bluuebunny/crossref_metadata_2025 huggingface dataset:
Laser emission from high-gain media of dye-doped dendrimer
Authors: Yokoyama Shiyoshi , Otomo Akira , Mashiko Shinro
Published: July 2002

We prepared high-gain media for laser emission by using a conventional laser-dye (DCM) and dendrimer. A dendrimer can encapsulate the laser dye, and therefore increase its concentration up to 9.0 mM with limited self-aggregation and intermolecular quenching. The optical confinement is attributed to gain guiding under optical excitation. The laser spectrum clearly indicated periodic resonant modes though the laser feedback was caused in an unusual way. Above the lasing threshold, the spectral linewidth became less than 0.1 nm. The laser-dye-doped dendrimer showed a particular ability to decrease the lasing threshold intensity by increasing the dye concentration.
Monte Carlo simulation of the crystallization and annealing of a freestanding thin film of n-tetracontane
Authors: Xu Guoqiang , Mattice Wayne L.
Published: July 2002

A freestanding thin film of n-tetracontane chains is simulated by a Monte Carlo (MC) method on a high coordination lattice. The coarse-grained chains, represented by 20 beads each, can be reverse-mapped into the fully atomistic description, C40H82. The Hamiltonian includes a short-range interaction based on a rotational isomeric state model and a long-range interaction obtained from a Lennard-Jones potential energy function. When the melt is instantaneously quenched from 473 to 298 K, crystallization initiates in the surface region and propagates into the interior of the film, as was found in a prior molecular dynamics simulation of a united atom model of polyethylene [M. Ito, M. Matsumoto, and M. Doi, Fluid Phase Equilibria, 144, 395 (1998)]. Several repetitions of the MC simulation, starting from different configurations of the melt at 473 K, reveal that two distinctly different structures can be obtained. Usually the independently initiated crystals at the two surfaces of the thin film produce a disordered grain boundary when they impinge on one another as a consequence of propagation into the interior of the film. This grain boundary was also observed by Ito et al. However, if the MC simulation is repeated many times, there are a few instances in which the independently initiated crystals happen to have a similar orientation, and then crystallization propagates completely through the thin film without producing a grain boundary in the interior. A well-defined melting phenomenon is observed at about 390 K when the film without the grain boundary is heated. Annealing at 380 K of the film with the grain boundary causes growth of one crystal at the expense of the other. This growth eventually leads to a completely crystalline film, with elimination of the grain boundary. Therefore, the stable structure of the thin film is the one that is completely crystalline, with no grain boundary in the interior, even though rapid quenching is more likely to lead to a structure with a grain boundary. The MC simulation can anneal the imperfect structure into the more perfect one.
Two-photon absorption spectroscopy of As2S3 glass
Authors: Tanaka Keiji
Published: July 2002

Spectral dependence of two-photon absorption in a chalcogenide glass, As2S3, has been studied using a tunable light source. The result shows that two-photon and two-step absorptions occur, and the two-photon absorption spectrum appears to be exponential. These results and roles of nonlinear processes in photoinduced phenomena are discussed.
Selenium dimers and linear chains in one-dimensional cancrinite nanochannels: Structure, dynamics, and optical properties
Authors: Poborchii Vladimir V. , Lindner Gottlieb-Georg , Sato Mitsuo
Published: July 2002

Cancrinite crystals possessing parallel nanochannels are attractive for incorporation of guest materials and preparation of one-dimensional structures. In this work, we study variety of cancrinite crystals synthesized with Se inside their channels. Single crystal x-ray diffraction, polarized Raman, optical absorption, and luminescence spectra are investigated. It is shown that Se is stabilized in the form of Se22− and Se2− dimers located in the center of the channel and oriented along the channel. Different absolute and relative concentrations of Se22− and Se2− are obtained for different samples. The Se22− dimers at high concentration show tendency to organize linear chains. At low temperatures, quite strong interdimer bonding for both Se22− and Se2− is observed. Another important low-temperature effect is appearance of additional Raman bands, which are attributed to the vibrations of linear Se22− chains distorted by the incommensurate potential of cancrinite. Strong near-infrared polarized luminescence is observed for all samples. Photoionization of dimers is shown to be important step in the mechanism of the luminescence.
In situ observation of nickel metal-induced lateral crystallization of amorphous silicon thin films
Authors: Miyasaka Mitsutoshi , Makihira Kenji , Asano Tanemasa , Polychroniadis Efstathios , Stoemenos John
Published: July 2002

The lateral crystallization of amorphous silicon thin films induced by nickel was studied in detail, performing in situ annealing experiments with a transmission electron microscope. The nickel-induced crystallization starts with the fast growth of thin needle-like crystallites of [110] orientation, which advance along the 〈111〉 directions within the film plane. The fast growth rate and the small probability of the crystallite exhibiting the [110] orientation result in large crystalline grains. These grains are, however, composed of many small misorientated subgrains. It is thought that this is because the needle-like crystallite does not grow continuously but grows by successive jumps. Our model is that after the nickel disilicide precipitate grows a thin crystalline slice epitaxially at the leading edge of the needle-like crystallite, the nickel moves to the new leading edge and forms the new nickel disilicide precipitates to maintain the needle-like crystalline growth.
Phase separation in Nd60−xYxFe30Al10 melt-spun ribbons
Authors: Sun Z. G. , Löser W. , Eckert J. , Müller K.-H. , Schultz L.
Published: July 2002

