bluuebunny/crossref_metadata_2025 huggingface dataset:
Circuit simulation of resonant tunneling double-barrier diode
Authors: Liu H. C.
Published: July 2002

Circuit simulations of resonant tunneling double-barrier diodes (DBD) are reported. Small signal equivalent circuit considerations predict whether a circuit is stable when a DBD is biased in the negative differential resistance (NDR) region, and also predict the nature of an oscillation (sinusoidal-like or exponential) if unstable. Large signal analyses give the temporal behavior of an oscillation. The nonlinear differential equation is solved by a piecewise linear model to a high accuracy. Oscillations give rise to an apparent plateaulike structure or even a peak structure in the NDR region when measuring the dc current-voltage characteristic.
Critical current as a function of temperature in thin YBa2Cu3O7−δ films
Authors: de Vries J. W. C. , Gijs M. A. M. , Stollman G. M. , Baller T. S. , van Veen G. N. A.
Published: July 2002

The critical current Ic of several triode-sputtered and laser-ablated thin YBa2Cu3O7−δ films is investigated over the temperature range of 5–85 K. Near the critical temperature Tc it is found that Ic∝(1−T/Tc)γ with γ≂1.5–2. The value of γ depends very sensitively on the value determined for Tc. The low-temperature data, for temperatures below about 0.6Tc, can be well analyzed in terms of a proximity-effect model. The effective barrier thickness dN between superconducting parts of the samples is of the order of a few nanometers for all samples studied.
Extremely low nonalloyed and alloyed contact resistance using an InAs cap layer on InGaAs by molecular-beam epitaxy
Authors: Peng C. K. , Chen J. , Chyi J. , Morkoç H.
Published: July 2002

Extremely low alloyed and nonalloyed ohmic contact resistances have been formed on n-type InAs/In0.53Ga0.47As/In0.52Al0.48As structures grown on InP(Fe) by molecular-beam epitaxy. To insure the accuracy of the small contact resistances measured, an extended transmission line model was used to extrapolate contact resistances from test patterns with multiple gap spacings varying from 1 to 20 μm. For a 150-Å-thick InAs layer doped to 2×1018 cm−3 and a 0.1-μm-thick InGaAs layer doped to 1×1018 cm−3, a specific contact resistance of 2.6×10−8 Ω* cm2 was measured for the nonalloyed contact, while a resistance less than 1.7×10−8 Ω* cm2 is reported for the alloyed contact. Conventional Au-Ge/Ni/Au was used for the ohmic metal contact and alloying was performed at 500 °C for 50 s in flowing H2. Using a thermionic field emission model, the barrier height at the InAs/InGaAs interface was calculated to be 20 meV.
Perturbative analysis of planar phase change processes with time-dependent temperature at the boundary
Authors: Charach Ch. , Huleihil M. , Zarmi Y.
Published: July 2002

The perturbation method developed recently for analysis of planar phase change processes is extended to the case of time-dependent temperature boundary conditions. Approximate solutions uniformly valid in time are derived. The role of time scales, that of the driving signal and the (time-dependent) response time of the emerging phase, in the dynamics of a weakly nonlinear melting process is clarified. The short and long time asymptotics are studied in detail. The analytical results are compared with the numerical solution for the problem of melting in a semi-infinite slab induced by a periodically varying boundary condition.
Silicon oxynitride films: Ion bombardment effects, depth profiles, and ionic polarization, studied with the aid of the Auger parameter
Authors: Rivière J. C. , Crossley J. A. A. , Sexton B. A.
Published: July 2002

Thin films of Si3 N4, SiO2, and of silicon oxynitrides with compositions in the range 0.3≤O/N≤3.6 were deposited on silicon substrates to thicknesses between 20 and 150 nm by low-pressure chemical vapor deposition. Auger parameters of the films were measured during the study of ion bombardment effects and depth profiling, and as a function of the O/N ratios.

