bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure, lattice parameters, and thermal expansion anisotropy of C70 fullerite
Authors: Isakina A. P. , Prokhvatilov A. I. , Strzhemechny M. A. , Yagotintsev K. A.
Published: July 2002

X-ray diffraction studies of the two low-temperature phases of especially pure polycrystalline C70 fullerite are carried out in the temperature range 20–310 K. At room temperature a rhombohedral structure is established, and at T<276 K a monoclinic structure. The volume jump at the low-temperature phase transition is determined to be ∼8.5 cm3/mole, or 1.7%. The temperature dependence of the lattice parameters and of the linear and volume thermal expansion coefficients is investigated in the two phases. For both the intermediate and the low-temperature modifications a substantial anisotropy of the thermal deformation of the crystals along individual crystallographic directions is observed. The results are analyzed with the use of previous structural studies. It is shown that in the absence of rotations at low temperatures the lattice is unstable to monoclinic distortions, the estimated sign and order of magnitude of which agree with the observations.
Nature of the optical transition in polymethine dyes and J-aggregates
Authors: Egorov Vladimir V.
Published: July 2002

Using a new theory of photoinduced electron transfer beyond the Landau–Zenner ideology or, more precisely, electrodynamics of extended multiphonon transitions [V. V. Egorov, Chem. Phys. 269, 251 (2001)], we give an explanation for the well-known experimental data [L. G. S. Brooker et al., J. Am. Chem. Soc. 62, 1116 (1940)] for the absorption line shape in the vinylogous series of an ideal polymethine dye represented by a thiacarbocyanine. Then, using this explanation together with our earlier explanation [V. V. Egorov, Chem. Phys. Lett. 336, 284 (2001)] for the nature of the well-known intense narrow J-band due to an aggregation of polymethine dyes, we predict very intense narrow absorption lines for short optical transitions. Interpretation of these results is given on a basis of the Heisenberg uncertainty relation. A process of creation of the pure (quantum-mechanical) electron-transfer state is considered for the two complementary cases: the electron-transfer state is determined by interaction of the electron with its environment through spontaneous pumping of this state by an ordered or disordered environmental motion. The latter case corresponds to the Landau–Zenner-type picture of adiabatic and nonadiabatic electron transfers. The former case is used to account for the nature of the intense narrow bands.
Focused-ion-beam induced grain growth in magnetic materials for recording heads
Authors: Park C.-M. , Bain J. A.
Published: July 2002

It is reported that focused-ion-beam (FIB) processing causes considerable grain growth in magnetic thin films, which may be a concern for the use of this technique in recording head pole trimming. Two magnetic thin films (NiFe and FeCoN) were used to examine FIB-induced grain growth. NiFe showed more significant grain growth with grain size increasing from ∼20 to ∼500 nm after FIB etching with a dose of 0.15 nC/μm2. The grain size in the FeCoN films increased from ∼20 to ∼100 nm at the same dose. It was found that the NiFe films fibbed with the same ion dose (0.15 nC/μm2) and different beam currents (99, 672, and 1500 pA) experienced similar grain growth, suggesting that the grain growth is driven by momentum transfer rather than local heating. Grains in the NiFe films often had an elongated shape rather than circular and many crystallographic twins were observed in the FIB images, and confirmed with transmission electron microscopy. These twins tended to be parallel to the fast axis of beam rastering, while the axis of elongation, if present, tended to be along slow axis of rastering. It was found that the ratio of pixel space to beam diameter controls this behavior. It was revealed that values of this ratio less than 0.5 resulted in elongated or “acicular” grains. For fibbed FeCoN films, no acicular grains or twins were observed. This is believed to be due to the difference in crystal structure and twinning planes of the face-centered-cubic and body-centered-cubic structures. Both fibbed films showed magnetic degradation in the form of reduced moment and/or increased coercivity, as measured by Kerr imaging.
Realization of band gap above 5.0 eV in metastable cubic-phase MgxZn1−xO alloy films
Authors: Choopun S. , Vispute R. D. , Yang W. , Sharma R. P. , Venkatesan T. , Shen H.
Published: July 2002

We report on the realization of wide band gap (5–6 eV), single-phase, metastable, and epitaxial MgxZn1−xO thin-film alloys grown on sapphire by pulsed laser deposition. We found that the composition, structure, and band gaps of the MgxZn1−xO thin-film alloys depend critically on the growth temperature. The structural transition from hexagonal to cubic phase has been observed for (Mg content greater than 50 at. %) (1⩾x⩾0.5) which can be achieved by growing the film alloys in the temperature range of 750 °C to room temperature. Interestingly, the increase of Mg content in the film has been found to be beneficial for the epitaxial growth at relatively low growth temperature in spite of a large lattice mismatch between sapphire and cubic MgZnO alloys.
On the geometry of master–slave synchronization
Authors: Martens Marco , Pécou Elisabeth , Tresser Charles , Worfolk Patrick
Published: July 2002

In 1990, Pecora and Carroll reported the observation that one can synchronize the orbits of two identical dynamical systems, which may be chaotic, by feeding state variables of one of them to the other one with no feedback, a phenomenon often called master–slave synchronization. We report here some results on the theory of master–slave synchronization for maps and flows, which are all inspired by a similar geometric and coordinate independent point of view to the one introduced in master–slave synchronization by Tresser, Worfolk, and Bass. Our results are variations on the theme that projection often can compensate for expansion.
Thermally activated magnetization reversal in submicron magnetic tunnel junctions for magnetoresistive random access memory
Authors: Rizzo N. D. , DeHerrera M. , Janesky J. , Engel B. , Slaughter J. , Tehrani S.
Published: July 2002

