bluuebunny/crossref_metadata_2025 huggingface dataset:
Photoemission spectroscopy of size selected zinc sulfide nanocrystallites
Authors: Nanda J. , Sarma D. D.
Published: July 2002

We report photoemission spectroscopic studies of zinc sulfide nanocrystallites in the quantum size regime. The nanocrystallites studied have average sizes of 1.8, 2.5, and 3.5 nm and narrow size distributions as determined from UV-visible absorption spectroscopy, as well as x-ray diffraction and high-resolution transmission electron microscopy. Analysis of sulfur core levels from the nanocrystallites show the presence of the three types of sulfur species corresponding to the core, the surface, and the capping layer of the nanocrystallites. We show that a quantitative analysis of these different sulfur components can be used to estimate the sizes of the nanocrystallites; thus, the obtained sizes are in good agreement with the sizes determined independently from small angle x-ray diffraction and high-resolution electron microscopy.
Observation of proton rear emission and possible gigagauss scale magnetic fields from ultra-intense laser illuminated plastic target
Authors: Murakami Yoshihiro , Kitagawa Yoneyoshi , Sentoku Yasuhiko , Mori Michiaki , Kodama Ryosuke , Tanaka Kazuo A. , Mima Kunioki , Yamanaka Tatsuhiko
Published: July 2002

CR-39 film stacks are used to measure the energy and angular distribution of protons emitted from the rear surface of ultra-intense laser illuminated plastic foils. The experiment suggests that the energetic protons are dragged away from the rear surface, where the hot electron formed a virtual cathode. The two-dimensional particle in cell simulation supports this hypothesis. For 5 (100) μm thick target, 1.8×109 protons have a slope temperature of 3 (2) MeV. The ring structure of proton emission leads us to the hypothesis that a toroidal magnetic field associated with the hot electrons works on the fast ions and deflects them. From the logarithmic slope of the ring diameter versus the ion energy, the product of the magnetic field × the length over which it works on the ions is estimated to be up to 1000 MG⋅μm. The simulation shows that a strong toroidal magnetic field was excited at the target rear side with expansion of plasmas. The proton’s angular distribution from the rear surface has the logarithmic slope similar to the experimental results due to this magnetic field.
Piezoelectric instability in 〈011〉-oriented Pb(B1/3IB2/3II)O3–PbTiO3 crystals
Authors: Viehland D. , Amin A. , Li J. F.
Published: July 2002

Piezoelectric investigations of 〈011〉-oriented Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT) and Pb(Zn1/3Nb2/3)O3–PbTiO3 (PZN–PT) crystals have revealed high electromechanical properties. Piezoelectric constants of up to 5000–6000 pC/N and electromechanical coupling coefficients of up to 0.95 have been observed near the ferroelectric rhombohedral to ferroelectric tetragonal transformation. The data clearly demonstrate that the enhanced electromechanical performance of oriented piezocrystals is not constrained to the 〈001〉 orientation. Furthermore, analysis of the data has demonstrated the presence of piezoelectric instability between two ferroelectric states at a critical transformation temperature and/or electric field.
A general approach to the electronic spin relaxation of Gd(III) complexes in solutions. Monte Carlo simulations beyond the Redfield limit
Authors: Rast S. , Fries P. H. , Belorizky E. , Borel A. , Helm L. , Merbach A. E.
Published: July 2002

The time correlation functions of the electronic spin components of a metal ion without orbital degeneracy in solution are computed. The approach is based on the numerical solution of the time-dependent Schrödinger equation for a stochastic perturbing Hamiltonian which is simulated by a Monte Carlo algorithm using discrete time steps. The perturbing Hamiltonian is quite general, including the superposition of both the static mean crystal field contribution in the molecular frame and the usual transient ligand field term. The Hamiltonian of the static crystal field can involve the terms of all orders, which are invariant under the local group of the average geometry of the complex. In the laboratory frame, the random rotation of the complex is the only source of modulation of this Hamiltonian, whereas an additional Ornstein–Uhlenbeck process is needed to describe the time fluctuations of the Hamiltonian of the transient crystal field. A numerical procedure for computing the electronic paramagnetic resonance (EPR) spectra is proposed and discussed. For the [Gd(H2O)8]3+ octa-aqua ion and the [Gd(DOTA)(H2O)]− complex [DOTA=1,4,7,10-tetrakis(carboxymethyl)-1,4,7,10-tetraazacyclo dodecane] in water, the predictions of the Redfield relaxation theory are compared with those of the Monte Carlo approach. The Redfield approximation is shown to be accurate for all temperatures and for electronic resonance frequencies at and above X-band, justifying the previous interpretations of EPR spectra. At lower frequencies the transverse and longitudinal relaxation functions derived from the Redfield approximation display significantly faster decays than the corresponding simulated functions. The practical interest of this simulation approach is underlined.
Single-crystal, mesoscopic films of lead zinc niobate-lead titanate: Formation and micro-Raman analysis
Authors: Spanier Jonathan E. , Levy Miguel , Herman Irving P. , Osgood Richard M. , Bhalla Amar S.
Published: July 2002