The particular parameter used was based on the Si 2s photoelectron, and bremsstrahlung-excited Si KLL Auger lines. When normalized to constant ion dose, no significant differences in the effects of bombardment with argon ions in the energy range 0.5–4.0 keV could be found, and the equilibrium O/N ratio measured by x-ray photoelectron spectroscopy was close to that measured by Rutherford backscattering and elastic recoil detection analysis. During depth profiling no reduction of the oxynitride to elemental silicon was observed, in contrast to previous work using the low-energy Si LVV Auger peak with electron excitation; it is therefore recommended that to perform depth profiling of these materials while retaining artefact-free compositional and chemical information, the high kinetic energy Si spectral features should be used. At the oxynitride/silicon interfaces in some of the films additional features were found corresponding to intermediate Si chemical states. The widths of the Si 2s, 2p and KLL spectral features were measured at the stages in depth profiling at which equilibrium composition had been reached.

In every case there was an increase in width as a function of oxygen fraction O/(O+N), from Si3 N4, through a maximum at about O/(O+N)≊0.5, followed by a decrease to SiO2. The maximum increase for the KLL Auger peak was ≊0.9 eV, but for the 2s and 2p only ≊0.35 eV, and the peak broadening was uniformly distributed about the peak centroid. The broadening is attributed to the appearance of a number of additional chemical states of Si too closely spaced to be resolvable, and probably arising from a defective film structure. The Auger parameters of the various films, measured also once equilibrium composition had been reached during profiling, showed what appeared to be a monotonic dependence on O/(O+N), thus supporting the random bonding model in which nitrogen atoms are substituted randomly by oxygen in the SiNz O4−z (z=0, 1, 2, 3, 4) tetrahedra.

However, the Auger parameter versus composition plot was displaced slightly from the theoretical one in the direction of higher parameter than expected. When the Auger parameter was plotted against the function (n2−1 )/n2, proportional to the polarization energy, n being the measured refractive index of a film, a linear relationship was found within experimental error, but with a slope different from that for the standards SiO4/2 (silica), SiO6/3 (stishovite), Si3 N4, and Si. This divergence is interpreted in terms of a lower O ionic polarizability in the oxynitrides than in the oxides.
Structure and magnetism of transition-metal–magnesium thin films with an artificially layered structure
Authors: Sato Noboru
Published: July 2002

Transition-metal–magnesium (TM-Mg) thin films, Fe-Mg, Co-Mg, and Ni-Mg, with an artificially layered structure are formed by the magnetron sputtering method. TM and Mg grow epitaxially on the closest planes in films with a thicker Co layer than a Mg layer. Crystallographic structures of the films are the bcc with the 〈110〉 direction normal to the film plane for Fe-Mg films, the hcp with the 〈001〉 normal for Co-Mg films, and the fcc with the 〈111〉 normal for Ni-Mg films. The films, composed of a monatomic layer of Mg and then several atomic layers of TM in an alternating way, show a clearly separated spot pattern in the reflection high-energy electron diffraction analysis. The artificial superlattice structure of the films is thermally stable at temperatures up to 700 K. The films composed of a thicker TM layer than a Mg layer are ferromagnetic with an easy magnetization direction in the film plane. The magnetization of TM in all films is smaller than that of the bulk. The paramagnetism or the nonmagnetism is observed in the films composed of a thinner TM layer than a Mg layer. Small magnetization, i.e., paramagnetism or nonmagnetism, originates from decreases both in magnetization and in Curie temperature of the TM due to lattice deformation.
A high-sensitivity torsional magnetometer for two-dimensional electron systems
Authors: Templeton I. M.
Published: July 2002

A dc torsional magnetometer for use in high magnetic fields is described. It provides several significant advantages over an earlier device, both by its increased sensitivity and better damping, and also, in particular, because its design allows a torque to be introduced electrostatically. This makes it possible to compensate for monotonic background signals, and to provide a ‘‘transfer standard’’ between calibration and operating conditions. An example is given of its use in the study of the de Haas–van Alphen effect in a single-quantum-well two-dimensional electron system.
Lattice vibrations of cadmium manganese telluride alloys
Authors: Wrobel J. M. , Clayman B. P. , Becla P. , Sudharsanan R. , Perkowitz S.
Published: July 2002

Reflectivity spectra in the reststrahlen region (100–250 cm−1) are presented for bulk zinc blende Cd1−xMnxTe crystals at 2, 50, and 300 K and for x=0 to 0.64, covering nearly the entire possible composition range for cubic structure. Kramers-Kronig analysis gives the transverse and longitudinal phonon frequencies. The Lorentz model confirms these results and also gives mode strengths and damping constants. Compositional dependence of the phonon energies is predicted by the random element isodisplacement model.
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u investigation of the microcrystalline germanium nucleation and growth processes
Authors: Drevillon B. , Godet C.
Published: July 2002