We have measured thermally activated magnetization reversal of the free layers in submicron magnetic tunnel junctions to be used for magnetoresistive random access memory. We applied magnetic field pulses to the bits with a pulse duration tp ranging from nanoseconds to 0.1 ms. We have measured the switching probability as a function of tp with a fixed field amplitude H, and as a function of H for fixed tp. For both cases, we find good agreement with the switching probability predicted by the Arrhenius–Néel theory for thermal activation over a single energy barrier.
Successive bifurcations in a simple model of atmospheric zonal-flow vacillation
Authors: Koo Seongjoon , Ghil Michael
Published: July 2002

Low-frequency variability of the atmospheric flow in the Southern Hemisphere is dominated by irregular changes in the latitude and intensity of the mid-latitude eastward jet about its climatological mean state. This phenomenon, known as atmospheric zonal-flow vacillation, is characterized by the existence of two persistent states of the zonal (i.e., east–west oriented) jet and irregular transitions between them. Nonlinear interactions between the mean flow and the waves play a key role in the dynamics of this vacillation. In the present study, we develop a low-order, deterministic model for the nonlinear dynamics of atmospheric zonal-flow vacillation. Multiple equilibria arise in this model’s zonal-mean flow, that is, in the longitudinal flow averaged along a given latitude circle. These equilibria bear a strong resemblance to the two persistent flow regimes found in Southern Hemisphere observations. The two equilibrium states are maintained by wave forcing against surface drag, as in the observations. Successive bifurcations to periodic and chaotic zonal-mean flow regimes occur as the model’s dissipation parameter is reduced.
Noise-resistant chaotic maps
Authors: Carroll T. L.
Published: July 2002

Synchronized chaotic systems are highly vulnerable to noise added to the synchronizing signal. It was previously shown that chaotic circuits could be built that were less sensitive to this type of noise. In this work, simple chaotic maps are demonstrated that are also less sensitive to added noise. These maps are based on coupling a shift map to a digital filter. These maps are simple enough that they should help lead to an understanding of how noise-robust chaotic systems work.
Autocatalytic reactions of phase distributed active particles
Authors: Santoboni Giovanni , Nishikawa Takashi , Toroczkai Zoltán , Grebogi Celso
Published: July 2002

We investigate the effect of asynchronism of autocatalytic reactions taking place in open hydrodynamical flows, by assigning a phase to each particle in the system to differentiate the timing of the reaction, while the reaction rate (periodicity) is kept unchanged. The chaotic saddle in the flow dynamics acts as a catalyst and enhances the reaction in the same fashion as in the case of a synchronous reaction that was studied previously, proving that the same type of nonlinear reaction kinetics is valid in the phase-distributed situation. More importantly, we show that, in a certain range of a parameter, the phenomenon of phase selection can occur, when a group of particles with a particular phase is favored over the others, thus occupying a larger fraction of the available space, or eventually leading to the extinction of the unfavored phases. We discuss the biological relevance of this latter phenomenon.
Noise- and inertia-induced inhomogeneity in the distribution of small particles in fluid flows
Authors: Cartwright Julyan H. E. , Magnasco Marcelo O. , Piro Oreste
Published: July 2002

The dynamics of small spherical neutrally buoyant particulate impurities immersed in a two-dimensional fluid flow are known to lead to particle accumulation in the regions of the flow in which vorticity dominates over strain, provided that the Stokes number of the particles is sufficiently small. If the flow is viewed as a Hamiltonian dynamical system, it can be seen that the accumulations occur in the nonchaotic parts of the phase space: the Kolmogorov–Arnold–Moser tori. This has suggested a generalization of these dynamics to Hamiltonian maps, dubbed a bailout embedding. In this paper we use a bailout embedding of the standard map to mimic the dynamics of neutrally buoyant impurities subject not only to drag but also to fluctuating forces modeled as white noise. We find that the generation of inhomogeneities associated with the separation of particle from fluid trajectories is enhanced by the presence of noise, so that they appear in much broader ranges of the Stokes number than those allowing spontaneous separation.
Near-classical noise enhancement of microwave ionization of Rydberg atoms
Authors: Bayfield James E.
Published: July 2002

The ionization of the highly excited hydrogen atom in a strong external microwave field is a classically chaotic, near-classical quantum system for microwave frequencies somewhat below the initial Kepler electron orbit frequency. The addition of microwave noise is found to reduce the sinewave microwave field needed for ionization, modifying the near-classical fast process responsible for the microwave energy absorption. A classical numerical calculation based upon a many-frequency model of the noise qualitatively reproduces the observed noise enhancement.
Conditions of stochasticity of two-dimensional billiards
Authors: Bunimovich L. A.
Published: July 2002

There are two known mechanisms that produce chaos in billiard systems. The first one, discovered by Ya. G. Sinai, is called dispersing, the second, found by the author, is called defocusing. The same mechanisms produce chaos for geodesic flows. Some results on two-dimensional billiards, which indicate that only these two mechanisms can produce chaos in Hamiltonian systems, are discussed.
Complex dynamics of cardiac arrhythmias
Authors: Coumel Philippe , Maison-Blanche Pierre
Published: July 2002

