bluuebunny/crossref_metadata_2025 huggingface dataset:
Towards structural dynamics in condensed chemical systems exploiting ultrafast time-resolved x-ray absorption spectroscopy
Authors: Bressler Christian , Saes Melanie , Chergui Majed , Grolimund Daniel , Abela Rafael , Pattison Philip
Published: July 2002

We present the case for exploiting time-resolved x-ray absorption to study structural dynamics in the liquid phase. With this aim in mind and considering the large differences between absorption coefficients in the optical and the x-ray domains as well as the x-ray absorption cross sections due to unexcited species, we have estimated the anticipated signal-to-noise ratio (S/N) under realistic conditions with femtosecond laser pump pulses and synchrotron radiation x-ray probe pulses. As a model system, we examine I− photodetachment in water and detect the appearance of laser-generated neutral I atoms by their x-ray near-edge absorption structure (XANES) and by their extended x-ray absorption fine structure (EXAFS). While the S/N ratio critically depends on the photolysis yield, which itself is governed by the optical absorption cross section, the optimum sample concentration varies in a complex fashion as a function of pump laser intensity and optical absorption cross section. However, concentrations yielding near total absorption of the pump laser deliver quite optimum S/N ratios. The calculations presented here provide guidelines for the implementation of time-resolved x-ray absorption experiments in condensed phase chemical systems.
Electron magnetic resonance studies of magnetic inhomogeneities in crystalline and nanosized powders of La1−xSrxMnO3
Authors: Shames A. I. , Rozenberg E. , Gorodetsky G. , Arsenov A. A. , Shulyatev D. A. , Mukovskii Ya. M. , Gedanken A. , Pang G.
Published: July 2002

Electron magnetic resonance (EMR) spectra were measured on different La1−xSrxMnO3 manganites at the interval 100⩽T⩽420 K for studying an influence of the synthesis conditions on the magnetic homogeneity of the sample. The micron-sized crushed crystals La0.833Sr0.126MnO3 and La0.7Sr0.3MnO3 grown by fused-salt electrolysis and by floating zone melting with the Curie temperatures TC=320 K and 350 K, respectively, as well as nanosized sonochemically sintered powders La0.7Sr0.3MnO3 (annealed, TC=340 K) were studied. The results obtained show that samples originated from bulk crystals are less homogeneous than the nanocrystalline one. Except for higher homogeneity, the nanosized sample demonstrates also a weaker magnetic anisotropy. As a result, strongly pronounced magnetic phase separation (revealed as coexisting ferromagnetic and paramagnetic signals in EMR spectra) is observed in crushed crystals.
Edge localized modes and the pedestal: A model based on coupled peeling–ballooning modes
Authors: Snyder P. B. , Wilson H. R. , Ferron J. R. , Lao L. L. , Leonard A. W. , Osborne T. H. , Turnbull A. D. , Mossessian D. , Murakami M. , Xu X. Q.
Published: July 2002

A model based on magnetohydrodynamic (MHD) stability of the tokamak plasma edge region is presented, which describes characteristics of edge localized modes (ELMs) and the pedestal. The model emphasizes the dual role played by large bootstrap currents driven by the sharp pressure gradients in the pedestal region. Pedestal currents reduce the edge magnetic shear, stabilizing high toroidal mode number (n) ballooning modes, while at the same time providing drive for intermediate to low n peeling modes. The result is that coupled peeling–ballooning modes at intermediate n (3<n<20) are often the limiting instability which constrains the pedestal and triggers ELMs. These modes are characterized in shaped tokamak equilibria using an efficient new numerical code, and simplified models are developed for pedestal limits and the ELM cycle. Results are compared to several experiments, and nonideal MHD effects are briefly discussed.
Zonal flows in gyrofluid simulations of slab electron temperature gradient turbulence
Authors: Li Jiquan , Kishimoto Y.
Published: July 2002

Zonal flows can be generated by drift wave turbulence through the nonlinear interactions. Three-dimensional gyrofluid simulations show that in a turbulent system driven by electrostatic sheared slab electron temperature gradient (ETG) modes, the excitation of zonal flows is a slower process, which indicates the amplitudes grow up approximately proportionally to time. The zonal flows are very weak compared with the background turbulence and hardly work for suppressing the turbulent electron heat transport. The dynamics of zonal flows is numerically explored by starting the zonal flow component in the quasi-steady state. Meanwhile, the basic modulation processes between zonal flows and the pump waves as well as the sidebands in an ETG turbulence–zonal flows system are theoretically analyzed using a four-wave or five-wave (including two pump waves) coupling model. Through comparing the spectral evolution of both ETG turbulence and zonal flows in the simulations with the modulational instability analysis based on a five-wave model, it is identified that the zonal flows may originate from the beating of any two pump waves in the ETG turbulence, the evolution of zonal flows is determined by a modulational instability that strongly depends on the spectra of the turbulence, usually a marginal instability observed in ETG turbulence simulations. The saturation of zonal flows is testified to be subject to the same modulation process that becomes stable. Afterwards, the zonal flows enter a periodic oscillation phase.
Difference of secondary defect formation by high energy B+ and Al+ implantation into 4H–SiC
Authors: Ohno Toshiyuki , Kobayashi Naoto
Published: July 2002