A process to form thin films of lead zinc niobate–lead titanate (PZN–PT) from a bulk crystal for microelectronic and microelectromechanical device applications is presented. The structural phase transitions and ferroelectric ordering in unpoled crystalline bulk and thin-film relaxor PZN–PT are studied from −190 °C to 600 °C using polarized micro-Raman scattering. The structural phase transitions in this material are observed by distinct changes in the polarization selectivity. The results for the thin film and bulk crystal are in good agreement for a wide range of the temperatures studied, indicating that the thin-film PZN–PT retains much, if not all, of the structural and ferroelectric properties of the original bulk substrate.
Surface smoothing by energetic cluster impact
Authors: Rattunde Oliver , Moseler Michael , Häfele Andreas , Kraft Jürgen , Rieser Daniel , Haberland Hellmut
Published: July 2002

This article reports on experimental observations of surface smoothing by high energy cluster impact. Thin films have been produced by energetic cluster impact deposition (ECI), and the surface roughness and the power spectrum of the films have been measured by atomic force microscopy. By depositing large metal clusters onto an initially rough substrate, the surface roughness is significantly reduced. On the other hand, the roughness of initially smooth surfaces increases only logarithmically. Results for both surface roughness and power spectrum can be quantitatively explained by a recently developed mesoscopic model for the ECI process employing a stochastic differential equation. In this model the smoothing effect results from a downhill particle current (consisting of both cluster and surface atoms) transferring the higher lying parts of the surface profile into the valleys.
Reconstructing x-ray fluorescence microtomograms
Authors: Schroer Christian G.
Published: July 2002

X-ray fluorescence microtomography allows one to map element distributions inside a sample with high sensitivity and resolutions in the micrometer range. Quantitative reconstruction of the element concentrations from the fluorescence data requires correction for the attenuation inside the sample. However, the attenuation of the fluorescence radiation is not directly accessible by experiment. The method described self-consistently estimates this attenuation and allows one to reconstruct relative concentrations. This is demonstrated on numerical as well as experimental data. A measure for the quality of the reconstruction is given.
Lateral composition modulation in GaP/InP short-period superlattices grown by solid source molecular beam epitaxy
Authors: Song Jin Dong , Ok Young-Woo , Kim Jong Min , Lee Yong Tak , Seong Tae-Yeon
Published: July 2002

Transmission electron microscopy (TEM) is employed to investigate the structural properties of (GaP)/(InP) short-period superlattices (SPS) grown at temperatures in the range of 425–490 °C by solid source molecular beam epitaxy. TEM results show that lateral composition modulation (LCM) is formed in the SPS layers grown at temperatures above 460 °C. Transmission electron diffraction results show that CuPt ordering occurs in all samples. It is shown that the degree of order increases, reaches a maximum at 460 °C, and then decreases, as the growth temperature increases. Photoluminescence examination (at 9 K) shows that the samples experience a reduction in band gap, which is in the range of 55–221 meV, as the growth temperature increases. The reduction is attributed to the combined effects of the LCM and CuPt-type ordering.
Easy axis dispersion and micromagnetic structure of electrodeposited, high moment Fe–Co–Ni films
Authors: Liu Xiaomin , Zangari Giovanni
Published: July 2002

The magnetic properties of soft films are usually affected by extrinsic properties such as easy axis dispersion and domain structure, which are dependent on deposition conditions, microstructure, alloy composition, and internal stresses. In this work, the anisotropy dispersion of electrodeposited, high moment Fe–Co–Ni alloys of various compositions is investigated by measuring the angular dependence of the remanence, Mr. These data are correlated with the micromagnetic structure of the films as determined by magnetic force microscopy. Films of type A (saturation magnetostriction λs>0, high; internal stress σ>0, high), B (λs>0, low; σ>0, low), and C (λs<0; σ>0) have been grown at increasing current density in the presence of a magnetic field and show increasing Ni content. Type A films exhibit a large dispersion of the easy axis (α90∼51°). A relatively large coercivity (∼25 Oe) was obtained and high remanence was observed along the hard axis, correlated with a ripple structure. Type B films show well-defined uniaxial anisotropy with small easy axis dispersion (α90∼3.0°). Stripe domains were formed in type C films due to the stress anisotropy induced perpendicularly to the film plane. Type B films show the best potential for application in high frequency magnetic devices, in particular, recording write heads, due to the high saturation magnetization and well-defined uniaxial anisotropy.
Characteristics of laser-driven electron acceleration in vacuum
Authors: Wang P. X. , Ho Y. K. , Yuan X. Q. , Kong Q. , Cao N. , Shao L. , Sessler A. M. , Esarey E. , Moshkovich E. , Nishida Y. , Yugami N. , Ito H. , Wang J. X. , Scheid S.
Published: July 2002

The interaction of free electrons with intense laser beams in vacuum is studied using a three-dimensional test particle simulation model that solves the relativistic Newton–Lorentz equations of motion in analytically specified laser fields. Recently, a group of solutions was found for very intense laser fields that show interesting and unusual characteristics. In particular, it was found that an electron can be captured within the high-intensity laser region, rather than expelled from it, and the captured electron can be accelerated to GeV energies with acceleration gradients on the order of tens of GeV/cm. This phenomenon is termed the capture and acceleration scenario (CAS) and is studied in detail in this article. The accelerated GeV electron bunch is a macropulse, with duration equal to or less than that of the laser pulse, which is composed of many micropulses that are periodic at the laser frequency. The energy spectrum of the CAS electron bunch is presented. The dependence of the energy exchange in the CAS on various parameters, e.g., a0 (laser intensity), w0 (laser radius at focus), τ (laser pulse duration), b0 (the impact parameter), and θi (the injection angle with respect to the laser propagation direction), are explored in detail. A comparison with diverse theoretical models is also presented, including a classical model based on phase velocities and a quantum model based on nonlinear Compton scattering.
Quasielastic neutron scattering study of the methyl group dynamics in polyisoprene
Authors: Zorn R. , Frick B. , Fetters L. J.
Published: July 2002

