bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamic fragility in polymers: A comparison in isobaric and isochoric conditions
Authors: Huang Dinghai , Colucci Dina M. , McKenna Gregory B.
Published: July 2002

The dynamic fragilities of six polymeric materials have been compared for isobaric (constant pressure) and isochoric (constant volume) conditions. Data were taken from the published literature for dynamic experiments performed at elevated pressures. Published pressure–volume–temperature (PVT) data were then used to determine the dynamic response as a function of temperature for both the isochoric and isobaric conditions. It is found that the pressure and volume dependences of the dynamic fragility vary greatly among the polymers for which data were available. For poly(vinyl acetate) (PVAc) and poly(ethyl acrylate) (PEA), the dynamic fragility is independent of the pressure and specific volume and the isochoric and isobaric fragilities are almost the same. On the other hand for poly(vinylchloride) (PVC), polystyrene (PS) and poly(methyl acrylate) (PMA), the dynamic fragility is sensitive to changes of pressure and volume and the isobaric behavior is more fragile than the isochoric behavior when referenced to the same glass formation points (Pg,Vg,Tg). It is also remarked that the signature Vogel–Fulcher temperature dependence of the isobaric dynamics is lost for the isochoric behavior of both PMA and PVC.
Monte Carlo simulations of interacting magnetic nanoparticles
Authors: Lee H. K. , Schulthess T. C. , Landau D. P. , Brown G. , Pierce J. P. , Gai Z. , Farnan G. A. , Shen J.
Published: July 2002

Motivated by recent advances in synthesis techniques of nanometer size magnetic particles, we have performed Monte Carlo simulations of the magnetic properties of such assemblies of particles. The particles are assumed to be point dipoles, which interact magnetostatically and have uniaxial anisotropy. Spatial distributions of the particles are either generated numerically or are taken from experimental data sets. The properties, such as the remnant magnetization, are studied as a function of temperature for a wide range of parameters. The role of magnetostatic interaction is found to be important in many cases where it has typically been neglected. For example, we find that for a thin film of iron particles with 3.5 nm average diameter and only 13% area coverage, the magnetostatic interactions raise the blocking temperature by 20%, and the particles do not have Stoner–Wolfarth character.
Exchange bias in FexZn1−xF2/Co bilayers
Authors: Shi Hongtao , Lederman David , Fullerton Eric E.
Published: July 2002

The exchange bias (HE) was measured in FexZn1−xF2/Co bilayers grown on MgO (100) with and without a 1.0 nm layer of pure FeF2 at the interface between the FexZn1−xF2 and the Co layers. For x=0.83 samples with an additional 1.0 nm FeF2 interface layer HE increases by 65% when compared to the FeF2/Co bilayer. The FeF2 interface layer also causes a significant HE to appear for x<0.5. This enhancement of HE with an FeF2 interlayer cannot be explained by simply scaling the exchange bias with the Fe concentration. Presumably this is due to the formation of domain states inside the dilute antiferromagnetic film, and the pure interface layer increases the coupling between the ferromagnet and the antiferromagnet.
Fewest-switches with time uncertainty: A modified trajectory surface-hopping algorithm with better accuracy for classically forbidden electronic transitions
Authors: Jasper Ahren W. , Stechmann Samuel N. , Truhlar Donald G.
Published: July 2002

We present a modification of Tully’s fewest-switches (TFS) trajectory surface-hopping algorithm (also called molecular dynamics with quantum transitions) that is called the fewest-switches with time uncertainty (FSTU) method. The FSTU method improves the self-consistency of the fewest-switches algorithm by incorporating quantum uncertainty into the hopping times of classically forbidden hops. This uncertainty allows an electronic transition that is classically forbidden at some geometry to occur by hopping at a nearby classically allowed geometry if an allowed hopping point is reachable within the Heisenberg interval of time uncertainty. The increased accuracy of the FSTU method is verified using a challenging set of three-body, two-state test cases for which accurate quantum-mechanical results are available. The FSTU method is shown to be more accurate than the TFS method in predicting total nonadiabatic quenching probabilities and product branching ratios.
Outgoing multiphonon resonant Raman scattering and luminescence in Be- and C-implanted GaN
Authors: Sun W. H. , Chua S. J. , Wang L. S. , Zhang X. H.
Published: July 2002

We have performed outgoing resonant Raman scattering and photoluminescence measurements on as-grown, Be- and C-implanted GaN in the temperature range of 77–330 K. In implanted GaN after postimplantation annealing at 1100 °C, the A1(LO) multiphonons up to the seventh order were observed with the very strong four longitudinal optical (LO) and five LO modes at ∼2955 and ∼3690 cm−1, respectively, showing extraordinary resonance behavior. With the sample temperature, these two modes significantly decreased and increased in intensity, respectively. The phenomenon is attributed to the variation of resonant conditions due to the shift of the band gap energy. Meanwhile, the combination of E2(high) and quasi-LO phonons was strongly enhanced by quasi-LO phonon involvement and thus the corresponding overtones can be clearly observed even up to the sixth order (m=6). The mechanisms that such strong outgoing multiphonon resonance Raman scattering occurred to implanted GaN instead of high-quality as-grown GaN samples can be attributed to the strong Frohlich-induced scattering by LO phonons and exciton-mediated resonant Raman scattering with impurity inducement.
Continuum spin excitations in S=1 one-dimensional antiferromagnet
Authors: Zaliznyak Igor A.
Published: July 2002