In the case of a benign but very active cardiac arrhythmia often observed in healthy subjects, the behavior of the ventricular extrasystoles during long-term electrocardiographic recording was carefully scrutinized using the facilities of complex computerized programs of analysis. The presence or the absence of the extrasystoles, their distribution with respect to the normal beats, their time relationships, and the existence or the absence of a repetitive activity after the initial extrasystole follow complex, interactive rules that essentially depend on two determinants: the cardiac frequency as such (rate-dependence) and the autonomic nervous system (adrenergic dependence). It can be concluded, at variance from the random distribution suggested by a superficial examination, that an in-depth study allows the complex rules conditioning the various types and patterns of extrasystoles to be unearthed. Precise quantitative regularities underly the dynamics, and observing such rules gives the cardiologist hopes that one might distinguish various physiological mechanisms from a careful consideration of the dynamics. Collaboration of cardiologists and nonlinear dynamicists is needed to understand the origin of complex cardiac rhythms and to sort out the roles of chance and determinism in their genesis.
Transition to chemical turbulence
Authors: Ouyang Q. , Swinney Harry L.
Published: July 2002

Experiments have been conducted on Turing-type chemical spatial patterns and their variants in a quasi-two-dimensional open spatial reactor with a chlorite–iodide–malonic acid reaction. A variety of stationary spatial structures−hexagons, stripes, and mixed states−were observed, and transitions to these states were studied. For conditions beyond those corresponding to the emergence of patterns, a transition was observed from stationary spatial patterns to chemical turbulence, which is marked by a continuous motion of the pattern within a domain and of the grain boundaries between domains. The transition to chemical turbulence was analyzed by measuring the correlation length, the average pattern speed, and the total length of the domain boundaries. The emergence of chemical turbulence is accompanied by a large increase in the defects in the pattern, which suggests that this is an example of defect-mediated turbulence.
Coupled map models for chaos in extended systems
Authors: Bourzutschky M. S. , Cross M. C.
Published: July 2002

Coupled maps with conserved quantities are introduced as models for chaos in extended systems. The long-wavelength limit of a simple one-dimensional example is investigated in detail. A Langevin equation for this system is derived, and the applicability of the fluctuation-dissipation theorem is discussed. Several numerical experiments to investigate possible thermodynamic properties are described.
Unsteady processes in machines
Authors: Pfeiffer Friedrich
Published: July 2002

Couplings in machines and mechanisms always have play and friction. While under loading, stick–slip phenomena and impact events can take place. Such processes are modeled as multibody systems whose structure is time variant or unsteady. The time-variant number of degrees of freedom is due to stick–slip contacts. The coupling characteristics become unsteady, for instance there exist jumps in the loads, if impacts occur. For establishing a uniform theory for such phenomena we use a Lagrangian approach connecting the additional constraint equations and the equations of motion by Lagrange multipliers, which are proportional to the constraint forces. Stick–slip and impact events are evaluated by indicator functions leading to special numerical algorithms for the search of switching points. Contact problems are formulated as a complementarity problem which can be solved by efficient algorithms. The theory is applied to rattling in gears, impact drilling machines, turbine blade dampers, and a woodpecker toy. In some of these applications, chaos as a result of bifurcations is possible, which results from variations in the parameters.
Irregular phase slipping in a model for a laser with injected signal
Authors: Ellmer P. , Jung C.
Published: July 2002

In a model for a single mode laser with injected signal we find irregular pulsing of the output signal. During the turbulent time intervals the relative phase between the internal and the external field slips by an integer multiple of 2π, where the multiplicity varies irregularly from pulse to pulse. Between the pulses, in the laminar time intervals, the relative phase is locked. We compare this behavior with the results of various other laser models. By the investigation of this interesting mechanism for the creation of phase chaos in a driven oscillator, we learn which properties are responsible for the irregular interruption of the phase locking.
Chaotic transmission of waves and “cooling” of signals
Authors: Zaslavsky G. M. , Abdullaev S. S.
Published: July 2002

Ray dynamics in waveguide media exhibits chaotic motion. For a finite length of propagation, the large distance asymptotics is not uniform and represents a complicated combination of bunches of rays with different intermediate asymptotics. The origin of the phenomena that we call “chaotic transmission,’’ lies in the nonuniformity of the phase space with sticky domains near the boundary of islands. We demonstrate different fractal properties of ray propagation using underwater acoustics as an example. The phenomenon of the kind of Lévy flights can occur and it can be used as a mechanism of cooling of signals when the width of spatial spectra dispersion is significantly reduced.
Quasi-periodicity, global stability and scaling in a model of Hamiltonian round-off
Authors: Lowenstein John , Hatjispyros Spyros , Vivaldi Franco
Published: July 2002

We investigate the effects of round-off errors on the orbits of a linear symplectic map of the plane, with rational rotation number ν=p/q. Uniform discretization transforms this map into a permutation of the integer lattice Z2. We study in detail the case q=5, exploiting the correspondence between Z and a suitable domain of algebraic integers. We completely classify the orbits, proving that all of them are periodic. Using higher-dimensional embedding, we establish the quasi-periodicity of the phase portrait. We show that the model exhibits asymptotic scaling of the periodic orbits and a long-range clustering property similar to that found in repetitive tilings of the plane.
Control of chaos in excitable physiological systems: A geometric analysis
Authors: Christini David J. , Collins James J.
Published: July 2002

