bluuebunny/crossref_metadata_2025 huggingface dataset:
Role of boron on grain sizes and magnetic correlation lengths in recording media as determined by soft x-ray scattering
Authors: Hellwig Olav , Margulies D. T. , Lengsfield B. , Fullerton Eric E. , Kortright J. B.
Published: July 2002

We have measured the chemical grain sizes and magnetic correlation lengths in CoCr-based magnetic recording media films using resonant soft x-ray small-angle scattering. We find that the addition of boron, while leading to slightly smaller physical grains, dramatically reduces the magnetic correlation length. These results show that B additions effectively act to suppress intergranular magnetic exchange via segregation to the grain boundaries.
Structural dynamics in quantum solids. II. Real-time probing of the electronic bubble formation in solid hydrogens
Authors: Vigliotti Franco , Bonacina Luigi , Chergui Majed
Published: July 2002

The ultrafast dynamics of electronic bubble formation upon excitation of the A(3sσ) Rydberg state of NO trapped in solid H2 and D2 has been studied by femtosecond pump–probe spectroscopy. The evolution of the spherical bubble is followed in real time by means of a probe pulse, which maps the transient configurations via transitions to higher-lying Rydberg states. It is found that bubble formation is a one-way process and no oscillations of the bubble are observed. In addition, thermalization of the system occurs on the time scale of bubble formation. In the process, there is a net energy flow away from the excited center and 0.55–0.6 eV leave the first shell around the impurity. We directly extract from the experimental data the time dependence of the bubble radius, which we represent by a rising exponential with time constants of 300±50 fs in solid H2 and 410±30 fs in solid D2 to reach a final radius of ∼5 Å. This is confirmed by simulations of the transients. The different energy dissipation mechanisms in the expansion of the bubble are discussed and we suggest that emission of a sound wave is the dominant one.
Magnetization reversal in magnetostatically coupled dot arrays
Authors: Shima H. , Guslienko K. Yu. , Novosad V. , Otani Y. , Fukamichi K. , Kikuchi N. , Kitakami O. , Shimada Y.
Published: July 2002

The submicron permalloy dots with variable diameter and interdot distance were microfabricated into a rectangular lattice by means of e-beam lithography and lift-off techniques. The hysteresis loops exhibit characteristic magnetization reversal accompanied by “nucleation” and “annihilation” of magnetic vortices inside the dots. The magnetic response of the samples with well-separated elements is isotropic in the plane. The arrays with a small interdot distance show magnetic anisotropy with the easy axis along the shortest period in the array. This anisotropy is originated from the interdot magnetostatic interaction. In the closely packed (when interdot distance is smaller than dot radius) arrays so that d/R<1, the magnetostatic interaction decreases the vortex nucleation and annihilation fields, and increases the initial susceptibility.
Boundary structure for improving performances of single-sided linear induction motors
Authors: Kamiya Yushi , Akagi Masataka , Wakao Shinji
Published: July 2002

In this paper, we propose a boundary structure for improving performances of single-sided linear induction motors. In this study, by adopting a three-dimensional finite element method, detailed phenomena at the boundary between secondary sections have been clarified from the viewpoint of propulsive force and secondary current distributions. Further, to suppress the aggravation of the characteristic caused by the secondary structural transition, we connect the secondary sections electrically. Finally, the propulsive force characteristic has been improved in spite of the existence of the boundary.
Ion extraction from a laser-ionized plasma produced between parallel plate cathodes and an anode above them
Authors: Kurosawa Hitoshi , Hasegawa Shuichi , Suzuki Atsuyuki
Published: July 2002

We studied the behavior of a laser-photoionized plasma in atomic-vapor laser isotope separation under an external electric field with a two-dimensional one-fluid model, in which electrons are assumed to be in thermal equilibrium. Sheath-formation and ion-extraction processes are investigated in both a conventional parallel-electrode system and an M-type electrode system consisting of two parallel cathodes and one anode above them. The process of ion extraction in the M-type electrode was made clear and it is shown that ions are collected twice as fast as in the parallel-electrode system.
Structure of slow shocks in a magnetized plasma with heat conduction
Authors: Tsai C. L. , Tsai R. H. , Wu B. H. , Lee L. C.
Published: July 2002

The structure of slow shocks in the presence of a heat conduction parallel to the local magnetic field is simulated from the set of magnetohydrodynamic equations. In this study, a pair of slow shocks is formed through the evolution of a current sheet initiated by the presence of a normal magnetic field. It is found that the slow shock consists of two parts: The isothermal main shock and foreshock. Significant jumps in plasma density, velocity and magnetic field occur across the main shock, but the temperature is found to be continuous across the main shock. The foreshock is featured by a smooth temperature variation and is formed due to the heat flow from downstream to upstream region. The plasma density downstream of the main shock decreases with time, while the downstream temperature increases with time, keeping the downstream pressure constant. It is shown that the jumps in plasma density, pressure, velocity, and magnetic field across the main shock are determined by the set of modified isothermal Rankine–Hugoniot conditions. It is also found that a jump in the temperature gradient is present across the main shock in order to satisfy the energy conservation. The present results can be applied to the heating in the solar corona and solar wind.
Magnetic nanogranularity and spin-glass behavior in mechanically alloyed Fe35Al50B15
Authors: De Toro J. A. , López de la Torre M. A. , Riveiro J. M. , Bland J. , Goff J. P. , Thomas M. F.
Published: July 2002