The difference of secondary defect formation between high-energy B+ and Al+ implanted layers was investigated by transmission electron microscopy. At the same volume concentration of implanted ions, the density of secondary defects in the Al+ implanted layers is higher than that in the B+ implanted layers and the mean defect size in the B+ implanted layers is larger than that in the Al+ implanted layers. These secondary defects are formed by agglomeration of self-interstitials and the amount of interstitials stored in the defects roughly coincides with that of implanted ions. This correlation does not depend on the ion species. The activation energies of secondary defect formation are not significantly different for B+ and Al+ implanted layers. B+ and Al+ implanted layers have different agglomerations of interstitials, which cause the differences of defect size and density. The difference does not originate from the self-diffusion of interstitials in implanted layers but is probably due to the initial nucleation of defects.
Polytype homogeneity and doping distribution in homoepitaxial 4H SiC grown on nonplanar substrates
Authors: Nordell N. , Bowallius O. , Anand S. , Kakanakova-Georgieva A. , Yakimova R. , Madsen L. D. , Karlsson S. , Konstantinov A. O.
Published: July 2002

SiC was grown around stripe mesas, oriented along the 〈112̄0〉 and 〈11̄00〉 lattice directions on 4H SiC substrates. The grown layers were investigated with respect to polytype homogeneity by using cathodoluminescence. It was found that 3C inclusions rarely occur at normal growth conditions. However, 3C inclusions were prevalent at low growth temperatures and high C:Si ratios at the mesa tops, where the (0001) plane is revealed during growth. The doping distribution was recorded on cleaved mesa cross sections by using scanning capacitance spectroscopy. It was found that the p-type doping (using Al as the dopant) was considerably lower at the mesa walls than on the mesa tops or between the mesas, while the n-type doping (using N as the dopant) was independent of the substrate geometry.
Giant magnetic-field-induced strain in NiMnGa seven-layered martensitic phase
Authors: Sozinov A. , Likhachev A. A. , Lanska N. , Ullakko K.
Published: July 2002

Giant magnetic-field-induced strain of about 9.5% was observed at ambient temperature in a magnetic field of less than 1 T in NiMnGa orthorhombic seven-layered martensitic phase. The strain proved to be caused by magnetic-field-controlled twin boundary motion. According to an analysis of x-ray diffraction data, the crystal structure of this phase is nearly orthorhombic, having lattice parameters a=0.619 nm, b=0.580 nm, and c=0.553 nm (in cubic parent phase coordinates) at ambient temperature. Seven-layer shuffling-type modulation along the (110)[11̄0]p system was recorded. The results of mechanical tests and magnetic anisotropy property measurements are also reported.
Efficient exciplex emitting organic electroluminescent devices
Authors: Cocchi M. , Virgili D. , Giro G. , Fattori V. , Di Marco P. , Kalinowski J. , Shirota Y.
Published: July 2002

Organic electroluminescent (EL) single-layer (SL) and double-layer (DL) light-emitting diodes were fabricated based on 4,4′,4″-tris[3-methylphenyl(phenyl)amino] triphenylamine (m-MTDATA) and 2-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) molecules incorporated in bisphenol-A-polycarbonate (PC) matrix. The external EL quantum efficiency of the dominating exciplex emission of the indium tin oxide (ITO)/m-MTDATA:PBD:PC/Ca SL structure is ≅0.4% photon/electron and that from the ITO/m-MTDATA:PC/PBD/Ca DL device exceeds 1% photon/electron. The operating voltage of the DL device at maximum efficiency is 4.8 V and the onset voltage of the EL is as low as 2.0 V. The photoluminesce quantum efficiency of the exciplex is greater than 17%. The high exciplex emission-based EL yield from the DL device is attributed to the narrow recombination zone imposed by the high energy barriers for the electron and hole transfer at the (m-MTDATA:PC)/PBD interface.
Dispersive ionic space charge relaxation in solid polymer electrolytes. II. Model and simulation
Authors: Wagner Achim , Kliem Herbert
Published: July 2002

Dynamic Monte-Carlo simulations of ionic space charge relaxations are carried out using a three-dimensional model for thermally activated ion hopping in a multiwell energy structure. In this model a solid polymer electrolyte is embedded between two ideal blocking electrodes. The polymer is subdivided into 100×100×100 lattice cells. Positive ions (typically 1000) are distributed on the cells. To provide charge neutrality a negative background charge, constant in space and time, is introduced. The positive ions are able to hop between neighboring cells, surmounting energy barriers of distributed heights. The barrier heights consist of an intrinsic part due to the polymer structure, a part due to the Coulomb interaction of the ions, and a part due to the externally applied field. To calculate the interaction between the ions and the electrodes a method of images is used. Periodical boundary conditions are used for those lattice surfaces which are not in contact with the electrodes. The ionic space charge polarization process is simulated as dependent on the time, the sample thickness, the ion concentration, and the externally applied voltage. The polarization current after a step of the electric field shows dispersion due to distributed energy barrier heights in the short time range and a Kohlrausch behavior due to image charges in the long time range.
Low-temperature growth of NiMnSb thin films by pulsed-laser deposition
Authors: Giapintzakis J. , Grigorescu C. , Klini A. , Manousaki A. , Zorba V. , Androulakis J. , Viskadourakis Z. , Fotakis C.
Published: July 2002

