the thermal conductivity of alumina
Authors: White D. P.
Published: July 2002

The effects of ionizing and displacive radiation on the thermal conductivity of alumina at high temperatures have been studied. The phonon scattering relaxation times for several scattering mechanisms have been used to determine the effect on the thermal conductivity. The scattering mechanisms considered are scattering by electrons excited into the conduction band, vacancies, aluminum precipitates, and voids. It is found that under irradiation conditions where the electrical conductivity and dielectric loss tangent are greatly increased there is not a significant decrease in the thermal conductivity due to phonon-electron scattering. The conditions under which the scattering due to vacancies, aluminum precipitates, and voids each produces a significant reduction in the thermal conductivity are discussed.
The effective thermal conductivity of composites with coated reinforcement and the application to imperfect interfaces
Authors: Dunn Martin L. , Taya Minoru
Published: July 2002

An analytical model is developed to predict the effective thermal conductivity of multiphase composites reinforced with coated fillers. The coated fillers are modeled as confocal spheroids, thus enabling the simulation of a wide range of reinforcement geometry ranging from thin flake to continuous fiber. The orientation of the coated fillers is described by a density distribution function which can simulate completely random, in-plane random, and aligned distributions as special cases. The analytical approach appears to be the only one in the literature that renders closed-form predictions of the effective thermal conductivity of composites with misoriented coated reinforcement and thus recourse to a numerical scheme is not required. The analytical approach is specialized to consider the effects of a thermal resistance at the filler-matrix interface of a composite. Results of the proposed model are compared with those obtained by self-consistent and differential estimates [Y. Benveniste and T. Miloh, J. Appl. Phys. 69, 1337 (1991)] for the effective thermal conductivity of a composite reinforced by completely random coated short fibers. The proposed model is in close agreement with the other two models for low conductivity coatings, but differs appreciably for high conductivity coatings. Finally, good agreement is shown to exist between analytical predictions and experimental results obtained from the literature for a diamond particle/zinc-sulfide matrix composite with an interfacial thermal resistance.
Measurement of magnetic anisotropy constant for magneto-optical recording media: A comparison of several techniques
Authors: Wu Te-ho , Fu Hong , Hajjar R. A. , Suzuki T. , Mansuripur M.
Published: July 2002

Experimental data of the intrinsic perpendicular magnetic anisotropy energy constant Ku are presented for amorphous rare earth-transition metal (RE-TM) Tbx(FeCo)1−x and multilayered Co/Pt thin film samples. These data were independently measured using five techniques based on torque magnetometry, the extraordinary Hall effect, and the magneto-optic Kerr effect. In the Hall effect measurement, the external field was applied to the sample in three different ways: fixed at 45° from the film normal; rotating around the sample; and fixed along the in-plane direction. The results obtained with these techniques agree with each other for the Co/Pt samples. However, we do find systematic differences in the measured Ku for the Tbx(FeCo)1−x samples. For example, Ku given by the Hall effect and Kerr effect is always larger (by up to a factor of 3) than that given by torque technique. Another interesting fact is that Ku given by the Hall effect technique drops as x approaches the compensation point xc in the TM-dominant case, but increases as x approaches xc in the RE-dominant case. These experimental results are explained by taking into account the canting between RE and TM subnetworks.
Polycrystalline silicon thin films processed with silicon ion implantation and subsequent solid-phase crystallization: Theory, experiments, and thin-film transistor applications
Authors: Yamauchi Noriyoshi , Reif Rafael
Published: July 2002

A review is presented of the self-implantation method which has been developed to achieve high-quality polycrystalline silicon thin films on insulators with enhanced grain sizes and its applications to thin-film transistors (TFTs). In this method, silicon ions are implanted into an as-deposited polycrystalline silicon thin film to amorphize most of the film structure. Depending on ion implantation conditions, some seeds with 〈110〉 orientation remain in the film structure due to channeling. The film is then thermally annealed at relatively low temperatures, typically in the range of 550–700 °C. With optimized process conditions, average grain sizes of 1 μm or greater can be obtained. First, an overview is given of the thin-film transistor technology which has been the greatest motivation for the research and development of the self-implantation method. Then the mechanism of selective amorphization by the silicon self-implantation and the crystallization by thermal annealing is discussed. An analytical model and experimental results are described. Polycrystalline silicon TFTs fabricated using the self-implanted polycrystalline silicon thin-films are then reviewed. The high-quality polycrystalline silicon thin films processed with the self-implantation method results in excellent TFT characteristics for both n- and p-channel devices thereby allowing complementary metal-oxide-semiconductor integrated circuits. High mobilities of around 150 cm2/V s for n-channel TFTs and around 50 cm2/V s for p-channel TFTs as well as on-to-off current ratios of 1×108 have been achieved. Fabrication and characterization of polycrystalline silicon TFTs with channel dimensions comparable to or smaller than the grain size of polycrystalline silicon films are also described to present a case study to discuss the self-implantation process and associated technologies. Finally, new approaches that extend the self-implantation method to control grain-boundary locations are discussed. If grain-boundary locations can indeed be controlled, the self-implantation method will become even more valuable in developing future high-performance TFT integrated circuits.
Excitation mechanisms and gain modeling of the high-pressure atomic Ar laser in He/Ar mixtures
Authors: Shon Jong W. , Kushner Mark J.
Published: July 2002

