bluuebunny/crossref_metadata_2025 huggingface dataset:
PtMn-based spin-dependent tunneling materials with thin alumina barrier fabricated by two-step natural oxidation
Authors: Moon Ki-Seok , Chen Yingjian , Huai Yiming
Published: July 2002

Spin-dependent tunneling (SDT) materials with bottom-pinned structure (substrate/Ta/NiFeCr/PtMn/CoFe/t Al2O3/CoFe/NiFe/Ta) are fabricated by magnetron sputtering in ultrahigh vacuum. In this study, a two-step natural oxidation was used, in which the second Al layer was deposited and naturally oxidized after the natural oxidation of the first Al layer. The top and bottom leads were also patterned into bow-tie shaped structures. The two-step oxidation process results in a perfectly decoupled pinned and free layer in a film with a total as-deposited aluminum thickness of 7 Å, whereas, the one-step oxidation process gives rise to strongly coupled magnetic layers in a film with this thickness of aluminum. By using this two-step natural oxidation technique, an optimum tunneling magnetic resistance (TMR) ratio of 29.3% and resistance×area (RA) product of 34 Ω μm2 were achieved in junctions with 8 Å barrier (5+3 Å). In conclusion, a two-step oxidation process was used to fabricate spin-dependent tunneling devices with low RA product and high TMR ratio for head applications.
Interaction evaluation in particulate media using the integral generalized ΔM plot
Authors: Cerchez M. , Bissell P. R. , Stancu Al.
Published: July 2002

This paper presents a method of estimating the parameters of the generalized moving Preisach model by simple dc measurements. Two points on the integral generalized ΔM (IGDM) plot are used, which are very sensitive to choice of the parameters describing the interactions. The technique allows a refinement of parameters once that the coercive field distribution is approximated by using the magnetic hysteresis loop (MHL), dc magnetization curve (DCD), and the isothermal remanent magnetization curve starting from the dc demagnetized state of the system (IRMdc). The method may also be used to test the parameters provided by different identification techniques by considering the two points on the IGDM curve as check points for the simulations.
Radiation science using Z-pinch x rays
Authors: Bailey J. E. , Chandler G. A. , Cohen D. , Cuneo M. E. , Foord M. E. , Heeter R. F. , Jobe D. , Lake P. W. , MacFarlane J. J. , Nash T. J. , Nielson D. S. , Smelser R. , Torres J.
Published: July 2002

Present-day Z-pinch experiments generate 200 TW peak power, 5–10 ns duration x-ray bursts that provide new possibilities to advance radiation science. The experiments support both the underlying atomic and plasma physics, as well as inertial confinement fusion and astrophysics applications. A typical configuration consists of a sample located 1–10 cm away from the pinch, where it is heated to 10–100 eV temperatures by the pinch radiation. The spectrally-resolved sample-plasma absorption is measured by aiming x-ray spectrographs through the sample at the pinch. The pinch plasma thus both heats the sample and serves as a backlighter. Opacity measurements with this source are promising because of the large sample size, the relatively long radiation duration, and the possibility to measure opacities at temperatures above 100 eV. Initial opacity experiments are under way with CH-tamped NaBr foil samples. The Na serves as a thermometer and absorption spectra are recorded to determine the opacity of Br with a partially-filled M-shell. The large sample size and brightness of the Z pinch as a backlighter are also exploited in a novel method measuring re-emission from radiation-heated gold plasmas. The method uses a CH-tamped layered foil with Al+MgF2 facing the radiation source. A gold backing layer that covers a portion of the foil absorbs radiation from the source and provides re-emission that further heats the Al+MgF2. The Al and Mg heating is measured using space-resolved Kα absorption spectroscopy and the difference between the two regions enables a determination of the gold re-emission. Measurements are also performed at lower densities where photoionization is expected to dominate over collisions. Absorption spectra have been obtained for both Ne-like Fe and He-like Ne, confirming production of the relevant charge states needed to benchmark atomic kinetics models. Refinement of the methods described here is in progress to address multiple issues for radiation science.
Antiferro- to ferromagnetic transition and large magnetoresistance in YMn6Sn6−xTix (x=0–1.0) compounds
Authors: Zhang Shao-ying , Zhao Peng , Li Run-wei , Zhang Hong-wei , Shen Bao-gen
Published: July 2002

Magnetic and transport properties of YMn6Sn6−xTix (0⩽x⩽1.0) compounds with HfFe6Ge6-type structure were investigated in the temperature range of 5–400 K. The compounds with low Ti content (x⩽0.2) display antiferromagnetic behavior (317 K⩽TN⩽327 K). The Ti-rich compounds (0.8⩽x⩽1.0) behave as ferromagnetic in their whole magnetic ordered range (293 K⩽TN⩽334 K). The intermediate compounds (0.4⩽x⩽0.6) undergo an antiferro- to ferromagnetic transition with increasing temperature. The metamagnetic transition from antiferro- to ferromagnetism can be also induced by an applied field for the antiferromagnetic compounds. The metamagnetic transition field decreases with increasing Ti concentration. The large magnetoresistance of up to 35% is observed at 5 K under 50 kOe for the compounds (0.4⩽x⩽0.6) with metamagnetic behavior. The evolution of the magnetic properties is discussed.
Avalanche breakdown voltage of In0.53Ga0.47As
Authors: Ng J. S. , David J. P. R. , Rees G. J. , Allam J.
Published: July 2002