The structure of melt-spun Nd60−xYxFe30Al10 (x=0, 10, and 30) ribbons has been investigated by x-ray diffraction, isochronal differential scanning calorimetry (DSC), isothermal DSC, and thermomagnetic measurements. Precipitation of nanocrystalline Nd embedded in an amorphous matrix is found upon quenching. The amorphous phase further separates into Fe-rich regions and Fe-depleted regions. The growth of Nd nanocrystals upon annealing was identified by the characteristic isothermal calorimetric signal. The Fe-depleted amorphous phase crystallizes at 580 K, while the Fe-rich amorphous phase crystallizes at about 740, 779, and 750 K for x=0, 10, and 30, respectively. Phase separation in the amorphous matrix is also revealed by the thermomagnetic measurements, which show ferromagnetic transitions at about 60 and 480 K.
Dynamic high throughput screening of chemical libraries using acoustic-wave sensor system
Authors: Potyrailo Radislav A. , May Ralph J.
Published: July 2002

We report a novel sensor-based high throughput screening (HTS) system for identification and quantitation of volatile substances in combinatorial chemical libraries. The measurement method employs a combination of a periodic introduction of a minute amount of a liquid sample into the HTS system, rapid evaporation of volatile components in the sample at room temperature, and dynamic measurement of a generated vapor pulse. These measurements are performed using an array of four 10 MHz acoustic-wave thickness-shear mode sensors coated with different chemically sensitive films. Developed HTS system is applied for screening of multiple samples such as those created in combinatorial chemical libraries of catalyst candidates in an industrially important arene oxidation process. The temporal modulation of the concentration of analyte vapors and measurement of both the temporal profile and the magnitude of the response improves sensor selectivity and makes possible robust identification and quantitation of arene oxidation components such as cresol and benzoquinone in multicomponent combinatorial mixtures with reduced number of sensors in the array. Different solvents such as water, acetonitrile, benzene, and toluene do not alter the response of sensors to analytes. Depending on the gas flow rate, quantitative measurements are performed 10–150 s after the sample introduction and provide significant throughput advantage over gas-chromatographic instruments. Determinations of mixtures of analytes in a variety of solvents are performed using multivariate locally weighted regression. This data analysis method provides the root mean squared error of prediction of less than 2 μg when measurements of cresol and benzoquinone amounts ranging from 0 to 50 μg are performed in 2 μL samples. This method of dynamic sensor-based measurements allows for instrument miniaturization and increases the usefulness of the instrument in space-limited applications. Upon operation of multiple sensors in parallel, effective analysis of dense combinatorial libraries of materials is possible.
The possibility of single C–H bond activation in CH4 on a MoO3-supported Pt catalyst: A density functional theory study
Authors: Zhang C. J. , Hu P.
Published: July 2002

Methane activation is a crucial step in the conversion of methane to valuable oxygenated products. In heterogeneous catalysis, however, methane activation often leads to complete dissociation: If a catalyst can activate the first C–H bond in CH4, it can often break the remaining C–H bonds. In this study, using density functional theory, we illustrate that single C–H bond activation in CH4 is possible. We choose a model system which consists of isolated Pt atoms on a MoO3(010) surface. We find that the Pt atoms on this surface can readily activate the first C–H bond in methane. The reaction barrier of only 0.3 eV obtained in this study is significantly lower than that on a Pt(111) surface. We also find, in contrast to the processes on pure metal surfaces, that the further dehydrogenation of methyl (CH3) is very energetically unfavorable on the MoO3-supported Pt catalyst.
ZnRh
2
O
4
:
A p-type semiconducting oxide with a valence band composed of a low spin state of Rh3+ in a 4d6 configuration
Authors: Mizoguchi Hiroshi , Hirano Masahiro , Fujitsu Satoru , Takeuchi Tomonari , Ueda Kazushige , Hosono Hideo
Published: July 2002

It is demonstrated that a ZnRh2O4 normal spinel with a band gap of ∼2.1 eV is a unique material as a p type wide-gap semiconductor. The electrical conductivity of the sputtered film was 0.7 S cm−1 at 300 K with no intentional doping. The electronic structure was investigated by photoemission and inverse photoemission measurements and indicated that the band gap is composed mainly of ligand field splitting of an octahedrally coordinated Rh3+ octahedron between fully occupied t2g6 and empty eg0 sets.
Pattern formation on anisotropic and heterogeneous catalytic surfaces
Authors: Bär Markus , Meron Ehud , Utzny Clemens
Published: July 2002

We review experimental and theoretical work addressing pattern formation on anisotropic and heterogeneous catalytic surfaces. These systems are typically modeled by reaction-diffusion equations reflecting the kinetics and transport of the involved chemical species. Here, we demonstrate the influence of anisotropy and heterogeneity in a simplified model, the FitzHugh–Nagumo equations. Anisotropy causes stratification of labyrinthine patterns and spiral defect chaos in bistable media. For heterogeneous media, we study the situation where the heterogeneity appears on a length scale shorter than the typical pattern length scale. Homogenization, i.e., computation of effective medium properties, is applied to an example and illustrated with simulations in one (fronts) and two dimensions (spirals). We conclude with a discussion of open questions and promising directions that comprise the coupling of the microscopic structure of the surface to the macroscopic concentration patterns and the fabrication of nanostructures with heterogeneous surfaces as templates.
Effects of the Ewald sum on the free energy of the extended simple point charge model for water
Authors: Arbuckle Brendan W. , Clancy Paulette
Published: July 2002