The combination of Mössbauer spectroscopy and magnetization measurements is demonstrated to be efficient in the determination of the phases responsible for the glassy magnetic dynamics of the highly disordered alloy Fe35Al50B15 produced by mechanical alloying. Its spin-glass behavior is apparent from the FC-ZFC magnetization irreversibility, the thermoremanent magnetization, and the slightly frequency-dependent peak in the temperature dependence of the ac susceptibility. Furthermore, the maximum at Tp≈22 K in the real component of the ac susceptibility is accompanied by a sudden onset of the imaginary component. The fit of the frequency dependence of Tp to the critical slowing down law yielded a critical exponent zν=8±1. However, Mössbauer spectroscopy uncovered the existence of two different Fe environments, of which only the minor one freezes at low temperatures into a distribution of magnetic sextets. This result, combined with the superparamagnetic behavior detected at moderate fields, allowed us to ascribe the spin glass behavior to the presence of interacting, very fine, ferromagnetic clusters (estimated size ∼1 nm). The origin of this nanoscale compositional heterogeneity is argued to be related to regions of partially unalloyed Fe. The conclusions are contrasted with other mechanically alloyed systems with reported spin glass behavior or compositional inhomogeneity.
Electrical characterization of metal–insulator–semiconductor capacitors with xerogel as dielectric
Authors: Manjari E. Anulekha , Subrahmanyam A. , DasGupta N. , DasGupta A.
Published: July 2002

Xerogel films have been prepared on p-type silicon (pSi) substrates by the sol–gel process using hexamethyldisilazane for surface modification. The dielectric constants of the films are in the range of 1.9–2.5. Detailed electrical characterization has been carried out using an aluminum–xerogel–pSi metal–insulator–semiconductor structure. Low values of fixed oxide charges, mobile oxide charges, and interface state densities have been obtained. The low leakage current density and high breakdown field strength of these films make them suitable for intermetal isolation. Very little degradation of the film properties was observed even after 40 days without any capping layer.
Generation and annihilation of boron–oxygen related defects in boron-doped Czochralski-grown Si solar cells
Authors: Vu Tuong Khanh , Ohshita Yoshio , Araki Kenji , Yamaguchi Masafumi
Published: July 2002

Defects that reduce the minority-carrier lifetime in silicon crystal are produced by minority-carrier injection (forward bias or light illumination) when the boron-doped Czochralski-grown silicon (Cz-Si) is used as a solar cell material. The number of induced defects is determined from changes in open-circuit voltage (VOC) of the cells. It increases with the carrier injection time, and then becomes saturated. The saturated value increases as the ambient temperature increases, during the carrier injection. These defects are observed to be vanished by thermal annealing at 200 °C for 20 min, indicating that they are in an unstable state and that some of them are annihilated even during the carrier injection. Therefore, the total number of induced defects to be determined by the difference between the generation and the annihilation rates. The activation energies for the generation process and annihilation process are evaluated to be 0.77 eV and 0.32 eV, respectively.
Disorder-induced photoluminescence up-conversion in InAs/GaAs quantum-dot samples
Authors: Cassabois G. , Kammerer C. , Sopracase R. , Voisin C. , Delalande C. , Roussignol Ph. , Gérard J. M.
Published: July 2002

Photoluminescence up-conversion under cw excitation in semiconductor quantum-dot structures is systematically studied in a sample exhibiting a crossover between two-dimensional and three-dimensional (3D) growth modes. We probe the existence of carrier up-conversion by using ultrathin quantum wells close to the quantum-dot layer. We show that the efficiency of the up-conversion is closely related to the disorder induced by the 3D-growth mode of the quantum dots.
Near-field spectroscopy of porous silicon microcavity samples
Authors: Fuso F. , Labardi M. , Sbrana F. , Pardi L. , Allegrini M. , Gaburro Z. , Pavesi L.
Published: July 2002

Near-field optical spectroscopy has been used to investigate photoluminescence features of porous silicon microcavity samples with a subwavelength space resolution. The emission is found to be markedly dependent on the lateral position, with the presence of relatively narrow spectral features peaked at different wavelengths in the range 610–690 nm. Furthermore, the spectrum obtained by summing up spectra taken at different sampling points (relative displacement ∼100 nm) recovers the standard (macroscopic) photoluminescence spectrum of porous silicon, except for the presence of a dip around the resonance wavelength of the microcavity resonator.
Transonic and subsonic dynamics of the current-vortex sheet
Authors: Antognetti A. , Einaudi G. , Dahlburg R. B.
Published: July 2002

Simulations of the magnetohydrodynamic (MHD) dynamics of a current-vortex sheet for low values of the Alfvén number (A=VA/V0) in both the subsonic and transonic regimes are described. It is shown that, in agreement with previous linear results, the shear layer is unstable for all values of the sonic Mach number (M). However, the subsonic and transonic disturbances which develop differ significantly in many of their properties. In the subsonic case the evolution of the system is similar to the incompressible one since the growing perturbation is symmetric and evanescent in the cross stream direction. The inclusion of compressible effects mainly permits the study of its thermodynamic behavior. In the transonic case the growing perturbation appears to be overstable, asymmetric, oscillatory, and weakly evanescent in the cross stream direction. In their nonlinear evolution such modes, which we identify as fast MHD waves, lead to the formation of shocks and to a very different dynamics of the current-vortex system.
Domain walls in ferromagnetic nanoconstriction
Authors: Labaye Y. , Berger L. , Coey J. M. D.
Published: July 2002

