bluuebunny/crossref_metadata_2025 huggingface dataset:
Sb enhancement of lateral superlattice formation in GaInP
Authors: Fetzer C. M. , Lee R. T. , Jun S. W. , Stringfellow G. B. , Lee S. M. , Seong T. Y.
Published: July 2002

Epitaxial layers of GaInP were grown by organometallic vapor phase epitaxy with small amounts of TESb added to control the surface bonding. Above a concentration of Sb/III(v)=0.016, 12 K photoluminescence measurements show that the band gap is reduced, as compared to completely disordered GaInP, by Sb addition and that polarization along the [1̄10] direction is as much as 41 times larger than along [110]. Transmission electron microscopy results show a lamellar domain structure in the [1̄10]-zone axis dark-field images with a period of 120 nm. Atomic force microscopy shows surface undulations with the same period along the [1̄10] direction. The results demonstrate an increase in the magnitude of the presence of lateral composition modulation with increasing Sb concentration.
Effects of GaAlN barriers and of dimensionality on optical recombination processes in InGaN quantum wells and quantum boxes
Authors: Lefebvre P. , Taliercio T. , Morel A. , Allègre J. , Gallart M. , Gil B. , Mathieu H. , Damilano B. , Grandjean N. , Massies J.
Published: July 2002

We compare several InGaN-based low-dimensional systems, by time-resolved photoluminescence (PL), versus temperature (8<T<280 K). We investigate the influence of growing or not an AlGaN barrier on top of the active layer. We address the differences between quantum wells and quantum boxes 5–10 nm in diameter and 2 nm in height. Our results are consistent with carrier localization on potential fluctuations with spatial extension much smaller than the size of the quantum boxes. Growing an AlGaN barrier reduces the carrier mobility between fluctuations, thus maintaining an effective PL dominated by localized carriers up to room temperature.
A three-dimensional ray-tracing code dedicated to x-ray laser amplification simulation
Authors: Temporal M. , Jacquemot S. , Bonnet L. , Decoster A.
Published: July 2002

A three-dimensional (3D) ray-tracing code has been developed to simulate the x-ray intensity produced in recent experiments where a silver target was driven by two laser beams. The code is used as a postprocessor of a detailed atomic physics code, which provides emissivities and opacities for inverted transitions. The hydrodynamics of the plasma is calculated with a 1D1/2 hydrocode where transverse profiles of temperature and density follow a self-similar solution. The 3D ray-tracing code accounts for progressive target illumination and calculates the x-ray laser output by solving the eikonal equation. Once 3D paths are determined, a steady-state transport solution is used to calculate the output intensity. The ray-tracing package is discussed first, then the present 3D results are compared with 2D calculations, as well as with collected experimental data.
Stochastic modeling of solid nucleation in supercooled liquids
Authors: Leonard J. P. , Im James S.
Published: July 2002

We have formulated a nucleation model that can simulate nucleation of solids in supercooled liquid in a manner that is consistent with the stochastic nature and kinetic aspect of the phenomenon. This is accomplished, even under highly transient and nonuniform thermal conditions, by (1) calculating the probability of nucleation in each and every liquid node during each time step using the Poisson expression, and (2) triggering nucleation if and only when the random number assigned to a node for the time step is less than the calculated nucleation probability. In contrast with previous models, no empirical or deterministic conditions for nucleation are imposed. We demonstrate the effectiveness of the model by analyzing nucleation-initiated solidification as encountered in pulsed laser-induced irradiation of thin Si films, and discuss the generality and utility of the model.
Crystalline beam emulations in a pulse-excited linear Paul trap
Authors: Kjærgaard Niels , Drewsen Michael
Published: July 2002

This paper considers a pulsed voltage excitation of the quadrupole electrodes of a linear Paul ion trap. The transverse dynamics of ions in this time-varying electric field is analogous to that of charged particles in the strong focusing magnetic lattice of a storage ring. By laser cooling ions stored in a pulse-excited linear Paul trap theoretical results on the stability of crystalline ion beams in storage rings can be tested. The stability of ion motion in a pulse-excited trap is derived in (q,a)-parameter formalism and we show where in (q,a) space to expect the formation of Coulomb crystals according to the theory of crystalline ion beams.
Control of magnetic anisotropy in Fe1−xCox films on vicinal GaAs and Sc1−yEryAs surfaces
Authors: Isakovic A. F. , Berezovsky J. , Crowell P. A. , Chen L. C. , Carr D. M. , Schultz B. D. , Palmstro/m C. J.
Published: July 2002

We demonstrate that two distinct surface contributions to the magnetocrystalline anisotropy can be used to control the magnetic properties of thin films of bcc Fe1−xCox grown on GaAs (100) and Sc1−yEryAs (100). The bare GaAs (100) surface has twofold symmetry, and Fe1−xCox films grown directly on it show a strong uniaxial magnetic anisotropy. Fourfold symmetry is restored in films grown on interlayers of Sc1−yEryAs, in which the rock-salt structure provides a fourfold symmetric surface. A uniaxial magnetic anisotropy can be induced in this case by miscutting the substrate towards a {111} plane, so that vicinal steps run along a 〈011〉 direction. A simple Néel pair-bonding model describes the evolution of the anisotropy with the degree of miscut. For miscut GaAs (100) surfaces without interlayers, both the intrinsic anisotropy originating from the surface bonding and a step-induced term contribute to the total magnetic anisotropy. Depending on the orientation of the miscut, the step contribution can either enhance or partially suppress the intrinsic uniaxial anisotropy.
Exact calculation of demagnetization field in micromagnetic simulation of shielded read heads
Authors: Wang Lei , Giusti James H. , Fernandez-de-Castro Juan
Published: July 2002