In this paper the microscopic dynamics of methyl side groups in polyisoprene is studied by means of inelastic neutron scattering. By combining time-of-flight and backscattering technique a range of four decades can be obtained (0.2 ps–2 ns). The two experimental results were combined in the time domain by using an inverse Fourier transform. Multiple scattering effects were treated by a novel procedure acting on the time-dependent intermediate scattering function S(Q,t). In the description of the data incoherent and coherent scattering from “fixed” atoms was taken into account, i.e., atoms in the main chain that move too slow to be observed in the dynamical window of the experiment. In this way good agreement with the rotation rate distribution model of a threefold jump could be obtained. Seeming discrepancies of the elastic incoherent structure factor vanish after the corrections mentioned above. The distribution of activation energies can be expressed as a Gaussian with an average of 9.7 kJ/mol and a width of 30%. It turns out that the width calculated by the model fit of the data depends on the use of the multiple scattering correction while the average activation energy can also be obtained reliably without that correction from high Q spectra.
Giant photoresistivity and optically controlled switching in self-assembled nanowires
Authors: Kouklin N. , Menon L. , Wong A. Z. , Thompson D. W. , Woollam J. A. , Williams P. F. , Bandyopadhyay S.
Published: July 2002

We report the observation of giant photoresistivity in electrochemically self-assembled CdS and ZnSe nanowires electrodeposited in a porous alumina film. The resistance of these nanowires increases by one to two orders of magnitude when exposed to infrared radiation, possibly because of real-space transfer of electrons from the nanowires into the surrounding alumina by photon absorption. This phenomenon has potential applications in “normally on” infrared photodetectors and optically controlled switches.
High voltage pulse discharge for ion treatment of metals
Authors: Dougar-Jabon V. D. , Moreno J. Dulce , Tsygankov P. A.
Published: July 2002

It is well known that the production of semiconductors and chips with required parameters and properties is based on ion beam technology. However, the industrial use of ion beams still remains a problem because it requires a high rate and homogeneous treatment of the samples of complex configuration. Plasma technologies can make an efficient contribution to improving the tribological and corrosion characteristics of the most important workpieces for industrial machines. In the present report, consideration is given to the high voltage discharge of 3DII type at the pressure of 0.2–0.9 Pa with 10–30 kV pulse length of 0.25 ms and the repetition frequency of 30 Hz. We have defined such characteristics of the discharge as the volt-ampere curve at the mentioned pressure range, the range of discharge existence, and the thickness of the cathode sheath. The comparison of the profiles of implanted nitrogen ions at 10 kV pulse voltage obtained by the secondary ion mass spectroscopy CAMECA method with the results of the TRIM simulation, as well as the analysis of the voltage and current discharge pulses demonstrate that the ion flux incident on the workpiece is quasimonoenergetic and the angle of surface bombardment is normal.
Electrochromic semiconductor nanocrystal films
Authors: Wang Congjun , Shim Moonsub , Guyot-Sionnest Philippe
Published: July 2002

Electron injection into nanocrystal thin films is reversibly controlled by applying an electrochemical potential. Complete bleach of the visible interband transition and strong midinfrared intraband absorption are observed upon electron injection into the nanocrystal films. Quenching of the photoluminescence of the nanocrystal film is also observed.
Long-range superluminal pulse propagation in a coaxial photonic crystal
Authors: Haché Alain , Poirier Louis
Published: July 2002

We study the propagation of brief electric pulses along a coaxial line having a spatially periodic impedance. The periodicity causes anomalous dispersion and the appearance of a stop band in transmission near 10 MHz. Group velocities of up to three times the speed of light are observed in that spectral region, in accordance with calculations based on an effective index theory.
Experimental study on the feasibility of hot plasmas as stripping media for MeV heavy ions
Authors: Shibata K. , Tsubuku K. , Nishimoto T. , Hasegawa J. , Ogawa M. , Oguri Y. , Sakumi A.
Published: July 2002

The feasibility of using hot plasmas as strippers for low-energy ion beams has been investigated by measuring charge state distributions of 350 keV/u ions (12C,16O,19F) after their passage through a laser-produced plasma target. The plasma target was produced by irradiating a small pellet of lithium hydride with a Nd-glass laser. The profiles of electron densities of the plasma target were estimated from the intensity profiles of an Ar laser refracted by the plasma. The intensities of ions with different charge states were simultaneously measured using a time-resolved magnetic spectrograph. It was found that this plasma can yield higher charge states than conventional gaseous or solid strippers. Results of a numerical analysis are compared with the experimental data to explain the observed stripping capability of the plasma.
Thermomechanical analysis of the Taylor impact test
Authors: Celentano Diego J.
Published: July 2002