We report the growth of thin films of the ferromagnetic half-Heusler alloy NiMnSb by pulsed-laser deposition on Si (111) and polycrystalline InAs substrates. Highly crystalline films are grown using low substrate temperatures (190–210 °C), without any postdeposition annealing. The structural, magnetic, and transport properties of the films are relatively consistent with those of the bulk NiMnSb used as target, which suggests that thin layers of this material grown by laser ablation could be used in multilayer structures as effective spin-polarized conducting layers.
Evaluation of the coating–substrate adhesion by laser-ultrasonics: Modeling and experiments
Authors: Rosa Gaélle , Oltra Roland , Nadal Marie-Hélène
Published: July 2002

The aim of the present study is to investigate a laser-ultrasonic method to determine the oxide-coating’s adhesion strength on a metallic substrate. In order to demonstrate this method a completely contactless system is used, combining laser generation and detection of ultrasonic waves. A nanosecond pulsed Nd:YAG laser is used to irradiate the transparent oxide coatings, while the normal displacement of the rear surface of the metallic substrate is detected at the epicenter by a laser heterodyne interferometer. Increasing the laser beam energy, the generated acoustic wave forms are correlated with the thermoelastic regime, the fracture of the coating–substrate interface, and finally the expulsion of the coating. The generation of ultrasonic signals in the thermoelastic regime is described by a two-dimensional model suitable to calculate the in- and out-of-plane components of the mechanical displacement versus time. The values of the so-called practical adhesion found are in agreement with those obtained by classic contact techniques (tensile adhesion test, indentation, bending test). This work demonstrates the suitability of this quantitative and contactless test to evaluate the coating–substrate adhesion.
Size and grain-boundary effects of a gold nanowire measured by conducting atomic force microscopy
Authors: Bietsch Alexander , Michel Bruno
Published: July 2002

The resistivities of thin metal films and wires are highly sensitive to their polycrystalline structure and surface morphology because grain boundaries and surfaces provide additional scattering sites compared to bulk materials. Here, we investigated polycrystalline gold wires of nanometer-scale diameter that were—at some locations—connected through single grain boundaries. A detailed topography of the wires was recorded by atomic force microscopy. A Pt-coated tip in a conducting atomic force microscopy setup served as a mobile electrode to probe the resistance of a wire. Analyzing the topographical cross section and the resistance data allowed us to evaluate the effective specific resistivity of the wire as well as reflection coefficients of single grain boundaries.
The viscous damping of capillary-gravity waves in a brimful circular cylinder
Authors: Nicolás José A.
Published: July 2002

The natural frequency and damping rate of surface capillary-gravity waves in a circular cylinder with pinned-end edge conditions and clean surfaces depend on the inverse gravitational Reynolds number C, the Bond number B, the aspect ratio of the cylinder Λ, and the radial and azimuthal wave numbers of each excited mode. A semianalytical method valid for arbitrary values of C, B, and Λ is used. It is shown that the damping rate has a weak dependence on the Bond number and also that the oscillations become overdamped for large values of C or low values of Λ. The results are compared with recent asymptotic calculations for small viscosity (which include viscous dissipation in the boundary layers and in the bulk), showing that the approximations yield good results if C≲10−3 and Λ≳0.1, for all values of B. The results are also compared with experimental measurements (which include values of C larger than 10−3), showing a very good agreement.
Absolute equation of state measurements of iron using laser driven shocks
Authors: Benuzzi-Mounaix A. , Koenig M. , Huser G. , Faral B. , Batani D. , Henry E. , Tomasini M. , Marchet B. , Hall T. A. , Boustie M. , De Rességuier Th. , Hallouin M. , Guyot F. , Andrault D. , Charpin Th.
Published: July 2002

First absolute equation of state measurements obtained for iron with laser driven shock waves are presented. The shock velocity and the free surface velocity of compressed iron have been simultaneously measured by using a VISAR diagnostic, and step targets. The pressure range 1–8 Mbar has been investigated, which is directly relevant to planetary physics. The experiments have been performed at the Laboratoire pour l’Utilisation des Lasers Intenses of the Ecole Polytechnique.
Comparative magneto-optical investigation of d–d charge–transfer transitions in Fe3O4, CoFe2O4, and NiFe2O4
Authors: Kim Kwang Joo , Lee Han Seung , Lee Myoung Hee , Lee Sung Ho
Published: July 2002

Magneto-optical transitions in CoFe2O4 and NiFe2O4 spinel ferrites are investigated in comparison with those in isostructural Fe3O4. The Kerr angle and ellipticity of the ferrites have been measured by magneto-optic Kerr effect (MOKE) spectroscopy in the 1.5–5 eV region. The results indicate that the d–d intervalence charge–transfer (IVCT) transitions, Co2+(t2g)→Fe3+(eg), and Ni2+(t2g)→Fe3+(eg), in the octahedral sites in CoFe2O4 and NiFe2O4 are located at about 3.6 and 4.5 eV, respectively, shifted to higher energies by about 1.5 and 2.4 eV, respectively, from the corresponding IVCT transition in Fe3O4, Fe2+(t2g)→Fe3+(eg). The MOKE spectra of CoFe2O4 and NiFe2O4 also indicate that the IVCT transitions, Co2+(t2g)→Fe3+(t2g) and Ni2+(t2g)→Fe3+(t2g), exist at about 2.2 and 3.1 eV, respectively. On the other hand, the intersublattice charge-transfer (ISCT) transitions between the tetrahedral and the octahedral Fe3+ sites in CoFe2O4 and NiFe2O4 are observed at about 2.6 and 4.0 eV, same as the corresponding ISCT transitions in Fe3O4. The observed IVCT and ISCT transitions involving octahedral Fe3+ ion consistently give the crystal-field energy splitting of about 1.4 eV between its t2g and eg orbitals for all the compounds.
Measurement of thermal conductivity of silicon dioxide thin films using a 3ω method
Authors: Yamane Tsuneyuki , Nagai Naoto , Katayama Shin-ichiro , Todoki Minoru
Published: July 2002