In micromagnetic simulation of shielded GMR heads, the self-demagnetizing field is usually calculated under the assumption that the two shields extend to infinity beyond the air bearing surface. Under this assumption, the demagnetization tensor for rectangular cells in free space and the method of images are used to calculate the self-demagnetizing field in the sensor. An unanswered question for this approach is how accurate it is. In this work, the Fourier series method is used to calculate the self-demagnetizing field of the sensor in the shielded environment exactly. Simulation results from this rigorous approach and the common approach are compared for both simple spin valve and synthetic antiferromagnet (SAF) biased spin valve heads. It is shown that the error of the common approach is very small for a SAF spin valve head and can be as large as 10% for a simple spin valve head.
Oxygen-enhanced IrMn spin valves deposited by ion-beam and magnetron sputtering
Authors: Lee W. Y. , Carey M. , Toney M. F. , Rice P. , Gurney B. , Chang H.-C. , Allen E. , Mauri D.
Published: July 2002

Enhancement of giant magnetoresistance properties of single (bottom) and dual IrMn-based spin valves through exposure of part of the CoFe pinned layer to O2 is reported. Under optimal conditions, a ΔR/R of 10.4% [Hua=460 Oe, Hf=5.1 Oe, and Hc=4.7 Oe for a free and pinned layer thickness (permalloy equivalent) of 50 Å each] for an ion beam sputtered single spin valve, and a ΔR/R of as high as 20.5% for a magnetron sputtered dual spin valve having a 30 Å thick CoFe free layer are observed, compared to a value of 6.5% and 10.6% for the corresponding spin valve without O2 exposure, respectively. Transmission electron microscopy results reveal the presence of a thin (10 Å) crystalline oxygen-containing layer near the IrMn–CoFe pinned layer interface as a result of O2 exposure. X-ray reflectivity data show smoother interfaces for the spin valves subjected to O2 exposure, consistent with the lower Hf and smaller sheet resistance observed for these samples. The enhanced ΔR/R thus can be attributed to improved growth after O2 exposure.
Magnetic and electrical properties of spin valve with single and double specular oxide layers
Authors: Hong Jongill , Noma Kenji , Kanai Hitoshi , Kane Junichi
Published: July 2002

Appropriate oxide capping on a spin valve significantly improved electrical and magnetic properties. The interlayer exchange coupling oscillated in the thickness range of a Cu spacer (between 20 and 30 Å). The coupling was antiferromagnetic and it allowed us to reduce the Cu spacer down to 20 Å without sacrificing the good properties of the spin valve. The improvement is due to enhanced specular reflection at the interface between the magnetic and the oxide layer and to less current shunting through the Cu spacer. In particular, the Cu in the capping acts as a filter controlling the diffusion of oxygen, which has led to the soft magnetic properties. Embedding an additional thin oxide layer into the pinned layer further improved the magnetoresistance response of the spin valve. Confinement of electrons between two oxides helps increase the occurrence of spin-dependent scattering. As a result, high giant magnetoresistance values resulted. The coupling oscillated from ferromagnetic to antiferromagnetic as a function of thickness of the Cu spacer. No significant bias in the coupling was observed and this lack of bias can be attributed to the smoothness of the interfaces. The oscillations were observable due to amplified Ruderman–Kittel–Kasuya–Yoshida-like coupling by strong reflection at the interfaces of the oxides.
Enhancement of exchange bias in Mn–Ir/Co–Fe based spin valves with an ultrathin Cu underlayer and in situ Mn–Ir surface modification
Authors: Yagami Kojiro , Tsunoda Masakiyo , Takahashi Migaku
Published: July 2002

Enhancement of exchange bias induced at the interface of the antiferromagnetic (AF)/ferromagnetic (F) layers was studied using the bottom “spin-valve films” (SVs) with the Mn–Ir/Co–Fe exchange coupled films. Exchange bias increased using an ultrathin Cu underlayer. Meanwhile, both exchange bias field, Hex, and blocking temperature, TB, increased intensively by heating specimens after depositing Mn–Ir film in a high vacuum. These two enhancement effects worked in an additive. As a result, an unidirectional anisotropy constant, JK, of 0.39 erg/cm2 (Hex of 1.3 kOe) and TB of ∼325 °C were obtained for the bottom SVs with a total thickness of 233 Å including an AF layer of 68 Å Mn74Ir26 and a pinned layer of 20 Å Co90Fe10, where the SVs were field annealed at 320 °C. A microstructural analysis using x-ray diffraction revealed that Hex did not depend on the diffraction intensity from Mn–Ir (111) for the SVs with various underlayers, and no remarkable changes occurred in the microstructure of the SVs with the heating treatment in a vacuum. Therefore, the enhancement effects might result from some changes in the microstructure and/or the morphology of the interface of AF/F layers.
Colossal magnetoresistive La0.7(Pb1−xSrx)0.3MnO3 films for bolometer and magnetic sensor applications
Authors: Lisauskas A. , Bäck J. , Khartsev S. I. , Grishin A. M.
Published: July 2002