180° domain walls in ferromagnetic nanoconstrictions are investigated by classical atomic Monte Carlo simulations. Two types of constrictions are considered; one is a uniform circular cylinder (isthmus), the other is a double-truncated cone (hour glass). The wall width is determined by the effective length of the constriction, which may be as little as a nanometer. The wall can have a Néel-like configuration for constrictions much narrower than the normal wall width, but there is a crossover region with vortex-type walls before reaching a Bloch-type wall at larger diameters. In very narrow constrictions, effects of the atomic-scale structure become evident as the domain-wall structure depends on the number of atoms in the cross section. The simulations confirm the prospect of creating very narrow domain walls in a nanoconstricted soft magnetic material, and they indicate the possibility of spontaneous thermal fluctuations between different magnetic modes in walls smaller than about 10 nm.
Wavelength modulation spectroscopy of single quantum dots
Authors: Stievater T. H. , Li Xiaoqin , Guest J. R. , Steel D. G. , Gammon D. , Katzer D. S. , Park D.
Published: July 2002

We demonstrate that external cavity diode lasers with large mode-hop-free tuning ranges (up to 80 GHz) together with wavelength modulation spectroscopy can be used to study excitonic transitions in semiconductor nanostructures. Such transitions are characterized by homogeneous linewidths typically on the order of a few GHz. Wavelength modulation spectroscopy offers a high signal-to-noise method for the determination of resonance line shapes. We have used this technique to accurately measure dipole moments and dephasing rates of single semiconductor quantum dot eigenstates. These measurements are important for the use of quantum dots in semiconductor cavities and quantum logic gates, and for an improved understanding of the physics of exciton confinement.
Apparent suppression of turbulent magnetic dynamo action by a dc magnetic field
Authors: Montgomery David C. , Matthaeus W. H. , Milano L. J. , Dmitruk P.
Published: July 2002

Numerical studies of the effect of a dc magnetic field on dynamo action (development of magnetic fields with large spatial scales), due to helically-driven magnetohydrodynamic turbulence, are reported. The apparent effect of the dc magnetic field is to suppress the dynamo action, above a relatively low threshold. However, the possibility that the suppression results from an improper combination of rectangular triply spatially-periodic boundary conditions and a uniform dc magnetic field is addressed: heretofore a common and convenient computational convention in turbulence investigations. Physical reasons for the observed suppression are suggested. Other geometries and boundary conditions are offered, for which the dynamo action is expected not to be suppressed by the presence of a dc magnetic field component.
Temperature dependence of the Raman shift in GaAs conformal layers grown by hydride vapor phase epitaxy
Authors: Ardila A. M. , Martínez O. , Avella M. , Jiménez J. , Gérard B. , Napierala J. , Gil-Lafon E.
Published: July 2002

Raman spectra between room temperature and 350 °C were measured in GaAs layers grown by hydride vapor phase epitaxy on Si substrates using the selective conformal growth method. The contributions of the thermal expansion, anharmonic phonon decay, and strain are considered in order to analyze the Raman data. The tensile strain in the conventional GaAs/Si seed and in the conformal GaAs layers was determined from the Raman spectra. It is shown that the thin SiO2 layer between the GaAs and the Si substrate is a compliant layer that plays an important role in the reduction of the dislocation density in the conformal layers. The tensile strain in conformal layers was higher than in the conventional GaAs/Si layers, in which strain is relieved by the high density of dislocations.
Mini-conference on plasma turbulence in the corona, heliosphere and interstellar medium
Authors: Matthaeus W. H. , Dmitruk P. , Milano L. J.
Published: July 2002

This paper provides a summary of some major physics issues discussed in the Mini-conference on plasma turbulence in the corona, heliosphere and interstellar medium. This is one of two Mini-conferences sponsored by the Topical Group on Plasma Astrophysics held as part of the American Physical Society Division of Plasma Physics Fall 2001 Meeting, 30 October–2 November 2001
Acetone n-radical cation conformational preference and torsional barrier
Authors: Pophristic Vojislava , Goodman Lionel , Gorb Leonid , Leszczynski Jerzy
Published: July 2002

The ab initio architecture and torsional barrier for acetone n-radical cation are obtained. The 923 cm−1 MP4/6-311+G(3df,2p) barrier is calculated to be 30% higher than for neutral acetone. This increase is largely attributed to correlation effects and less importantly to increased hyperconjugative stabilization of the equilibrium cation conformer. Ionization is predicted to cause opening of the central CCC angle by 7° and cause the methyl groups to lose the C3v symmetry that they possess in neutral acetone. The torsional coordinate for the infrared active b1 (gearing) rotation is predicted to not lie purely on the torsional potential surface, but to be contaminated by puckering of the CCCO skeleton in both the neutral and cation species, thereby making the b1 infrared torsional frequencies only partially suitable for sampling the torsional potential surface.
Segmental dynamics in a blend of alkanes: Nuclear magnetic resonance experiments and molecular dynamics simulation
Authors: Budzien Joanne , Raphael Colleen , Ediger Mark D. , de Pablo Juan J.
Published: July 2002