The amorphous (a) to microcrystalline (μc) transition, induced by the increase of the ion bombardment energy Eion and the substrate temperature Ts , is investigated on glow-discharge deposited germanium thin films. This transition and the deposition of μc-Ge on metallic substrates are observed in situ by kinetic and spectroscopic phase modulated ellipsometry (SPME) over the range 1.7–4.5 eV. The ion bombardment energy corresponding to the transition a-μc (Eion∼120 eV) decreases as a function of Ts in the range 160–280 °C. Decreasing Eion (or Ts) results in the reverse transition. The μc-a transition is found at a ∼50 eV (or ∼120 °C) lower value, giving evidence of an epitaxial growth effect. The kinetics of the growth processes is deduced from the SPME data by using an effective medium approximation. The early stage of the growth is accurately described by the nucleation of microcrystalline islands at a 50 Å level; the time dependence being coherent with a low surface mobility of the diffusing species. During the growth, a composition depth profile is observed; the ∼150 Å thick surface layer being more amorphous than the bulk.
Layer disordering of GaAs-AlGaAs superlattices by diffusion of laser-incorporated Si
Authors: Epler J. E. , Ponce F. A. , Endicott F. J. , Paoli T. L.
Published: July 2002

Recently, a laser-scanning technique for patterning layer disordering has been reported in which silicon impurities are incorporated by laser irradiation into GaAs-AlGaAs heterostructures. The sample is subsequently annealed to diffuse the Si deep into the heterostructure. The GaAs-AlGaAs layers are locally intermixed by diffusion of the silicon impurities. In this report, this two-step process of laser-assisted disordering is studied in detail with several material characterization techniques. Scanning electron microscopy and secondary ion mass spectrometry are used to correlate the extent of the layer-disordered region with the presence of laser-incorporated Si and oxygen. Transmission electron microscopy (TEM) is used to obtain high resolution images of the disordered/as-grown interface and to evaluate the crystallinity of the laser-affected material. TEM images allow the determination of the distribution of the Al and Ga constituents at the interface between the impurity-disordered alloy and the as-grown crystal. The data indicate a more rapid Si diffusion in the GaAs layers relative to the Al-rich layers. The data are discussed in the context of device fabrication.
Optical properties at the metal-insulator transition in thermochromic VO2−xFx thin films
Authors: Khan K. A. , Niklasson G. A. , Granqvist C. G.
Published: July 2002

Thermochromic VO2−xFx thin films were prepared by reactive rf magnetron sputtering followed by annealing post-treatment. Electrical and optical properties were measured as a function of temperature. The transmittance and reflectance were essentially wavelength independent at 65 °C, i.e., at the metal-insulator transition.
Effects of N2, O2, and H2O on GaAs passivated by photowashing or coating with Na2S⋅9H2O
Authors: Wilmsen C. W. , Kirchner P. D. , Woodall J. M.
Published: July 2002

The photoluminescence (PL) from the surface region of GaAs passivated by photowashing or coating with Na2S⋅9H2O is shown to be sensitive to the gas ambient. Both water vapor and oxygen must be present in order to obtain a large PL signal. The effects are activated by the measuring laser light.
Microtwin morphology and volume fraction for silicon on sapphire
Authors: Twigg M. E. , Richmond E. D.
Published: July 2002

In order to understand the influence of various growth and processing procedures on the quality of silicon grown on sapphire (SOS), one needs a way to measure microtwin density as a function of distance from the interface. In previous studies, however, the technique best suited for determining such a density profile, transmission electron microscopy (TEM), has been applied to this problem in a relatively qualitative manner. We have chosen to apply TEM in a more quantitative fashion to this problem by keying our observations to microtwin morphology. These observations indicate that the microtwins occurring in SOS grown by chemical vapor deposition are faceted and grow contiguously from the interface. The volume fraction of silicon occurring as microtwins does not decrease away from the interface at as large a rate as indicated by previous TEM studies. Instead, the decrease in the number of twins away from the interface appears to be balanced by an increase in twin size. It also appears that the greatest volume fraction of twinned material occurs at least several hundred angstroms away from the interface rather that at the interface.
Vapor concentration measurement with photothermal deflectometry
Authors: Banish R. Michael , Xiao Rong-Fu , Rosenberger Franz
Published: July 2002