A coupled thermomechanical analysis of the Taylor impact test considering flat-ended cylindrical specimens of different materials fired at several striking velocities is presented. To this end, a large strain rate-dependent thermoelasto–plasticity model is used to define the constitutive relationships appearing in the momentum and energy equations both solved in the context of the finite element method. The influence of the strain, temperature, and strain rate effects on the material response is particularly analyzed. The numerical predictions are experimentally validated and compared with results provided by previously reported simulations and simplified analytical expressions.
Focusing multilayer mirror detection system for carbon K edge soft x-ray absorption spectroscopy (invited)
Authors: Fischer D. A. , Sambasivan S. , Kuperman A. , Platonov Y. , Wood J. L.
Published: July 2002

Fluorescence yield carbon K edge soft x-ray absorption measurements of mixed element samples are impeded by background scattered x-rays and fluorescence from noncarbon atoms within the sample induced by the incident x-ray beam (first and higher orders). We describe a high efficiency near-normal incidence focusing multilayer mirror detection system for carbon K edge fluorescence yield soft x-ray absorption spectroscopy. A spherical focusing multilayer mirror collects nearly 60% of the available solid angle with a reflectivity of 6% and a resolution of 2% (dE/E) for carbon K radiation in combination with a high efficiency proportional counter detector. The focusing multilayer system is able to effectively discriminate the background fluorescence and scattered light signals originating at the sample during a carbon K edge soft x-ray absorption experiment. A signal-to-background ratio in excess of 100 is possible for very dilute carbon samples, enabling a whole new class of in situ photon-in photon-out soft x-ray absorption measurements of carbon chemistry in catalysts to be made, even in a reactive chemical environment.
Depolarizing-field-mediated 180° switching in ferroelectric thin films with 90° domains
Authors: Roelofs A. , Pertsev N. A. , Waser R. , Schlaphof F. , Eng L. M. , Ganpule C. , Nagarajan V. , Ramesh R.
Published: July 2002

Switching of the out-of-plane and in-plane polarizations in polydomain epitaxial PbZr0.2Ti0.8O3 thin films is studied using three-dimensional piezoresponse force microscopy (PFM). It is found that, under an electric field induced between the PFM tip and the bottom electrode, the 180° switching occurs in both c and a domains. After the removal of this field, the spontaneous reversal of the out-of-plane and in-plane polarizations back to the initial orientations takes place, evolving via heterogeneous development of antiparallel 180° domains. The switching of in-plane polarization inside a domains and the preferential formation of reversed 180° domains at 90° domain walls are explained by the effects of the depolarizing fields caused by transient polarization charges appearing on these domain walls.
Fluorescence properties of divalent and trivalent europium ions in aluminosilicate glasses
Authors: Song Hongwei , Lu Shaozhe , E Shulin , Gao Renxi , Zhang JiaHua , Chen Baojiu , Xia Haiping , Zhang Jianlin , Ni Qiuhua
Published: July 2002

Optical absorption, fluorescence, excitation and time-resolved spectra, and fluorescence dynamics were studied in Eu2+- and Eu3+-codoped aluminosilicate glasses. The results indicated that the energy transfer occurred between Eu2+ and Eu3+ ions, in which the energy corresponding to the 4f65d to S87/2 transition of Eu2+ ions caused excitation of the Eu3+ ions to the D5J level. Some of the electrons in the ground state F70 were thermally excited into the F71 state at room temperature, and thermal populations of the F70 level had great effects on the spectral shape of the D30−F72 emission. The effect of light irradiation at different wavelengths on fluorescence was systemically studied. The visible, ultraviolet and x-ray irradiation caused a decrease in the fluorescence of Eu2+ and/or Eu3+ ions. Frequency selective excitation experiments were performed at 10 K and room temperature. Instead of forming a spectral hole, the intensity of the F50−D70 transition decreased as a whole. This was attributed to the interaction of Eu3+ ions with the two-level systems existing in the glass network.
Spinodal decomposition in BxGa1−xN and BxAl1−xN alloys
Authors: Teles L. K. , Scolfaro L. M. R. , Leite J. R. , Furthmüller J. , Bechstedt F.
Published: July 2002

We present first-principles calculations of the structural and thermodynamic properties of cubic BxGa1−xN and BxAl1−xN alloys. The calculations are based on the generalized quasichemical approach to disorder and composition effects and a pseudopotential-plane-wave approximation. The bulk moduli and lattice constants are found to vary linearly with the alloy composition. Due to the large lattice mismatch between BN and binaries GaN and AlN, the phase diagrams display large miscibility gaps in the temperature range usually adopted to grow the corresponding alloys. This explains the difficulties reported in growing these alloys with boron content higher than 0.1.
Optical control of gallium nanoparticle growth
Authors: MacDonald K. F. , Fedotov V. A. , Pochon S. , Ross K. J. , Stevens G. C. , Zheludev N. I. , Brocklesby W. S. , Emel’yanov V. I.
Published: July 2002

We report that low-intensity light can dramatically influence and regulate the nanoparticle self-assembly process: Illumination of a substrate exposed to a beam of gallium atoms results in the formation of gallium nanoparticles with a relatively narrow size distribution. Very low light intensities, below the threshold for thermally induced evaporation, exert considerable control over nanoparticle formation.
One-stage identification algorithm and two-step compensation method of Hammerstein model with application to wrist force sensor
Authors: Xu Ke-Jun , Jia Lin
Published: July 2002