Avalanche breakdown voltages were determined for a series of In0.53Ga0.47As p-i-n diodes with i-region thicknesses ranging from 0.4 to 4.79 μm using measurements of reverse dark current and phase-sensitive photomultiplication. Despite its narrower bandgap In0.53Ga0.47As is found to have a very similar breakdown voltage to GaAs.
Generalized coherent and squeezed states based on the h(1)⊕su(2) algebra
Authors: Alvarez M. Nibaldo , Hussin Véronique
Published: July 2002

States which minimize the Schrödinger–Robertson uncertainty relation are constructed as eigenstates of an operator which is an element of the h(1)⊕su(2) algebra. The relations with supercoherent and supersqueezed states of the supersymmetric harmonic oscillator are given. Moreover, we are able to compute general Hamiltonians which behave like the harmonic oscillator Hamiltonian or are related to the Jaynes–Cummings Hamiltonian.
Theoretical characterization of the lowest-energy absorption band of pyrrole
Authors: Roos Björn O. , Malmqvist Per-Åke , Molina Vincent , Serrano-Andrés Luis , Merchán Manuela
Published: July 2002

The lowest-energy band of the electronic spectrum of pyrrole has been studied with vibrational resolution by using multiconfigurational second-order perturbation theory (CASPT2) and its multistate extension (MS–CASPT2) in conjunction with large atomic natural orbital-type basis sets including Rydberg functions. The obtained results provide a consistent picture of the recorded spectrum in the energy region 5.5–6.5 eV and confirm that the bulk of the intensity of the band arises from a ππ* intravalence transition, in contradiction to recent theoretical claims. Computed band origins for the 3s,3p Rydberg electronic transitions are in agreement with the available experimental data, although new assignments are suggested. As illustrated in the paper, the proper treatment of the valence–Rydberg mixing is particularly challenging for ab initio methodologies and can be seen as the main source of deviation among the recent theoretical results as regards the position of the low-lying valence excited states of pyrrole.
On stable Taylor vortices above the transition to wavy vortices
Authors: Antonijoan J. , Sánchez J.
Published: July 2002

The transition from Taylor to wavy vortices is revisited for parameter values in the range of new laboratory experiments [Lin et al., Phys. Fluids 10, 3233 (1998)]. The dependence of the critical Reynolds number with the axial wavelength of the Taylor vortices is obtained for azimuthal wave numbers from 1 to 5, and for five different values of the radius ratio. We show how islands of stable Taylor vortices above the transition to wavy vortices form.
Determination of the humidity of soil by monitoring the conductivity with indium tin oxide glass electrodes
Authors: Fabregat-Santiago Francisco , Ferriols Noemı́ S. , Garcia-Belmonte Germà , Bisquert Juan
Published: July 2002

A number of approaches to the determination of water content of soil use a dielectric permittivity measurement, which is accurate but involves expensive and complex devices. We explore the use of indium tin oxide, a wide-band gap semiconductor with high conductivity, long-term stability, and low cost, to build a cheap sensor of low humidity values for automatic irrigation systems. The electrical resistance determined by an ac impedance method discounts the effects of electrode contacts and provides a reliable measure of the water content, even when the chemical conditions of water vary widely. The reason for this is that, at low values of humidity, the conductivity is controlled by a surface conduction mechanism that is largely insensitive to the electrolytic properties of added water. The results appear promising for applications that require the detection of very low levels of humidity in soil.
Velocity modulation laser absorption spectroscopy of the A 2Πi←X 2Σ+ transition of the CS+ cation
Authors: Liu Yuyan , Duan Chuanxi , Liu JinJun , Wu Ling , Xu Chaoxiong , Chen Yangqin , Hamilton P. A. , Davies P. B.
Published: July 2002

The laser absorption spectrum of the (5,0) and (6,0) bands of the A 2Πi←X 2Σ+ transition of CS+ has been recorded using velocity modulation spectroscopy enhanced by optical heterodyne and magnetic rotation effects. First, improved molecular constants for the ground state of the ion were determined by a global linear least-squares fit of combination differences from the (1,0) (5,0), and (6,0) bands. Then a standard model was used to analyze the perturbations between the vibrational levels of the A 2Πi state and high vibrational levels of the X 2Σ+ state, leading to molecular constants and perturbation parameters for the υA=5∼υX=13 and υA=6∼υX=14 complexes. A new set of equilibrium parameters has been derived for the A and X states by combining all the available spectroscopic data, weighted according to the accuracy of the input parameters. The equilibrium internuclear distances from the new analysis are X 2Σ+re=1.492 156(78) Å and A 2Π re=1.639 55(10) Å. The Rydberg–Klein–Rees potential energy curves for the A 2Πi and X 2Σ+ states were constructed using the improved equilibrium constants, and the Franck–Condon factors calculated for all vibrational bands up to υ″=11 and υ′=10 of the A 2Πi−X 2Σ+ system. The overlap integrals calculated from the RKR turning points were used to find the interaction parameters a and b from the experimental perturbation parameters ξ and η. The r dependence of the a and b values relevant to the perturbations in the (1,0), (5,0), and (6,0) bands is discussed.
Resonant-cavity-enhanced heterostructure metal–semiconductor–metal photodetector
Authors: Chen Xiying , Nabet Bahram , Quaranta Fabio , Cola Adriano , Currie Marc
Published: July 2002