We present detailed measurement of the nonhydrodynamic part of the spin excitation spectrum in the model quasi-one-dimensional S=1 antiferromagnet CsNiCl3 by inelastic magnetic neutron scattering. In the better part of the Brillouin zone the effect of interchain coupling is negligible, and the spin dynamics reflects that of a single Haldane chain. We find that at q≲0.6π this quantum spin system ceases to support coherent propagating excitation, which gradually turns into a continuum band of states, whose width grows with decreasing q. This finding is consistently verified under different resolution conditions, obtained with two different high-luminosity experimental setups.
The SiP molecule: The first observation and spectroscopic characterization
Authors: Jakubek Zygmunt J. , Nakhate S. G. , Simard Benoit
Published: July 2002

SiP molecules have been produced in a molecular free jet apparatus by laser vaporizing a silicon rod in the presence of He doped with phosphine (PH3) gas. Excitation spectra have been observed in the 21200–22350 cm–1 range by monitoring laser induced fluorescence. Dispersed fluorescence spectra have been recorded out of the upper states of the excitation bands. The ground electronic state is X 2Πi with the spin–orbit splitting A=−186.774(11) cm−1, the harmonic wavenumber ωe=615.7(6) cm−1, and the equilibrium internuclear distance re=2.0775(17) Å. Two excited electronic states, A 2Σ+ and B 2Σ+, are observed at T0=427.4(5) and 21317.1(1) cm−1, respectively. The harmonic wavenumbers, ωe, and the equilibrium internuclear distances, re, are 680(2) cm−1 and 1.9658(13) Å, respectively, for the A 2Σ+ state and 455(1) cm−1 and 2.1278(8) Å, respectively, for the B 2Σ+ state. For the X 2Πi, A 2Σ+, and B 2Σ+ electronic states, RKR potentials have been generated. Franck–Condon factors for the B–X, B–A, and A–X systems have been calculated. A 2Σ+, v∼X 2Π1/2, v+1 perturbations have been observed. A deperturbation procedure has yielded the off-diagonal spin–orbit electronic matrix element equal to 51(8) cm−1. This work represents the first spectroscopic investigation of the SiP molecule.
Microwave surface resistance of MgB2
Authors: Zhukov A. A. , Purnell A. , Miyoshi Y. , Bugoslavsky Y. , Lockman Z. , Berenov A. , Zhai H. Y. , Christen H. M. , Paranthaman M. P. , Lowndes D. H. , Jo M. H. , Blamire M. G. , Hao Ling , Gallop J. , MacManus-Driscoll J. L. , Cohen L. F.
Published: July 2002

The microwave power and frequency dependence of the surface resistance of MgB2 films and powder samples were studied. Sample quality is relatively easy to identify by the breakdown in the ω2 law for poor-quality samples at all temperatures. The performance of MgB2 at 10 GHz and 21 K was compared directly with that of high-quality YBCO films. The surface resistance of MgB2 was found to be approximately three times higher at low microwave power and showed an onset of nonlinearity at microwave surface fields ten times lower than the YBCO film. It is clear that MgB2 films are not yet optimized for microwave applications.
Band gap in epitaxial NaCl-structure CrN(001) layers
Authors: Gall D. , Shin C.-S. , Haasch R. T. , Petrov I. , Greene J. E.
Published: July 2002

B1–NaCl-structure CrN(001) layers were grown on MgO(001) at 600 °C by ultrahigh vacuum reactive magnetron sputter deposition in pure N2 discharges. X-ray diffraction analyses establish the epitaxial relationship as cube-on-cube, (001)CrN∥(001)MgO with [100]CrN∥[100]MgO, while temperature-dependent measurements show that the previously reported phase transition to the orthorhombic Pnma structure is, due to epitaxial constraints, absent in our layers. The resistivity increases with decreasing temperature, from 0.028 Ω cm at 400 K to 271 Ω cm at 20 K, indicating semiconducting behavior with hopping conduction. Optical absorption is low (α<2×104 cm−1) for photon energies below 0.7 eV and increases steeply at higher energies. In situ ultraviolet photoelectron spectra indicate that the density of states vanishes at the Fermi level. The overall results provide evidence for CrN exhibiting a Mott–Hubbard type band gap.
Intraband relaxation time in wurtzite InGaN/GaN quantum-well structures with (101̄0) crystal orientation
Authors: Park Seoung-Hwan
Published: July 2002

Intraband relaxation time is investigated theoretically for a wurtzite InGaN/GaN quantum well (QW) with a (101̄0) crystal orientation. The results are compared with those of (0001)-oriented QW structures. It is found that the linewidths for the h–h scattering of the (101̄0)-oriented QW is largely reduced compared to that of the (0001)-oriented QW. This is because the hole effective mass of the (101̄0)-oriented QW is reduced due to the inclusion of the anisotropic strain in the QW plane. In the case of the (0001)-oriented QW, considering only the h–h scattering near subband edges is a good approximation. On the other hand, in the case of the (101̄0)-oriented QW, linewidths for the e–e and e–h scatterings are comparable to that for the h–h scattering. The intraband relaxation times near band edges for (0001)- and (101̄0)-oriented QWs are about 20 and 25 fs at a sheet carrier density of 2×1012 cm−2, respectively. These values are shorter than those (40–100 fs) reported for InP and GaAs.
Kinetics of volume phase transition in poly(N-isopropylacrylamide) gels
Authors: Okajima Takaharu , Harada Ichiro , Nishio Kazufumi , Hirotsu Shunsuke
Published: July 2002