The segmental dynamics of a model miscible blend, C24H50 and C6D14, were investigated as a function of temperature and composition. The segmental dynamics of the C24H50 component were measured with C13 nuclear magnetic resonance T1 and nuclear Overhauser effect measurements, while H2 T1 measurements were utilized for the C6D14 component. Use of low molecular weight alkanes provides a monodisperse system in both components and allows differentiation of dynamics near the chain ends. From these measurements, correlation times can be calculated for the C–H and C–D bond reorientation as a function of component, position along the chain backbone, temperature, and composition. At 337 K, the segmental dynamics of both molecules change by a factor of 2 to 4 across the composition range, with the central C–H vectors of tetracosane showing a stronger composition dependence than other C–H or C–D vectors. Molecular simulations in the canonical and isobaric–isothermal ensembles were conducted with a united-atom force field that is known to reproduce the thermodynamic properties of pure alkanes and their mixtures with good accuracy. With a minor change to the torsion parameters, the correlation times for pure tetracosane are in good agreement with experiment. For pure hexane and its mixtures with tetracosane, the simulated dynamics are faster than experiment.
Chiral fluctuations in achiral clusters and liquids via molecular dynamics simulations
Authors: LaViolette Randall A. , Harris Robert A.
Published: July 2002

A chiral correlation function described previously [R. A. Harris, J. Chem. Phys. 115, 10577 (2001)] is employed here to detect chiral fluctuations in achiral systems. For the sake of illustration, classical constant volume and energy molecular dynamics simulations were employed in order to generate trajectories for each of the following systems: clusters of 4 to 119 water molecules in vacuum, a cluster of 6 water molecules in liquid carbon tetrachloride, the neat carbon tetrachloride liquid, and water itself. The power spectrum of the normalized chiral correlation function also has been calculated for each of these systems. The simulation results suggest that the chiral fluctuations, through their correlations, could give statistically significant signals from about 1 GHz for the liquids, to 10–100 GHz for either the solvated or the intermediate clusters, to 500 GHz–1 THz for the smallest clusters.
Laminar plasma dynamos
Authors: Wang Zhehui , Pariev Vladimir I. , Barnes Cris W. , Barnes Daniel C.
Published: July 2002

A new kind of dynamo utilizing flowing laboratory plasmas has been identified. The conversion of plasma kinetic energy to magnetic energy is verified numerically by kinematic dynamo simulations for magnetic Reynolds numbers above 210. As opposed to intrinsically-turbulent liquid-sodium dynamos, the proposed plasma dynamo corresponds to laminar flow topology. Modest plasma parameters (1–20 eV temperatures, 1019–1020 m−3 densities in 0.3–1.0 m scale-lengths driven by velocities on the order of the Alfvén critical ionization velocity) self-consistently satisfy the conditions needed for the magnetic field amplication. Growth rates for plasma dynamos are obtained numerically with different geometry and magnetic Reynolds numbers. Magnetic-field-free coaxial plasma guns can be used to sustain the plasma flow and the dynamo.
Manifestation of the double-exchange interaction in the Mössbauer spectrum of mixed-valence systems
Authors: Coropceanu V. , Brédas J. L. , Winkler H. , Trautwein A. X.
Published: July 2002

The dependence of the Mössbauer line shape on the double-exchange interaction is studied for dimeric mixed-valence systems in the framework of a model that treats the entire nuclear-electron system quantum-mechanically. The coherent tunneling motion of the “excess” electron between the metal sites produces a resonance splitting of the localized states. In the case of weak double-exchange interaction, this motion shows up as a splitting of Mössbauer lines. This effect however, is readily suppressed by an asymmetry distortion, which destroys the resonance conditions. The origin of different patterns in the temperature dependence of the Mössbauer line shape is discussed.
Instability of convective cells in ion temperature gradient turbulence
Authors: Dastgeer Sheikh , Weiland Jan , Majahan Sangeeta
Published: July 2002

It has been demonstrated using numerical simulation of two-dimensional ion temperature gradient (ITG) equations that a periodic array consisting of large scale convective cells is subject to shear flow instability. A dynamically global and self-consistent evolution of linearly unstable modes, close to marginal instability, leads to the formation of poloidal shear flow through the process of peeling-instability. The saturated state thus comprises radially localized and polloidally extended length scales, namely, zonal flows. The zonal flows are predominantly led by the perturbed shear flow, whose growth rate is larger than the linear ITG instability.
Studies of Mn/GaAs digital alloys using x-ray absorption fine structure and x-ray diffraction methods
Authors: Soo Y. L. , Kioseoglou G. , Kim S. , Chen X. , Luo H. , Kao Y. H. , Sasaki Y. , Liu X. , Furdyna J. K.
Published: July 2002

Local structure and effective chemical valency of Mn atoms in Mn/GaAs digital alloys have been investigated using the x-ray absorption fine structure techniques. The samples were prepared by molecular-beam epitaxy with different thickness of GaAs layers separating the nominal Mn monolayers. Lattice constants of the digital alloys are found by x-ray diffraction to increase linearly in a very narrow range (about 0.3%) with the Mn/GaAs ratio in the samples. Our data show that Mn atoms in the nominal Mn monolayers actually combine with GaAs to form (Ga, Mn)As alloys with Mn atoms substituting for the Ga sites in GaAs. This result clearly rules out the possibility of dominant MnAs formation.
Consideration of multifaceted asymmetric radiation from the edge (MARFE) as a dissipative structure
Authors: Tokar M. Z.
Published: July 2002