There is nonlinearity in the dynamic characteristics of sensors under some conditions. Previous researchers have presented the nonlinear dynamic models of sensors only using a block, which make it difficult to compensate the nonlinear dynamic responses of sensors. In this article a Hammerstein model is adopted to describe the nonlinear dynamic models of sensors, and a one-stage identification algorithm is proposed to simplify the calculation. On this basis a two-step compensation method is presented for the nonlinear dynamic responses of sensors. The one-stage identification algorithm has advantages as follows: (1) One-stage identification simplifies the algorithm; (2) it depends only on the data of input and output of the system, and does not need to introduce auxiliary variables that could not be measured in practice; (3) it only needs dynamic calibration experimental data of systems, and does not need to do static calibration experiments. The simulation results and its application in the wrist force sensor show the methods proposed by this article are effective.
Structural determination of titanium-oxide nanoparticles by x-ray absorption spectroscopy
Authors: Wu Z. Y. , Zhang J. , Ibrahim K. , Xian D. C. , Li G. , Tao Y. , Hu T. D. , Bellucci S. , Marcelli A. , Zhang Q. H. , Gao L. , Chen Z. Z.
Published: July 2002

To understand and improve the applications of titanium-oxide nanoparticles, it is extremely important to perform a detailed investigation of the surface and the interior structural properties of nanocrystalline materials, such as rutile and anatase with diameter of few nanometers. Here, x-ray absorption spectroscopy has been used to identify the local Ti environment and characterize the related electronic structure. We combine experimental results at the Ti K edge in both bulk and nanocrystal samples to determine the lattice distortion via the characteristic pre-edge features and the variation in the multiple-scattering region of the x-ray absorption near-edge structure spectra. The correlation between peak intensities and surface-to-volume ratio of nanoparticles is also discussed.
Origin of low-frequency noise in polycrystalline silicon thin-film transistors
Authors: Dimitriadis C. A. , Farmakis F. V. , Kamarinos G. , Brini J.
Published: July 2002

Low-frequency drain current fluctuation noise was measured in a series of polycrystalline silicon thin-film transistors (polysilicon TFTs), characterized by an average grain size Lg and in-grain defect density Din. In one type of polysilicon TFT, Din is low (about 5×108 cm−2) and Lg varies from about 67 to 145 nm. In another type of polysilicon TFT, Lg remains constant (about 2.5 μm) and Din varies from about 5×1012 to 5×1010 cm−2. It is demonstrated that the noise originates from fluctuations of carriers due to carrier capture/release processes: (a) in traps located near the polysilicon/SiO2 interface and (b) in traps located at the grain boundaries. The polysilicon/SiO2 interface is distinguished from that of the crystalline silicon/SiO2 by comprising the in-grain traps, in addition to the oxide traps. The proposed carrier number fluctuation model, sufficient to explain noise in polysilicon TFTs, is closely related to the structural parameters Lg and Din of the polysilicon layer.
Characteristics of MgO/GaN gate-controlled metal–oxide– semiconductor diodes
Authors: Kim Jihyun , Mehandru R. , Luo B. , Ren F. , Gila B. P. , Onstine A. H. , Abernathy C. R. , Pearton S. J. , Irokawa Y.
Published: July 2002

Gate-controlled n+p metal–oxide–semiconductor diodes were fabricated in p-GaN using MgO as a gate dielectric and Si+ implantation to create the n+ regions. This structure overcomes the low minority carrier generation rate in GaN and allowed observation of clear inversion behavior in the dark at room temperature. By contrast, diodes without the n+ regions to act as an external source of minority carriers did not show inversion even at measurement temperatures of 300 °C. The gated diodes showed the expected shape of the current–voltage characteristics, with clear regions corresponding to depletion and inversion under the gate. The MgO was deposited prior to the Si implantation and was stable during the activation annealing for the Si-implanted n+ regions.
Remark on the (non)convergence of ensemble densities in dynamical systems
Authors: Goldstein S. , Lebowitz J. L. , Sinai Y.
Published: July 2002

We consider a dynamical system with state space M, a smooth, compact subset of some Rn, and evolution given by Tt, xt=Ttx, x∈M; Tt is invertible and the time t may be discrete, t∈Z, Tt=Tt, or continuous, t∈R. Here we show that starting with a continuous positive initial probability density ρ(x,0)>0, with respect to dx, the smooth volume measure induced on M by Lebesgue measure on Rn, the expectation value of logρ(x,t), with respect to any stationary (i.e., time invariant) measure ν(dx), is linear in t, ν(logρ(x,t))=ν(logρ(x,0))+Kt. K depends only on ν and vanishes when ν is absolutely continuous with respect to dx.
Alternans and higher-order rhythms in an ionic model of a sheet of ischemic ventricular muscle
Authors: Arce Humberto , Xu Aoxiang , González Hortensia , Guevara Michael R.
Published: July 2002