The thermal conductivity of SiO2 thin films prepared using various procedures has been studied using a 3ω method. The thermal conductivity of SiO2 thin films of above approximately 500 nm thickness decreases as the porosity of the specimen, which is determined by infrared absorption spectroscopy, increases. Below approximately 250 nm thickness, the observed thermal conductivity of the SiO2 thin films systematically decreases as a function of film thickness. The data have been analyzed based on a SiO2-thickness-independent thermal conductivity and interfacial resistance. The total estimated interfacial resistance between the metal strip and the film, and between the film and the substrate is about 2×10−8 m2 KW−1.
Sub-500-fs soliton-like pulse in a passively mode-locked broadband surface-emitting laser with 100 mW average power
Authors: Garnache A. , Hoogland S. , Tropper A. C. , Sagnes I. , Saint-Girons G. , Roberts J. S.
Published: July 2002

We report on femtosecond operation of a broadband diode-pumped external-cavity surface-emitting semiconductor laser, passively mode locked with a fast quantum–well Semiconductor Saturable Absorber Mirror grown at 735 °C. We obtained 477 fs pulses at 1.21 GHz. The average output power is 100 mW at 1040 nm, the pulse peak power 152 W, with ∼1 W of 830 nm pump. The rf spectrum shows a linewidth <50 kHz at the noise level (−65 dB). We believe that the group-delay dispersion is compensated by the negative self-phase modulation in the absorber structure, leading to soliton-like mode locking. This system requires no additional technological step after the growth of the structures.
The quantized D-transformation
Authors: Saraceno M. , Vallejos R. O.
Published: July 2002

We construct a new example of a quantum map, the quantized version of the D-transformation, which is the natural extension to two dimensions of the tent map. The classical, quantum and semiclassical behavior is studied. We also exhibit some relationships between the quantum versions of the D-map and the parity projected baker’s map. The method of construction allows a generalization to dissipative maps which includes the quantization of a horseshoe.
Solving the traveling-salesman problem by a statistical-physics method
Authors: Yoshiyuki Usami , Yoshiki Kano , Masatoshi Kitaoka
Published: July 2002

We summarize approximate ways of solving the travelling-salesman problem (TSP) and discuss their relationship to methods of statistical physics. We also describe our new approach, based on real-space renormalization theory used in studies of critical phenomena approximate solutions of the TSP that are close to optimal are solved efficiently. (AIP) © 1996 American Institute of Physics.
The effect of titanium on microstructure and magnetic properties of Fe-Cr-Co hard magnetic materials
Authors: Sugimoto S. , Okada M. , Ohtani Y. , Tanaka T. , Homma M.
Published: July 2002

Fe-Cr-Co ductile alloys produce magnetic properties comparable to those of Alnico permanent magnets family, up to (BH)max=91.2 kJ/m3(11.4 MGOe). It is technologically known that the addition of Ti to the Fe-Cr-Co alloys extends the α-phase region of system and improves the magnetic properties. The purpose of this work is to study the effect of Ti on microstructures and magnetic properties of Fe-Cr-Co alloys. The microstructures of Fe-25 wt.% Cr-12 wt.% Co alloys with or without Ti melted in air or in vacuum, were studied by optical microscopy, electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). The alloys without Ti exhibit the precipitates of Cr23C6 carbide and Cr2N nitride a along the grain boundary or inside grain. These carbides and nitrides form the Cr-depleted zone around them. However, the addition of Ti suppressed the formation of Cr-depleted zone, in forming the Ti(C,N), and the magnetic properties are improved.
Electromagnetic missiles
Authors: Wu Tai Tsun
Published: July 2002

Under transient excitation the energy transmitted by an antenna of finite size to a faraway receiver can decrease much more slowly than the usual R−2. Instead, by a suitable choice of excitation, this quantity can decrease as slowly as one wishes, under the physical restriction that the total energy radiated by the antenna is finite. Such cases of slow decrease are referred to as electromagnetic missiles. Another way of stating the result is that the product of the following two quantities can approach zero as slowly as one wishes: (1) the energy transmitted per unit area of receiver, and (2) the total area of the receiver. Examples are constructed when either one, but not both, of these quantities remains finite at infinite distance.
Magnetic domains in amorphous metal ribbons (invited)
Authors: Livingston J. D. , Morris W. G.
Published: July 2002

The results of various static and dynamic studies of domains in amorphous metals by SEM and Bitter techniques are reviewed. Observations include domain widths and orientations, wall mobility and pinning, domain-wall energy measurements, and the effects of various distributions of residual or applied stresses on domain patterns. Domain structures resulting from longitudinal, transverse, and perpendicular anisotropies are contrasted.
Geometry of thin-film morphology
Authors: Messier Russell , Yehoda Joseph E.
Published: July 2002