Theoretical and experimental results for using the photothermal deflection technique to measure vapor species concentration, while minimizing the disturbance of the transport (material) parameters due to vapor heating, are developed and described. In contrast to common practice, the above constraints require using a pump-beam duty cycle of less than 50%. The theoretical description of the shortened heating time is based on a step-function formulation of the pumping cycle. The results are obtained as closed-form solutions of the energy equation for many chopping cycles until steady state is reached, by use of a Green’s-function method. The Euler formulation of the Fermat principle is used to calculate the deflection angle. The equations are expanded to include the effects of vapor velocity on both the temperature and temperature gradient profiles. The effects of finite (unfocused) pump and probe beams and thermal (Soret) diffusion are also accounted for. Excellent agreement between theory and experiment is obtained.
Transient analysis of bulk nitrogen glow discharge
Authors: Dhali S. K. , Low L. H.
Published: July 2002

The growth of the discharge current, reduced field (E/N), vibrational population, excited-state population, and nitrogen atom population in a nitrogen discharge is simulated using a self-consistent calculation of the electron distribution function and the vibrational population. The model developed includes diffusion losses to the wall and the external circuit parameters. The results discussed are for pressures of 1–100 Torr, discharge currents of a few amperes, and reduced field (E/N) in the range of 150–250 Td. A typical discharge in a tube of 2 cm diam at a current of 3.5 A requires a maintenance E/N of 65 Td at 10 Torr pressure, produces an electron density in the range of 0.5–1.0×1012 cm−3, and N2(A) density of 0.5–1.0×1014 cm−3.
Effect of pressure on the transport properties of (In, Ga)As
Authors: Patel D. , Spain I. L.
Published: July 2002

Results of room-temperature photoconductivity and Hall measurements on (In, Ga)As are presented for pressures up to 1.5 GPa in a diamond anvil cell. The increase in the energy band gap with increasing pressure is shown by the shift in the photoconductivity edge. The increase in the energy band gap with the electron effective mass has been calculated. Analysis of the Hall data in terms of scattering mechanisms show dominance of polar-optical scattering with only a small fraction of alloy scattering.
Analysis of temperature dependence of Hall mobility of nondoped and nitrogen-doped β-SiC single crystals grown by chemical vapor deposition
Authors: Suzuki Akira , Ogura Atsuko , Furukawa Katsuki , Fujii Yoshihisa , Shigeta Mitsuhiro , Nakajima Shigeo
Published: July 2002

Temperature dependencies of Hall mobility of nondoped and nitrogen-doped n-type β-SiC films have been analyzed using a conventional theoretical model. Considering acoustic, polar optical, and piezoelectric lattice scatterings, as well as ionized and neutral impurity scatterings, theoretical calculations well fitted to the experimental results are obtained at 70–1000 K. Contributions of acoustic, polar optical, and piezoelectric scatterings to the whole lattice scattering are 84%, 14%, and 2% at 300 K, respectively. Impurity compensation ratio NA/ND of nondoped films increases from 0.45 to 0.96 with increasing Si/C ratio in the source gases. Nitrogen-doped films show constant compensation ratios of 0.25–0.30 with various doping amounts. These values are different from the previous results obtained by the analysis of temperature dependencies of carrier concentration.
Frequency-dependent negative photoconductivity and related dielectric behavior in single-crystal proustite Ag3AsS3
Authors: Taylor K. N. R. , Yang S. R.
Published: July 2002

Ultralow-frequency (10−2–103 Hz) electrical conductivity and dielectric constant measurements have been made on single-crystal proustite at room temperature under various illumination conditions. An unusual phenomenon of frequency-dependent negative photoconductivity and negative photocapacitance was observed at very low frequencies. We believe that both these negative photoeffects can be attributed to space-charge effects and the results are interpreted using a two-layer dielectric structure model.
Extended bulk defects induced by low-energy ions during partially ionized beam deposition
Authors: Lee W. I. , Wong J. , Borrego J. M. , Lu T.-M.
Published: July 2002