We report on electrical and magnetic properties of a continuous series of solid solutions La0.7(Pb1−xSrx)0.3MnO3 prepared by the pulsed laser deposition technique on LaAlO3 and SrTiO3 single crystals. Strict compositional control enables us to tailor the metal-to-semiconductor phase transition from 266 to 327 K, the maximum of temperature coefficient of resistance from 10.2% K−1 to 3.2% K−1, and maximum of magnetoresistance ratio at 7 kOe from 41% to 17% for x=0 and x=1 correspondingly. The ferromagnetic resonance linewidth ranges from 124 to 300 Oe, indicating low microwave loss and the films uniformity. Noise spectroscopy performed in the 2 Hz–20 kHz range reveals two components: Johnson noise (independent of frequency and bias current) and excess 1/f noise proportional to the square of the bias current. Very low excess noise (normalized value γ/n varying in the range from 10−20 to 10−22 cm3) has been achieved due to the epitaxial quality of the fabricated films. Using these films, an infrared radiation bolometer and weak magnetic field sensor have been built and tested. The bolometer resolves the noise equivalent temperature difference as low as 120 nK/√Hz at 30 Hz frame frequency, while the magnetic field sensor shows the noise equivalent magnetic field difference of 50 μOe/√Hz at 1 kHz and optimum bias magnetic field applied.
Temperature dependence of exchange bias in La1/3Ca2/3MnO3/La2/3Ca1/3MnO3 epitaxial multilayers
Authors: Krivorotov I. N. , Nikolaev K. R. , Dobin A. Yu. , Kobrinskii A. L. , Wentzcovitch R. , Goldman A. M. , Dahlberg E. D.
Published: July 2002

A unidirectional magnetotransport anisotropy observed in thin films of exchange-biased ferromagnetic manganites is used to measure temperature dependence of exchange bias in these structures. Values of both the exchange field and the blocking temperature obtained from magnetotransport measurements significantly exceed those obtained from the hysteresis loop shift.
Ferromagnetism at room temperature in La–Ba–Ca–Mn–O thin films
Authors: Hong Nguyen Hoa , Sakai Joe , Imai Syozo
Published: July 2002

The effects of substitution of Ca by Ba in La1−xCaxMnO3 (LCMO) with x<0.5 were investigated systematically in order to clarify the role of the size of the A cations. As for the La1−x(Ba–Ca)xMnO3 (LBCMO) films of ferromagnetic metallic (FMM) region (x=0.2; 0.3; and 0.4), by doping Ba, the ferromagnetic transition temperature (TC) and the insulator-to-metal (IM) transition temperature (TIM) were improved about 30–60 K, compared with those of LCMO thin films with the same ratio of Mn3+:Mn4+. Especially, La0.7Ba0.1Ca0.2MnO3 thin films have an anomalously high TC (about room temperature) and a TIM of 275 K under zero field. In the ferromagnetic insulating (FMI) region (x=0.15; 0.1), the Ba doping enables the IM transition and remarkably heightens the TC as well. The phase diagram shows that in the slightly doped region (x<0.5), Ba doping has made the FMM phase significantly expanded.
Muon spin relaxation examination of transverse spin freezing (invited)
Authors: Ryan D. H. , van Lierop J. , Pumarol M. E. , Roseman M. , Cadogan J. M.
Published: July 2002

Muon spin relaxation has been used to study transverse spin freezing in a-FexZr100−x and a-Fe90−xRuxZr10. The derived phase diagrams, while generally consistent with model calculations, exhibit some significant differences. Comparison between zero-field and applied field measurements shows that Txy is strongly affected by a field and falls as B−1, contrary to mean field predictions.
Ab initio study of hexagonal Fe/Ru multilayers
Authors: Spišák D. , Lorenz R. , Hafner J.
Published: July 2002

Calculations of structural and electronic properties of hexagonal Fe/Ru multilayers, as well as of pure Fe and Ru metals, were performed using the spin-polarized density functional technique. Apart from the conventional hexagonal AB stacking of close-packed planes, an AB′ stacking with atoms in a B′ plane shifted over the triangle edges formed by atoms in an A plane was examined in the Fe part of the multilayers. It was concluded that the most stable solution is a ferromagnetic configuration with an AB′ stacking within Fe layers with the axial ratio cFe/aRu=1.47 and with an AB stacking in Ru layers with cRu/aRu=1.60. Interfacial interdiffusion effects and antiferromagnetic order were also investigated in connection with the reported disappearance of a magnetism at the Fe/Ru interfaces. The equilibrium lattice parameters of hexagonal close-packed Fe metal, aFe=2.44 Å, cFe/aFe=1.60, were obtained for a nonmagnetic state. At an only slightly larger lattice constant an antiferromagnetic state appears. For bulk Ru the parameters aRu=2.73 Å, cRu/aRu=1.58 were found for a nonmagnetic phase with the onset of antiferromagnetism beginning at highly dilated interatomic distances.
Liquid–solid and solid–solid phase transitions of oxygen in a single cylindrical pore
Authors: Morishige Kunimitsu , Ogisu Yutaka
Published: July 2002