We report a GaAs-based high-speed, resonant-cavity-enhanced, heterostructure metal–semiconductor–metal photodetector with Al0.24Ga0.76As/Al0.9Ga0.1As distributed Bragg reflector operating around 850 nm. The photocurrent spectrum shows a clear peak at this wavelength with full width at half maximum (FWHM) of around 30 nm. At resonance wavelength, a seven-fold increase can be achieved in quantum efficiency compared to a detector of the same absorption depth. The top reflector is a delta modulation doped Al0.24Ga0.76As that also acts as the barrier enhancement layer thus providing very low dark current values. The breakdown voltage is above 20 V. Time response measurements show rise time, fall time, and FWHM of 8.8 ps, 9 ps, and 8.1 ps, respectively, giving a 3-dB bandwidth of about 33 GHz. Combination of low dark current, fast response, wavelength selectivity, and compatibility with high electron mobility transistors makes this device especially suitable for short haul communications purposes.
Vibronic structure and photodissociation dynamics of the Ã state of jet-cooled ammonia
Authors: Bach Andreas , Hutchison J. Matthew , Holiday Robert J. , Crim F. Fleming
Published: July 2002

Vibrationally mediated photodissociation action spectroscopy provides vibronic spectra of the Ã state of jet-cooled ammonia by detecting the H-atoms produced by the photodissociation of vibrationally excited molecules. Initial vibrational excitation to selected rotation-inversion levels in the N–H stretching fundamental changes the Franck–Condon factors for the subsequent electronic transition markedly. Analysis of the vibronic structure in the Ã state reveals a progression in both the umbrella and the bending modes and provides fundamental frequencies for the symmetric and antisymmetric stretching motions. Additional state selectivity in infrared–ultraviolet optical double resonance excitation combined with photofragment detection allows rovibronic analysis of the rapidly predissociating levels in the Ã state of ammonia. The lifetime for NH3(Ã) excited to four quanta of bending motion is as short as 13±4 fs.
Fully leaky guided mode study of an orthoconic antiferroelectric liquid crystal cell deviating from perfect horizontal surface stabilization
Authors: Scalia Giusy , Rudquist Per , Hermann David S. , D’havé Koen , Lagerwall Sven T. , Sambles J. Roy
Published: July 2002

The optical properties of an antiferroelectric liquid crystal (AFLC) cell slightly deviating from the ideal orthoconic condition, i.e., 45° tilt angle and fully horizontal surface stabilization (the molecular tilt plane parallel to the glass plates), have been investigated using the fully leaky guided mode (FLGM) technique. In contrast to standard optical investigative methods, such as polarization microscopy, the FLGM data give information about the orientation of the index ellipsoid or the molecular tilt plane, not only in the plane of the cell but in three dimensions. The sensitivity of the FLGM technique thus allows a detailed optical characterization on a mesoscopic scale and, in particular, has allowed us to discriminate, in the investigated cell, a tilt out of the substrate plane in the anticlinic state, thus a deviation from the horizontal surface stabilization condition. Orthoconic surface-stabilized AFLCs are, in the ideal case, uniaxial negative with the optic axis perpendicular to the glass plates and therefore provide a perfect dark state between crossed polarizers even if the direction of the smectic layer normal is not homogeneous in the plane of the cell. Small deviations from the ideal case lead to the appearance of a very small birefringence in the cell plane which, however, should not compromise the attractive optical properties of this class of AFLCs. Using laser diffraction we have verified that, even with the deviation from the horizontal surface stabilization detected by the FLGM study, a horizontal chevron geometry, clearly visible in the synclinic state, is not optically detectable in the anticlinic state of orthoconic AFLCs. We conclude that the attractive optical properties are preserved in a reasonably large interval around the ideal orthoconic conditions allowing for the practical exploitation of these materials as key components in liquid crystal displays and other electro-optic devices.
Manufacture of specific structure of aluminum-doped zinc oxide films by patterning the substrate surface
Authors: Jiang X. , Jia C. L. , Szyszka B.
Published: July 2002

(0001)-oriented aluminum-doped zinc oxide films were prepared using a magnetron sputtering technique. High-resolution transmission-electron-microscopic images show that the oriented grains nucleate directly on the substrate surface and grow with equal lateral dimensions through the film thickness. A surface-energy-driven self-texture mechanism was proposed on the basis of process modes. A method for manufacturing specific film structure by patterning the substrate surface is tested and discussed.
Molecular weight dependence of segmental alignment in a sheared polymer melt: A deuterium nuclear magnetic resonance investigation
Authors: Cormier Ryan J. , Callaghan Paul T.
Published: July 2002

H 2 NMR quadrupole interaction spectroscopy has been used to measure the molecular weight dependence of poly(dimethylsiloxane) chain deformation under shear in a cylindrical Couette cell while NMR velocimetry has been used to directly measure shear rates. The signals were acquired from a perdeuterated benzene probe molecule, which provides a motionally averaged sampling of the entire segmental ensemble. We have measured the dependence on shear rate of the SXX (velocity), SYY (velocity gradient), and SZZ (vorticity) elements of the segmented alignment tensor, fitting the data using the standard Doi–Edwards theory and modified to allow for convected constraint release. Our results suggest that the tube disengagement times scale as molecular weight to the power 3.5±0.1, consistent with the usual 3.4 power law. Our velocimetry measurements indicate a reproducible and consistent slip occurring at high molecular weights (>1 M Dalton), a phenomenon which is independently observed in a lower than expected chain deformation.
Kinetics of fluctuational deliquescence
Authors: Djikaev Y. S.
Published: July 2002