The study of possible defects generated by low-energy ions during partially ionized beam (PIB) depositions was performed. No defects were observed when acceleration voltage was set lower than 1 kV. Surprisingly, several deep levels were detected up to the depth of 4000 Å in the 3-kV sample. However, these levels can be annealed out at a relatively low temperature of 400 °C. It is concluded in this study that, by properly choosing the ion energy range, PIB deposition will not cause severe damage to the substrate and can be a viable technique for growing heterostructures.
On the rate of thinning of thin liquid films on a rotating disk
Authors: McConnell William H.
Published: July 2002

Thin liquid films are often used to coat rotating magnetic recording disks so as to enhance their durability. These rotating films are subject to inertial and wind shear forces that induce radial motion thereby thinning them. An experiment is discussed here to study the relative importance of inertial forces and wind shear. For a 28-nm-thick layer of perfluoropolyether, air shear dominates inertial effects. Furthermore, the thinning of these films without wind shear does not deviate from the classical models of nearly parallel, slow viscous flow using the bulk viscosity and no slip.
Microscopic transient simulation of semiconductors and insulators
Authors: Shapiro Finley R. , Bar-Yam Yaneer
Published: July 2002

We have developed a general-purpose simulator for transient experiments in semiconductors and insulators based on microscopic transport of carriers and including an arbitrary distribution of localized states in the band gap. As a first application of the simulator we show the results of simulated time-of-flight experiments on amorphous silicon hydride. We demonstrate that due to the large dynamic range of microscopic transit times, different definitions of the mean transit time result in different observed mobilities. We also show that such experiments may underestimate the extended state mobility.
Minority-carrier lifetime in ITO/InP heterojunctions
Authors: Ahrenkiel R. K. , Dunlavy D. J. , Hanak T.
Published: July 2002

The relationship between photoluminescence lifetime and minority-carrier lifetime is derived for window/absorber heterojunctions by the method of Laplace transforms. The model includes the effects of diffusion to the depletion region and self-absorption of the emitted radiation. The model is applied to InP photoluminescence generated by pulsed laser excitation of indium-tin-oxide (ITO)/InP heterojunctions. The photoluminescence lifetime of a device with NA=1×1016 cm−3 is about 21 ns. The bulk lifetime of the device can be fit with a lifetime of about 30 ns. The lifetime in the unprocessed substrate exceeds 200 ns indicating that recombination is induced by ITO processing.
Further studies of liquid-metal flows and power losses in ducts with a moving conducting wall and a skewed magnetic field
Authors: Walker John S. , Brown Samuel H. , Sondergaard Neal A.
Published: July 2002

In a previous paper the authors initiated studies of fully developed laminar liquid-metal flows, currents, and power losses in a rectangular channel with a moving perfectly conducting wall and with a skewed homogeneous external magnetic field for high Hartmann numbers, high interaction parameters, low magnetic Reynolds numbers, and different aspect ratios. The channel had insulating side walls that were skewed to the external magnetic field, while the perfectly conducting moving top wall with an external potential and the stationary perfectly conducting bottom wall at zero potential acted as electrodes. These electrodes were also skewed to the external magnetic field. A mathematical solution was obtained for high Hartmann numbers by dividing the flow into three core regions, two free shear layers, and six Hartmann layers along the channel walls. The free shear layers were treated rigorously and in detail with fundamental magnetohydrodynamic theory. The previous work, however, left the solution for the velocity profiles in terms of a complex integral equation which was not solved. In the present work the integral equation is solved numerically

by the method of quadratures to give the velocity profiles, viscous dissipation and Joulean losses in the free shear layers. In addition, expressions for the viscous dissipation in the six Hartmann layers are presented. The best approximation to the viscous dissipation in the channel is the sum of the O(M3/2) contributions from the two free shear layers, the O(M3/2) contributions from the two Hartmann layers separating the free shear layers from the insulators, and the O(M) contributions from three of the Hartmann layers separating core regions from the walls. The best approximation to the Joulean power losses in the channel is the sum of the O(M2) contribution from the central core region which carries an O(1) current between the electrodes and the O(M3/2) contributions from the free shear layers. The expressions for the viscous dissipation and Joulean losses in each region involve the products of universal constants, electrical potentials and geometric factors. The theoretical magnetohydrodynamic model presented here was developed to provide data to help in the design of liquid-metal current collectors.
Microwave spectroscopy and magnetization measurements of flux trapping and hysteresis in YBa2Cu3O7−δ
Authors: Rubins R. S. , Drumheller John E. , Hutton Stuart L. , Rubenacker Gerald V. , Jeong D. Y. , Black T. D.
Published: July 2002