To study the liquid–solid and solid–solid phase transitions of a confined O2, we performed x-ray diffraction measurements of O2 confined inside the cylindrical pores of six kinds of regular mesoporous adsorbents (MCM-41 and SBA-15) with different pore radii (R=2.1, 2.9, 3.9, 4.5, 5.3, and 7.0 nm) as a function of temperature. Hysteresis effects between the γ-to-β and β-to-γ solid–solid phase transitions are considerably larger than those between freezing and melting and the depression of the β-to-γ transition point is slightly smaller than that of the melting point. This suggests that the β-to-γ rather than the γ-to-β transition point represents the thermodynamic phase transition temperature between the γ and β solids. The nature of the phase transitions between the γ and β solids is almost unchanged by confinement. The depression of the melting point is analyzed based on a simple capillary theory by invoking an empirical relation between liquid–solid interfacial free energy and enthalpy of fusion proposed by Turnbull. The O2 confined to the cylindrical mesopores of R=2.1 nm gives rise to an amorphous diffraction pattern even at the lowest temperature studied (27 K). Unusual behavior of the peak width against temperature strongly suggests that the amorphous pattern originates from the formation of the γ-phase clusters rather than the vitrification of a confined liquid.
Direct numerical simulation of reacting scalar mixing layers
Authors: de Bruyn Kops S. M. , Riley J. J. , Kosály G.
Published: July 2002

Understanding the passive reaction of two chemical species in shear-free turbulence with order unity Schmidt number is important in atmospheric and turbulent combustion research. The canonical configuration considered here is the reacting scalar mixing layer; in this problem two initially separated species mix and react downstream of a turbulence generating grid in a wind tunnel. A conserved scalar in this flow is, with some restrictions, analogous to temperature in a thermal mixing layer, and considerable laboratory data are available on the latter. In this paper, results are reported from high resolution, direct numerical simulations in which the evolution of the conserved scalar field accurately matches that of the temperature field in existing laboratory experiments. Superimposed on the flow are passive, single-step reactions with a wide range of activation energies and stoichiometric ratios (r). The resulting data include species concentrations as a function of three spatial dimensions plus time, and statistical moments and spectra of all species. Several aspects of the flow are investigated here with the conclusions that (1) reactions in which r≠1 are more accurately modeled by frozen and equilibrium chemistry limits than are reactions in which r=1, (2) an existing definition of a reduced Damköhler number that includes temperature and stoichiometry effects is a useful measure of reaction rate, and (3) existing theoretical models for predicting the coherence and phase of fuel-oxidizer cross-spectra and the spectrum of the equilibrium fuel mass fraction when r=1 yield accurate predictions.
Nonuniversal k−3 energy spectrum in stationary two-dimensional homogeneous turbulence
Authors: Kaneda Yukio , Ishihara Takashi
Published: July 2002

A spectral closure analysis and numerical simulations suggest that there may be a class of two-dimensional turbulence in which the energy spectrum E(k) scales with the wave number k like E(k)=Ak−3 in the enstrophy transfer range in accordance with the Kraichnan–Leith–Batchelor (KLB) spectrum, but the prefactor A is different from the KLB spectrum and depends in a nontrivial way on the flow conditions at large scales.
Semirigid vibrating rotor target calculation for reaction H+HOD→H2+OD, HD+OH
Authors: Li Yi-Min , Wang Ming-Liang , Zhang John Z. H. , Zhang Dong H.
Published: July 2002

The semirigid vibrating rotor target (SVRT) model is applied to study the branching reaction H+HOD→H2+OD, HD+OH on the Schatz–Elgersma potential energy surface. Using the SVRT model, the time-dependent wave packet calculation is carried out in four-mathematical dimensions with the two additional internal coordinates fixed at/near transition state geometries. The reaction probabilities for producing two product branches are calculated from two separate dynamics calculations. Comparison with results from the six-dimensional dynamics calculation shows that the SVRT reaction probabilities and cross sections for both branching products are accurate within a wide range of collision energy. This shows that the SVRT model is capable of giving quantitatively accurate dynamics information for polyatomic reactions.
BaM/Pt multilayered films with fine grains and large perpendicular magnetic anisotropy for high density recording media
Authors: Matsushita Nobuhiro , Feng Jie , Funabashi Nobuhiko , Nakagawa Shigeki , Naoe Masahiko
Published: July 2002

A magnetoplumbite type of Ba ferrite (BaM) thin layer was deposited on a 9 nm thick Pt underlayer, and excellent c-axis orientation was observed even for an 8 nm thick BaM layer, which corresponds to only three or four BaM unit cells. The grain size was almost in the same range of 60–85 nm even when the BaM layer thickness tBaM decreased from 60 to 17 nm, and tBaM should be reduced below 10 nm to make a grain size smaller than 50 nm. However, the perpendicular coercivity Hc⊥ and squareness S⊥ decreased drastically from 2.6 to 0.5 kOe and from 0.6 to 0.2, respectively, with the decrease of tBaM from 60 to 8 nm because of higher demagnetizing field and susceptibility to thermal fluctuation. On the other hand, the [BaM(5–24 nm)/Pt(9 nm)]3 multilayer exhibited higher Hc⊥ and larger S⊥ than the BaM/Pt bilayer for the same BaM layer thickness and Hc⊥ and S⊥ of the [BaM(8 nm)/Pt(9 nm)]3 multilayer was 2.0 kOe and 0.6, respectively. It was clarified that the deposition of the BaM/Pt multilayer was very effective for achieving a high perpendicular magnetic anisotropy constant even for the ultrathin BaM layer.
Fingering instabilities in driven thin nematic films
Authors: Ben Amar M. , Cummings L. J.
Published: July 2002