The effects of including the Ewald sum on several key system properties for the extended simple point charge (SPC/E) model of water, in the temperature range of 140–300 K have been investigated using molecular dynamics simulations. The original SPC/E parameters are used without alteration since this study is intended to determine whether the Ewald sum affects the model sufficiently to warrant reparameterization. Overall comparison to experiment has been improved for the liquid phase as evidenced by several factors. The effect of temperature on the self-diffusivity coefficient has improved in the range from 300 to 200 K. There is a shift in the temperature at which the density maximum occurs (260 K from 255 K in previous simulations of SPC/E water without the Ewald) and the value of ρmax has decreased to 1.0003 g/ml (versus 0.99997 g/ml for experiment at 277 K) from 1.0082 g/ml for the SPC/E model. However, the shape of the ρ(T) profile is worse than without the Ewald sum in comparison to experiment, but is qualitatively similar to Monte Carlo data from Harrington et al. for SPC/E modeled water including a reaction field method. For the solid, while the ρ(T) profile is unchanged, values are worse than without an Ewald sum in comparison to experiment (≈3% higher than experiment without the Ewald and ≈5% with the Ewald). Calculation of free energies of various low pressure ices and liquid water using thermodynamic integration has produced the major finding, the determination of a melting point for low pressure ices. The melting points of proton-disordered ices Ih and Ic were found to be 279±5 K and 291±5 K, respectively. Melting points of their proton-ordered counterparts are 284±5 K (Ih) and 281±5 K (Ic)
A portable stray-immune capacitance meter
Authors: Yang San Xu , Yang Wu Qiang
Published: July 2002

Most existing portable capacitance meters are not stray-immune, i.e., their measurements are affected by stray capacitance between the measurement terminals and earth. Therefore, it is difficult to measure small capacitance, say less than 10 pF, using those meters. This article describes a portable stray-immune capacitance meter based on a four-phase charge transfer measuring circuit. It has two measurement ranges, 2 and 20 pF. The resolution for the two different measurement ranges is 0.001 and 0.01 pF, respectively. Together with a current-to-voltage converter, it can also be used as an industrial capacitance transducer with 0–20 mA current output.
Effects of boron substitution on the structural and magnetic properties of melt-spun Sm(Co,Fe,Zr)7.5 and Sm(Co,Fe,Zr,Cu)7.5 magnets
Authors: Makridis S. S. , Litsardakis G. , Panagiotopoulos I. , Niarchos D. , Hadjipanayis G. C.
Published: July 2002

In this work we examine the effect of small boron substitution (x=0, 0.005, 0.010, 0.015) on the structural and magnetic properties of Sm(Co0.86−xFe0.1Zr0.04Bx)7.5 and Sm(Co0.74−xFe0.1Cu0.12Zr0.04Bx)7.5 melt-spun samples, as a function of wheel speed and annealing conditions. Boron substituted as-spun ribbons are found to have increased coercivity, Hc>5 kOe, and small grain size of 60–100 nm. For copper containing samples, the highest coercivity (Hc=16.3 kOe) was obtained in as-spun ribbons with x=0.015. In samples without copper the coercivity increased after short annealing (Hc=12 kOe for x=0.015). The large coercivities are attributed to a fine microstructure consisting mainly of hexagonal TbCu7-type phase and a small amount of soft-phase grains.
Electronic structure of ytterbium-doped strontium fluoroapatite: Photoemission and photoabsorption investigation
Authors: Nelson A. J. , van Buuren T. , Bostedt C. , Schaffers K. I. , Terminello L. J. , Engelhard Mark , Baer Don
Published: July 2002

X-ray photoemission and x-ray photoabsorption were used to study the composition and the electronic structure of ytterbium-doped strontium fluoroapatite (Yb:S-FAP). High resolution photoemission measurements on the valence band electronic structure and Sr 3d, P 2p and 2s, Yb 4d and 4p, F 1s, and O 1s core lines were used to evaluate the surface and near surface chemistry of this fluoroapatite. Element specific density of unoccupied electronic states in Yb:S-FAP were probed by x-ray absorption spectroscopy at the Yb 4d (N4,5 edge), Sr 3d (M4,5 edge), P 2p (L2,3 edge), F 1s, and O 1s (K edges) absorption edges. These results provide measurements of the electronic structure and surface chemistry of this material.
Two shear flow regimes in nematic liquid crystals: Near a charged surface and in the bulk
Authors: Zakharov A. V. , Dong Ronald Y.
Published: July 2002

Laminar and tumbling flow regimes in a high shear flow, as well as transition from a flow alignment regime to a tumbling situation are investigated in the polar liquid crystal p-n-hexyloxybenzylidenep′-aminobenzonitrile (HBAB) near a charged surface. The relaxation of the alignment angle θ(τ) to its equilibrium θbulk is calculated using the conventional Erickson–Leslie theory for the case of homeotropic alignment of HBAB at a charged indium tin oxide-coated glass plate and for the temperature range in which HBAB exhibits a nematic phase. In accordance with earlier Couette flow experiments, our calculations of the relaxation process for temperatures below 365 K show that the alignment angle is characterized by relaxing to a θbulk of zero value, indicating that the steady laminar flow may break up into a tumbling flow. The nature of the hydrodynamic instability under high shear flow rates is also discussed for HBAB.
Density functional theory investigation of H adsorption and H2 recombination on the basal plane and in the bulk of graphite: Connection between slab and cluster model
Authors: Ferro Y. , Marinelli F. , Allouche A.
Published: July 2002