Model-independent chaos control techniques are inherently well-suited for the control of physiological systems for which quantitative system models are unavailable. The proportional perturbation feedback (PPF) control paradigm, which uses electrical stimulation to perturb directly the controlled system variable (e.g., the interbeat or interspike interval), was developed for excitable physiological systems that do not have an easily accessible system parameter. We develop the stable manifold placement (SMP) technique, a PPF-type technique which is simpler and more robust than the original PPF control algorithm. We use the SMP technique to control a simple geometric model of a chaotic system in the neighborhood of an unstable periodic orbit (UPO). We show that while the SMP technique can control a chaotic system that has UPO dynamics which are characterized by one stable manifold and one unstable manifold, the success of the SMP technique is sensitive to UPO parameter estimation errors.
Eulerian mean flow from an instability of convective plumes
Authors: Childress Stephen
Published: July 2002

The dynamical origin of large-scale flows in systems driven by concentrated Archimedean forces is considered. A two-dimensional model of plumes, such as those observed in thermal convection at large Rayleigh and Prandtl numbers, is introduced. From this model, we deduce the onset of mean flow as an instability of a convective state consisting of parallel vertical flow supported by buoyancy forces. The form of the linear equation governing the instability is derived and two modes of instability are discussed, one of which leads to the onset of steady Eulerian mean flow in the system. We are thus able to link the origin of mean flow precisely to the profiles of the unperturbed plumes. The form of the nonlinear partial differential equation governing the Eulerian mean flow, including nonlinear effects, is derived in one special case. The extension to three dimensions is outlined.
Statistical properties of actions of periodic orbits
Authors: Sano Mitsusada M.
Published: July 2002

We investigate statistical properties of unstable periodic orbits, especially actions for two simple linear maps (p-adic Baker map and sawtooth map). The action of periodic orbits for both maps is written in terms of symbolic dynamics. As a result, the expression of action for both maps becomes a Hamiltonian of one-dimensional spin systems with the exponential-type pair interaction. Numerical work is done for enumerating periodic orbits. It is shown that after symmetry reduction, the dyadic Baker map is close to generic systems, and the p-adic Baker map and sawtooth map with noninteger K are also close to generic systems. For the dyadic Baker map, the trace of the quantum time-evolution operator is semiclassically evaluated by employing the method of Phys. Rev. E 49, R963 (1994). Finally, using the result of this and with a mathematical tool, it is shown that, indeed, the actions of the periodic orbits for the dyadic Baker map with symmetry reduction obey the uniform distribution modulo 1 asymptotically as the period goes to infinity.
Spectral properties and anomalous transport in a polygonal billiard
Authors: Artuso Roberto , Guarneri Italo , Rebuzzini Laura
Published: July 2002

We analyze a class of polygonal billiards, whose behavior is conjectured to exhibit a variety of interesting dynamical features. Correlation functions are numerically investigated, and in a subclass of billiard tables they give indications about a singular continuous spectral measure. By lifting billiard dynamics we are also able to study transport properties: the (normal or anomalous) diffusive behavior is theoretically connected to a scaling index of the spectral measure; the proposed identity is shown to agree with numerical simulations.
Stalactite basin structure of dynamical systems with transient chaos in an invariant manifold
Authors: Dronov Vasily , Ott Edward
Published: July 2002

Dynamical systems with invariant manifolds occur in a variety of situations (e.g., identical coupled oscillators, and systems with a symmetry). We consider the case where there is both a nonchaotic attractor (e.g., a periodic orbit) and a nonattracting chaotic set (or chaotic repeller) in the invariant manifold. We consider the character of the basins for the attracting nonchaotic set in the invariant manifold and another attractor not in the invariant manifold. It is found that the boundary separating these basins has an interesting structure: The basin of the attractor not in the invariant manifold is characterized by thin cusp shaped regions (“stalactites”) extending down to touch the nonattracting chaotic set in the invariant manifold. We also develop theoretical scalings applicable to these systems, and compare with numerical experiments.
An algorithm for finding spurious points in turbulent signals
Authors: Roberts D. Aaron
Published: July 2002

An algorithm is presented for finding spurious points in time series consisting of data from a turbulent process. The usual statistical criterion of eliminating points a certain number of standard deviations (σ) from the mean is shown to work well provided that (1) running means and standard deviations of about 100 points are used, (2) the interval for testing is moved by one point at a time, and (3) spurious points are eliminated from the statistics as the run progresses. This procedure allows the use of a criterion that points be 4 or 5σ from the running mean, and it preserves near discontinuities and other desired features in the time series. An automated procedure such as this is essential for the processing of certain types of data, such as those obtained from some spacecraft instruments.
Mantle convection modeling on parallel virtual machines
Authors: Bunge Hans-Peter , Baumgardner John R.
Published: July 2002