Motivated by experimental work (Cazabet et al., unpublished), we consider the possibility of fingering instabilities in thin films of nematic liquid crystals. We use lubrication theory on the flow equations for nematic liquid crystals to derive a simple model describing the evolution of the film height. As far as we are aware, this is the first time such a systematically derived, time-dependent thin film model for nematics has been presented. Simple “leading-order” solutions (depending on only one spatial coordinate) are found for two different flow driving mechanisms: (i) gravity perpendicular to the film and (ii) gravity parallel to the film (capillarity is also included in both cases). The effect of imposing two-dimensional perturbations to these solutions is studied. We find that for case (i) instability is possible, depending on whether or not there is complete wetting (i.e., whether or not the equilibrium contact angle of the droplet with the substrate is zero). For case (ii) we always have instability, as we would expect from the analogous result for Newtonian fluids [Phys. Fluids 8, 460 (1996); Europhys. Lett. 10, 25 (1989)].
Electron density and recombination rate measurements in CO-seeded optically pumped plasmas
Authors: Palm Peter , Plönjes Elke , Buoni Matt , Subramaniam Vish V. , Adamovich Igor V.
Published: July 2002

Electron production rate and electron density in cold optically pumped CO–Ar and CO–N2 plasmas in the presence of small amounts of O2 and NO have been measured using a Thomson discharge probe and microwave attenuation. Nonequilibrium ionization in the plasmas is produced by an associative ionization mechanism in collisions of highly vibrationally excited CO molecules. It is shown that adding small amounts of O2 or NO (50–100 mTorr) to the baseline gas mixtures at P=100 torr results in an increase of the electron density by up to a factor of 20–40 (from ne<1010 cm−3 to ne=(1.5–3.0)×1011 cm−3). This occurs while the electron production rate either decreases (as in the presence of O2) or remains nearly constant within a factor of 2 (as in the presence of NO). It is also shown that the electron–ion recombination rates inferred from these measurements decrease by two to three orders of magnitude compared to their baseline values (with no additives in the cell), down to β≅1.5×10−8 cm3/s with 50–100 mTorr of oxygen or nitric oxide added to the baseline CO–Ar mixture, and β≅(2 to 3)×10−7 cm3/s with 75–100 mTorr of O2 or NO added to the baseline CO–N2 mixture. The overall electron–ion removal rates in the presence of equal amounts of O2 or NO additives turn out to be very close, which shows that the effect of electron attachment to oxygen at these conditions is negligible. These results suggest a novel method of electron density control in cold laser-sustained steady-state plasmas and open a possibility of sustaining stable high-pressure nonequilibrium plasmas at high electron densities and low plasma power budget.
Quantum calculations of conduction properties of metal/InAlAs/InGaAs heterostructures
Authors: Podevin F. , Vanbésien O. , Lippens D.
Published: July 2002

Quantum calculations of the current–voltage characteristics of metal/wide-gap/narrow-gap semiconductor heterostructures have been performed in order to analyze the pure tunneling and thermally assisted current contributions. The InAlAs/InGaAs material system lattice matched to an InP substrate, with the InAlAs layer acting as a single semiconductor barrier, shows pronounced quantum size effects which yield resonant tunneling paths. On the basis of the nonlinear current–voltage characteristics, it is shown that an optimum barrier configuration can be found. Some consequences from the device viewpoint are finally discussed.
Overlayer-induced anisotropic alignment of Nd2Fe14B nanograins
Authors: Shima T. , Kamegawa A. , Hono K. , Fujimori H.
Published: July 2002

Highly anisotropic Nd2Fe14B thin film has been produced by triggering crystallization of sputter-deposited amorphous Nd–Fe–B layer by deposition of crystalline Cr overlayer. A downward growth of strongly textured Nd2Fe14B columnar grains was observed, starting from the interface between the Nd–Fe–B amorphous layer and the Cr overlayer. Cross-sectional transmission electron microscope observation results suggest that the possible mechanism of this peculiar crystallization is the heterogeneous nucleation at the surface of Nd–Fe–B amorphous layer, triggered by the crystalline Cr overlayer.
Scattering anisotropy and giant magnetoresistance in magnetic multilayers
Authors: Binder J. , Zahn P. , Mertig I.
Published: July 2002

We present full ab initio calculations of giant magnetoresistance (GMR) in Co/Cu (001) multilayers including self-consistent impurity scattering potentials. Starting from density functional theory the electronic structure of the multilayer and the scattering at impurities are described by means of a new Green function method. It will be demonstrated that impurity scattering in magnetic multilayers is strongly anisotropic in comparison to impurity scattering in bulk systems. Concerning transport the anisotropy of scattering leads to a formation of highly conducting channels which give rise to short circuits and cause strong variation of GMR as a function of the impurity position in the multilayer.
Oscillatory thermocapillary flow in encapsulated liquid column
Authors: Majima S. , Kawamura H. , Otsubo F. , Kuwahara K. , Doi T.
Published: July 2002

The thermocapillary flow in a half zone model of an encapsulated liquid column was studied experimentally. The encapsulated liquid column was formed successfully with the use of two immiscible liquids, the silicone oil and Fluorinart. The thermocapillary convection was induced in the inner and outer columns by temperature difference imposed between top and bottom disks. With the increase of the temperature difference, the flow exhibited the transition from axisymmetric steady to nonaxisymmetric oscillatory flow. The oscillation started in the lower viscosity fluid and influenced the higher one. The critical Marangoni number was determined for different combinations of inner and outer column liquids. If the lower viscosity fluid was encapsulated with the higher one, the start of the oscillation was significantly retarded compared to the transition of the single column.
Surfactant mediated growth of MnSi1.7 on Si(001)
Authors: Teichert S. , Hortenbach H. , Hinneberg H.-J.
Published: July 2002