The scope of this work is the study of hydrogen atom interaction with the graphite surface taken as a model of the interactions that occur in the tokamaks (magnetic confinement fusion devices) between the carbon covered wall and the hydrogen ions (H+ or D+ or T+) coming out of the plasma. This study is performed at the atomic scale in the framework of the density functional theory. The graphite surface is modeled by the (0001) layer in either a periodic or a molecular approach. The clusters best reproducing the periodic two-dimensional results were selected to investigate hydrogen–graphite interaction. One- and two-layer clusters were used to model the basal plane and the bulk of graphite. It was found that hydrogen atoms could be bonded to the surface and in the bulk with an exothermic energy. The potential-energy barriers corresponding to the over crossing of the first surface layer by an atomic hydrogen have been determined. The H+H recombination (Eley–Rideal mechanism) was investigated on the surface and in the bulk. The quantitative results concerning the ability of hydrogen atoms to penetrate into the bulk through the basal plane of graphite is linked to the hydrogen uptake at the walls of the tokamak during the plasma discharges.
Very high hole mobilities in modulation-doped Ge quantum wells grown by low-energy plasma enhanced chemical vapor deposition
Authors: Känel Hans von , Kummer Matthias , Isella Giovanni , Müller Elisabeth , Hackbarth Thomas
Published: July 2002

We report on the fabrication of modulation-doped compressively strained Ge quantum wells by low-energy plasma enhanced chemical vapor deposition. A virtual substrate consisting of a thick linearly graded SiGe buffer layer and a cap layer of constant composition is first grown at a high rate (>5 nm/s). The active layer stack, grown at a reduced rate, contains strain compensating cladding layers with modulation doping above the channel. Mobilities of up to 3000 cm2/V s and 87 000 cm2/V s have been achieved at room temperature and liquid He temperature, respectively.
Magnetic field calibration of a transmission electron microscope using a permanent magnet material
Authors: Volkov V. V. , Crew D. C. , Zhu Y. , Lewis L. H.
Published: July 2002

A new method of assessing the magnitude of the magnetic field in a transmission electron microscope using a permanent magnetic material is described. The approach is versatile and simple enough to be implemented for certain scientific or engineering situations in which the exact calibration of the magnetic field in the microscope column using a small Hall probe may be a problem. We have applied this approach to obtain the magnetic field calibration inside a JEM 3000F field emission electron microscope as a function of the objective lens potential. In the course of this in situ calibration it was not necessary to disassemble the microscope or interrupt its operation. The procedure used is versatile and accurate enough to measure magnetic fields up to 20 kOe (μ0H=2 T in SI units) as was confirmed by subsequent Hall-probe field measurement of the same electron microscope. The calibration method described in this article does not require any special Hall-probe holder adaptations and can be applied to any transmission electron microscope or similar instrument. To illustrate the utility of the technique and its results, quantitative analysis of magnetic domain images obtained with Lorentz microscopy during magnetic reversal of a sample subjected to an in situ magnetic field in the JEM3000F microscope is presented and discussed.
Ab initio theory and rotational spectra of linear carbon chains SiCnS
Authors: Botschwina Peter , Eugenia Sanz M. , McCarthy Michael C. , Thaddeus Patrick
Published: July 2002

On the basis of extensive coupled cluster calculations, the rotational spectra of the linear silicon- and sulfur-containing carbon chains SiC2nS (n=1–3) in their singlet electronic ground state and SiC3S in its triplet electronic ground state have been detected and characterized by means of molecular beam Fourier transform microwave spectroscopy. Rotational and centrifugal distortion constants have been determined to high accuracy as well as the spin-spin coupling constant for triplet SiC3S. In addition, the Si29, S34, and both C13 isotopic species of SiC2S have been detected, allowing the determination of both an effective r0 structure, as well as a mixed experimental–theoretical structure resulting from the combination of the measured rotational constants with the vibration–rotation coupling constants calculated ab initio. Several rotational satellite lines have also been observed for SiC2S and, on the basis of their predicted vibration–rotation and l-type doubling constants, were assigned to the two highest-frequency stretching modes v1 and v2, and to a progression in the second lowest-frequency bending mode v4 up to 3v4. Equilibrium structures and various spectroscopic properties are predicted for all SiCnS species with n=1–8.
Plasma parameters analysis of various mixed gas inductively coupled plasmas
Authors: Bai K. H. , You S. J. , Chang H. Y. , Uhm H. S.
Published: July 2002

The electron energy distribution functions and plasma parameters in various gas mixture discharges (N2,O2,CF4/He,Ar,Xe) are measured. When He is mixed, the electron temperature increases but the electron density is almost constant. The electron temperature increases rapidly near a He mixing ratio of 1, but it is almost constant when the mixing ratio is small. In Ar mixture discharge, the electron temperature is almost constant; the electron density increases rapidly near a mixing ratio of 1, but increases slightly when the mixing ratio is small. Mixing Xe increases the electron density and decreases the electron temperature. The electron density varies in a similar way with that of the Ar mixing case. A simple two-ion-species global model is used to analyze the plasma parameter variations as a function of mixing ratio, and it agrees well with the experimental results.
Anion–aromatic molecule complex. Ab initio study of the benzene⋅O2 anion
Authors: Jalbout A. F. , Adamowicz L.
Published: July 2002