The columnar morphologies commonly found in all vapor-deposited thin films prepared under low mobility conditions have been classified by several variations of what have been termed structure zone models. Such morphological structures are found to have a strong similarity in shape and form over six orders of magnitude in film thickness and three orders of magnitude in magnification for films of a given thickness. Thick (45-mm) pyrolytic graphite films are shown to be a good demonstration of the continuous growth evolution of conical-shaped units. Due to competition for growth each cone eventually goes through a death stage. A model based upon these general structural observations is presented and is shown to be a geometric construction similar to a Sierpinski gasket. The origin of this morphology seems to be the natural clustering which occurs due to the random process of ballistic aggregation.
Metalorganic chemical vapor deposition of III-V semiconductors
Authors: Ludowise M. J.
Published: July 2002

The metalorganic chemical vapor deposition (MOCVD) of epitaxial III-V semiconductor alloys on III-V substrates is reviewed in detail. The emphasis is placed on both practical and theoretical knowledge of the equipment and deposition process. The chemistry of the source alkyls and the dynamics of the transport process are discussed. The growth of the GaAs and AlxGa1−xAs systems are treated as prototypical examples (and the most studied) of the III-V materials. Latter sections review InP, Ga1−xInxAs, and related alloys. Finally, the antimonide and the other systems are reviewed. Electronic and optical devices fabricated from MOCVD-grown materials are used as examples of the capabilities of the growth technique.
A theoretical analysis of electron transport in ZnSe
Authors: Ruda H. E.
Published: July 2002

Boltzmann’s transport equation was solved using a variational method (including all major scattering mechanisms and screening) yielding electron mobilities in n-ZnSe. The electron concentration dependence of the mobility at 300 and 77 K was calculated as functions of ionized impurity concentration and compensation ratio. A routine method for determining acceptor and donor densities (total and ionized) and compensation ratio from given experimental mobility and carrier concentration values is outlined; mobilities reported for samples grown by various techniques are discussed. Inherent limits to the mobility are given as about 800 cm2/V s (n∼3×1018 cm−3) and about 1×104 cm2/V s (n≤4×1015 cm−3) at 300 and 77 K, respectively. The calculated temperature dependence of the mobility for η=−6 is used to explain qualitative trends in such behavior for material grown by different techniques, and to provide an inherent mobility limit at this η.
SiO2-induced substrate current and its relation to positive charge in field-effect transistors
Authors: Weinberg Z. A. , Fischetti M. V. , Nissan-Cohen Y.
Published: July 2002

Experimental data are presented for the substrate current (holes), which accompanies electron injection into the oxide of n-channel field-effect transistor structures, in the tunneling regime. Dependencies of the effect on oxide thickness and on the metal gate material were investigated. An inverse relation was found between the initial rise time of oxide current transients and both the electron and hole currents. It is shown that these initial current increases are related to positive charge, therefore a correlation exists between the positive charge and electron or hole currents. The strength of impact ionization in SiO2 is discussed on the basis of band-structure arguments and it is concluded that there are difficulties in explaining the substrate current by impact ionization. A technique for fast measurements of capacitance-voltage shifts at the end of an applied high field pulse is described.
Long-range surface plasmon-polaritons in asymmetric layer structures
Authors: Wendler L. , Haupt R.
Published: July 2002

We analyze the effect of asymmetric embedding of thin silver layers on the propagation length and the power flow distribution of long-range surface plasmon-polaritons (LRSP). We show that in such configurations LRSP can achieve propagation length which exceeds the value for the symmetric case by up to 3 orders of magnitude.
Interpretation of 29Si nuclear magnetic resonance spectra of amorphous hydrogenated silicon
Authors: Hayashi Shigenobu , Hayamizu Kikuko , Yamasaki Satoshi , Matsuda Akihisa , Tanaka Kazunobu
Published: July 2002

2 9Si nuclear magnetic resonance (NMR) spectra have been measured for amorphous hydrogenated silicon as well as amorphous-microcrystalline mixed-phase hydrogenated silicon using cross polarization and magic-angle sample spinning. 1H NMR spectra have been measured as well. The peak position of the 29Si signal has been found to correlate well with the line shape of the 1H NMR spectra, and mechanisms determining the position have been discussed.
Ga1−xAlxAs-Ga1−yAlyAs-GaAs double-barrier structures
Authors: Schulman J. N.
Published: July 2002

The effect on negative resistance of a small aluminum concentration in the outer GaAs layers of a GaAs-Ga1−xAlxAs double-barrier tunneling structure is calculated. The aluminum concentration can be chosen so that the alloy conduction-band minimum is slightly below the energy of the resonant tunneling level of the GaAs central well region. Less voltage is then needed to raise the incoming electrons into the resonant energy level. The decreased voltage across the structure has several beneficial effects. The electron transmission coefficient through the barriers on resonance increases, the total applied voltage across the device is reduced, and the current through parasitic parallel resistances is reduced. Significant improvements in peak-to-valley ratios are predicted, the improvement being dependent on the parameters of the structure.
Compressional wave propagation in liquid and/or gas saturated elastic porous media
Authors: Garg S. K. , Nayfeh A. H.
Published: July 2002