Multifaceted asymmetric radiation from the edge (MARFE) is considered as an example of dissipative structures which develop under critical conditions in different physical and technical systems. The model proposed results in a system of algebraic equation including a relation similar to Maxwell’s Rule that determines such characteristic parameters as the plasma temperature in MARFE, its extent in poloidal and radial directions. Predictions of this approximate approach are compared with the results of one- and two-dimensional numerical simulations.
Short-wavelength (λ<2 μm) intersubband absorption dynamics in ZnSe/BeTe quantum wells
Authors: Akimoto R. , Akita K. , Sasaki F. , Kobayashi S.
Published: July 2002

We report on linear and nonlinear short-wavelength (λ<2 μm) intersubband (ISB) absorption characteristics in ZnSe/BeTe quantum wells by means of an interband pump and ISB pump/probe technique. The ISB absorption saturates with a hole burning effect, indicating the absorption band is broadened inhomogeneously. The saturation intensity is as low as 4.3 MW/cm2 at λ=1.76 μm. The direct ISB energy relaxation time increases gradually from 0.20 to 0.38 ps with decreasing λ from 2.2 to 1.8 μm, while the saturation recovery is replaced by another slow relaxation process with a time constant of a few ps. The Γ(ZnSe)–X(BeTe) electron transfer is a relevant mechanism for this slow relaxation.
Excitation of accelerating plasma waves by counter-propagating laser beams
Authors: Shvets Gennady , Fisch Nathaniel J. , Pukhov Alexander
Published: July 2002

The conventional approach to exciting high phase velocity waves in plasmas is to employ a laser pulse moving in the direction of the desired particle acceleration. Photon downshifting then causes momentum transfer to the plasma and wave excitation. Novel approaches to plasma wake excitation, colliding-beam accelerator (CBA), which involve photon exchange between the long and short counter-propagating laser beams, are described. Depending on the frequency detuning Δω between beams and duration τL of the short pulse, there are two approaches to CBA. First approach assumes (τL≈2/ωp). Photons exchanged between the beams deposit their recoil momentum in the plasma driving the plasma wake. Frequency detuning between the beams determines the direction of the photon exchange, thereby controlling the phase of the plasma wake. This phase control can be used for reversing the slippage of the accelerated particles with respect to the wake. A variation on the same theme, super-beatwave accelerator, is also described. In the second approach, a short pulse with τL≫ωp−1 detuned by Δω∼2ωp from the counter-propagating beam is employed. While parametric excitation of plasma waves by the electromagnetic beatwave at 2ωp of two co-propagating lasers was first predicted by Rosenbluth and Liu [M. N. Rosenbluth and C. S. Liu, Phys. Rev. Lett. 29, 701 (1972)], it is demonstrated that the two excitation beams can be counter-propagating. The advantages of using this geometry (higher instability growth rate, insensitivity to plasma inhomogeneity) are explained, and supporting numerical simulations presented.
Local potato-plateau transport fluxes and a unified plateau theory
Authors: Shaing K. C. , Houlberg W. A. , Strand P. I.
Published: July 2002

A local potato-plateau transport theory is presented. It is a nonradial averaged version of the original theory [Phys. Plasmas 4, 4331 (1997); 5, 953 (1998)]. The theory unifies conventional plateau theory and the potato-plateau theory. It is applicable at any radius. It is found that the ion heat conductivity is the same as that in the conventional theory in the region close to the magnetic axis.
Thermodynamic calculations of the effect of B and Ta on magnetically induced phase separation in Co–Cr–Pt alloys
Authors: Oikawa K. , Qin G. W. , Okamoto S. , Kitakami O. , Shimada Y. , Fukamichi K. , Ishida K.
Published: July 2002

In order to clarify the relation between the magnetically induced phase separation and the recording media characteristics, the thermodynamic calculations of Co–Cr–Pt–B and Co–Cr–Pt–Ta systems have been carried out by using the available binary assessment data and Miedema’s semiempirical values. B is segregated to the boundary in a similar manner as Cr, which makes the boundary region paramagnetic. This result is consistent with available data that B weakens the intergranular magnetic coupling and increases the magnetic anisotropy in Co–Cr–Pt recording media. By adding Ta, the Cr content in the paramagnetic phase is also increased, reducing the intergranular magnetic coupling. However, the Ta content in the ferromagnetic phase is higher than in the paramagnetic phase, decreasing the magnetic anisotropy. Accordingly, the thermodynamic calculations successfully explain experimental magnetic data for Co–Cr–Pt–B and Co–Cr–Pt–Ta recording media.
Interaction between low-energy electrons and defects created by hot holes in ultrathin silicon dioxide
Authors: Vogel Eric M. , Heh Dawei , Bernstein Joseph B.
Published: July 2002

The interaction of injected electrons with defects created by hot holes in 2.0-nm-thick silicon dioxide is studied using substrate hot hole injection and concomitant electron tunneling. The ratio of injected hot holes to electrons is varied from approximately 10−3 to 100 by changing the forward biased substrate hot hole injector voltage. Increased bulk and interfacial degradation caused by the interaction of concomitantly injected electrons with defects created by hole injection is not observed for the experimental conditions studied. The ability of defects to produce catastrophic breakdown is also not affected. The results suggest that the interaction of tunneling electrons with defects created by hot holes is not a viable mechanism for explaining the catastrophic breakdown of the oxide layer in field-effect transistors.
Crystal structures of polymerized fullerides AC60, A=K, Rb, Cs, and alkali-mediated interactions
Authors: Verberck B. , Michel K. H. , Nikolaev A. V.
Published: July 2002