A series of ab initio calculations were carried out for the O2⋅benzene complex and its anions to determine their structures and the O2⋅benzene adiabatic electron affinity. The calculations revealed interesting differences between the electronic and molecular structures of the anion and the neutral complex, and between the free and complexed O2−. While for O2⋅benzene the calculations predict two equilibrium structures, one planar and one with O2 located above the benzene and interacting with its π electrons, for [O2⋅benzene]− only the planar structure was predicted to be stable. Analysis of the [O2⋅benzene]− wave function shows some delocalization of the excess electron from O2− to the region of the σ electrons in benzene. This delocalization may explain the predicted increase of the adiabatic electron affinity (A.E.A) by 0.59 eV upon O2 complexation with benzene. This increase is consistent with A.E.A. shift reported in the accompanying paper by Le Barbu et al. [J. Chem. Phys. 116, 9663 (2002)].
Vanishing of mode B in the wake behind a rotationally oscillating circular cylinder
Authors: Poncet Philippe
Published: July 2002

This Brief Communication presents a numerical investigation of the dimensionality of a flow behind a three-dimensional rotationally oscillating circular cylinder. These computations are performed at a Reynolds number Re=500, which is at a level where the mode B is the dominant three-dimensional instability. We show that a rotation of high enough amplitude makes the 3D instabilities vanish and brings the flow back to its nominal two-dimensional state.
Directed placement of suspended carbon nanotubes for nanometer-scale assembly
Authors: Nagahara Larry A. , Amlani Islamshah , Lewenstein Justin , Tsui Raymond K.
Published: July 2002

Single-wall carbon nanotubes (SWNTs) suspended in an aqueous solution have been placed selectively between two metal electrodes separated by a few tens of nanometers. After the initial patterning of the metal electrodes by electron beam lithography, no further fine-line lithography steps are necessary to achieve directed placement of SWNTs at these dimensions. An ac bias is applied between the two electrodes and the “nanoscale wiring” is completed within seconds. An additional advantage of using ac bias is the enhancement for selectively placing SWNTs between the electrode gap over competing contaminant species in the solution.
Photoluminescence quenching of a low-pressure metal-organic vapor-phase-epitaxy grown quantum dots array with bimodal inhomogeneous broadening
Authors: Saint-Girons G. , Sagnes I.
Published: July 2002

The photoluminescence (PL) behavior of a bimodal In(Ga)As/GaAs quantum dots (QDs) array grown by low-pressure metal-organic-vapor-phase-epitaxy is studied as a function of the temperature. The PL quenching is attributed to the thermal escape of charge carriers out of the QDs for the high-energy emitting QDs population, and to the presence of nonradiative defects in the immediate vicinity of the lower-energy emitting QDs population. The PL intensity behavior of both QDs population is investigated, and the experimental results are fitted with the help of a rate equations model. The nonradiative mechanisms activation energies are found to be about 180 and 40 meV for the high- and low-energy emitting QDs population, respectively. A charge carriers transfer mechanism between the two QDs populations is also evidenced, and the results are discussed in terms of laser applications.
Convection driven zonal flows and vortices in the major planets
Authors: Busse F. H.
Published: July 2002

The dynamical properties of convection in rotating cylindrical annuli and spherical shells are reviewed. Simple theoretical models and experimental simulations of planetary convection through the use of the centrifugal force in the laboratory are emphasized. The model of columnar convection in a cylindrical annulus not only serves as a guide to the dynamical properties of convection in rotating sphere; it also is of interest as a basic physical system that exhibits several dynamical properties in their most simple form. The generation of zonal mean flows is discussed in some detail and examples of recent numerical computations are presented. The exploration of the parameter space for the annulus model is not yet complete and the theoretical exploration of convection in rotating spheres is still in the beginning phase. Quantitative comparisons with the observations of the dynamics of planetary atmospheres will have to await the consideration in the models of the effects of magnetic fields and the deviations from the Boussinesq approximation.
Self-organizing dynamics of the human brain: Critical instabilities and Šil’nikov chaos
Authors: Kelso J. A. Scott , Fuchs Armin
Published: July 2002

Using a sensorimotor coordination task in conjunction with an array of SQUIDs (Superconducting Quantum Interference Devices) we demonstrate critical instabilities in human brain activity patterns. Analysis of the dominant spatial pattern of the brain and its time-varying amplitude displays a task-dependent geometry characteristic of Šil’nikov-like chaos, which changes qualitatively at the transition.
On small stationary localized solutions for the generalized 1-D Swift–Hohenberg equation
Authors: Glebsky L. Yu. , Lerman L. M.
Published: July 2002

We prove the existence of small localized stationary solutions for the generalized Swift–Hohenberg equation and find under some assumption a part of a boundary of their existence in the parameter plane. The related stationary equation creates a reversible Hamiltonian system with two degrees of freedom that undergoes the Hamiltonian–Hopf bifurcation with an additional degeneracy. We investigate this bifurcation in a two-parameter unfolding by means of the sixth-order normal form for the related Hamiltonian. The region where no localized solutions exist has been pointed out as well.
Ventricular fibrillation and atrial fibrillation are two different beasts
Authors: Gray R. A. , Jalife J.
Published: July 2002