Concepts from the theory of interacting continua are employed to develop constitutive relations for liquid and/or gas saturated elastic porous media. The model is formulated by defining intrinsic stress tensors and densities in terms of the partial stress tensors, partial densities, and actual volume fractions occupied by each component. It is assumed that the constitutive law for each component as a single continuum relates intrinsic pressure to intrinsic deformation. Relative motion between the constituents is allowed through simple Darcy-type expressions. The governing equations together with the constitutive relations are used to investigate the propagation of both harmonic and transient pulses. In general three modes of wave propagation exist. In the case of a transient pulse, these modes lead to a three-wave structure. Laplace transform techniques are used to derive closed-form solutions for transient loading for two limiting values of viscous coupling (i.e., weak viscous coupling, strong viscous coupling). Strong viscous coupling results in the coalescence of the three wave fronts into a single front. Solutions for the general case of transient loading are obtained by numerical inversion of the Laplace transforms.
Mössbauer spectroscopy investigation of K2FeF5⋅H2O in the helium-3 temperature range
Authors: Takács L. , Reiff W. M.
Published: July 2002

Mössbauer spectroscopy investigations were performed on the quasi-one-dimensional complex K2FeF5⋅H2O between 0.3 and 1.5 K. The results indicate three-dimensional antiferromagnetic ordering below 1 K in agreement with recent susceptibility data. The saturation hyperfine field is about 520 kOe, typical of an ionic ferric compound.
Activation field, fatigue, and waiting-time effects in KNO3 thin-film memories
Authors: Scott J. F. , Pouligny B. , Dimmler K. , Parris M. , Butler D. , Eaton S.
Published: July 2002

By analyzing current transient data i(t) from KNO3 thin-film memories in terms of the Ishibashi–Takagi parametrization of the Avrami theory, we have been able to extract detailed information concerning the presence of an activation field (and the absence of a threshold), the relationship between characteristic switching time and spontaneous polarization when both are decreased either by increasing temperature or by repetitive read-write cycles (fatigue), and the behavior of switching kinetics as a function of the delay after application of a dc bias field (‘‘waiting-time’’).
Random-walk treatment of carrier diffusion with surface recombination
Authors: Holloway H.
Published: July 2002

The random-walk formulation of diffusion of a particle in a lossy medium is extended to include partially reflecting boundaries. A relationship between the reflection coefficient and the surface recombination velocity for minority carriers in a semiconductor is derived. This result is verified by numerical test using a one-dimensional problem that has an exact algebraic solution.
Electronic structure and transport properties of GaAs-GaAlAs superlattices in high perpendicular electric fields
Authors: Austin E. J. , Jaros M.
Published: July 2002

We present a study of the effect of high (∼1×105 V/cm) perpendicular electric fields on the electronic structure of a GaAs-Ga0.7Al0.3As superlattice represented by five quantum wells separated by 70-Å barriers. An exact solution is obtained by using the transfer-matrix technique to match the Airy functions at the interfaces. The method of phase-shift analysis is employed to evaluate the energy levels, the width of the Stark resonances, the real-space wave functions, and the time of flight associated with the field-induced phenomena. Wells of 50- and 70-Å width are used to elucidate the relationship between level depth and field. The quantum states above the semiclassical confining barrier, which have so far been represented by a continuum, are correctly accounted for. These states are mainly localized in the barrier material. It is shown that the real-space localization and bandwidth of these states are significantly altered by the external field. In particular, we evaluate the time delay for electrons above the barrier due to the field-induced change in the density of states, which is related to the presence of the above higher-lying confined states and which would be observed in current–voltage measurements.
A model for the effect of stress on the low-frequency harmonic content of the magnetic induction in ferromagnetic materials
Authors: Sablik M. J. , Burkhardt G. L. , Kwun H. , Jiles D. C.
Published: July 2002

A simple model used previously by the authors to explain stress variation of magnetic hysteresis is now employed to explain the effect of stress on the amplitudes of the first- and third-order harmonics of the magnetic induction signal resulting from application of an ac magnetic field of low frequency to a steel specimen. An improved expression for the effective field contribution Hσ due to stress has been derived from thermodynamic considerations.
Quantum energy gap in two quasi-one-dimensional S=1 Heisenberg antiferromagnets (invited)
Authors: Renard J. P. , Verdaguer M. , Regnault L. P. , Erkelens W. A. C. , Rossat-Mignod J. , Ribas J. , Stirling W. G. , Vettier C.
Published: July 2002

Following the Haldane conjecture, the antiferromagnetic (AF) Heisenberg chain of integer spins has a singlet ground state separated from the excited states by an energy gap. Recent numerical calculations on finite AF chains with S=1 supported this conjecture and provided an approximate value for the energy gap: EG≂0.4‖ J‖, where J is the intrachain exchange interaction. We report experimental studies on two Ni (II) quasi-one-dimensional (1D) AF with large intrachain interaction, J/k≂−50 K, Ni(C2H8N2)2NO2(ClO4) (NENP) and Ni(C3H10N2)2NO2(ClO4), (NINO). In both compounds, the magnetic susceptibility along the three crystal axes steeply decreases below T≂15 K and no 3D long-range magnetic order could be detected down to 1.2 K. These features are consistent with the Haldane conjecture. Inelastic neutron scattering experiments performed on NENP show two energy gaps, with an average value of about 0.4‖ J‖, which are explained by a splitting of the Haldane gap by single-ion anisotropy.
Very low interface recombination velocity in (Al,Ga)As heterostructures grown by organometallic vapor-phase epitaxy
Authors: Molenkamp L. W. , van’t Blik H. F. J.
Published: July 2002