Sb is used as a surfactant for the growth of MnSi1.7 by reactive deposition of Mn on Si(001). It is found that the presence of Sb during the growth strongly increases the island density and changes the crystalline orientation of the MnSi1.7. The morphology and structure of the resulting silicide are the same both for the deposition of Mn only on a Sb-terminated Si(001) surface and for the codeposition of Mn and Sb on Si(001). A residual Sb coverage close to one monolayer at the sample surface has been determined for both of the preparation conditions.
Determination of the surface potential in thin-film transistors from C–V measurements
Authors: Migliorato P. , Tam S. W.-B. , Lui O. K. B. , Shimoda T.
Published: July 2002

In this article we present a method for the determination of the gate voltage versus surface potential (VGS−ψS) relationship in thin-film transistors (TFTs), from low frequency capacitance–voltage (C–V) characteristics. This information is very important for device design, process characterization, and modeling of TFTs and provides the basis for extracting the gap density of states. The accuracy of the method is demonstrated by applying it to the analysis of C–V data generated by two-dimensional simulations. Its application to laser recrystallized polysilicon TFTs is presented.
Theoretical study of first-row transition metal oxide cations
Authors: Nakao Yoshihide , Hirao Kimihiko , Taketsugu Tetsuya
Published: July 2002

The equilibrium geometries, dissociation energies, and electronic structures of the ground and low-lying excited states for the first-row transition metal oxide cations, MO+ (M=Sc to Zn), have been studied using the multireference singles and doubles configuration interaction (MR–SDCI) and the multireference second-order Møller–Plesset methods. To investigate the applicability of the density functional theory (DFT) to an electronic structure system with a multiconfigurational character, the Becke exchange functional with the Lee–Yang–Parr correlation functional, the Becke exchange functional with the one-parameter progressive correlation functional (BOP), and the Becke three-parameter hybrid exchange functional with the Lee–Yang–Parr correlation functional (B3LYP) methods have also been applied. The DFT predicts the ground state M–O bond lengths in good agreement with the multireference-based methods except for MnO+ and CuO+, which have a multiconfigurational electronic structure. With respect to the dissociation energies, the B3LYP results are in good agreement with the multireference-based methods, while the DFT with pure functionals overestimates the energetics by about 20 kcal/mol compared to the MR–SDCI method.
Generation of plasma rotation in a tokamak by ion-cyclotron absorption of fast Alfvén waves
Authors: Perkins F. W. , White R. B. , Bonoli P. T. , Chan V. S.
Published: July 2002

A mechanism is proposed and evaluated for driving rotation in tokamak plasmas by minority ion-cyclotron heating, even though this heating introduces negligible angular momentum. The mechanism has two elements: First, angular momentum transport is governed by a diffusion equation with a boundary condition at the separatrix. Second, Monte Carlo calculations show that ion-cyclotron energized particles will provide a torque density source which has a zero volume integral but separated positive and negative regions. With such a source, a solution of the diffusion equation predicts that ion-cyclotron heating will cause a rotational shear layer to develop. The corresponding jump in plasma rotation ΔΩ is found to be negative outwards when the ion-cyclotron surface lies on the low-field side of the magnetic axis and positive outwards with the resonance on the high-field side. The magnitude of the jump ΔΩ=(4qmaxWJ2*) (eBR3a2ne(2π)2)−1(τM/τE) where |J2*|≈2–4 is a nondimensional rotation frequency calculated by the Monte Carlo ORBIT code [R. B. White and M. S. Chance, Phys. Fluids 27, 2455 (1984)]. For a no-slip boundary condition when the resonance lies on the low-field side of the magnetic axis, the sense of predicted axial rotation is co-current and overall agreement with experiment is good. When the resonance lies on the high-field side, the predicted rotation becomes countercurrent for a no-slip boundary while the observed rotation remains co-current. The rotational shear layer position is controllable and of sufficient magnitude to affect microinstabilities.
Magnetotransport of La0.7Sr0.3MnO3/SrTiO3 multilayers with ultrathin manganite layers
Authors: Dörr K. , Walter T. , Sahana M. , Müller K.-H. , Nenkov K. , Brand K. , Schultz L.
Published: July 2002

The thickness dependence of electrical transport and magnetization of ultrathin La0.7Sr0.3MnO3 films has been investigated using epitaxial La0.7Sr0.3MnO3(LSMO)/SrTiO3 multilayers prepared by pulsed laser deposition. Layer thicknesses range from 1.9 to 10 nm for both constituents. Microstructural characterization by x-ray diffraction and transmission electron microscopy has confirmed a well-defined superlattice structure, coherent growth, and fairly smooth interfaces of the samples. Magnetization measurements show a pronounced drop of the ferromagnetic Curie temperature below a LSMO thickness of d=3 nm, accompanied by a strong rise of the coercive field. The in-plane electrical transport changes from metallic (dρ/dT>0) to insulating at d∼2.3 nm. Three different types of magnetoresistance (MR) have been observed for d<3 nm. In the sequence of decreasing d, the first resembles the grain boundary MR of polycrystalline LSMO, the second is positive, and the third is associated with a magnetic field-induced insulator-to-metal transition. Magnetic and transport results are discussed in the framework of a mixed-phase state of ferromagnetic metallic and less magnetically ordered insulating clusters in the ultrathin LSMO films.
Fluid and kinetic stability of virtual cathodes for the periodically oscillating plasma sphere
Authors: Nebel R. A. , Finn J. M.
Published: July 2002