Although the mechanisms of fibrillation are no doubt multi-faceted, the geometry of the heart may play a major role in the dynamics of wave propagation during fibrillation [A. T. Winfree, Science 266, 1003–1006 (1994)]. The ventricles are thick chambers made up of sheets of parallel muscle fibers with the direction of fibers rotating across the ventricular walls (rotational anisotropy). The thick walls of the ventricles allow reentry to develop transmurally, provided the wavelength is sufficiently small. Depending on the kinetics of heart cells, the dynamics of rotating waves in three dimensions may be fundamentally different than in two dimensions, leading to destabilization of reentry and ventricular fibrillation (VF) in thick ventricles. The atria have an intricate geometry comprised of a thin sheet of cardiac tissue attached to a very complex network of pectinate muscles. The branching geometry of the pectinate muscles may lead to destabilization of two-dimensional reentry via “long-distance” electrical connections giving rise to atrial fibrillation (AF). Therefore, although fibrillation occurs via complex three-dimensional wave propagation in the ventricles and the atria, the underlying mechanisms and factors that sustain VF and AF are probably different.
Transport in finite size systems: An exit time approach
Authors: Castiglione P. , Cencini M. , Vulpiani A. , Zambianchi E.
Published: July 2002

In the framework of chaotic scattering we analyze passive tracer transport in finite systems. In particular, we study models with open streamlines and a finite number of recirculation zones. In the nontrivial case with a small number of recirculation zones a description by means of asymptotic quantities (such as the eddy diffusivity) is not appropriate. The nonasymptotic properties of dispersion are characterized by means of the exit time statistics, which shows strong sensitivity on initial conditions. This yields a probability distribution function with long tails, making impossible a characterization in terms of a unique typical exit time.
Thermal analysis of the catastrophic mirror damage in laser diodes
Authors: Nakwaski Wlodzimierz
Published: July 2002

A new thermal analysis of a catastrophic mirror damage in laser diodes is presented in the present paper. Three-dimensional heat spreading and temperature dependence of a thermal conductivity is taken into account in the model. Both an active layer heating and a nonradiative recombination of carriers generated by absorbed radiation in an active area close to a facet mirror are considered as heat sources. The model is strictly correct for times ranging from 6.2 to 500 nsec for a standard stripe laser diode, i.e., for currents not higher than 4.5 A and not lower than 0.8 A. The analytical solution of the thermal conduction equation gives a dependence of a catastrophic-degradation time, i.e., a permissible length of current pulses from a point of view of the catastrophic mirror damage, on the amplitude of the pulses for the standard stripe laser diode.
Lead telluride doped with rare-earth elements
Authors: Partin D. L.
Published: July 2002

This study was undertaken to investigate the potential advantages of certain rare-earth elements as n-type dopants for lead chalcogenide diode lasers grown by molecular beam epitaxy. It was found that holmium (Ho), dysprosium (Dy), and erbium (Er) are n-type dopants in PbTe, producing maximum electron concentrations of 1×1019 cm−3 at 77 K. However, annealing under tellurium-rich conditions caused PbTe samples doped with these elements to convert to p-type under some annealing conditions. The diffusion coefficient of Ho in PbTe at 370 °C is less than 1×10−16 cm2/s at high concentration (>1×1018 cm−3). At low concentration (∼1017 cm−3), fast diffusing Ho was observed. Significant degradation of contacts to Ho-, Dy-, and Er-doped PbTe was observed after prolonged aging.
Effects of space charge and lens aberrations in the focusing of an electron beam by a solenoid lens
Authors: Loschialpo P. , Namkung W. , Reiser M. , Lawson J. D.
Published: July 2002

The effects of space charge and lens aberrations on a 5-keV, 80–200-mA electron beam focused by a solenoid lens are investigated theoretically and experimentally. A third-order radial trajectory equation is derived with space charge added in the form of discrete annular current elements. This equation is numerically solved for the trajectories of focused electrons in a short solenoid lens. Trajectories launched near the axis are ‘‘reflected’’ from the axis at the waist due to space charge, and trajectories launched farther from axis, cross the axis. The resulting discontinuity is best illustrated in a transverse phase-space plot where it may be described as a ‘‘tearing’’ of the electron distribution. Due to the combined action of the nonlinear lens forces and the space charge, the initially uniform radial current density distribution becomes markedly nonuniform (hollow or peaked depending on distance) downstream from the lens. Detailed experimental measurements of the density profiles at different positions are found to be in very good agreement with computations.
Mechanisms of electroluminescence during aging of polyethylene
Authors: Laurent C. , Mayoux C. , Noel S.
Published: July 2002