We show that measurements of minority-carrier lifetimes in thin layer (Al,Ga)As double heterostructures grown by organometallic vapor-phase epitaxy can be severely influenced by modulation doping of the layer studied. Samples where this effect has been eliminated exhibit a dominantly nonradiative luminescence decay, from which we deduce an interface recombination velocity of only 18 cm s−1, which is among the lowest ever reported for a GaAs/(Al,Ga)As interface.
Antiphase domains in GaAs grown by metalorganic chemical vapor deposition on silicon-on-insulator
Authors: Chu S. N. G. , Nakahara S. , Pearton S. J. , Boone T. , Vernon S. M.
Published: July 2002

Distinct antiphase domain structures in GaAs epitaxial layers grown on a Si/SiO2/Si-substrate structure by metalorganic chemical vapor deposition have been revealed by using a silicon etchant (HF/HNO3). The antiphase is characterized by the [011]-oriented etching textures which rotate 90° between adjacent domains. The corresponding lattice rotation is further confirmed by a convergent beam electron diffraction technique. The size of the antiphase domains is found to increase with increasing film thickness and to grow upon annealing at temperatures above 700 °C. The maximum size of the domain, however, is found to be limited by the film thickness. The majority of the domain boundary lines revealed by chemical etching on the (100) surface do not correspond to any crystalline orientation. Only small segments are found to orient along [011], [010], [021], and, occasionally, [031] and [041] directions. Cross-sectional transmission electron microscopy studies confirmed that the boundaries are generally in curved configurations or zigzag configurations constituted of (011), (010), and (121) planes. All the boundaries are initiated at the interface and propagate through the film in the growth direction. Diffraction contrast experiments show a stacking-faultlike contrast of intrinsic type, indicating an inward relaxation of the lattice planes at the boundary. The rapid chemical reaction of the boundary with the silicon etchant, the intrinsic nature of the lattice distortion at the boundary, and the curved configuration of the boundary indicate that the boundary atoms are replaced by Si atoms. The higher concentration of Si atoms at the antiphase boundary has further been verified by energy dispersive x-ray analysis. In view of the reduction of bond distortion energy, the segregation of Si atoms at the antiphase boundary eliminates the highly distorted GaGa and AsAs bonds and is, therefore, an energetically favorable process. The possible antiphase boundary structure and a mechanism for its migration are discussed. Spatially resolved photoluminescence and cathodoluminescence studies reveal that both the antiphase boundaries and the defective interfacial regions contain nonradiative recombination centers. The luminescence efficiency of the domains increases strongly after annealing.
Sol-gel preparation of high-T
c Bi-Ca-Sr-Cu-O and Y-Ba-Ca-O superconductors
Authors: Fransaer J. , Roos J. R. , Delaey L. , Van Der Biest O. , Arkens O. , Celis J. P.
Published: July 2002

High-Tc superconducting oxides have been prepared by a liquid-mix technique using etylene-diamine-tetra-acetic acid (EDTA) as a complexing agent. Bi-Sr-Ca-Cu oxides and Y-Ba-Cu oxides were made using this technique giving a better compositional homogeneity, a more precise control of the cation stoichiometry and a decreased firing temperature as compared to conventional material produced by a solid-state reaction. This technique extends the amorphous citrate process to those systems where no citrate complex exists. EDTA binds with most metallic elements of the periodic table, making this technique a versatile tool in the production and study of these new ceramic materials. Therefore, the method is easily adapted to the preparation of new superconducting oxides.
Optical absorption and phase transitions in CuInSe2 and CuInS2 single crystals at high pressure
Authors: González J. , Rincón C.
Published: July 2002

The optical absorption edge of CuInSe2 and CuInS2 single crystals was measured as a function of hydrostatic pressure up to 10 GPa. In both cases the direct energy gap increases linearly with pressure at the rate of 30 and 24 meV GPa−1 for CuInSe2 and CuInS2 , respectively. From changes in the light-transmission characteristic of the samples under pressure, a structural first-order irreversible transition was observed at 7.1 GPa for CuInSe2 and 9.6 GPa for CuInS2 . Also, a new metastable semiconducting phase was formed in pressure-quenched CuInS2 samples.
Simulated behavior of the magnetically insulated oscillator
Authors: Marder B. M.
Published: July 2002

The magnetically insulated oscillator (MILO) is a high-power microwave generating device whose behavior has been investigated experimentally and by numerical simulation. This paper describes MILO operation and its dependence on various parameters as predicted by these simulations. The simulations are compared with both linear theory and experimental data.
Reaction of nonaqueous halogen solutions with YBa2Cu3O7−x
Authors: Vasquez R. P. , Foote M. C. , Hunt B. D.
Published: July 2002

X-ray photoelectron spectroscopy (XPS) has been used to investigate the reaction of YBa2Cu3O7−x films with solutions of HF, HCl, Br2, HBr, I2, and HI in absolute ethanol (EtOH). The XPS core level and x-ray excited Auger spectra from untreated and halogen-treated surfaces are used to identify surface species by comparison with XPS data from the literature and with XPS spectra from more than 20 Y, Ba, and Cu halides, oxides, hydroxides, and carbonates measured in this work. XPS measurements on a number of these materials are being reported for the first time. Treatment of films with HF/EtOH results in the formation of an oxyfluoride with Y:Ba:Cu relative concentrations of 1:4:3. Additional features in the XPS spectra from HF-treated films are also consistent with the formation of CuF, a compound which does not exist in bulk form. Treatment of films with HCl/EtOH results primarily in the formation of BaCl2 (∼75%), with smaller amounts of YCl3, CuCl, and CuCl2. Treatment of films with Br2/EtOH or HBr/EtOH results in the formation of YBr3, BaBr2, and CuBr with relative concentrations 1:4:3. YBa2Cu3O7−x films were found to have no discernible reaction with I2/EtOH solutions, but treatment of films with HI/EtOH results in the formation of CuI on the surface.
Applicability of the classical curvature-stress relation for thin films on plate substrates
Authors: von Preissig F. J.
Published: July 2002