Starting from a model of rigid interacting C60 polymer chains on an orthorhombic lattice, we study the mutual orientation of the chains and the stability of the crystalline structures Pmnn and I2/m. We take into account (i) van der Waals interactions and electric quadrupole interactions between C60 monomers on different chains as well as (ii) interactions of the monomers with the surrounding alkali atoms. The direct interactions (i) always lead to an antiferrorotational structure Pmnn with alternate orientation of the C60 chains in planes (001). The interactions (ii) with the alkalis consist of two parts: translation-rotation (TR) coupling where the orientations of the chains interact with displacements of the alkalis, and quadrupolar electronic polarizability (ep) coupling, where the electric quadrupoles on the C60 monomers interact with induced quadrupoles due to excited electronic d-states of the alkalis. Both interactions (ii) lead to an effective orientation-orientation interaction between the C60 chains and always favor the ferrorotational structure I2/m, where C60 chains have a same orientation. The structures Pmnn for KC60 and I2/m for Rb- and CsC60 are the result of a competition between the direct interaction (i) and the alkali-mediated interactions (ii). In Rb– and CsC60 the latter are found to be dominant, the preponderant role being played by the quadrupolar electronic polarizability of the alkali ions.
Fast time-resolved x-ray diffraction in BaTiO3 films subjected to a strong high-frequency electric field
Authors: Zolotoyabko E. , Quintana J. P. , Hoerman B. H. , Wessels B. W.
Published: July 2002

The pulsed synchrotron radiation from the Advanced Photon Source of Argonne National Laboratory was used to measure the dynamic structural response in 200-nm-thick BaTiO3 ferroelectric films, in situ, under the application of a high-frequency electric field. X-ray diffraction measurements were performed in the stroboscopic mode, i.e., by synchronizing the x-ray bursts with the electric-field periodicity. Time-dependent variations of lattice parameters were derived from the electric-field-induced distortions of the diffraction profiles. Drastic reduction of the relaxation time, from 6.9 ns at 71.69 MHz down to 0.7 ns at 521.36 MHz, was found with an increase of the electric-field frequency.
Surface microstructuring and long-range ordering of silicon nanoparticles
Authors: Fowlkes J. D. , Pedraza A. J. , Blom D. A. , Meyer H. M.
Published: July 2002

Pulsed-laser irradiation was used to induce the formation of linear arrays of nanoparticles that can extend over millimeter distances. On flat surfaces, the irradiation induces the fragmentation and clustering of a thin silicon film pulsed-laser deposited on silicon into nanoparticles that grow to 30–40 nm in diameter. The nanoparticles aggregate into clusters that migrate, forming short curvilinear groups that exhibit a short-range ordering. If a region containing a microscopic roughness is introduced, the nanoparticles are forced to align into long and remarkably straight lines with line spacing very close to the laser wavelength. A close connection is established between the nanoparticle alignment and the evolution of laser-induced periodic surface structures. The microscopic roughness solely serves as a trigger to produce the alignment.
Ultraefficient cooling in ferromagnet–superconductor microrefrigerators
Authors: Giazotto Francesco , Taddei Fabio , Fazio Rosario , Beltram Fabio
Published: July 2002

A promising scheme for electron microrefrigeration based on ferromagnet–superconductor contacts is presented. In this setup, cooling power densities up to 600 nW/μm2 can be achieved, leading to electronic temperature reductions largely exceeding those obtained with existing superconductor–normal metal tunnel contacts. Half-metallic CrO2/Al bilayers are indicated as ideal candidates for the implementation of the device.
Electron lifetime of heavily Be-doped In0.53Ga0.47As as a function of growth temperature and doping density
Authors: Vignaud D. , Lampin J. F. , Lefebvre E. , Zaknoune M. , Mollot F.
Published: July 2002

The electron lifetime has been studied by a pump–probe optical transmission technique in heavily Be-doped InGaAs lattice matched to InP as a function of the growth temperature (350⩽Tg⩽500 °C) and doping (2×1019⩽[Be]⩽2.6×1020 cm−3). Reduction of the growth temperature to 350–400 °C induces the creation of electron recombining centers, efficient at the lowest doping studied here. But, for higher dopings, these defects have negligible effects compared to intrinsic Auger processes: the high diffusion of Be can thus be limited by growing heterostructures at reduced temperatures without compromising the electron lifetime. Subpicosecond electron lifetimes have been measured at the highest doping.
Two-dimensional model of a kicked oscillator: Motion with intermittency
Authors: Abdullaev S. S.
Published: July 2002

The phenomenon of intermittency, which arises near a point of degeneracy of an unperturbed Hamiltonian under the influence of a discontinuous perturbation function, is studied in the example of a two-dimensional (2-D) model of a kicked oscillator. This example describes the dynamics of a particle in a cylindrically symmetric potential well subjected to radial kicks which occur periodically in time. The problem is reduced to a Hamiltonian system with N=3/2 degrees of freedom, whose unperturbed Hamiltonian has a degeneracy point. The intermittency is studied numerically and analytically.
Contacting and forming singularities: Distinguishing examples
Authors: Steen Paul H. , Chen Yi-Ju
Published: July 2002