Deliquescence is the very first stage of heterogeneous unary condensation on solid soluble nuclei; at this stage, a solid nucleus dissolves in a liquid film formed by the molecules of the vapor condensing on the nucleus. As recently shown [Djikaev et al., J. Phys. Chem. B 105, 7708 (2001)], deliquescence in the atmosphere may occur in a fluctuational mode at relative humidities lower than the “deliquescence point” (at this point deliquescence becomes barrierless). Developing the kinetics of fluctuational deliquescence, we derive a two-dimensional kinetic equation, governing the time evolution of the droplet distribution. When solid soluble nuclei consist of molecules of a single species, in the vicinity of the saddle point of the free energy surface of deliquescence this equation has the form of the kinetic equation of binary nucleation. A quasisteady-state solution for this equation is obtained by using the method of complete separation of variables. An expression for the average deviation of the stable variable of state of a droplet from its equilibrium value is also derived. It shows that the film of liquid solution, forming around the nucleus during its deliquescence, is not in equilibrium neither with the vapor nor with the solid core. A recipe is proposed to construct the time evolution of the quasisteady state caused by the depletion of undeliquesced nuclei in the system. The theoretical results are illustrated by numerical calculation for the deliquescence of model particles in a water vapor.
Initiation and evolution of phase separation in heteroepitaxial InAlAs films
Authors: Shin B. , Lin A. , Lappo K. , Goldman R. S. , Hanna M. C. , Francoeur S. , Norman A. G. , Mascarenhas A.
Published: July 2002

We have investigated the initiation and evolution of phase separation in heteroepitaxial InAlAs films. In misfit-free InAlAs layers, cross-sectional scanning tunneling microscopy (XSTM) reveals the presence of isotropic nanometer-sized clusters. For lattice-mismatched InAlAs layers with 1.2% misfit, quasiperiodic contrast modulations perpendicular to the growth direction are apparent. Interestingly, these lateral modulations are apparently initiated within the first few bilayers of film growth, and both the amplitude and wavelength of the modulations increase with film thickness. The saturation value of the modulation wavelength determined from XSTM coincides with the lateral superlattice period determined from (002) x-ray reciprocal space maps, suggesting that the lateral modulation wavelength represents a periodic composition variation. Together, these results suggest that phase separation in the heteroepitaxial InAlAs thin-film system is a misfit-driven kinetic process initiated by random compositional nonuniformities, which later develop into coupled compositional and surface morphological variations.
Stability of one-dimensional electromagnetic solitons in relativistic laser plasmas
Authors: Hadžievski Lj. , Jovanović M. S. , Škorić M. M. , Mima K.
Published: July 2002

Existence and stability of one-dimensional electromagnetic solitons formed in a relativistic interaction of a linearly polarized laser light with an underdense cold plasma are discussed. In a weakly relativistic model, the original equation of the nonlinear Schrödinger type, with local and nonlocal cubic nonlinearities, is derived. Standing electromagnetic soliton solutions are analytically shown to be stable in agreement with the model simulation. A difference in soliton stability for linear and circular polarization is discussed. Finally, by fully relativistic fluid–Maxwell simulations, a family of large relativistic solitons is revealed, while analytical estimates for the maximum amplitude and the soliton eigenfrequency come close to simulation results.
Field emission microscopy of the cluster and subcluster structure of a Zr–Ti–Cu–Ni–Be bulk metallic glass
Authors: Bakaı̆ A. S. , Mikhailovskij I. M. , Mazilova T. I. , Wanderka N.
Published: July 2002

The cluster structure of a Zr–Ti–Cu–Ni–Be bulk metallic glass is investigated by the methods of high-resolution field emission microscopy. Internal interfaces are revealed, having a density of ∼106 cm−1 and a width of approximately 1 nm. It is shown that the binding energy of the atoms at the intercluster boundaries is lower by 0.13–0.43 eV than in the interior of a cluster. Subcluster heterogeneities of the structure of the bulk amorphous alloy are observed, with a characteristic scale of the short-range order equal to 1.5–3 nm, attesting to the diversity of the short-range order in glassy melts.
Detailed atomistic Monte Carlo simulation of grafted polymer melts. I. Thermodynamic and conformational properties
Authors: Daoulas Kostas Ch. , Terzis Andreas F. , Mavrantzas Vlasis G.
Published: July 2002