The applicability in various circumstances of the classical equation relating substrate curvature to film stress for the case of a thin film on a plate substrate is examined. Theoretical treatments, based on elementary plate-bending theory and general elasticity theory, are given of the effects of gravity, substrate shape, film nonuniformity, and substrate crystallinity on substrate curvature. Formulas describing the effect of gravity and of film nonuniformity are confirmed experimentally using a laser-beam reflection technique. It is shown that gravity effects can cause significant errors in stress calculations based on the classical curvature versus stress equation, but these effects are largely avoidable or subtractable. Within limits, curvature is independent of substrate shape. For a film of nonuniform thickness, the classical equation does not apply, but in certain cases a simple analog does. Appropriate interpretation of elastic moduli appearing in the classical equation allows the equation to apply when the substrate is elastically anisotropic (e.g., a single crystal) with third-order or higher symmetry about its z axis. These studies augment thin-film stress measurement experimental technique.
XeCl laser ablation of polyimide: Influence of ambient atmosphere on particulate and gaseous products
Authors: Singleton Donald L. , Paraskevopoulos George , Irwin Robert S.
Published: July 2002

The gaseous and particulate products of the XeCl (308 nm) laser ablation of polyimide (Kapton H) are quantitatively determined and compared with the mass loss of the polymer in atmospheres of He, N2, air, or O2. In air and in pure O2, the observed mass balance is about 90%, but is lower for inert atmospheres. With increasing oxygen content in the atmosphere, the yield of CO2 increases at the expense of particulates and acetylene. The influence of laser fluence and nature of the ambient atmosphere on the product distribution is interpreted in terms of ejection of small reactive species which are involved in the competitive reactions of particulate formation and oxidation to CO2.
The crystallization of amorphous Fe60Co20Si10B10 and its effect on the electrocatalytic activity for H2 evolution
Authors: Trudeau M. L. , Huot J.-Y. , Schulz R.
Published: July 2002

The amorphous alloy Fe60Co20Si10B10 has been shown to display a high electrocatalytic activity for the hydrogen evolution in 30 wt. % KOH at 70 °C. We present, in this paper, a structural study of the crystallization process and an attempt to correlate the observed changes to the electrocatalytic behavior. The results for crystalline samples of the same average composition obtained from the melt at a lower cooling rate are also presented. Our results show that the amorphous structure is the best suited for the production of hydrogen and that the changes observed can be related in some part to the different processes of surface oxidation.
Effect of partial substitution of Ge4+ for Zn2+ on the microstructure and temperature stability of Mn-Zn ferrites
Authors: Liu C. S. , Shu W. B. , Tung M. J. , Ke M. Y.
Published: July 2002

The effects of substitution of GeO2 for ZnO on the microstructure and magnetic properties of Mn-Zn ferrite have been studied. Results suggest that, to a very limited extent, by the substitution of Ge and a proper choice of sintering condition, good temperature coefficients can be obtained in a temperature range from 20 to 80 °C. Experimental results show that the substitution of GeO2 promotes discontinuous grain growth, and the lattice parameter is slowly reduced due to the substitution of from 0.5 to 2.0 mol % GeO2 for ZnO. The temperature coefficient is extremely process sensitive. For appropriate Ge substitution (0.5%), optimum initial permeability and the temperature coefficient are dependent on the sintering conditions. Samples with 0.5 mol % GeO2 sintered at 1290 °C using a rapid heating rate and short-time sintering yielded the best temperature coefficient of 0.1×10−6 °C−1 over the range 20–80 °C.
Power dissipation in capacitively coupled rf discharges
Authors: Beneking C.
Published: July 2002

The power dissipation in capacitively coupled rf discharges (Ar, 400 Pa) has been studied by impedance measurements in the 10–50 MHz frequency range. The article focuses on electrodeless discharge configurations in which the field-supplying electrodes are separated from the discharge volume by a dielectric wall, but the study of a classical electroded rf discharge is also included for comparison. The power share between bulk plasma and sheath regions is determined quantitatively using an equivalent circuit model which reflects the discharge structure. It depends strongly on both rf discharge current and frequency. For the electrodeless discharge configurations and equally for the electroded one, the fraction of power dissipated in the sheaths generally increases with increasing current and decreasing frequency. The sheath thickness derived from the impedance measurements decreases with frequency. It is nearly independent of the current for the electrodeless discharge configurations, but decreases with increasing current for the discharge configuration with internal metallic electrodes. A model for the power dissipation in the sheath is based on the current-voltage characteristics of a space-charge-limited ion current. It allows the calculation of the dissipated power Ps for operator set discharge conditions such as rf current, frequency, electrode area, and gas pressure, and predicts that Ps should vary with current I and frequency ω as (I/ω)5/2. The model is in good agreement with the experimental data. The contribution to the power dissipation from further processes besides the ion current in the sheath is also examined.