Recent theoretical work [R. A. Nebel and D. C. Barnes, Fusion Technol. 38, 28 (1998); D. C. Barnes and R. A. Nebel, Phys. Plasmas 5, 2498 (1998)] has suggested that a tiny oscillating ion cloud (referred to as the periodically oscillating plasma sphere or POPS) may undergo a self-similar collapse that can result in the periodic and simultaneous attainment of ultrahigh densities and temperatures. However, a major uncertainty in this plasma system is the behavior of the electron cloud that forms a virtual cathode. Here it is demonstrated that the required electron cloud (which forms a harmonic oscillator potential) is susceptible to an instability related to buoyancy-driven modes present in compressible fluids. Although it is demonstrated that no absolutely stable profiles with uniform electron density exist, stable profiles that are close to the required harmonic oscillator potential are found. A simple two-stream analysis indicates that kinetic effects lead to a critical limit in λD/a above which the virtual cathodes are stable. This result is consistent with previous experimental observations.
Prebreakdown phenomena and formation process of the glow discharge in low-pressure Ar gas
Authors: Hosokawa Tatsuzo , Goto Kazuhiro , Ohuchi Mikio , Kaneda Teruo
Published: July 2002

The prebreakdown phenomena and the formation process of the glow discharge in a low-pressure Ar gas were investigated under a uniform field gap. Prebreakdown phenomena were observed for 0.5 Torr cm⩽pd⩽2 Torr cm (where p is pressure, d the gap distance) in Ar gas under conditions of a slowly increasing voltage. It was observed that the prebreakdown phenomena formed pulse discharges up to the transition to the glow discharge. The amplitudes of the photon and current pulses due to the pulse discharge increased with time, and then decreased as soon as the transition to a steady glow discharge occurred. When the overvoltage or external series resistance was increased, the pulse amplitudes increased with the applied voltage and decreased with the resistance. The characteristics of the prebreakdown phenomena were changed by the shape of the electrodes. The formation mechanism of the glow discharge can be qualitatively explained by that of the streamer in a high-pressure discharge. The transient glow discharge was observed, and its duration increased with an increase in resistance. The instability of the glow discharge was controlled by three factors, namely, Kaufmann’s criterion, the Child–Langmuir law, and the density balance between the production and removal rates of electrons.
Molecular excitation energies computed with Kohn–Sham orbitals and exact exchange potentials
Authors: Veseth L.
Published: July 2002

Exact local exchange potentials are computed for the diatomic molecules N2, O2, and CO, based on expansions in terms of molecular orbitals. Kohn–Sham orbitals and orbital energies are obtained for the exact exchange potentials, with correlation effects neglected. The ionization potential is in all cases found to be accurately predicted by the orbital energy of the highest occupied orbital. Limited configuration interaction calculations are performed based on the Kohn–Sham orbitals, and are found to yield accurate excitation energies for a series of singly excited states, in particular for N2 and CO. Clearly inferior results are obtained from similar calculations by use of Hartree–Fock orbitals. Thus Kohn–Sham orbitals obtained with exact exchange potentials tend to have an interesting potential as basis for sophisticated many-body methods.
Controlled modifications of electron injection on Au/Si and Au/SiO2/Si contacts using ballistic electron emission microscopy
Authors: Chahboun A. , Coratger R. , Pascale A. , Baules P. , Ajustron F. , Zorkani I. , Beauvillain J.
Published: July 2002

In this article, ballistic electron emission microscopy (BEEM) induced modifications on Au/Si and Au/SiO2/Si contacts are presented. BEEM current can be locally enhanced or reduced in a controlled manner. These observations are attributed to tip induced modifications on the gold surface. According to Au thickness, x-ray reflectivity experiments show different surface evolutions correlated to the size variations of the modifications introduced as a function of time.
Onset of homogeneous oscillations in reactive systems
Authors: Mansour M. Malek , Dethier J. , Baras F.
Published: July 2002

The statistical properties of a one-dimensional reaction–diffusion system undergoing a Hopf bifurcation are studied using the master equation approach. The analysis reveals nontrivial interferences between macroscopic dynamics and mesoscopic local fluctuations that eventually wipe out any trace of homogeneous oscillations, even though the latter are asymptotically stable solution of the deterministic equations. The comparison with the corresponding Langevin formulation leads to quantitative agreement. Analytical calculations are carried out using the stochastic Poincaré model. The onset of the desynchronization mechanism and its relation with the dimensionality of the embedding system is clarified.
Nonequilibrium high-frequency noise and responsivity in multiple quantum well infrared photodetectors
Authors: Ershov M.
Published: July 2002

A theory of the nonequilibrium high-frequency noise and responsivity in multiple quantum well infrared photodetectors (QWIPs) is presented. The nonequilibrium regime dominates at frequencies higher than the inverse characteristic response time of the injecting contact. The nonequilibrium generation–recombination (GR) noise and responsivity are determined by the fast component of the transient current (primary current) in response to impulse excitation. The nonequilibrium GR noise power and responsivity are lower than their low-frequency values due to the cutoff of the injection from the emitter at high frequencies. Analytical formulas for the noise and responsivity are obtained. These expressions are consistent with the conventional formulas of the standard model of unipolar photoconductors if the transit time across the QWIP is longer than the capture time by the quantum wells.
Transferring vibrational population between electronic states of diatomic molecules via light-induced-potential shaping
Authors: Chang B. Y. , Solá I. R. , Santamarı́a J. , Malinovsky V. S. , Krause J. L.
Published: July 2002