The thermodynamic and conformational properties of polymer melts grafted on a solid substrate as obtained from detailed, atomistic Monte Carlo simulations with the end-bridging algorithm are presented. The interface between a basal graphite plane (as well as a non-interacting hard surface) and a bulk polyethylene (PE) melt, a few or all chains of which are grafted on the plane, has been studied. Three different PE melts, of mean molecular length C78, C156, and C250, have been investigated, at grafting densities ranging from 0.54 to 2.62 nm−2. For melts composed of grafted and free chains, it is observed that, at moderate to high surface densities (σ⩾1 nm−2), the region close to the substrate is fully occupied by segments belonging to grafted chains, which are forced by their chemical grafting to have their first segment on the interface. As the grafting density increases, free chains are progressively expelled from the surface region, in agreement with scaling arguments and the predictions of lattice-based self-consistent mean-field (SCF) theory. For melts grafted on a graphite plane, it is also seen that the local melt density in the region closest to the interface is systematically higher than in the bulk, exhibiting distinct local maxima due to polymer adsorption. Results for the chain conformation tensor demonstrate that chains are significantly stretched in the direction perpendicular to the surface, even for moderate surface densities. For the C250 (PE) melt at a grafting density σ=1.31 nm−2, for example, the average chain dimension perpendicular to the interface is 1.9 times larger than its equilibrium value in the bulk. The profile of the chain end density is seen to exhibit universal behavior in agreement with the predictions of the SCF theory. Additional results concerning the mean chain conformational path and the structure of the interfacial area for both systems studied (fully grafted and mixtures of grafted and free chain systems) are also presented.
Stochastic molecular dynamics: A combined Monte Carlo and molecular dynamics technique for isothermal simulations
Authors: Attard Phil
Published: July 2002

A hybrid Monte Carlo molecular dynamics technique is developed that gives equations of motion for an isothermal system. Test results for a Lennard-Jones fluid are shown to be in good agreement with the known equation of state. The physical interpretation of the procedure and the extension to isothermal–isobaric systems is also discussed.
A first-order transition in the charge-induced conformational changes of polymers
Authors: Mao Yi , Burin Alexander L. , Ratner Mark A. , Jarrold Martin F.
Published: July 2002

Analytical mean-field theories and lattice model simulations have been used to study the charge-induced conformational changes of single polymer molecules. The compact-to-extended transition induced by charge is found to be first-order (i.e., two-state transition with a transition state) in the presence of strong short-range interactions at low temperatures. Short-range interactions decay much faster than electrostatic energy so expansion below a minimal value cannot produce electrostatic compensation for short-range energy loss. This is the origin of a free energy barrier (transition state) between the compact and the extended states. If the short-range interactions are weak in comparison with attractive and repulsive Coulomb interactions, the transition is expected to be second-order (one-state transition without a transition state). The prediction is compared to the computer simulation of the exhaustive enumeration of all 12-mer cubic lattice polymer conformations using different potentials, and qualitative agreement is found. Implications for protein folding and unfolding are discussed.
Growth of nanocrystalline diamond protective coatings on quartz glass
Authors: Yang W. B. , Lü F. X. , Cao Z. X.
Published: July 2002

Transparent diamond films with a crystallite size well controlled below 70 nm were grown by hydrogen and methane microwave plasma-enhanced chemical vapor deposition on quartz glass substrates, which had been scratched with 0.5 μm diamond powder. A complementary set of analyzing tools was employed to study the microstructure, the optical and mechanical properties of the deposits. Transmission electron microscopy revealed a nucleation density generally larger than 1011/cm2, which is of the same order of magnitude as the spotlike defects on the pretreated surface of the substrates. The Vicker’s hardness of the deposits scatters between 61 and 95 GPa. An optimal transmittance of 65% in the visible light range is achieved in coatings of 1.0 μm in thickness when the surface roughness measures about 10 nm or less. The nanocrystalline diamond films thus prepared can meet the requirements on transparent protective coatings for optical components.
Self-organization and the dynamical nature of ventricular fibrillation
Authors: Jalife José , Gray Richard A. , Morley Gregory E. , Davidenko Jorge M.
Published: July 2002

This article reviews recent data supporting the conjecture that, in the structurally and electrophysiologically normal heart, cardiac fibrillation is not a totally random phenomenon. Experimental and numerical studies based on the theory of excitable media suggest that fibrillation in the mammalian ventricles is the result of self-organized three-dimensional (3-D) electrical rotors giving rise to scroll waves that move continuously (i.e., drift) throughout the heart at varying speeds. A brief review of studies on the dynamics of rotors in two-dimensional (2-D) and 3-D excitable media is presented with emphasis on the experimental demonstration of such dynamics in cardiac muscle of various species. The discussion is centered on rotor dynamics in the presence and the absence of structural heterogeneities, and in the phenomena of drifting and anchoring, which in the electrocardiogram (ECG) may manifest as life-threatening cardiac rhythm disturbances. For instance, in the rabbit heart, a single electrical rotor that drifts rapidly throughout the ventricles gives rise to complex patterns of excitation. In the ECG such patterns are indistinguishable from ventricular fibrillation. On the other hand, a rotor that anchors to a discontinuity or defect in the muscle (e.g., a scar, a large artery or a bundle of connective tissue) may result in stationary rotating activity, which in the ECG is manifested as a form of so-called “monomorphic” ventricular tachycardia. More recent data show that ventricular fibrillation occurs in mammals irrespective of size or species. While in small hearts, such as those of mice and rabbits, a single drifting or meandering rotor can result in fibrillation, in larger hearts, such as the sheep and possibly the human, fibrillation occurs in the form of a relatively small number of coexisting but short-lived rotors. Overall, the work discussed here has paved the way for a better understanding of the mechanisms of fibrillation in the normal, as well as diseased human heart.
Epitaxial growth of VSi2 on (111) Si
Authors: Chien C. J. , Cheng H. C. , Nieh C. W. , Chen L. J.
Published: July 2002