Kinetics of volume phase transition in poly(N-isopropylacrylamide) (NIPA) gels jumped from a low-temperature swollen phase to a high-temperature shrunken phase was studied as functions of NIPA monomer and crosslinker concentrations. We found for the first time a clear kinematical boundary at which the shrinking relaxation time of gels changes discontinuously by 102–104 times, and that the profile of the boundary correlates with the sol-gel transition line and the contour line of turbidity of gels. A “morphological” boundary which characterizes the emergence of the bubble formation on gel surface was also determined. The theoretical calculation of the phase diagram on the basis of the mean field theory shows qualitatively that the shrinking speed of gels could be connected with the depth of the thermodynamic region of the spinodal instability (K+4μ/3=0) into which they are transferred where K and μ are the bulk and the shear moduli, respectively. A mechanism of discontinuous change of the shrinking speed is discussed in connection with the thermodynamic properties as well as the inhomogeneity of network structure.
Effects of long-range interactions in cyclic cluster calculations of metal oxides
Authors: Janetzko Florian , Bredow Thomas , Jug Karl
Published: July 2002

The effect of long-range Coulomb interactions on bulk and surface properties calculated within the cyclic cluster model is investigated for the oxides MgO, TiO2, V2O5, and α-Al2O3. The Madelung potential is calculated with the Ewald summation method. Results of semiempirical calculations using free clusters, cyclic clusters with and without Madelung potential are compared with each other and with available experimental data from the literature. For the cubic system MgO the effect of long-range interactions on calculated binding energies and structure parameters is small, indicating that the Madelung sum is rapidly converging. On the other hand, the inclusion of lattice sums is of high importance for the simulation of anisotropic systems using the cyclic cluster model. The convergence of calculated properties with increasing cluster size is considerably improved by introducing Madelung sums. Therefore, it is possible to use moderately small cyclic clusters to model the bulk and surface properties of a variety of systems with reasonable accuracy.
Nonlinear evolution equation for the stress-driven morphological instability
Authors: Xiang Yang , E Weinan
Published: July 2002

A flat surface of stressed solid is unstable to small perturbations. This morphological instability is called Asaro–Tiller–Grinfield instability. Nonlinear evolution of this instability will result in the formation of cusp singularities. This instability can be described by a continuum model with surface diffusion driven by a stress-dependent chemical potential. The stress and strain in the solid are coupled with surface morphology and an elasticity problem must be solved numerically. We derive a nonlinear approximation equation governing the evolution of this instability in which the stress-dependent chemical potential is expressed explicitly as a function of the surface morphology. Linear instability analysis using our equation shows the same results as the well-known Asaro–Tiller–Grinfeld instability result. Compared with the exact solution of the elasticity problem for cycloid surface obtained by Chiu and Gao, our nonlinear approximation has a much wider range of applicability than linear approximation. Numerical simulation using our nonlinear evolution equation shows that the surface evolves towards a cusplike morphology from small perturbations, which agrees very well with results obtained by solving the full elasticity problem.
Chaotic stirring by a mesoscale surface-ocean flow
Authors: Abraham Edward R. , Bowen Melissa M.
Published: July 2002

The horizontal stirring properties of the flow in a region of the East Australian Current are calculated. A surface velocity field derived from remotely sensed data, using the maximum cross correlation method, is integrated to derive the distribution of the finite-time Lyapunov exponents. For the region studied (between latitudes 36°S and 41°S and longitudes 150°E and 156°E) the mean Lyapunov exponent during 1997 is estimated to be λ∞=4×10−7 s−1. This is in close agreement with the few other measurements of stirring rates in the surface ocean which are available. Recent theoretical results on the multifractal spectra of advected reactive tracers are applied to an analysis of a sea-surface temperature image of the study region. The spatial pattern seen in the image compares well with the pattern seen in an advected tracer with a first-order response to changes in surface forcing. The response timescale is estimated to be 20 days.
Coercivity mechanisms in magneto-optical recording media
Authors: Fu Hong , Giles R. , Mansuripur M.
Published: July 2002

The sources for the nucleation coercivity and wall motion coercivity in magneto-optical recording thin films are investigated based on the Connection Machine simulations. It was assumed that the thin films consist of nanoscale patches which are magnetic structure other than columnar structure or crystal grains. The postulated inhomogeneities have not been observed directly, but we believe that magnetic structure must exist in view of the fact that the media suitable for practical applications do not have columnar structure and crystal grains, yet they exhibit coercivity phenomena which can only be caused by nanoscale inhomogeneities. The nucleation and wall motion processes were simulated in the patchy films with different types of inhomogeneities and with the average patch size ranging from 60 to 200 Å. The simulation results show that the nucleation coercivity depends sensitively on the patch size characterizing the fluctuation of the anisotropy constant, but weakly on the exchange at the patch borders and the fluctuation of easy-axis orientation. To account for the observed nucleation coercivity in magneto-optical thin films the average patch size should be on the order of 100 Å. In contrast, the wall motion coercivity is mostly caused by the fluctuation in the exchange stiffness constant and the patch-to-patch variations of the easy-axis orientation. It was found that the domain wall, when encountering a high anisotropy region, first encircles it and then reverses its magnetization due to the domain wall force. This suggests a scenario other than thermal fluctuation by which a wall can go over a local energy barrier.
Transient cathodoluminescence of semiconductors in a scanning electron microscope
Authors: Jakubowicz A.
Published: July 2002