The high-pressure (≥0.5 atm) atomic Ar laser (3d→4p) oscillates on four infrared transitions (1.27–2.4 μm). Quasicontinuous oscillation on the 1.79 μm transition has been obtained using electron beam and fission-fragment excitation over a wide range of power deposition and gas pressure. In this regard, a computer model has been developed to investigate excitation mechanisms of the Ar laser. Results from the model suggest that the upper laser level of the 1.79 μm transition [Ar(3d[1/2]1)] is dominantly populated by dissociative recombination of HeAr+. In contrast, the dissociative recombination of Ar+2 is believed to predominantly produce Ar(4s) states. Electroionization from Ar metastables at moderate to high pump rates is likely to be responsible for the high efficiency of the Ar laser. Gain and laser oscillation are discussed and compared to experiments for He/Ar gas mixtures using various Ar mole fractions and total pressures. These results show that the optimum Ar mole fractions in He/Ar mixtures are ∼0.1%–5% for quasicontinuous pumping.
Decay of ultraviolet-induced fiber Bragg gratings
Authors: Erdogan T. , Mizrahi V. , Lemaire P. J. , Monroe D.
Published: July 2002

We report measurements of thermally induced decay of fiber Bragg gratings patterned by ultraviolet irradiation in germanium-doped silica fiber. The decay is well characterized by a ‘‘power-law’’ function of time with a small exponent, which is consistent with the rapid initial decay followed by a substantially decreasing rate of decay. We propose a decay mechanism in which carriers excited during writing are trapped in a broad distribution of trap states, and the rate of thermal depopulation is an activated function of the trap depth. This model is consistent with the observed power-law behavior. An important consequence of this mechanism is that the decay of the induced index change can be accelerated by increasing temperature. A decelerated-aging experiment verifies this prediction. This result demonstrates that it is possible to preanneal a device incorporating ultraviolet-induced refractive-index changes, wiping out the portion of the index change that would decay over the lifetime of the device, and keeping only the very stable portion of the index change.
Fiber-matrix interphase characterization in composites using ultrasonic velocity data
Authors: Chu Y. C. , Rokhlin S. I.
Published: July 2002

A nondestructive method is introduced to determine the effective elastic moduli of fiber-matrix interphases from measured composite moduli. The composite moduli are obtained by measuring the angular dependences of ultrasonic longitudinal and transverse phase velocities in planes along and perpendicular to the fibers. Three independent interphase effective moduli are found using micromechanical multiphase models which were applied after averaging composite transverse moduli to account for the composite orthotropy. Sensitivity analysis shows that errors in the calculated interphasial moduli are approximately ten times those in the composite moduli. Experiments are performed on Si3N4 ceramic and Ti-24Al-11Nb intermetallic matrix composites reinforced with carbon coated silicon carbide fibers. The experimental interphasial moduli for the intermetallic composites agree with literature data after accounting for the interphase microstructure. The interphasial moduli for ceramic composites are lower than those for the intermetallic composite due to imperfect mechanical contact between the interphase and the porous matrix. The use of the method to assess the interphase degradation is demonstrated for interphasial oxidation damage. The analysis helps to determine the morphology of the damaged interphase.
The application of scattering cross sections to ion flux models in discharge sheaths
Authors: Phelps A. V.
Published: July 2002

We suggest consistent sets of Ar++Ar and Ar+++Ar differential and integral cross sections for modeling ion scattering that take into account differential scattering data and the fact that symmetric charge transfer collisions are one aspect of elastic scattering collisions. These suggestions make possible a considerable improvement in the accuracy of future Monte Carlo calculations of the angular, energy, and temporal distributions of Ar+ and Ar++ ions passing through the electrode sheaths of low-pressure, rf, and dc discharges in Ar. The cross sections necessary for a proper modeling of the energy dissipation in the gas and at the electrodes by fast neutral Ar atoms formed in symmetric-charge-transfer collisions of Ar+ and Ar++ with Ar are also reviewed.