Life-threatening arrhythmias such as ventricular tachycardia and fibrillation often occur during acute myocardial ischemia. During the first few minutes following coronary occlusion, there is a gradual rise in the extracellular concentration of potassium ions ([K+]0) within ischemic tissue. This elevation of [K+]0 is one of the main causes of the electrophysiological changes produced by ischemia, and has been implicated in inducing arrhythmias. We investigate an ionic model of a 3 cm×3 cm sheet of normal ventricular myocardium containing an ischemic zone, simulated by elevating [K+]0 within a centrally-placed 1 cm×1 cm area of the sheet. As [K+]0 is gradually raised within the ischemic zone from the normal value of 5.4 mM, conduction first slows within the ischemic zone and then, at higher [K+]0, an arc of block develops within that area. The area distal to the arc of block is activated in a delayed fashion by a retrogradely moving wavefront originating from the distal edge of the ischemic zone. With a further increase in [K+]0, the point eventually comes where a very small increase in [K+]0 (0.01 mM) results in the abrupt transition from a global period-1 rhythm to a global period-2 rhythm in the sheet. In the peripheral part of the ischemic zone and in the normal area surrounding it, there is an alternation of action potential duration, producing a 2:2 response. Within the core of the ischemic zone, there is an alternation between an action potential and a maintained small-amplitude response (∼30 mV in height). With a further increase of [K+]0, the maintained small-amplitude response turns into a decrementing subthreshold response, so that there is 2:1 block in the central part of the ischemic zone. A still further increase of [K+]0 leads to a transition in the sheet from a global period-2 to a period-4 rhythm, and then to period-6 and period-8 rhythms, and finally to a complete block of propagation within the ischemic core. When the size of the sheet is increased to 4 cm×4 cm (with a 2 cm×2 cm ischemic area), one observes essentially the same sequence of rhythms, except that the period-6 rhythm is not seen. Very similar sequences of rhythms are seen as [K+]0 is increased in the central region (1 or 2 cm long) of a thin strand of tissue (3 or 4 cm long) in which propagation is essentially one-dimensional and in which retrograde propagation does not occur. While reentrant rhythms resembling tachycardia and fibrillation were not encountered in the above simulations, well-known precursors to such rhythms (e.g., delayed activation, arcs of block, two-component upstrokes, retrograde activation, nascent spiral tips, alternans) were seen. We outline how additional modifications to the ischemic model might result in the emergence of reentrant rhythms following alternans.
Magnetic properties of compounds R2Fe14B
Authors: Abache C. , Oesterreicher H.
Published: July 2002

A spin reorientation occurs in Nd2Fe14B starting around Ts= 150 K. We present data on tendencies in spin reorientations in systems (Nd1−xRx)2Fe14B and Nd2Fe14−xAxB where R=different rare earths and A=Co, Al. The insensitivity of Ts to some of these substitutions appears to indicate that Nd alone is responsible for the transition. We discuss a model according to which this transition may be triggered primarily by a competition amongst the anisotropies of the two Nd sites. According to a crystal field analysis the Nd 4f site should be more susceptible to plane preference while the Nd 4g site should have axial anisotropy. At lower temperatures the 4f site gains in relative importance and triggers the spin reorientation. The absence of a similar spin reorientation in analogous systems however indicates a complex situation.
Determination of alloy scattering potential in Ga1−xAlxAs alloys
Authors: Saxena A. K. , Adams A. R.
Published: July 2002

Room-temperature Hall mobility as a function of pressure (0–8 kbar) has been measured for high-purity liquid-phase-epitaxy-grown Ga1−xAlxAs layers. GaAs-like band structure of low-composition alloys has also been converted to Si-like band structure at high pressures and the Hall mobility measured as a function of temperature (77≲T≲300 °K) with crystals locked under constant pressures. The data have been analyzed to identify and distinguish the presence of space charge and alloy scatterings both characterized by mobilities limited by T−1/2. The space charge scattering has been found to be absent in all the crystals studied except x=0.047. The alloy scattering potential for electrons in the Γ minimum has been shown to depend on the alloy composition with a maximum value of 1.56 eV at x=0.19. For electrons in the X minima, this potential has been found to be independent of composition with a value of only 0.4 eV.
Structural and magnetic characterization of the lithiated iron oxide LixFe3O4
Authors: Fontcuberta J. , Rodríguez J. , Pernet M. , Longworth G. , Goodenough J. B.
Published: July 2002

The Rietveld profile-analysis method is used to investigate the x-ray diffraction pattern of lithiated Fe3O4. It is shown that, after exposure to air, pure magnetite coexists with a lithium-inserted LixFe3O4 phase. The Mössbauer spectra at 300 and 4.2 K have been used to estimate the lithium content of the sample, the pure magnetite concentration, and the iron distribution over the available 16c and 16d sites of the spinel structure. Magnetization measurements from 4.2 to 120 K with an external magnetic field up to 150 kOe have been used to obtain the saturation magnetic moment, the magnetic anisotropy constants, and the susceptibility. It is concluded that a noncollinear spin structure should be present in Li0.5Fe3O4. These results indicate that there is no room-temperature extrusion of iron even for x→2.0, but that on exposure to air LixFe3O4 samples with x>0.5 are oxidized at room temperature by delithiation.
Pressure-time profile of multiply shocked carbon disulfide
Authors: Sutherland G. T. , Gupta Y. M. , Bellamy P. M.
Published: July 2002

An experimental method was developed to measure the pressure-time profile of a liquid in a reverberation or multiple-shock experiment. Profiles, with peak pressures to 30 kbars, were measured for carbon disulfide using shorted quartz gauges (25.4 mm diameter by 3.15 mm thick); these gauges formed the back surfaces of cells which contained the carbon disulfide. Sapphire plates were used both as impactors and as the front surfaces of the cell. Up to six pressure steps were clearly observed in the quartz-gauge output. Measured pressure-time profiles were compared to profiles calculated with available equations of state. The experiments agreed well with profiles predicted with an equation of state proposed by Sheffield [J. Chem. Phys. 79, 1981 (1983)]. Calibration experiments were performed to characterize both the initial current response and the subsequent current ramping of the shorted quartz gauges used in this study.
Frequency-temperature behavior of thickness vibrations of doubly rotated quartz plates affected by plate dimensions and orientations
Authors: Lee P. C. Y. , Yong Y. K.
Published: July 2002