A theoretical analysis of the cathodoluminescence versus time decay is presented when the electron beam in a scanning electron microscope is turned off. It is shown that the initial part of the decay depends on the surface recombination velocity, the absorption coefficent, and the range of primary electrons. At longer times after the cutoff of the electron beam, the decay approaches a near-exponential behavior, which is controlled only by the bulk lifetime.
Variation of refractive index in strained InxGa1−xAs-GaAs heterostructures
Authors: Das Utpal , Bhattacharya Pallab K.
Published: July 2002

InxGa1−xAs-GaAs heterostructures and strained-layer superlattices can be used as optical waveguides. For such applications it is important to know explicitly the refractive index variation with mismatch strain and with alloying in the ternary layer. Starting from the Kramers-Kronig integral dispersion relations, we have developed a model from which the refractive index change in the ternary layer of InxGa1−xAs-GaAs heterojunctions can be calculated. The results are presented and discussed. The expected changes in a superlattice have been qualitatively predicted.
Stability of amorphous Cu/Ta and Cu/W alloys
Authors: Nastasi M. , Saris F. W. , Hung L. S. , Mayer J. W.
Published: July 2002

Although both the Cu/W and Cu/Ta alloy systems are known to be immiscible, the response of coevaporated amorphous films of a-CuxW1−x (x=0.45–0.72) and a-CuxTa1−x (x=0.10–0.55) alloys to thermal annealing and ion irradiation is very different. Alloys of a-CuxTa1−x crystallize between 600 and 800 °C, with the crystallization temperature increasing with Ta concentration, while a-CuxW1−x alloys crystallize below 200 °C. Likewise in Xe- and Ne-ion irradiation studies, a-Cu/Ta can still be observed after ∼13 displacements per atom (DPA), whereas a-Cu/W alloys completely transform to solid solutions after ∼0.08 DPA. The results of this study, in combination with previous investigations of amorphous alloys of immiscible systems, show that the crystallization temperature Tc is not predicted equally well for systems with a positive or negative heat of formation. It appears that the kinetic model of crystallization only applies if the transformation proceeds through long-range diffusion. Amorphous alloys which have a positive heat of mixing are predicted to undergo a polymorphic transformation at lower temperatures.
Causality and the Keller approximation for acoustic waves
Authors: Beltzer Abraham I.
Published: July 2002

A method is presented for specifying the physically meaningful high-frequency wave velocity (the geometric limit) in the context of the Keller approximation, which describes wave propagation in a weakly inhomogeneous medium. The root regularly chosen is shown to violate causality of the coherent wave.
New intrinsic gettering process in silicon based on interactions of silicon interstitials
Authors: Nauka K. , Lagowski J. , Gatos H. C. , Ueda O.
Published: July 2002

A new intrinsic gettering process was found in silicon crystals subjected to a three-step annealing sequence. The process involves native point defects (silicon interstitials) rather than residual oxygen impurity, and thus it can be realized in crystals with low or virtually zero oxygen concentration. The key characteristics of the process (i.e., denuding and gettering efficiency and the role of annealing ambient) are discussed in conjunction with a kinetic model involving diffusion of silicon interstitials. The identity of the intrinsic gettering centers was pursued by transmission electron microscopy.
An analytical study of etch and etch-stop reactions for GaAs on AlGaAs in CCl2F2 plasma
Authors: Seaward K. L. , Moll N. J. , Coulman D. J. , Stickle W. F.
Published: July 2002

We have studied selective reactive ion etching of GaAs on AlGaAs in CCl2F2 plasma in situ by optical emission spectroscopy and mass spectrometry and have analyzed etched surfaces, before and after air exposure, by x-ray photoelectron spectroscopy. Data from etching GaAs samples indicate that volatile arsenic fluorides, chlorides and fluorochlorides, and gallium chlorides are the products formed, leaving a stoichiometric GaAs surface with adsorbed F and Cl for the particular plasma conditions we used. Data from samples etched to AlGaAs, where the etching process stops, demonstrate that the stopping is due to formation of nonvolatile AlF3 and GaClxFy, leaving a surface nearly depleted of arsenic. This etch-stop ‘‘layer’’ is between 4 and 10 monolayers in thickness. After air exposure this surface consists of gallium and aluminum oxides and a small percentage of arsenic oxide with about the same quantities of Ga, Al, and As as on the surface before exposure to air. This differs from a wet-etched (in dilute ammonium hydroxide and hydrogen peroxide), air-exposed AlGaAs surface for which the composition is rich in arsenic oxide.
Evidence of near-ultraviolet emission during electrical-tree initiation in polyethylene
Authors: Bamji S. S. , Bulinski A. T. , Densley R. J.
Published: July 2002

The spectra of light emitted during electrical-tree initiation in low-density polyethylene used in high-voltage cables were investigated. Light in the visible and in the near-ultraviolet range was detected. Low-density polyethylene subjected to highly divergent fields at voltages below the light-inception level did not develop an electrical tree, and it is suggested that the light-inception voltage is the threshold for insulation degradation. The ultraviolet light emitted at voltages above the light-inception level could cause degradation of the polymer. A model is proposed to explain the mechanism of electroluminescence. Ultraviolet stabilizers added to the polymer prolonged the time to electrical-tree initiation by preventing photodegradation of the polymer.
a-Si:H prepared by ion bombardment activated hydrogen incorporation during Si evaporation
Authors: Strauven H. , Stesmans A. , Winters J. , Spinnewijn J. , Verbeke O. B.
Published: July 2002