Epitaxial VSi2 has been successfully grown on (111)Si for the first time. Best epitaxy was found in samples substrate heated at 400 °C during electron gun deposition of V thin films, followed by 400–1000 °C two-step annealing in vacuum. The orientation relationships were determined to be (0001)VSi2∥(111)Si, (224̄0)VSi2∥(224̄)Si and (202̄0)VSi2∥(202̄)Si. Interfacial dislocations were identified to be of edge type with (1)/(6)  〈112〉 Burgers vectors. The average dislocation spacing was measured to be about 250 Å. The disparity between calculated and measured values of dislocation spacing is attributed to the sharing of lattice mismatch between dislocations and elastic strains at the VSi2/Si interface.
Heterojunction CdS/CdTe solar cells based on electrodeposited p-CdTe thin films: Fabrication and characterization
Authors: Bhattacharya R. N. , Rajeshwar K.
Published: July 2002

The electrodeposition technique previously developed by the present authors for in situ growth of p-type CdTe thin films, has been further improved using As as an acceptor dopant. Photovoltaic-quality films were grown on conducting glass and Mo substrates and characterized by scanning electron microscopy, electron probe microanalyses, and Auger electron spectroscopy. These films were subsequently utilized in the construction of inverted photovoltaic cell structures comprising the n-CdS/p-CdTe heterojunction. Efficiencies approaching the 5% level (AM1 insolation) have been obtained thus far on small-area (∼0.20 cm2) devices.
Investigation of the electro-optical properties of 270° chiral nematic layers in the birefringence mode
Authors: Scheffer T. J. , Nehring J.
Published: July 2002

Numerical results are presented on transmission, reflection, and color of the recently described, supertwisted birefringence effect for highly multiplexed liquid-crystal displays. The electro-optical properties, which are investigated for normal and oblique incidence of light assuming a multiplexed duty cycle of 1/100, are compared to those of a 270° layer operating in the Mauguin mode and to 270° twisted Heilmeier and White–Taylor guest-host schemes.
Behavior of high-voltage gaps in the presence of large space-charge fields
Authors: Friedman M. , Serlin V.
Published: July 2002

It is shown in this paper that a high-voltage gap exhibits a nonlinear capacitance behavior in the presence of large space-charge fields. The existence of this nonlinear capacitor is quasi dc in nature and does not result from transit time effects, i.e., high-frequency effects. This nonlinear behavior can be used to tailor the shape of bunched intense relativistic electron beams.
Magneto-optical measurements of hysteresis loop and anisotropy energy constants on amorphous TbxFe1−x alloys
Authors: Wolniansky P. , Chase S. , Rosenvold R. , Ruane M. , Mansuripur M.
Published: July 2002

Measurements of the Kerr magneto-optic effect, coercivity, and anisotropy energy constants are performed in a magneto-optical system that combines the Kerr rotation angle and ellipticity to enhance the strength of the signal. The temperature dependence of the polar Kerr effect is compared with the magnetization of the iron subnetwork in the mean-field approximation and good agreement is obtained in all cases. Coercivity is found to be a strong function of the sweeping frequency and the saturating field and the results are found to be in qualitative agreement with the magnetization reversal model based on the existence of initial nuclei in a saturated film and the subsequent growth of these nuclei under a reverse field. Finally, perpendicular magnetic anisotropy energy is studied by magneto-optical techniques and the first two coefficients in the expansion of anisotropy energy in powers of the angle of deviation from the easy axis are determined.
Numerical modeling of double-injection Si:In devices
Authors: Hurm V. , Hornung J. C. R. , Manck O.
Published: July 2002

A numerical model for double injection in Si:In devices is presented, which allows the simulation of the electrical behavior up to breakdown. It takes into account mobile and trapped space charge, carrier drift and diffusion, and a Shockley–Read–Hall type of recombination. The field dependencies of mobilities, capture coefficients, and thermal emission rates can be included. One-dimensional calculations reproduce quantitatively the experimentally determined current-voltage characteristics of n+-p-p+ devices. The main effort was concentrated upon the temperature range around 75 K, where the breakdown voltage shows a distinct temperature dependence. The discussion of the calculated local variations of carrier densities, ionized impurity concentrations, and the electrical field leads to the understanding of the comparably low breakdown voltages. Well before breakdown inside the p-type substrate, zones of local neutrality marked by high densities of free carriers and low values of the electric field are forming, in addition to the regular space-charge regions.
Effect of dipolar coupling on the magnetism of disordered solids
Authors: Cullen J.
Published: July 2002

A study is made of the stability of ferromagnetic long-range order under fluctuations due to random easy magnetic axes, including the dipole–dipole coupling. This coupling produces two effects. There is a random dipole field that acts on the spins in the same way as does the random uniaxial anisotropy. Secondly, there is an average dipole field that acts to prevent the appearance of spin-density gradients. The net result is very dependent on the number of spin components. For the planar model in three dimensions, there is a logarithmic divergence of the magnetization reversal and power-law spatial dependence of the correlation function. Thus the lower critical dimension is reduced from four to three by the presence of dipole interactions in the planar (x-y) version of the random easy axis model. For the Heisenberg model the random magnetic anisotropy destroys the long-range ferromagnetic order. There is a correlation length that depends on the direction of the local magnetization. Results of a calculation of the spin–spin correlation function are used to predict qualitatively different ordered regions (domains) for small random anisotropy as compared to large random anisotropy.
The Preisach model and hysteretic energy losses
Authors: Mayergoyz I. D. , Friedman G.
Published: July 2002