Three-dimensional linear equations of motion for small vibrations superposed on thermal deformations induced by steady, uniform temperature change in quartz are obtained. The material properties of quartz, such as the elastic stiffnesses and thermal expansion coefficients, are assumed temperature dependent and expressible by third-degree polynomials in temperature change. From the solutions of third-order perturbations of these equations for the thickness resonances of infinite quartz plates, six values of the effective third temperature derivatives of elastic stiffnesses C̃(3)pq are calculated by the use of the measured temperature coefficients of frequency by Bechmann, Ballato, and Lukaszek [Proc. IRE 50, 1812 (1962)] for various doubly rotated cuts and the values of the first temperature derivatives C(1)pq and the effective second temperature derivatives C̃(2)pq obtained in a previous study. An infinite system of two-dimensional equations of motion is derived by Mindlin’s method of power-series expansion for crystal plates subject to a steady, uniform temperature change. Four equations, governing the coupled thickness-shear, thickness-twist, thickness-stretch, and flexural vibrations, are extracted from the infinite set and employed to study the frequency-temperature behavior of thickness vibrations of finite SC-cut quartz plates with a pair of free edges. Changes in the thickness-shear resonance frequencies as a function of temperature are predicted and plotted for various values of orientation angles θ and φ, and length-to-thickness ratio a/b. Effects on the frequency-temperature behavior of the plates due to changes in the values of θ, φ, and a/b are observed and discussed.
Case-II diffusion in polymers. I. Transient swelling
Authors: Hui C.-Y. , Wu K.-C. , Lasky Ronald C. , Kramer Edward J.
Published: July 2002

The swelling of a polymer glass by sorption of a small molecule penetrant is considered in a regime characterized by so-called case-II diffusion. Attention is focused on the polymer so that the swelling process can be investigated apart from diffusion. The model of Thomas and Windle (TW) is used to predict the surface swelling as a function of exposure time. This model assumes that the swelling is driven by the osmotic pressure which relaxes to zero as the surface penetrant volume fraction φs approaches its equilibrium value φe. The rate-controlling factor of the swelling process is the viscosity of the polymer η, which decreases with increasing surface sorption according to η=η0 exp(−mφ) where η0 is the viscosity of the unswollen polymer. For large values of M=mφe, φs is very small until a time τ is reached beyond which the swelling then accelerates rapidly towards its equilibrium value. This feature is absent if M<e. The time τ is estimated by asymptotic analysis. Rutherford backscattering spectrometry is used to investigate the surface swelling kinetics of polystyrene by iodohexane. The TW model tends to underestimate the swelling rate when φs is low and to overestimate it when φs is high. Nevertheless, the time for φs to approach its equilibrium value φe is approximated well by the TW model.
Magnetic properties of diluted magnetic semiconductors: A review (invited)
Authors: Furdyna J. K. , Samarth N.
Published: July 2002

Diluted magnetic semiconductors (DMS) are semiconducting alloys containing a random distribution of substitutional magnetic ions (e.g., Mn++ in Cd1−xMnxSe). Magnetic properties of DMS—such as spin glass behavior due to lattice frustration, magnon excitations in a random system of spins, superexchange, and short range antiferromagnetic order—are of considerable interest in their own right. In addition, understanding these properties is important because they strongly influence the electronic phenomena in DMS via the sp-d exchange interaction between the band electrons and the localized magnetic moments. We review the most recent results concerning magnetic susceptibility, Mn++-Mn++ nearest-neighbor exchange, and magnetic short-range order in these materials. On this basis, we can present a fairly complete and unified picture of magnetic properties of DMS. In addition, we point out some of the issues and challenges that lie ahead.
Elastoplastic properties under shock compression of Al2O3 single crystal and polycrystal
Authors: Mashimo Tsutomu , Hanaoka Yasuharu , Nagayama Kunihito
Published: July 2002

Elastoplastic properties of Al2O3 single crystal (14–38 GPa) and polycrystal (6–33 GPa) under shock compression were studied by the measurements of Hugoniot compression curves and stress histories. The Hugoniot-elastic limit (HEL) stresses were determined to be 14.4–17.3 GPa for single crystals and 6.7–9.6 GPa for polycrystals with 2.7%–3.2% porosity. In the plastic region, the single-crystal Hugoniot stresses, at a given density, exceed the isothermal x-ray static data and its extrapolation by 6.5–8 GPa. And, the polycrystalline Hugoniot data exceeds, at a given density, the single-crystal Hugoniot data by about 1 GPa. These Hugoniot offsets are not caused by temperature increase, but rather by the shear strength and the remaining porosity. The stress histories of a single crystal show very similar shapes to that of a perfect elastoplastic material. The apparent yield stresses of single crystal at the HEL and in the plastic region are estimated to be 9.5–12 GPa and 4–6 GPa, from the HEL data and the quasirarefaction shock in the stress histories, respectively. As a result, it is concluded that both Al2O3 single crystal and polycrystal behave as quasi-elastoplastic material with some loss of shear strength, unlike many other brittle materials (elastoisotropic material with large catastrophic loss) such as Si, SiO2, etc. Furthermore, the present results suggest that the packing state and bonding state of crystal should be also ranked among the factors deciding the heterogeneous shock yielding of brittle materials, in addition to the thermophysical properties, which are related to the shear banding phenomenon or the other yielding mechanism.
Giant intrinsic coercivities in as-cast RFe2Ni2B alloys
Authors: Strzeszewski J. , Nazareth A. , Hadjipanayis G. C.
Published: July 2002