a-Si:H is prepared by vacuum evaporation of Si from a crucible kept at high positive potential relative to the substrate. The concomitant bombardment of the growing film surface with ionized vapor atoms (Si+) is found to activate the incorporation of hydrogen atoms cracked off from residual water molecules (pressure=2×10−7 Torr); a-Si:H containing up to 25 at. % of H may thus be obtained. The dark conductivity changes from activated behavior (low conductivity) to hopping (high conductivity) behavior by screening off this bombardment, while the dangling bond content (2×1017<Ns <1018 cm−3), as determined by electron-spin resonance, does not change within an order of magnitude. The hydrogen incorporation into the Si structure is studied by infrared absorption, revealing most of the incorporated hydrogen to prevail near the void surfaces in the Si–H2 configuration. The results point at the existence of an inhomogeneous film structure (microstructure), the specific features of which are controlled by the ion bombardment.
Depth profiling of the minority-carrier diffusion length in intrinsically gettered silicon by electron-beam-induced current
Authors: Donolato C. , Kittler M.
Published: July 2002

A new electron-beam-induced current method is proposed for the depth profiling of nonuniform minority-carrier diffusion lengths in semiconductors. An induced current scan is obtained on a Schottky diode formed on the surface of a beveled sample and converted into a collection efficiency profile η(z). From this profile the depth distribution of the diffusion length L(z) is deduced using a direct reconstruction algorithm. To check consistency, a propagation-matrix based method is developed to calculate η(z) for arbitrary L(z). The model underlying these measurements, in its more general form, holds for an extended generation and includes the presence of the depletion layer. The proposed method is tried both on artificial data and experimental η(z) profiles obtained in intrinsically gettered silicon samples. The results show, in particular, that the trend of L(z) does not, in general, follow that of η(z). The connection between charge-collection efficiency and the diffusive component of the reverse current of a diode with nonuniform L(z) is also discussed.
Growth of thin-film niobium and niobium oxide layers by molecular-beam epitaxy
Authors: Petrucci M. , Pitt C. W. , Reynolds S. R. , Milledge H. J. , Mendelssohn M. J. , Dineen C. , Freeman W. G.
Published: July 2002

The ultrahigh vacuum technique of molecular-beam epitaxy (MBE) has been successfully employed in growing highly oriented polycrystal and single-crystal niobium oxide layers on z-cut LiNbO3 substrates. A new variant of the monoxide has been grown and characterized, and the potential advantages and limitations of MBE for growing the higher oxides, such as Nb2O5, are put forward. Niobium metal layers have also been grown on z-cut LiNbO3. The deposition of excellent quality niobium oxide on α-alumina (z-cut sapphire) has been demonstrated, providing an optical waveguiding structure of large differential refractive index. Crystallinity and compositional data are presented for all the above layers. The post-deposition oxidation of oxygen-deficient films to the pentoxide has been studied in detail, and system requirements for the in situ growth of Nb2O5 proposed.
Ultrahigh magnetic fields produced in a gas-puff Z pinch
Authors: Felber F. S. , Wessel F. J. , Wild N. C. , Rahman H. U. , Fisher A. , Fowler C. M. , Liberman M. A. , Velikovich A. L.
Published: July 2002

Controlled, ultrahigh axial magnetic fields have been produced and measured in a gas-puff Z pinch. A 0.5-MA, 2-cm-radius annular gas-puff Z pinch with a 3-min repetition rate was imploded radially onto an axial seed field, causing the field to compress. Axial magnetic field compressions up to 180 and peak magnetic fields up to 1.6 MG were measured. Faraday rotation of an Argon laser (5154 Å) in a quartz fiber on-axis was the principal magnetic field diagnostic. Other diagnostics included a nitrogen laser interferometer, x-ray diodes, and magnetic field probes. The magnetic field compression results are consistent with simple snowplow and self-similar analytic models, which are presented here. Even small axial fields help stabilize the pinches, some of which exhibit several stable radial bounces during a current pulse. The method of compressing axial fields in a gas-puff Z pinch is extrapolable to the order of 100 MG. Scaling laws are presented. Potential applications of ultrahigh axial fields in Z pinches are discussed for x-ray lasers, inertial confinement fusion, gamma-ray generators, and atomic physics studies.
Temperature-dependent aluminum incorporation in AlxGa1−xAs layers grown by metalorganic vapor phase epitaxy
Authors: van Sark W. G. J. H. M. , Janssen G. J. H. M. , de Croon M. H. J. M. , Tang X. , Giling L. J. , Bik W. M. Arnold , Dunselman C. P. M. , Habraken F. H. P. M. , van der Weg W. F.
Published: July 2002

The temperature-dependent behavior of the solid composition xs of AlxGa1−xAs has systematically been studied as a function of gas phase composition xg in an optimized horizontal metalorganic vapor phase epitaxy reactor at atmospheric pressure. Up to a temperature of 660 °C the Al incorporation is constant but slightly exceeds the Ga incorporation. Above this temperature the Al incorporation strongly increases with temperature. This behavior is most probably related to a change in growth mechanism from mass transport limited growth to a regime where the growth is controlled by thermodynamics, especially for the gallium species.
Magnetic force microscopy: Some remarks from the micromagnetic point of view
Authors: Hartmann U.
Published: July 2002