The study of electrical aging of polyethylene subjected to a 50 Hz ac voltage has shown that electroluminescence occured in the bulk of the polymer. These light emissions (detected in the visible range of wavelength) were due to the buildup of space charge around the electrodes. In a previous paper [J. Appl. Phys. 54, 1532 (1983)] results concerning the role of dissolved gases in polyethylene were presented, where particular attention was paid to electroluminescence effects. The present work aims to answer the two following questions: (1) what are the mechanisms causing the electroluminescence? (2) what is the nature of the luminescent centers? Study of the light emission with respect to the applied voltage form, the frequency, (when ac voltage is applied) and the voltage amplitude, shows that the light emission is due to radiative recombinations of carriers injected through the metal-dielectric interface following the Fowler–Nordheim effect. This process requires a double injection of carriers. The influence of electrode material on the light emission intensity is then shown, which is in good agreement with the injection mechanism proposed. The luminescent centers are carriers stabilized by deep traps in the polyethylene.
Technique for measuring the ac susceptibility of portions of the human body or other large objects
Authors: Bastuscheck C. M. , Williamson S. J.
Published: July 2002

Measurement of the ac magnetic susceptibility of a large, weakly magnetic object has advantages over dc techniques in an unshielded environment having substantial magnetic fluctuations at low frequencies. We discuss the principles and practical details of ac techniques using both large- and small-diameter field coils with a second-order gradiometer and rf SQUID in a standard biomedical dewar. It is possible to make measurements in which the magnetic field of the susceptibility signal at the gradiometer is as small as 5×10−9 of the applied field, which was about 0.04 mT in these experiments. This sensitivity is sufficient to be of use in determining the susceptibility of tissues inside the human body.
Degradation of active region in InGaAsP/InP buried heterostructure lasers
Authors: Fukuda Mitsuo , Iwane Genzo
Published: July 2002

In InGaAsP/InP buried heterostructure lasers, a failure mode connected to the active region, not to the burying region, is observed. The degradation is related to a decrease in the carrier lifetime or a decrease in nonradiative lifetime due to degradations at the edges of the active region. Practically, the bad devices may be screened out by the hard screening method, however, the failure mode could determine the reliability of the devices under high-temperature or high-power operation, so care should be taken during forming the mesa and a regrowth of the burying layer.
A review of frequency measurements of optically pumped lasers from 0.1 to 8 THz
Authors: Inguscio M. , Moruzzi G. , Evenson K. M. , Jennings D. A.
Published: July 2002

We present a list of more than 800 far-infrared laser lines emitted by optically pumped molecular lasers whose frequencies have been measured. For each line, frequency, wavelength, wave number, lasing molecule, CO2 pump line, and, if available, the assignment of the lasing transition, are given. The list is accompanied by a survey of the techniques of frequency measurement in the far infrared. Accuracies and limitations of the various techniques are also discussed.
Scalar and vector Mathieu transform pairs
Authors: Habashy T. M. , Kong J. A. , Chew W. C.
Published: July 2002

A scalar and a vector Mathieu transform pair are developed and their properties are discussed. The orthogonality relationships of the transform kernels are derived in both the spatial and the spectral domains. Finally, Parseval’s theorems for the two transforms are derived.
Electrorefraction in GaAs and InGaAsP and its application to phase modulators
Authors: Alping A. , Coldren L. A.
Published: July 2002

A Kramers–Kronig analysis is used to calculate the refractive index change induced by changes in the shape of the fundamental absorption edge when a strong electric field is applied to a semiconductor. Calculations are made for GaAs and InGaAsP over the 0.9 to 1.55 μm wavelength range. For GaAs, we find that the electrorefraction is Δn=7.9×10−4 at λ=1.06 μm when the electric field is 400 kV/cm, but decreases to Δn=1.3×10−4 at λ=1.55 μm for the same electric field strength. For wavelengths far below the band gap, the refractive index change has a quadratic dependence on the applied electric field. The electrorefraction effect will have important applications for optical waveguide phase modulators.
Axisymmetric instability model for shaped charge jets
Authors: Curtis J. P.
Published: July 2002

As a shaped charge jet stretches it breaks up into a series of particles along the jet axis of symmetry. These particles are frequently of approximately the same length, suggesting the existence of a critical wavelength for which the growth of an initial surface disturbance is greatest. Following the failure of earlier one-dimensional models to reproduce this critical wavelength effect, this paper addresses the problem using the axisymmetric equations of motion and boundary conditions of continuum mechanics. Certain simplifying assumptions are made, which enable the analysis of the growth of a small initial sinusoidal disturbance for a range of wavelengths including that suggested by observation of break-up by flash radiographic experiments. The disturbance growth equation derived is of a complicated and singular nature. Under certain initial conditions it can approximately predict from the experimental data the observed critical wavelength.
Phase diagrams of three-dimensional Blume–Emery–Griffiths model
Authors: Wang Yung-Li , Wentworth Chris
Published: July 2002

The Monte Carlo simulation technique is used to obtain the complete phase diagram of the three-dimensional Blume–Emery–Griffiths (BEG) model. A staggered quadrupolar phase occurs with the competing bilinear and biquadratic interactions. Similar to the two-dimensional BEG model, the phase boundary line of the staggered quadrupolar phase and that of the ferromagnetic phase are extremely close to each other at low temperatures.
Cobalt-titanium substituted barium ferrite films for magneto-optical memory
Authors: Nakamura H. , Ohmi F. , Kaneko Y. , Sawada Y. , Watada A. , Machida H.
Published: July 2002