Microwave spectroscopic measurements at 9.2 GHz on a superconducting sample of YBa2Cu3O7−δ have established the occurrence of flux trapping and hysteresis associated with a low-field signal. The position of the signal peak is related to the field at which an abrupt change in the slope of the magnetization curve is observed. The data are consistent with a superconducting glass model.
Reliability of the diffusion approximation for the ballistic relocation function in atomic mixing by ion beams
Authors: Collins R. , Jimenez-Rodriguez J. J. , Wadsworth M. , Badheka R.
Published: July 2002

A simple idealized ballistic relocation function is considered for the case of a single marker layer. Material collectively displaced by recoiling atoms is represented by a Kirkendall drift velocity. The mixing equations are solved numerically both in the diffusion approximation and with the exact relocation function. For small mean relocation distance it is shown that the diffusion approximation gives very good agreement with the exact solution, but for larger mean relocations the diffusion approximation gives solutions with qualitatively wrong features, in some cases reversing the direction of motion of the peak of the marker layer.
Influence of magnetic field on 1/f noise in GaAs resistors without surface effects
Authors: Song M. H. , Birbas A. N. , van der Ziel A. , van Rheenen A. D.
Published: July 2002

The influence on magnetic field on 1/f noise in a planar GaAs resistor grown by molecular-beam epitaxy and without surface effects was investigated experimentally. The experimental results can be explained by the number fluctuation model but not by the mobility fluctuation model. Previously, experimental results indicating number fluctuation type of 1/f noise were mostly attributed to the surface effects associated with the particular structures used for the experiments. In our device the surface effects were diminished so that the fluctuations of the bulk current could be considered to produce the 1/f noise.
Investigation of the amorphous-to-microcrystalline transition of hydrogenated silicon films by spectroscopic ellipsometry
Authors: Herak T. V. , Schellenberg J. J. , Shufflebotham P. K. , Kao K. C.
Published: July 2002

Spectroscopic ellipsometry and x-ray diffraction measurements have been used to obtain structural information on hydrogenated amorphous and microcrystalline silicon thin films. The films were deposited onto quartz substrates from a microwave plasma in SiH4/H2 gas mixture. For ellipsometric data analysis, the films were modeled as multilayer structures with the dielectric response of each layer calculated as a function of the amorphous, crystallite, and void volume fractions through an effective-medium approximation. Results indicate that the transition from amorphous-to-microcrystalline films is accompanied by a reduction in the material density and a significant increase in the surface roughness overlayer. X-ray diffraction measurements estimate a higher volume fraction of crystallites as compared to that obtained from optical data.
Analysis of mechanism for resonant tunneling via localized states in thin SiO2 films
Authors: Gu B. , Mangiantini M. , Coluzza C.
Published: July 2002

In this work we use transfer matrix and Airy function approaches to investigate the distribution of carriers within thin SiO2 films containing localized states for Si/SiO2/Si structure in detail. We show that the distribution of the probability density ‖ψ‖2 within oxide films has a shape with, at first, a monotonically exponential decrease from the Si/SiO2 interface, then arriving at a valley, and followed by a peak at the center for the case of ‘‘off’’ resonance. When the carrier energy matches that of the eigenstate the probability density is exponentially decreased from the localized center on both sides. We show that there are two kinds of probability densities corresponding to resonant tunneling via a localized state within the barrier. On increasing the reverse applied field, the localization degree of the wave function decreases.
Triplet extinction coefficients of some laser dyes I
Authors: Pavlopoulos Theodore G. , Golich Daniel J.
Published: July 2002