The present communication focuses on a critical micromagnetic discussion of the promising analytical possibilities with regard to ferromagnetic fine structures offered by the recent invention of magnetic force microscopy. As a result, some limiting constraints for the analysis of soft and hard ferromagnetic media are derived which have to be taken into account for an interpretation of the experimental results.
X-ray and Raman characterization of AlSb/GaSb strained layer superlattices and quasiperiodic Fibonacci lattices
Authors: Macrander A. T. , Schwartz G. P. , Gualtieri G. J.
Published: July 2002

Double-crystal rocking curves of samples grown on (001)-oriented GaSb substrates by molecular-beam epitaxy have been analyzed by fitting computer simulations to data for the symmetric (004) and (002) reflections and for asymmetric (115) reflections. Rocking curves revealed a multiplicity of superlattice diffraction peaks. Dynamical diffraction theory using Abeles matrix method [D. W. Berreman, Phys. Rev. B 14, 4313 (1976)] was applied for the symmetrical reflections. We compare our results to standard kinematical simulations, and we find that there are significant differences. For the asymmetric reflections a new dynamical computer simulation code [D. W. Berreman and A. T. Macrander, Phys. Rev. B 37, 6030 (1988)] involving an 8×8 matrix solution of Maxwell’s equations was used. Lattice incoherency was determined from measurements of the in-plane mismatch. Dramatic diffraction peak broadening was observed for incoherent superlattices, and this broadening was attributed to a mosaic structure formed by misfit dislocations. Peak broadening was used to infer both a growth direction as well as an in-plane coherence length. The in-plane coherence length was found to be somewhat less than the mean distance between misfit dislocation lines. Mosaic broadening of diffraction peaks of a quasiperiodic lattice was found to be qualitatively similar to that observed for the periodic superlattices. Raman measurements of zone-folded acoustic phonon spectra yielded superlattice periods that agreed with the x-ray measurements within a few percent. Analysis of Raman peak intensities to yield individual layer widths was not found to be quantitative within the confines of current analytic models.
Electronic structures of In1−xGaxAs-InP strained-layer quantum wells
Authors: Houng Mau-Phon , Chang Yia-Chung
Published: July 2002

The band structures of In1−xGaxAs-InP strained-layer quantum wells are investigated theoretically in the bond-orbital model. For small x, the well material is subject to a compressive biaxial strain which lifts the HH1 subband further apart from the LH1 subband, resulting in smaller in-plane effective mass for holes. For large x, the strain becomes tensile and the LH1 subband is lifted upward with respect to the HHl subband. For x near the critical value, where the HHl and LHl energy levels cross each other, the valence-band structure undergoes a direct-to-indirect transition.
Long-term stability in the operation of a chemical oxygen-iodine laser for industrial use
Authors: Fujii Hiroo , Yoshida Sanichiro , Iizuka Masahiro , Atsuta Toshio
Published: July 2002

The influence of solution composition to the generation of the excited oxygen O2(1Δ) for a chemical oxygen-iodine laser is studied. Moderate power operation of 100 W for a duration of more than 3 h was achieved. For the achievement of long-term operation, new types of devices for water vapor trapping and new NaCl separation were developed.
Electrical transport and defect properties of Ca- and Mg-doped yttrium aluminum garnet ceramics
Authors: Schuh L. , Metselaar R. , de With G.
Published: July 2002

Electrical transport properties of a sintered yttrium aluminum garnet (YAG) doped with Ca or Mg have been investigated as a function of temperature and partial pressure of oxygen. Conductivity and ionic transport measurements show Ca- and Mg-doped YAG to be mixed ionic-electronic p-type conductors exhibiting an activation energy of about 2 eV. Thermally stable inclusions of alumina, which were found to be present in sintered YAG powders, act as a source of Al vacancies. Al vacancies are dominating the defect chemistry at high oxygen partial pressure. The electronic contribution to total conductivity becomes more pronounced when the temperature increases. At low pO2 a hole-release from O⋅O centers (trapped holes) influences the transport behavior. We developed a possible defect model which explains the experimental data qualitatively. This model considers the influence of dispersed particles of constant activity over a pO2 regime ranging from 105 to 10−12 Pa and their influence on the defect chemistry.
Dependence of the photorefractive properties of KTa1−xNbxO3:Cu,V on Cu valence state concentration
Authors: Leyva Victor , Agranat Aharon , Yariv Amnon
Published: July 2002

Absorption measurements and electron microprobe analysis are used to determine the density of Cu1+ and Cu2+ ions in a photorefractive KTa1−x Nbx O3 :Cu,V sample. Photorefractive measurements are made over a wide range of Cu1+ and Cu2+ concentrations, altered by a series of oxidation and reduction treatments. Diffraction efficiencies were varied by over an order of magnitude while erasure rates were varied by over two orders of magnitude.
Temperature dependence of transition-metal magnetism
Authors: Schwartzman K. , Hartford E. J. , Fry J. L.
Published: July 2002

A first-principles tight-binding formulation of the paramagnetic spin susceptibility is developed to study the temperature dependence of the magnetic properties of transition metals. The formulation, which includes many-body enhancements, has been successful in predicting magnetic ground-state properties. The present study introduces a temperature-dependent analytical tetrahedron method to perform Brillouin zone sums, and employs accurate Slater–Koster band structures for convenience. Critical temperatures and the temperature dependence of the spin-density wave characterizing antiferromagnetism are determined from instabilities of the paramagnetic state toward magnetic order. The Néel temperature of Cr and the Curie temperature of Fe found in this way are in good agreement with measured values. Nonzero critical temperatures are also found for alternate cubic structures of some transition metals, further supporting the prediction of ferromagnetism in bcc Mn, and antiferromagnetism in fcc Mn and expanded bcc Ti. The computed wave vector of the spin-density wave of Cr is found to increase along the Δ line with increasing temperature following observed trends.
Exponential critical-state model for magnetization of hard superconductors
Authors: Chen D.-X. , Sanchez A. , Muñoz J. S.
Published: July 2002