Thermally driven convection within the earth’s mantle determines one of the longest time scales of our planet. Plate tectonics, the piecewise continuous movement of the earth’s surface, is the prime manifestation of this slow deformational process but ultimately all large scale geological activity and dynamics of the planet involves the release of potential energy in the mantle. Massively parallel supercomputers are now allowing us to construct models of mantle convection with unprecedented complexity and realism. Here we present results from an approach to parallel computation that relies on explicit message-passing and distributed computing. Connecting workstations together as a single parallel machine over a network and using the parallel virtual machine software, we are able to perform computations in the hundreds of Mflops range with a demonstrated total parallel overhead of less than ten percent. We have run high-resolution thermal convection calculations for the earth’s mantle on the Los Alamos 16-node cluster of IBM RS/6000 workstations employing a finite element mesh with more than 1.3 million grid points. These results indicate this approach to parallel computing offers a practical and efficient means of utilizing a broad spectrum of parallel hardware. © 1995 American Institute of Physics.
How good are one-dimensional Josephson junction models?
Authors: Lomdahl P. S. , Olsen O. H. , Eilbeck J. C. , Samuelsen M. R.
Published: July 2002

A two-dimensional model of Josephson junctions of overlap type is presented and shown to reduce to the usual one-dimensional (1D) model in the limit of a very narrow junction. Comparisons between the stability limits for fluxon reflection obtained from the two models suggest that the many results obtained from the one-dimensional model can be used for large-area junctions, thus explaining the remarkable agreement between 1D theory and experiments.
A note on the Fermi–Dirac integral function
Authors: Aguilera-Navarro V. C. , Estévez G. A. , Kostecki Allyn
Published: July 2002

A constructive method based on the standard Padé approximant technique is employed for deducing an implicit analytical expression for the Fermi–Dirac integral function. The simple equation thus obtained has been found to be remarkably accurate and has shown a wide range of applicability.
Coercivity and microstructure of melt-spun Sm(Fe11Ti)
Authors: Sun Hong , Otani Yoshichika , Coey J. M. D. , Meekison C. D. , Jakubovics J. P.
Published: July 2002

Amorphous Sm(Fe11Ti) ribbons were prepared by melt spinning. After heat treatment, very fine grains (≊50 nm) of the ThMn12-structure phase were obtained with no signs of a separate intergranular phase. Each grain appears to be a single domain. The maximum room-temperature coercivity is 0.56 T, rising to 1.20 T at 11 K. A model is proposed for the coercivity which is based on an analogy with the random anisotropy model of amorphous magnetism. It gives the correct magnitude of the coercivity, and suggests it should vary as the reciprocal of the crystallite size.
Effects of ion bombardment and chemical reaction on wafer temperature during plasma etching
Authors: Durandet A. , Joubert O. , Pelletier J. , Pichot M.
Published: July 2002

Measurements of wafer surface temperature during plasma etching, performed with a new optical contactless thermometer method, are presented. The respective effects of ion bombardment and chemical reaction on the increase in wafer temperature during plasma etching are evaluated separately. The time dependence of the silicon surface temperature is shown, as a function of the ion bombardment energy and flux on the surface, for both rf and dc applied biases on the wafer. Sample heating resulting from the exothermic chemical reaction of silicon etching by fluorine is demonstrated.
The effect of substrate on the morphology of CuInSe2 films prepared by chemical spray pyrolysis for CuInSe2/CdS solar cells
Authors: Brown Brian J. , Bates Clayton W.
Published: July 2002

This communication provides a detailed examination of the morphology of thin films of CuInSe2 prepared by chemical spray pyrolysis on borosilicate glass, molybdenum-coated glass, and CdS substrates. The latter two are used in thin-film heterojunction solar cells of CuInSe2/CdS in the backwall and reverse backwall (RBW) configurations, respectively, and hence are of great technological interest. The pseudoepitaxial growth of CIS on CdS and its low deposition temperature (250 °C) which inhibits interdiffusion suggest that one should prepare CuInSe2/CdS solar cells in the RBW configuration.
Ion extraction characteristics on a barium plasma produced between grounded parallel plate electrodes and a positively biased upper plate electrode
Authors: Yamada Kimio , Tetsuka Tsutomu , Deguchi Yoshihiro
Published: July 2002

When a barium plasma was suddenly produced between parallel plate electrodes and an upper plate electrode with a higher potential, the ion extraction was investigated in comparison with characteristics of the usual parallel plate electrode configuration. The parameter dependencies of ion extraction time by an external electric field, measured under a variety of experimental conditions, led to the conclusion that the ion extraction time can be expressed as the product of exponents of the applied voltage, plasma width, and ion density as well as the parallel plate electrodes. This scaling relation estimates the ion extraction time with an accuracy of 15% for a given parameter set. Comparison of the scaling relations for the new electrode concept with a highly biased upper plate electrode and the parallel plate electrodes showed that the former improves the ion extraction time by a maximum factor of 2.6.
Tunnel deep level transient spectroscopy on a single quantum well
Authors: Letartre X. , Stiévenard D. , Lannoo M. , Barbier E.
Published: July 2002

Using deep level transient spectroscopy (DLTS), we have studied a GaAs/GaInAs/GaAs single quantum well. The DLTS spectra exhibit both a peak and a plateau: the peak is associated with the thermionic emission of electrons and the plateau corresponds to a quasiconstant emission rate associated with a tunneling emission of electrons from the well towards the conduction band. A modelization of the tunneling effect is proposed, based on Oppenheimer’s approach as to the WKB approximation.
Nitrogenation of R2Fe17 compounds: R=rare earth
Authors: Coey J. M. D. , Lawler J. F. , Sun Hong , Allan J. E. M.
Published: July 2002