Using Preisach’s model, general expressions for hysteretic energy losses are derived. These expressions are valid for arbitrary (not necessarily periodic) variations of magnetic field. Moreover, these expressions are given in terms of Preisach’s function as well as in terms of experimentally measured ‘‘first-order transition curves.’’ A formula is also found which relates the hysteretic energy losses for arbitrary field variations to the losses ocurring for certain periodic variations of magnetic field. This formula allows for easy measurement of hysteretic losses occurring for arbitrary field variations.
Oxidation of Fe-R-B powders during preparation of permanent magnets
Authors: Christodoulou C. N. , Schlup J. , Hadjipanayis G. C.
Published: July 2002

Differential scanning calorimetry (DSC) has been employed to study the changes which occur in the powders used to prepare Fe-R-B permanent magnets upon heating. Three exothermic reactions are observed upon heating the powders. The correlation of DSC data with the magnetic properties of Fe-R-B powders shows that these reactions cause a significant deterioration of the hard magnetic properties of the resulting magnets. The most probable explanation of these exothermic reactions is the reaction between chemisorbed oxygen and rare-earth metals to form bulk oxides. It appears that there is a critical oxygen concentration associated with a critical particle size which limits the development of hard magnetic properties.
Magnetostructural correlations in copper (II) bromide salts
Authors: Scott Brian , Willett Roger D.
Published: July 2002

Magnetic susceptibility measurements have been made on three ACuBr3 or A′Cu2Br6 salts containing bibridged Cu2Br2−6 dimers. All show antiferromagnetic (AFM) coupling. With A=piperidinium, chains of dimers exist, defining an alternating AFM chain with J/k=−9 K and α=0.9. In the salts with A=ethanolammonium and A′=melaminium, stacks of dimers occur and the coupling within the dimers is found to be J/k=−64 and −113 K, respectively. The value of J/k in these Cu2Br62− dimers is found to correlate roughly with the deviation from planarity of the dimers with maximum AFM coupling occurring when the dimers are planar. The antiferromagnetic contribution to the exchange coupling is found to be considerably larger than in the corresponding Cu2Cl2−6 dimers. This is related to the lowering of the ligand-to-metal charge-transfer energy when chloride is replaced by bromide.
Photorefractivity in GeO2-doped silica fibers
Authors: Stone J.
Published: July 2002

Photorefractivity has been observed in several GeO2-doped fibers. The effect is manifested by formation of a grating in the fiber core introduced by laser radiation in the green part of the spectrum. The grating is a distributed Bragg reflector which selectively reflects light at the writing wavelength. Reflections of up to 50% in a 15 cm length of fiber have been observed. The grating persists for at least five days after being written.
Ion irradiation induced grain growth in Pd polycrystalline thin films
Authors: Liu Joyce C. , Nastasi M. , Mayer J. W.
Published: July 2002

Ion irradiation induced grain growth in Pd polycrystalline thin films was studied by transmission electron microscopy for 100-keV Ne+, 185-keV Ar+, and 560-keV Xe++ irradiations over a wide range of doses. Grain growth from 9 to 60 nm was observed after ion irradiations. Initially, the amount of growth is approximately linear with the irradiation dose; the growth saturates in the high dose region. The saturated grain size scales with the dimension of the individual collision cascade and increases with the mass of the irradiating ion. In the proposed physical picture of ion irradiation induced grain growth, a heavily damaged region created by an individual collision cascade is surrounded by relatively undamaged grains. The growth of those surrounding grains into the heavily damaged region leads to the grain growth in the irradiated Pd film.
Characterization of nanometer-scale epitaxial structures by grazing-incidence x-ray diffraction and specular reflectivity
Authors: Lucas C. A. , Hatton P. D. , Bates S. , Ryan T. W. , Miles S. , Tanner B. K.
Published: July 2002

X-ray rocking curve analysis is a powerful and nondestructive technique for the characterization of heteroepitaxial structures. Conventionally, measurements are performed in symmetrical scattering geometry using a double-crystal x-ray diffractometer but the technique can be extended to the study of very thin layers (<200 Å) by the use of glancing-incidence scattering geometry and a triple-crystal diffractometer. These structures can also be studied by the technique of total external x-ray reflectivity. This is sensitive to the electron-density profile of the heterostructure as a function of depth. By combining the above techniques we have found it possible to obtain structural information on layers as thin as 20 Å. Such measurements permit accurate measurement of individual layer thicknesses and interface roughnesses on the angstrom level. The lattice parameter strain can be obtained by modeling of the intensity distribution of the crystal truncation rod.
Pulse method of measuring thermal diffusivity and optical absorption depth for partially transparent materials
Authors: Tischler M. , Kohanoff J. J. , Rangugni G. A. , Ondracek G.
Published: July 2002

Data analysis in the laser-pulse method of measuring thermal diffusivity is based on the solution of the heat diffusion equation, assuming several boundary and initial conditions. The initial condition generally results from considering the material opaque to the laser’s radiation. This requires building special absorbing layers on the sample’s surface when the material is transparent. A more general solution of the heat diffusion equation was obtained in this paper, in order to simplify sample preparation, and in some cases (high temperatures, sample contamination, etc.) even make possible the measurement of partially transparent materials. Based on an exponential-like absorption of the laser’s radiation, the solution includes an absorption-depth parameter p, which describes the material’s degree of transparency. A computer-programmable data-reduction procedure, including a least-squares estimation algorithm of nonlinear parameters, was also developed. It permits the simultaneous determination of both the thermal diffusivity and the absorption depth from a single laser-pulse experiment. The accuracy of the method was verified by measuring a sintered glass sample, both with the usual and the new procedures. The results indicate an accuracy level compatible with the current state of the art.
Magnetoelastic-surface-acoustic-wave attenuation peaks in thin ferromagnetic films
Authors: Walikainen D. , Wiegert R. F. , Levy M.
Published: July 2002