We have calculated the initial magnetization curves and hysteresis loops for hard type-II superconductors based on the exponential-law model, Jc(Hi) =k exp(−‖Hi‖/H0), where k and H0 are constants. After discussing the general behavior of penetrated supercurrents in an infinitely long column specimen, we define a general cross-sectional shape based on two equal circles of radius a, which can be rendered into a circle, a rectangle, or many other shapes. With increasing parameter p (=ka/H0), the computed M-H curves show obvious differences with those computed from Kim’s model and approach the results of a simple infinitely narrow square pulse Jc(Hi). For high-Tc superconductors, our results can be applied to the study of the magnetic properties and the critical-current density of single crystals, as well as to the determination of the intergranular critical-current density from magnetic measurements.
Numerical simulation of mesoscopic systems with open boundaries using the multidimensional time-dependent Schrödinger equation
Authors: Register Leonard F. , Ravaioli Umberto , Hess Karl
Published: July 2002

A numerical procedure based on the time-dependent Schrödinger equation for the modeling of multidimensional mesoscopic devices is presented. The primary features of this numerical method are a tight-binding formulation of the quantum mechanical Hamiltonian, an alternating direction implicit generalization of the Crank–Nicholson method for solving the discretized multidimensional time-dependent Schrödinger equation, and a numerical implementation of absorbing boundary conditions for propagating wavefunctions at the open boundaries of the simulated region. To show the capabilities of the absorbing boundary scheme, numerical results are presented for the diffusion of localized wavepackets out of open simulation regions. As an application to mesoscopic systems, results are presented for the switching in a T-structure quantum modulated transistor with a continuous input wavefunction.
Thermoelectric properties of pressure-sintered Si0.8Ge0.2 thermoelectric alloys
Authors: Vining Cronin B. , Laskow William , Hanson Jack O. , Van der Beck Roland R. , Gorsuch Paul D.
Published: July 2002

The thermoelectric properties of 28 sintered Si0.8 Ge0.2 alloys, heavily doped with either boron or phosphorus and prepared from powders with median particle sizes ranging from about 1 μm to over 100 μm, have been determined from 300 to 1300 K. The thermal conductivity decreases with decreasing particle size, however, the figure of merit is not significantly increased due to a compensating reduction in the electrical conductivity. The thermoelectric figure of merit is in good agreement with results of Dismukes et al. [J. Appl. Phys. 10, 2899 (1964)] on similarly doped alloys prepared by zone-leveling techniques. The electrical and thermal conductivity are found to be sensitive to preparation procedure while the Seebeck coefficient and figure of merit are much less sensitive. The high-temperature electrical properties are consistent with charge carrier scattering by acoustic or optical phonons.
Use of Fabry–Pérot devices for the characterization of polymeric electro-optic films
Authors: Eldering Charles A. , Knoesen André , Kowel Stephen T.
Published: July 2002

Electro-optic properties of thin polymeric films can be studied by incorporating the films into the spacer layers of transmission-mode Fabry–Pérot devices formed using metal mirrors. The metal mirrors, which also serve as electrodes, are used for the application of an electric field for poling the polymeric material and for subsequently modulating the index of refraction through the electro-optic effect. A model for transmission modulation is developed, and experimental characterization of the devices by angles tuning at a fixed wavelength is presented. Suitable approximations can be made to allow determination of the net trapped charge and the electro-optic coefficients of the spacer layer. This paper demonstrates the use of polymeric thin-film étalon devices for the study of new materials and indicates their potential use as electro-optic devices.
The thermal stability of SiGe films deposited by ultrahigh-vacuum chemical vapor deposition
Authors: Stiffler S. R. , Comfort J. H. , Stanis C. L. , Harame D. L. , de Frésart E. , Meyerson B. S.
Published: July 2002

The thermal stability of SiGe films deposited by ultrahigh-vacuum chemical vapor deposition was studied. Various Ge compositional profiles, including boxes, trapezoids, and triangles were examined. Planar-view transmission electron microscopy was performed following growth and after furnace annealing at 950 °C for 30 min to determine the presence and density of misfit dislocations. All profiles showed very similar stability behavior when expressed in terms of the total thickness of the film, heff, and the effective strain present in the layer, εeff. Following the anneal, misfit dislocations were observed when heff exceeded the critical thickness, as defined by Matthews and Blakeslee [J. Cryst. Growth 27, 118 (1974)], by a factor of ∼2.
The effects of grating size on acoustic charge transport properties on GaAs
Authors: Schmukler Bruce C.
Published: July 2002

Gratings in the channels of acoustic charge transport devices on GaAs create perturbing electric fields that can upset the charge transport process, principally by reducing the charge carrying capacity. An experimental device is designed to test the effect of 1.0, 1.3, and 1.6 μm gratings on the acoustic charge transport properties. The measured charge capacity losses are found to be less than the measurement tolerances, for transport parameters typical of delay line devices. In addition, for parameters typical of charge storage devices, the measured charge capacity losses are higher, but are still low. Furthermore, an approximate theory to predict the charge capacity loss is presented; its predictions are in reasonable agreement with experimental results.
Thermal fluctuations in a pair of magnetostatically coupled particles
Authors: Lyberatos A. , Chantrell R. W.
Published: July 2002