A thin film bridge breaks in a way that starts at one equilibrium state and ends at another equilibrium state. The dynamical trajectory that carries it from connected to disconnected provides rare evidence regarding the singularity of passage through topological change. This nonequilibrium trajectory, called a “forming” flow, is discussed in an attempt to frame it within the larger class of singularities for which bounding surfaces do not remain material surfaces. As a contrast, the weaker “contacting” singularity is illustrated by a stagnation flow where material points reach the stagnation point in finite time. A classification scheme based on pathology of the nonunique Lagrangian motions is suggested. New results for the disconnection example include healing of surgery in post-disconnection simulations, different dynamical scalings of the just-disconnected components and a comparison of post-disconnection simulation to experiment.
Using multiple attractor chaotic systems for communication
Authors: Carroll T. L. , Pecora L. M.
Published: July 2002

In recent work with symmetric chaotic systems, we synchronized two such systems with one-way driving. The drive system had two possible attractors, but the response system always synchronized with the drive system. In this work, we show how we may combine two attractor chaotic systems with a multiplexing technique first developed by Tsimring and Suschick to make a simple communications system. We note that our response system is never synchronized to our drive system (not even in a generalized sense), but we are still able to transmit information. We characterize the performance of the communications system when noise is added to the transmitted signal.
Detecting dynamical nonstationarity in time series data
Authors: Yu Dejin , Lu Weiping , Harrison Robert G.
Published: July 2002

Nonlinear time series analysis is becoming an ever more powerful tool to explore complex phenomena and uncover underlying patterns from irregular data recorded from experiments. However, the existence of dynamical nonstationarity in time series data causes many results of such analysis to be questionable and inconclusive. It is increasingly recognized that detecting dynamical nonstationarity is a crucial precursor to data analysis. In this paper, we present a test procedure to detect dynamical nonstationarity by directly inspecting the dependence of nonlinear statistical distributions on absolute time along a trajectory in phase space. We test this method using a broad range of data, chaotic, stochastic and power-law noise, both computer-generated and observed, and show that it provides a reliable test method in analyzing experimental data.
Shock-induced luminescence from Z-cut lithium niobate
Authors: Brannon P. J. , Morris R. W. , Asay J. R.
Published: July 2002

Shock-induced luminescence from lithium niobate has been studied in the stress range 1.6–21.0 GPa. Both fast-framing photography and five-channel optical pyrometry were used to observe the luminescence. The framing photography showed that the emission pattern is heterogeneous for stresses just above the dynamic yield point. A further increase of the stress resulted in a pattern which was essentially homogeneous to within the experimental spatial resolution of about 30 μm. Narrowband filters and photomultiplier tubes were used in the optical pyrometry experiments. A broadband spectrum with a peak near 700 nm was observed. A plot of the energy dissipated by the shock versus shock stress correlates very well with a plot of the 700-nm intensity versus shock stress. The mechanism for light emission in lithium niobate appears to be closely related to the dynamic yielding process.
Study of broadband emission of an ultraviolet preionized 20-atm CO2 laser
Authors: Wan Chong-Yi , Werling U. , Renk K. F.
Published: July 2002

A study of pulse shape and frequency distribution of the broadband emission from a 20-atm gas discharge CO2 laser, operated by use of a simple UV preionization scheme, is reported.
Hysteresis, metastability, and time dependence in d=2 and d=3 random-field Ising systems
Authors: Belanger D. P. , Rezende S. M. , King A. R. , Jaccarino V.
Published: July 2002

The hysteretic properties of random-field Ising model (RFIM) systems were studied. Using neutron scattering, a relatively narrow metastability boundary TF(H) in the d=2 (d=dl) RFIM system Rb2Co0.85Mg0.15F4 was found to lie well below the rounded specific heat peak ‘‘Tc(H)’’ of the (destroyed) phase transition. TF(H) scales as TN−TF(H)∝H2/φ with the RF crossover exponent φ=1.74±0.02. At TF(H), equilibrium is approached logarithmically with time. In the d=3 (d>dl) RFIM system Fe0.6Zn0.4F2 AF order was found to be stable against changes of T and H below the observed sharp phase transition boundary Tc(H), while the field-cooled domain state was not. Capacitance measurements in Fe0.68Zn0.32F2 established that equilibrium prevails for T≥Teq(H) which is slightly above Tc(H). Pronounced logarithmic time dependence is observed at T≲Tc(H).
Probing of impurity potential well at the Si/SiO2 interface by electric-field-enhanced emission
Authors: Rosencher E. , Coppard R. , Bois D.
Published: July 2002

A transient capacitance method is presented, which enables the study of electric-field-enhanced emission from a continuum of surface states at the Si/SiO2 interface. The enhancement of trap emission is measured in Al-SiO2-Si structures by the variation in the transient capacitance signal detected under a constant applied electric field, the trap emission having been previously activated by application of a higher electric field (excitation bias). The dipolar relaxation contribution to the transient signal observed in these structures is substracted during the experiment. The decrease in the resulting transient signal with the excitation bias clearly reveals the donor nature of the interface traps studied. Since the surface traps are highly localized at the Si-SiO2 interface, the electric field on the impurity is perfectly known. Assuming the sole presence of the Frenkel-Poole effect in our experimental conditions, the barrier lowering is obtained as a function of the applied electric field. From these data, the shape of a portion of the impurity potential well is deduced for different levels in the Si gap, in the direction normal to the Si-SiO2 interface. The results are in good agreement with a coulombic potential barrier at some distance from the impurities (∼10 nm).
Negative charge, barrier heights, and the conduction-band discontinuity in AlxGa1−xAs capacitors
Authors: Hickmott T. W. , Solomon P. M. , Fischer R. , Morkoç H.
Published: July 2002