The reaction between nitrogen gas and the series of R2Fe17 intermetallic compounds is studied by thermopiezic analysis on micron-size powders. Nitrogen diffusion in the range 300–550 °C can be represented by an activated interstitial diffusion process D=D0e−Em/kT with D0=1.95×10−10 m2 s−1 and an activation energy Em=0.81 eV. This stage of the nitrogenation reaction leads to the formation of R2Fe17N3−δ nitrides which have structures related to those of the parent compounds. Heating above about 800 °C leads to complete disproportionation of the compounds into a mixture of rare-earth nitride and iron.
Process dependence of the SiO2/Si(100) interface trap density of ultrathin SiO2 films
Authors: Fukuda Hisashi , Yasuda Makoto , Iwabuchi Toshiyuki , Kaneko Satoshi , Ueno Tomo , Ohdomari Iwao
Published: July 2002

The interface trap density of states Dit of ultrathin SiO2 film has been investigated as a function of process variables. The process parameters used were oxidation temperature (1000–1200 °C), oxide thickness (4–10 nm), and annealing condition, including ambient (deuterium, argon, and in vacuum), temperature (500 and 900 °C), and time (10–120 s). Analysis of as-grown SiO2 films showed that the Dit decreases with increasing oxidation temperature and/or oxide thickness. With annealing in argon or in vacuum at 900 °C, the Dit decreases exponentially at the initial stage, then starts to increase linearly with increasing time. Similar behavior was observed for 900 °C deuterium annealing. Deuterium annealing at 500 °C was more effective in the reduction of the Dit, whereas argon annealing at the same temperature did not affect the density at all. A possible model for annealing kinetics is proposed to explain the experimental results.
Surface phase decomposition in Bi2Sr2CaCu2Oy single crystals
Authors: Wu Wenbin , Li Fanqing , Li Xiao-Guang , Shi Lei , Zhou Guien , Qian Yitai , Qin Qiaonan , Zhang Yuheng
Published: July 2002

Phase decomposition in high-quality Bi2Sr2CaCu2Oy single crystals has been induced by annealing treatments at a temperature over 500 °C. X-ray diffraction and scanning electron microscope studies show that this change of structure is mainly limited in the surface part of the annealed single crystals and the 2201 phase together with a new phase of cell parameter c=9.6 Å segregated from the 2212 phase. The phase decomposition is directly associated with the oxygen content in the annealing atmospheres and evaporation of the Bi is detected and is suggested to be the main factor involving the phase segregation in the annealing process and an important factor for the BiO bilayers in the as-grown single crystals.
Magnetic fine structure of domain walls in iron films observed with a magnetic force microscope
Authors: Proksch Roger , Foss Sheryl , Dahlberg E. Dan , Prinz Gary
Published: July 2002

The submicron magnetic structure of domain walls in a single-crystal iron film has been studied using a magnetic force microscope (MFM). The MFM tip was sensitized to the component of the field perpendicular to the film plane. The sample examined was a 500-nm-thick single-crystal film of iron, grown by molecular-beam epitaxy (MBE). Before it was imaged, the film was magnetized along its (in-plane) easy axis in a 2000-Oe field. Studies of the domain structure at numerous locations on the film surface revealed a rich variety of micromagnetic phenomena. Parallel domain walls, determined to be Bloch walls with a width of 70–100 nm, were seen along the easy axis, spaced roughly 30 μm apart. These appeared to be Bloch walls. Bloch lines were also observed in the walls with an average periodicity of 1.5 μm. This is a value smaller than that predicted for Bloch wall-line structures. In addition, a pronounced zig–zag structure was observed, as expected from previous Fe whisker observations.
Video detection and mixing performance of GaAs Schottky-barrier diodes at 30 THz and comparison with metal-insulator-metal diodes
Authors: Hübers H.-W. , Schwaab G. W. , Röser H. P.
Published: July 2002

Video detection experiments from 0.7 to 30 THz and mixing experiments at 30 THz have been performed with GaAs Schottky-barrier diodes and W-Ni metal-insulator-metal (MIM) diodes. Above approximately 12 THz the MIM diode is the more sensitive video detector (a factor of 5–10 at 30 THz). Difference frequencies up to 34 GHz were generated by mixing the radiation of two adjacent CO2 laser lines and a microwave source in a Schottky-barrier diode. The dependence of the mixing signal on different diode parameters (doping density, plasma frequency, cutoff frequency) and on bias current was measured. Compared with MIM diodes at 30 THz the Schottky-barrier diodes are less efficient for mixing (a factor of 104). The results suggest that thermionic emission is the dominant physical mechanism responsible for video detection and mixing at 30 THz.
Quantum efficiency and spectral response of compositionally graded HgCdTe P-n heterojunction photodiodes
Authors: Rosenfeld D. , Garber V. , Bahir G.
Published: July 2002