The influence of the thermal agitation on the switching dynamics for a pair of identical uniaxially anisotropic dipoles is studied for the case of the applied field parallel to the bond direction and the common anisotropy axis. A set of Langevin equations was derived from the micromagnetic energy expression and solved numerically. The switching behavior resembles a random walk over the energy barrier arising from the anisotropy of the system. The relaxation time is computed as a function of temperature, applied field, and coupling strength. The temperature dependence of the maximum energy of the fluctuations provides a method of evaluating the energy barrier of reversal. The thermal agitation is shown to reduce the symmetry of the ‘‘fanning’’ reversal mode.
X-ray diagnostics of a plasma-jet–liquid interaction in electrothermal guns
Authors: Arensburg A. , Wald S. , Goldsmith S.
Published: July 2002

The penetration of a plasma jet into liquid water was observed at successive time intervals by means of x-ray shadowgraphy. The plasma jet was generated by producing in a polyethylene capillary tube a high-current pulsed discharge. The tube inner diameter and length were 0.64 and 22 cm, respectively. A pulse-forming network delivered 121–182 kJ of electrical energy in 500 μs. The plasma jet emerged from the open end of the tube and interacted with a column of water gel. Jet velocities around 250 m/s were measured by x-ray shadowgraphy. The water ablation rate, estimated by calculating the radiative energy flux emitted by the plasma and reaching the water surface, is approximately 0.0155 kg/s cm2. This rate is significantly lower than the measured average flux of mass lost by the water: 3.5 kg/s cm2. It is proposed, but not proven conclusively, that the main mechanism for water loss is in the form of droplets pulled apart by shear forces impressed by the plasma-water direct interaction. These droplets are entrained in the back-flowing part of the plasma jet, which streams out of the interaction chamber. Peak pressures up to 3.5×108 Pa were measured during the process.
Considerations on the physical interpretation of the Preisach model of ferromagnetic hysteresis
Authors: Bertotti G. , Basso V.
Published: July 2002

The physical meaning of the standard Preisach model (PM) and of the Preisach distribution p(α,β), particularly for what concerns the problem of the characterization of magnetic interactions, is discussed. It is shown that the standard PM can only describe interactions having a net demagnetizing effect, and that, when the factorization p(α,β)=f(α)f(−β) holds, the simple relation Id/I∞ = 1 − 2√Ir/I∞, independent of f(x), exists between the Henkel plot remanences Ir and Id. These predictions are compared with recent experimental results. Finally, the relation between magnetic interactions and hysteresis loop shapes is discussed in the frame of a generalized moving PM, where the relation between the applied field Ha and the internal PM field H is of the form H=Ha+Hi(I), where Hi(I) is an odd function of the magnetization I. The shape of p(α,β) and of Hi(I) for nonoriented SiFe alloys is determined and analyzed.
Two-dimensional simulation of switch-on speeds in hydrogenated amorphous silicon thin-film transistors
Authors: Huang J. S. , Wu C. H.
Published: July 2002

We report accurate two-dimensional simulations of switch-on speeds in hydrogenated amorphous silicon thin-film transistors. The trap charge density along, or transverse to, the direction of semiconductor channel is highly nonuniform and the trap filling time dominates the switching time as compared to the transit time, which is about four orders of magnitude smaller. Near both contacts, direction of the transverse current is always upwards toward the insulator-semiconductor interface due to the strong electric fields. However, at the central region of the channel, the transient current is quite complex and is discussed here. When the channel length varies from 2 to 10 μm, the switching-on time is of the order of 10−3 s. The occupation function everywhere displays a partial filling of higher-energy trap states during the switch-on. This is in contrast to results presented by other investigators. Finally, the relationship between the transit time and the switch-on time with respect to the amount of trap states is discussed.
Optical properties of bulk AlxGa1−xAs
Authors: Lin Chih-Hsiang , Meese J. M.
Published: July 2002

An analytical, semiempirical model is presented for the optical properties of AlxGa1−xAs for photon energies from 1.2 to 6 eV. For GaAs, the discrepancy between the calculated refractive index and the experimental data is about 0.003 at 1.8 eV, and is about 0.012 at 1.2 eV. For all the other AlxGa1−xAs alloys, the calculated optical properties are in satisfactory agreement with the experimental data for photon energies from 1.2 to 6 eV.
Bicrystal advanced thin-film media for high density recording
Authors: Min Tai , Zhu Jian-Gang
Published: July 2002

A new type of thin-film recording media with advanced microstructure: the bicrystal film, has been studied experimentally. The easy axes of the bicrystal films are oriented along two specified orthogonal directions in the film plane. In this study, an LiF single crystal substrate with (100) orientation is utilized to obtain the Cr monocrystal underlayer with (100) orientation, and on top of the (100)Cr underlayer the CoCrTa with (112̄0) texture was grown epitaxially. X-ray and transmission electron microscopy (TEM) diffraction studies confirm the fabricated thin-film media has the bicrystal microstructure. The magnetic properties of the bicrystal films have demonstrated very high coercivity (HC∼3000 Oe) and coercive and remanent squareness (S*∼0.9, S∼0.9), all desired for high areal density magnetic recording. The in-plane hysteresis loops of the fabricated films show fourfold symmetry which is the characteristic of the bicrystal microstructure. The effects of Cr underlayer thickness on the magnetic properties of the bicrystal film has also been studied.