We have combined current-voltage (I-V) and capacitance-voltage (C-V) measurements on n− GaAs–Al0.4Ga0.6As–n+ GaAs capacitors with different thicknesses of Al0.4Ga0.6As to determine the conduction-band discontinuity at the Al0.4Ga0.6As–n+ GaAs interface. Undoped AlxGa1−xAs deposited by molecular-beam epitaxy contains negative charge. We calculate the effect of band bending in undoped AlxGa1−xAs due to negative charge on measured barrier heights and on C-V curves. The temperature dependence of I-V curves is analyzed in terms of thermionic emission to derive barrier heights at the n+ GaAs–Al0.4Ga0.6As interface φG. Measured values of φG are proportional to the square of the insulator thickness, w, showing that negative charge is uniformly distributed through undoped Al0.4Ga0.6As. Combining values of Fermi energy and band bending at zero bias with φG at zero Al0.4Ga0.6As thickness, we find that the value of the barrier discontinuity for n+ GaAs–Al0.4Ga0.6As is ∼0.30 V. This corresponds to a ratio of conduction-band discontinuity ΔEC to band-gap difference ΔEG of 0.63±0.03, which is appreciably less than the value 0.85±0.03 that is widely accepted. We find that the dielectric constant of Al0.4Ga0.6As is more temperature dependent than that of GaAs between 77 and 250 K.
Propagation of envelope soliton of ionization waves
Authors: Ohe Kazuyuki , Hashimoto Masashi
Published: July 2002

Simplified basic equations describing ionization waves excited in the positive columns of glow discharges are numerically solved as an initial value problem under a certain boundary condition. The nonlinear dispersion relation with the finite wave amplitude in the basic equations is also calculated. The frequency decrease with increasing wave amplitude is obtained from the relation. We can predict the evolution of envelope solitons by evaluating the coefficients of the nonlinear Schrödinger equation from the backward wave property and the frequency decrease. By applying an envelope solutions as an initial value, the soliton propagates by keeping its initial waveform in contrast to spreading the width of an arbitrary wave packet with a small amplitude.
X-ray preionization of self-sustained, transverse excitation CO2 laser discharges
Authors: Jayaram Krishnaswamy , Alcock A. J.
Published: July 2002

X rays with a mean energy of ∼50 keV produced by a flash x-ray source have been used to preionize high-pressure CO2 laser mixtures so as to obtain uniform glow discharges. The x-ray exposure measured in the active volume was 1–2 mR/pulse with a pulse duration of 100 nsec (FWHM) which permitted specific energy inputs of up to 300 J/l atm. Glow discharges were obtained for a variety of pressures and gas mixtures including some which were helium-free or in which helium was replaced by argon. Further, a realistic comparison of UV and x-ray schemes was made under sealed-off conditions. X-ray preionization appears to be advantageous and the resulting discharges are shown to tolerate oxygen (up to 2.6%) better. Additives such as xenon or organic compounds were not necessary for obtaining uniform glow discharges.
Low-threshold disorder-defined buried-heterostructure AlxGa1−xAs-GaAs quantum well lasers
Authors: Deppe D. G. , Hsieh K. C. , Holonyak N. , Burnham R. D. , Thornton R. L.
Published: July 2002

Two different quantum well heterostructure wafers are used to fabricate buried-heterostructure AlxGa1−xAs-GaAs quantum well lasers using Si-induced layer disordering (via Si diffusion). In contrast to the first wafer (QWH1), the second quantum well wafer (QWH2) utilizes Zn instead of Mg as the p-type dopant in the top AlxGa1−xAs confining layer and yields, because of concentration mismatch in acceptor and donor doping in the confining layers (nZn>nSe), inferior laser diodes owing to Zn diffusion from the p-type to the n-type confining layer during high temperature processing (850 °C Si diffusion). The first quantum well heterostructure, however, employs a lower concentration Mg doping for its p-type confining layer (nMg<nSe) and yields high performance devices when used with the Si-induced layer-disordering process. For QWH1 the p-n junction and injection is not displaced (as for QWH2) from the QW active region during Si-induced layer disordering (850 °C annealing). A fabrication process is presented in which quantum well laser diodes are built with active regions as narrow in width as 0.6 μm, cw room-temperature laser threshold currents as low as 3 mA, and pulsed current thresholds as low as 1.5 mA.
Study of MBE growth of GexSi1−x on {111} vicinal surfaces of Si substrates
Authors: Brasen D. , Nakahara S. , Bean J. C.
Published: July 2002

GexSi1−x thin films were grown by molecular beam epitaxy on Si substrates which were purposely miscut ∼3° off the (111) face, toward the [011] pole. Rutherford backscattering channeling angular scans show that the miscut causes the Ge-rich alloy films to grow with an apparent crystallographic tilt relative to the substrate. Transmission electron microscopy micrographs reveal that these films contain many slip-induced stacking faults along a preferential direction. These results are explained in terms of a difference in the magnitude of the resolved shear stress, caused by the lattice mismatch and the 3° miscut.