We report on the in-plane magnetic field dependence of the attenuation of 600-MHz Rayleigh-mode surface acoustic waves (SAW) propagating through rf-sputtered Fe80B20 and Fe78Si10B12 and evaporated FeNi3 films. The films vary in thickness from 200 to 2000 Å. The attenuation has narrow (width≊15 Oe) peaks≊25 dB/cm at fields≊30 Oe. This is qualitatively different behavior than previously reported for Ni. Since the hysteresis in these peaks is very small they may be useful in magnetically controlled SAW filters.
Electron spin injection and detection at a ferromagnetic-paramagnetic interface (invited)
Authors: Johnson Mark , Silsbee R. H.
Published: July 2002

A new technique for investigating conduction electron magnetization is presented. A coupling between electronic charge and spin transport in ferromagnetic metals follows from generalizing the linear transport equations. In practice, this means that ferromagnetic films act as conduction electron spin polarizers. At an interface between a ferromagnetic and a paramagnetic metal, one can inject and detect nonequilibrium magnetization in the paramagnet, and use sensitive electric measurements to probe spin transport. Application of a small external magnetic field dephases the spins. The result is a new technique for measuring conduction spin relaxation times, which is demonstrated on bulk aluminum samples. Three advantages of the technique make it applicable to a wide range of studies. (1) The measurement can be made in the limit of zero field so that systems with interesting properties which are altered or destroyed by magnetic fields can now be probed (spin glasses, superconductors, metals with g anisotropies). (2) A SQUID voltmeter with picovolt noise floor provides a sensitivity that allows detection of one nonequilibrium spin in a background of 1012 equilibrium spins. (3) The experimental geometry utilizes microfabrication techniques and is ideal for application to small-size systems that are inaccessible to conventional electron spin resonance methods. The geometry conveniently needs but one good surface; other sample surfaces are available for alteration in situ for the study of surface relaxation effects. Finally, there are obvious applications to the study of ferromagnets and interfaces.
Nonlinear excitation and dissociation kinetics in discharges through mixtures of rare and attaching gases
Authors: Scheller Geoffrey R. , Gottscho Richard A. , Intrator T. , Graves D. B.
Published: July 2002

Using space-time-resolved laser-induced fluorescence and plasma-induced emission spectroscopy, we find metastable and dissociation product densities to vary nonlinearly as Ar is diluted with SF6, Cl2, and BCl3. Because rare gases are relatively inert in the ground state, it is often assumed that rare gases change discharge chemistry through metastable interactions. Surprisingly, the measured changes cannot be accounted for by considering only direct metastable energy transfer. A model is proposed in which Ar metastable states indirectly enhance molecular dissociation by enhancing the ion density via Penning reactions; the increase in ion density in turn causes an increase in the dissociative recombination rate. This rate varies nonlinearly with concentration because both ion-producing metastable states and recombining electrons are quenched by addition of the attaching, molecular gases. The model satisfactorily explains discharge changes when small concentrations of molecular gases are added to an Ar discharge. Less well understood are changes over a broader concentration range, where dissociation and excitation rates decrease with decreasing Ar concentration because of electron attachment and cooling.
Experimental investigations on an x-ray preionizer test bed
Authors: Scott S. J.
Published: July 2002

An experimental test bed has been used to extensively investigate the operating characteristics of various types of cathode and anode in an x-ray preionizer. The temporal development, intensity, and uniformity of x-ray beams generated by a carbon fiber cold cathode and corona plasma cathode were measured and compared. Electron densities of >108 cm−3 in N2 (1 atm) and >109 cm−3 in Ar (1 atm) could be generated. Factors affecting the high repetition rate operation of both cathodes were examined. The effect of transmitting target material and thickness on x-ray output were investigated.
Interaction of a fluxon with an inductance step in a long Josephson junction
Authors: Kivshar Yuri S. , Malomed Boris A.
Published: July 2002

Within the framework of the sine-Gordon model of a long Josephson junction, we have calculated a threshold value of bias current density which admits capture of a fluxon by an effective potential well generated by a step of inductance in the junction. Along with a kinematic approximation, a nontrivial dynamical correction is found. We have also calculated analytically energy emitted by a fast fluxon moving past the step.
Silicon nitride produced by catalytic chemical vapor deposition method
Authors: Matsumura Hideki
Published: July 2002

The catalytic chemical vapor deposition method is applied to obtain silicon nitride insulating films usable in large-scale integrated circuits. A N2 H4, N2 , and SiH4 gas mixture is decomposed by the catalytic or pyrolytic reaction with a heated catalyzer placed near substrates, and thus silicon nitride films are deposited at low temperatures around 300 °C with deposition rates higher than several hundreds Å/min, without any help from plasma and photochemical excitation. It is found that the resistivity, the breakdown electric field, and also the hydrogen content of the films can be almost equivalent to those of the films deposited by the thermal chemical vapor deposition at 700 °C or more, and that their step coverage appears sufficient to apply them on device fabrication.