We have studied Faraday rotation and magnetic properties of Co-Ti substituted barium ferrite films with perpendicular anisotropy, which were prepared by targets-facing-type sputtering method on (111) oriented GGG substrates. Faraday rotation (θF) of 500–830 nm was measured by using the Faraday modulation method. It was found that θF increased proportionally to the content of cobalt in the wavelength range 700–830 nm. In the BaFe10.42Co0.78Ti0.8O19 film, Faraday rotation and magnetic properties were as follows: θF=0.75 deg/μm at 780 nm, anisotropy field: Hk=5.2 kOe, coercive force: Hc=800 Oe, Curie temperature: TC=380 °C. Thermomagnetic recording was achieved on this film by using a 780-nm laser diode. As a result, 2.0-μm bit patterns were observed by Faraday effect.
Thermomagnetic switching in amorphous rare-earth transition metal alloys
Authors: Hansen P.
Published: July 2002

Conditions for writing and erasure have been investigated for different amorphous rare-earth transition metal alloys using a simple temperature distribution evaluated for a three-layer structure. The radial and time dependence of the magnetic parameters relevant for the switching process have been calculated by means of the mean-field theory based on experimental data of the saturation magnetization, uniaxial anisotropy, and the coercivity, for alloys of composition (Gd,Tb)1−xFex and Tb1−x(Fe,Co)x. From the radial profiles of the coercive field, maximum domain diameters are extracted in the limit of large domain velocities. Domain-wall stabilities are calculated from the force equation for writing and erasure conditions as a function of applied field yielding margins for stable domains and collapse fields. In addition to bubblelike domains, ring domain configurations are considered, which imply the possibility of direct overwrite for alloys with a compensation temperature above ambient temperature. The switching with a continuous read power level is discussed.
Schottky barrier junctions of hydrogenated amorphous silicon-germanium alloys
Authors: Matsuura Hideharu , Okushi Hideyo
Published: July 2002

The current-transport mechanisms of amorphous Schottky barrier junctions are investigated using both undoped and phosphorus-doped hydrogenated amorphous silicon-germanium alloys (a-Si1−xGex:H) fabricated by rf glow discharges in diode- and triode-reactor systems with and without the dilution of hydrogens. From the experimental results, the forward-current mechanisms for amorphous Schottky barrier junctions are generally discussed and classified into three categories: (1) field emission, (2) diffusion-field emission, and (3) diffusion. The diffusion-field-emission model (multistep tunneling through a part of the Schottky barrier) used for the first time in this work can interpret the current-voltage characteristics and their temperature dependence on amorphous Schottky barrier junctions to a high degree. Moreover, the flow chart proposed here classifies the material quality of amorphous semiconductors according to their junction properties. We concluded that one of the better methods to fabricate high-quality a-Si1−xGex:H for Schottky barrier junctions is the triode-reactor technique using hydrogen-diluted starting-gas materials.
Grain growth and grain size distributions in thin germanium films
Authors: Palmer J. E. , Thompson C. V. , Smith Henry I.
Published: July 2002

We have observed and characterized normal and secondary grain growth in thin films of germanium on silicon dioxide. Films were deposited on thermally oxidized silicon wafers, encapsulated with 1000-Å-thick sputtered SiO2 films and annealed in evacuated ampoules at 900 and 915 °C. After 5 min, the films had developed a columnar grain structure as a result of normal grain growth. The grain size distributions were lognormal with mean grain diameters of about 2.5 times the film thickness. The standard deviation of the normal grain size distribution did not change significantly with annealing time and temperature. Secondary grain growth, which can lead to grains that are much larger than the film thickness, occurred in films that were annealed for longer periods of time. The normal grain size distribution remained stationary, i.e., the peak height and width did not increase with annealing time. Secondary grains were few in number compared to normal grains, and were manifest as a small tail on the normal grain size distribution. The rate of secondary grain growth was constant and largest in the thinnest films.
Local spin-density functional theory of noncollinear magnetism (invited)
Authors: Kübler J. , Höck K.-H. , Sticht J. , Williams A. R.
Published: July 2002

We apply spin-density functional theory (SDF) to describe noncollinear magnetism; i.e., self-consistent energy-band calculations based on the local approximation to SDF theory are presented in which the magnetization associated with different atoms in a unit cell is allowed to point along different, noncollinear directions. In contrast to older work (e.g., by You and Heine and by Oguchi, Terakura, and Hamada) the present calculations are (1) self-consistent, (2) provide the total energy, and (3) provide the spin-quantization axes. In our applications we deal with noncollinear antiferromagnets γ-FeMn and perovskites Mn3GaN and show that their total energies are minimized in tetrahedral and triangular magnetic structures, respectively, first proposed by Kouvel and Kasper.
Photoionization of deep traps in AlGaAs/GaAs quantum wells
Authors: Takikawa M. , Kelting K. , Brunthaler G. , Takechi M. , Komeno J.
Published: July 2002

We investigated photoionization of deep traps in AlGaAs/GaAs multiple-quantum-well layers and measured the photocurrent (PC) parallel to the layers under a small electric field. There is a small shoulder due to the photoionization of a deep trap on the low-energy side of the n=1 exciton resonance peak in the PC spectra taken as a function of the excitation energy ℏω. The excitation energy dependence f(ℏω) and amplitude A of the photoionization cross section, σ(ℏω)=Af(ℏω), are determined by the time constants of single-shot PC transients. The excitation energy dependence increases linearly with excitation energy. The photoionization threshold energy EMQWth and the amplitude A increase as well thickness decreases. These characteristics are explained well by our theoretical study on the photoionization of a deep trap to subbands.