Results are presented of a study of the quantum efficiency of HgCdTe heterojunction photodiodes. All heterojunctions considered in the study consist of a wide-band-gap P-type layer and a narrow-band-gap n-type layer, and are illuminated from the backside. The n-type layer is compositionally graded and therefore contains a built-in electric field. Due to the difference in band gaps photons are absorbed in the active n-type layer only and are collected by both drift and diffusion mechanisms. The one-dimensional continuity equation is first solved in the linearly graded n-type region under illumination conditions, and then the dependence of the quantum efficiency on the resulting built-in electric field is presented. Two different modes of illumination are compared: In the first mode, associated with n-on-P HgCdTe diodes, the light reaches the transparent P layer first; in the second mode, associated with P-on-n diodes, the light reaches the transparent P side after it passes through the opaque n-type layer. The superiority of the first mode is demonstrated: It consistently renders higher quantum efficiencies, and is also less sensitive to several properties of the diode such as the width of the absorbing layer and the quality of the backside interface.
Recent developments in modeling of the stress derivative of magnetization in ferromagnetic materials
Authors: Jiles D. C. , Devine M. K.
Published: July 2002

The effect of changing stress on the magnetization of ferromagnetic materials leads to behavior in which the magnetization may increase, or decrease, when exposed to the same stress under the same external conditions. A simple empirical law seems to govern the behavior when the magnetization begins from a major hysteresis loop. The application of the law of approach, in which the derivative of the magnetization with respect to the elastic energy supplied dM/dW is proportional to the magnetization displacement Man−M, is discussed.
Deflection of a substrate induced by an anisotropic thin-film stress
Authors: van de Riet E.
Published: July 2002

The deflection of a substrate caused by an anisotropic stress in a thin film that is deposited on it, is studied theoretically. An exact solution of the elastic equations for the three-dimensional cantilever problem is found. Equations for the deflection caused by anisotropic stresses due to, e.g., magnetostriction and by isotropic stresses are given. A comparison is made between these equations and the equations usually given in the literature for the determination of thin-film stresses.
Field representations in uniaxial bianisotropic-ferrite medium by cylindrical vector wave functions
Authors: Cheng Dajun , Lin Weigan , Zhao Yushen
Published: July 2002

The uniaxial bianisotropic-ferrite medium is a generalization of the well-studied magnetically biased ferrite and uniaxial material. It can be manufactured either by immersing randomly oriented short helices and Ω-shaped particles in a magnetically biased ferrite, or by arranging short conductive helices in a magnetized ferrite in a certain manner. It has potential applications in microwave technology, antenna design, and antireflection shielding. In the present consideration, based on the concept of characteristic waves and the method of angular spectral expansion, field representations in uniaxial bianisotropic-ferrite medium are developed. The analysis reveals the solutions of source-free Maxwell’s equations for uniaxial bianisotropic-ferrite medium can be represented in sum-integral forms of the circular cylindrical vector wave functions. The addition theorem of vector wave functions for uniaxial bianisotropic-ferrite medium can be straightforwardly derived from that of vector wave functions for isotropic medium. An application of the proposed theory in scattering is presented to show how to use these formulations in a practical way.
Low-noise dc superconducting quantum interference devices in Bi2Sr2CaCu2O8+x thin films
Authors: Amrein T. , Schultz L. , Urban K.
Published: July 2002

Low-noise dc superconducting quantum interference devices (SQUIDs) were fabricated by thin film deposition of Bi2Sr2Ca Cu2O8+x (BSCCO) on bicrystalline substrates of SrTiO3 with misorientation angles Θ from 24° to 45°. An optimized dc SQUID design (βL=1) was developed for Θ=24°. Therein the SQUID inductance was varied between 7 and 80 pH. Fabricated dc SQUIDs operated at 77 K as well as high-quality YBa2Cu3O7−y (YBCO) SQUIDs, especially the 1/f and white noise levels were in the lowest range of data reported so far for YBCO SQUIDs and are two orders of magnitude better than noise data published this far for BSCCO SQUIDs at 77 K.
Hot electron capture in highly doped GaAs
Authors: Stasch R. , Asche M. , Däweritz L. , Hey R. , Kostial H. , Ramsteiner M.
Published: July 2002

Hot carrier transport processes in GaAs with δ-like high Si doping have been investigated. In the high electric field region the current density decays with time by as much as 20%, an indication of electron trapping. The electron traps are metastable and the electrons can be released by light or thermal excitation. DX− centers can explain the observations, however, other types of localized electron states cannot be unambiguously excluded.
Unique spatial solitons in Gunn-instability semiconductors
Authors: Hayata K. , Koshiba M.
Published: July 2002

A novel and unique principle for generating spatial solitons is presented, which is based on a balance between the diffusion of carriers and their nonlinear velocity-field characteristic in a compound semiconductor such as GaAs. In striking contrast to conventional solitons for electromagnetic radiation, the present soliton is supported solely by the longitudinal electric field. The soliton parameters such as the peak amplitude and the beam width can be controlled with varying the concentration of ionized donors.
Exciton binding energy in a quantum wire in the presence of a magnetic field
Authors: Li Gang , Branis Spiros V. , Bajaj K. K.
Published: July 2002

A calculation of the ground-state energy of an exciton confined in a cylindrical quantum wire in the presence of a uniform magnetic field is reported as a function of wire radius, using a variational approach. It is assumed that the magnetic field is applied parallel to the axis of the wire. The calculations have been performed using a suitable variational wave function taken as a product of the appropriate confining confluent hypergeometric functions and a hydrogenic function for infinite and finite confining potentials. For a given value of the magnetic field, the binding energy is found to be larger than the zero-field case. This behavior is explained in terms of an average electron-hole separation, which depends on the wire radius, and the magnetic-field strength.
Characteristics of β-phase PdAl Schottky contacts to n-GaAs
Authors: Huang T. S. , Pang J. G.
Published: July 2002