The origin of high coercivities in as-cast RFe2Ni2B alloys is investigated by correlating the magnetic properties with the microstructure. Hysteresis loop measurements showed high coercivities in samples containing Sm and Er. In SmFe2Ni2B the coercivities could not be measured in a field of 80 kOe at 4.2 K but they decreased to 56 kOe at 50 K. The coercivities are lower in ErFe2Ni2B, reaching the value of 38 kOe at 4.2 K. Electron diffraction studies showed a multiphase structure with a majority phase having a crystal structure of CeCo4B type. In SmFe2Ni2B, many stacking faults were observed which may explain the high coercivities.
The enhancement of the magnetic properties of Fe-Cr-Co-Mo polycrystalline permanent magnet alloys by cold rolling and annealing
Authors: Sugimoto S. , Okada M. , Homma M.
Published: July 2002

Present works describe the development of Fe-Cr-Co-Mo alloys with favorable {110}〈100〉 texture, which will lie along the transverse direction (TD) of the rolled strips. The studied alloy composition is Fe-30 wt. % Cr-15 wt. %Co-3 wt. % Mo-0.1 wt. % Mn-0.05 wt. % S. The magnetic properties of the alloys have been enhanced along the TD and the best magnetic properties obtained with TD alloys are Br=1.2 T (12.0 kG), iHc=82 kA m−1 (1025 Oe), and BHmax=56 kJ m−3 (7.0 MGOe). The favorable texture is developed by secondary recrystallization, caused by a mixture of rolling and annealing. It can be considered that the process adopted in this investigation is useful for mass production of Fe-Cr-Co alloys with high energy products, without forming a columnar structure or a single crystal.
Physical model of photostimulated luminescence of x-ray irradiated BaFBr:Eu2+
Authors: von Seggern H. , Voigt T. , Knüpfer W. , Lange G.
Published: July 2002

A dynamical model of the temporal dependence of photostimulated luminescence of the storage phosphor BaFBr:Eu2+ as a result of preceeding x-ray irradiation is presented. The model is based on a monomolecular recombination mechanism. The commonly used bimolecular mechanism leads to contradictions with experimental observations. The monomolecular recombination is explained by the existence of a photostimulable luminescence complex with a recombination center and an electron trap in close proximity. Charge transfer after optical excitation occurs through tunneling. The simulation of the transient charge carrier dynamics is performed through rate equations. Good agreement wih experimentally determined temporal responses for different x-ray doses applied and stimulating light intensities is obtained.
Al,Ga substitution in RE2Fe17 (RE=Ce,Pr,Nd): Magnetic behavior of RE2Fe15(Al,Ga)2 alloys
Authors: Weitzer F. , Hiebl K. , Rogl P.
Published: July 2002

Alloys with the composition RE2Fe17 and RE2Fe15X2 (X=Al,Ga) have been synthesized by the arc-melting technique followed by an 125-h anneal at 900–1000 °C in evacuated silica capsules. The new compound Nd2Fe15Ga2 is unstable below 800 °C, and is only obtained by quenching from above 900 °C. From x-ray powder diffraction analysis all alloys except Pr2 Fe15 Ga2 were found to crystallize with the hexagonal Th2 Zn17 type; indications for a preferential occupation of the (6c) sites by the X atoms are small. Substitution of Al, Ga atoms generally leads to a large increase of the Curie temperature and of the magnetocrystalline anisotropy, thereby enhancing the remanence as well as the coercivity. These effects are a consequence of the strongly distance-dependent exchange interactions of the Fe-Fe and Fe-RE coupling mechanisms. As for binary Pr2 Fe17 , the easy axis of magnetization for Pr2 Fe15 Al2 was found to be in the basal plane.
Measurements of minority-carrier diffusion length in n-CulnS2 by electron-beam-induced current method
Authors: Scheer R. , Wilhelm M. , Lewerenz H. J.
Published: July 2002

The diffusion length of minority carriers in n-CuInS2 samples is examined with electron-beam-induced current (EBIC) technique in vertical and planar configuration. Theories for the interpretation of the EBIC data from the experiments are reported. The holes as minority carriers show a diffusion length in the range of 1 μm, which is promising for high efficient solar energy conversion.
Temperature dependence of photorefractive properties of strontium-barium niobate (Sr0.6Ba0.4Nb2O6)
Authors: Rytz D. , Wechsler B. A. , Schwartz R. N. , Nelson C. C. , Brandle C. D. , Valentino A. J. , Berkstresser G. W.
Published: July 2002

Beam coupling gain and response time together with reflectivity and response time of an external ring phase-conjugate mirror are studied as a function of temperature for a crystal of strontium-barium niobate. The photorefractive sensitivity is increased by a factor of 5 when the crystal is heated to 65 °C. Self-pumped reflectivities up to 19% are observed at room temperature. The reflectivity is smaller at elevated temperature.