We investigate two-photon, selective excitation of diatomic molecules with intense, ultrafast laser pulses. The method involves transfer of a vibrational population between two electronic states by shaping of light-induced potentials (LIPs). Creation and control of the LIPs is accomplished by choosing pairs of transform-limited pulses with proper frequency detunings and time delays. Depending on the sequence of pulses (intuitive or counter-intuitive) and on the sign of the detuning (below or above the first transition) four schemes are possible for population transfer by LIP shaping. We develop a simple analytic model to predict the optimal laser pulses, and to model the adiabatic dynamics in the different schemes. Based on a harmonic, three-state model of the sodium dimer we demonstrate numerically that all four schemes can lead to efficient, selective population transfer. A careful analysis of the underlying physical mechanisms reveals the varying roles played by the adiabatic and diabatic crossings of the LIPs. The detailed mechanisms influence the robustness and experimental applicability of the schemes.
Surface properties and field emission characteristics of chemical vapor deposition diamond grown on Fe/Si substrates
Authors: Hirakuri Kenji , Yokoyama Takahiro , Enomoto Hirofumi , Mutsukura Nobuki , Friedbacher Gernot
Published: July 2002

Electron field emission characteristics of diamond grains fabricated on iron dot-patterned silicon (Fe/Si) substrates at different methane concentrations have been investigated. The characteristics of the samples could be improved by control of the methane concentration during diamond fabrication. Etching treatment of the as-grown diamond has enhanced the emission properties both with respect to current and threshold voltage. In order to study the influence of etching effects on the field emission characteristics, the respective surfaces were studied by Raman spectroscopy, Auger electron spectroscopy, and electron spectroscopy for chemical analysis (ESCA). ESCA revealed intensive graphite and FeOx peaks on the sample surface grown at high methane concentration. For the etched samples, the peaks of diamond and silicon carbide were observed, and the peaks of nondiamond carbon disappeared. The experimental results show that the etching process removes graphitic and nondiamond carbon components.
Mound surface roughness effects on the thermal capacitance of thin films
Authors: Palasantzas G. , De Hosson J. Th. M.
Published: July 2002

We investigate the influence of roughness at a nanometer scale on the thermal properties of thin films. It is shown that the roughness causes an increase of the thermal capacitance. For mound rough surfaces the increase of the thermal capacitance depends strongly on the relative magnitude of the average mound separation λ and the system correlation length ζ. Indeed, a rather complex behavior develops for ζ>λ, while for ζ<λ a smooth decrease of the capacitance as a function of the average mound separation λ takes place. Finally, the roughness strongly affects the thermal capacitance as a function of the film thickness as long as ζ<λ, while a precise determination of the actual effect requires a more-detailed knowledge of the thickness dependence of the involved roughness parameters during film growth.
Pulling hairpinned polynucleotide chains: Does base-pair stacking interaction matter?
Authors: Zhou Haijun , Zhang Yang
Published: July 2002

Force-induced structural transitions both in relatively random and in designed single-stranded DNA (ssDNA) chains are studied theoretically. At high salt conditions, ssDNA forms compacted hairpin patterns stabilized by base pairing and base-pair stacking interactions, and a threshold external force is needed to pull the hairpinned structure into a random coiled one. The base-pair stacking interaction in the ssDNA chain makes this hairpin-coil conversion a discontinuous (first-order) phase transition process characterized by a force plateau in the force-extension curve, while lowering this potential below some critical level turns this transition into continuous (second-order) type, no matter how strong the base-pairing interaction is. The phase diagram (including hairpin-I, -II, and random coil) is discussed as a function of stacking potential and external force. These results are in quantitative agreement with recent experimental observations of different ssDNA sequences, and they reveal the necessity to consider the base-pair stacking interactions in order to understand the structural formation of RNA, a polymer designed by nature itself. The theoretical method used may be extended to study the long-range interaction along double-stranded DNA caused by the topological constraint of fixed linking number.
Self-annihilation of antiphase boundaries in GaAs epilayers on Ge substrates grown by metal-organic vapor-phase epitaxy
Authors: Hudait M. K. , Krupanidhi S. B.
Published: July 2002

The self-annihilation of antiphase boundaries (APBs) in GaAs epitaxial layers grown by low-pressure metal-organic vapor-phase epitaxy on Ge substrates is studied by several characterization techniques. Cross-sectional transmission electron microscopy shows that antiphase domain free GaAs growth on Ge was possible due to the proper selection of the growth parameters. The antiphase boundaries annihilate with each other after a thick 3 μm layer of GaAs growth on a Ge substrate as observed by scanning electron microscopy studies. Double crystal x-ray diffraction data shows a slight compression of GaAs on Ge, and the full width at half maximum decreases with increasing growth temperatures. This confirms that the APBs annihilate inside the GaAs epitaxial films. Low temperature photoluminescence measurements confirm the self-annihilation of the APBs at low temperature growth and the generation of APBs at higher growth temperatures.
Boson and differential realizations of polynomial angular momentum algebra
Authors: Ruan Dong , Jia Yufeng , Ruan Wei
Published: July 2002

The inhomogeneous multiboson and multivariable differential realizations of the polynomial angular momentum algebra of quadratic type, L2, which may be seen as the nonlinear extension of an ordinary angular momentum algebra su(2) or its noncompact form su(1,1), are obtained from Fock representations of L2, which are related to master representation of L2 on the space of its universal enveloping algebra U(L2) or to induced representations on their respective quotient spaces U(L2)/Ii with different left ideals Ii.