Metallurgical and electrical properties of β-phase PdAl Schottky metallizations on n-GaAs after rapid thermal annealing for 20 s in the temperature range 500–1000 °C have been investigated using x-ray diffraction, transmission electron microscopy, Auger depth profiling, film resistivity, and current–voltage measurement. The Al-rich contacts were stable up to 900 °C whereas the Pd-rich contacts were less stable. The thermal stability of β-PdAl exhibited a sharp variation near the stoichiometric composition. The thermal stability of Pd-rich contacts decreased with increasing Pd composition, and the interfacial reaction after high-temperature annealing resulted in the formation of PdGa compound. The interface between Al-rich PdAl film and GaAs substrate remained quite sharp even after 900 °C anneal. The variation of interfacial stability at high temperatures between β-PdAl film and GaAs substrate is correlated to the compositional dependence of Al and Pd activities in PdAl within the β-phase region. The Schottky barrier heights of Al-rich PdAl contacts increased with annealing temperature. The barrier height enhancement in the annealed Al-rich contacts can be attributed to the thin AlxGa1−xAs layer formed at the interface.
Preparation of self-aligned in-line tunnel junctions for applications in single-charge electronics
Authors: Götz M. , Blüthner K. , Krech W. , Nowack A. , Fuchs H.-J. , Kley E.-B. , Thieme P. , Wagner Th. , Eska G. , Hecker K. , Hegger H.
Published: July 2002

The self-aligned in-line technique has been applied to the preparation of ultrasmall low-capacitance metallic tunnel junctions. By using e-beam lithography the area of Al/AlOx/Al contacts has so far been reduced to less than 0.005 μm2. At low temperatures high-ohmic double junctions with a small metallic island between them show the Coulomb blockade effect. The current through such a device could be modulated by a voltage applied to a gate electrode capacitively coupled to the island (single-electron transistor). Both single-charge phenomena have been observed at temperatures up to 1 K.
Structural and magnetic properties of Co-Nb/Pd multilayers
Authors: Yan Shi-shen , Liu Yi-hua , Zheng Min , Mei Liang-mo , Chen Jin-geng , Huang Jie
Published: July 2002

Co-Nb/Pd multilayers were prepared by a radio-frequency sputtering method. As the thickness of Pd layers increases, the phases of Co-Nb layers periodically alternate from an amorphous state to a crystalline state. The oscillation of the strain of Pd layers and that of the coherent length versus the thickness of Pd layers with a period of about 10 Å are obtained. The strain is caused mainly by the degree of lattice mismatching of Co-Nb layers and Pd layers, but the oscillatory behavior of the strain may be related to the oscillatory interlayer coupling between Co-Nb layers through Pd layers. The oscillation of the saturation magnetization is also obtained. It corresponds to the oscillatory polarization of Pd layers and the structural change in Co-Nb/Pd multilayers.
Absorption of n-type Ge, Si quantum wells for normal incident radiation
Authors: Xu Wenlan , Shen S. C. , Fu Y. , Willander M.
Published: July 2002

The theory of electron intersubband infrared absorptions for n-type Si/GeSi and Ge/GeSi quantum wells in the waveguide structure is presented in order to compare with recently obtained experimental results. The absorption coefficient as a function of the polarization angle of incident light has been investigated by considering the degeneracies and the occupancies of the different valleys for conduction band.
An improved approach for the wave equation solution of graded-index waveguide
Authors: Kim In , Choe Byung-Doo , Jeong Weon Guk
Published: July 2002

An improved way to solve the wave equation for the graded-index planar waveguide is presented. An analytic solution correct up to the first order of the dielectric function gradient is obtained for a linearly graded medium and is applied to calculate the optical confinement factors in a number of graded-index waveguides of laser diode structure. For a multilayer sectioning method, the number of section layers required to get a reasonable error is greatly reduced by considering the gradient in each sectioned layer instead of using the average value, especially in the case of linear grading. The advantages and limits of the solution are discussed.
Laser-energy dependence of optical emission from radicals and atoms in laser-induced chemical-vapor deposition of SiC
Authors: Mizunami Toru , Toyama Naotake , Sakaguchi Takahiro
Published: July 2002

Optical emission spectroscopy has been employed to study the photolysis and the subsequent reaction processes of the mixture of Si2H6 and C2H2 irradiated with a 193 nm ArF excimer laser. The emission intensities of SiH*, Si*, and C2* were measured as a function of the laser energy up to 80 mJ (∼0.43 J/cm2). The formation processes of the radicals and atoms were estimated from the dependence of these intensities on the laser energy. The dependence of the SiH* intensity in the low-energy region up to 10 mJ was quadratic and revealed two-step processes. The dependence of the intensities of Si* and C2* was the 1.3–1.6 power of the laser energy, which revealed the saturation of the processes. The intensity of C2* increased when Si2H6 was added to C2H2. The origin of this increase is the production of the carbon-based radicals by the collisions of Si* with C2H2 and C2H.