We measured the triplet extinction coefficients εT over the laser action spectral region of Rhodamine 6G; Rhodamine B; Rhodamine 110; Fluorol-7GA; Coumarin 540A; Coumarin 522; Coumarin 1; Coumarin 120; 4,4′-diphenyl stilbene; and 2,7-bis-(4-methoxy-phenyl)-9,9-dipropylfluorene. We employed the different lines from an argon ion cw laser for excitation. McClure’s method was used to obtain the triplet extinction coefficients εT. The method requires the measurement of triplet optical densities ODT as a function of different cw laser excitation intensities (powers) Iex . The importance of triplet-state losses on dye laser efficiency is reviewed. The laser action properties of the laser dyes we studied are briefly discussed as they relate to the measured εT values.
Current multiplication during relativistic electron-beam propagation in a subTorr-pressure gas
Authors: Gupta G. P. , Rohatgi V. K.
Published: July 2002

The current multiplication observed when a relativistic electron beam is propagated in an initially neutral gas at subTorr pressures (0.1–0.3 Torr) is generally attributed to the two-stream instability. A general expression for this current gain is derived for a scattered beam propagating in a neutral gas, which takes into account a quasihydrodynamic expression for the beam energy loss. Moreover, the beam-to-plasma density ratio is obtained as a function of time for a given beam pulse by numerically solving the electron energy and continuity equations. It is found that at a given beam-to-plasma density ratio, the current gain increases with increasing the beam energy, with the increase being most rapid for the small scattering angles. A finite average scattering angle is seen to reduce the current multiplication. The experimentally observed peak current gains are in good agreement with the predicted peak gains obtained by taking into account the time-evolved beam-to-plasma density ratio.
Reproducible sputtering and properties of Y-Ba-Cu-O films of various thicknesses
Authors: Luborsky F. E. , Kwasnick R. F. , Borst K. , Garbauskas M. F. , Hall E. L. , Curran M. J.
Published: July 2002

One goal of this work was to develop a reproducible method of preparing high quality Y-Ba-Cu-O superconducting films and to study their properties versus thickness. This was accomplished by rf diode sputtering from a single target. Twenty-seven depositions were made using a target containing 8.9-at. % Y, 37.3-at. % Ba, and 53.8-at. % Cu. Film thicknesses ranged from 0.09 to 2.4 μm. The film compositions obtained were 15.6±1.0-at. % Y, 35.8±1.0-at. % Ba, and 48.7±1.7-at. % Cu for the mean and standard deviation. The films were amorphous as-deposited and crystallized by annealing in O2 at 915 °C. X-ray diffraction, transmission electron microscopy, and scanning electron microscopy indicated that, on (100)SrTiO3 substrates, films with thickness less than ∼0.25 μm were epitaxially oriented with their c axis perpendicular to the substrate. Films on (110)SrTiO3 were oriented with their c axis parallel to the substrate. On (100)SrTiO3, zero resistance was achieved in 30 samples from 27 runs at 85.6±1.4 K with a transition width (10%–90%) of 1.8±1.1 K independent of thickness. Results for deposition on a variety of other substrates were more variable. Diamagnetic shielding of up to 38% was calculated from the initial slope of M vs H. This low value was attributed to the presence of second phases and a significant diffusion layer thickness, both observed by transmission electron microscopy. Critical currents, measured by transport using a four-point probe, reached 8.1×105 A/cm2 at 77.35 K for a 0.2-μm film deposited on (100)SrTiO3 . Comparable values were obtained by calculation from the M-H hysteresis loop, from which we infer that there are either continuous epitaxial sheets of c axis oriented 123 or, if there are grain boundaries, the coupling across them is strong.
Metallurgical studies and optimization of sintering for the Bi-Sr-Ca-Cu-O superconductors
Authors: Numata K. , Mori K. , Yamamoto H. , Sekine H. , Inoue K. , Maeda H.
Published: July 2002

Metallurgical studies and optimization of sintering condition to obtain the maximum volume fraction of the high Tc phase (∼110 K) have been made for the Bi-Sr-Ca-Cu-O system. In the differential thermal analysis, double peaks of endothermic reaction at ∼870 and ∼910 °C were observed during the increase of temperature, and a weight loss accompanied each endothermic reaction. Optimum calcining and sintering conditions to obtain a large volume fraction of the high Tc phase were 820 °C×14 h+840 °C×6 h and 875 °C×100 to 200 h in air, respectively. As for the variation of Cu concentration, nominal composition of Bi1Sr1Ca1Cu2Ox gave the maximum volume fraction of the high Tc phase. (For these specimens, sintering time was varied up to 20 days.) Post annealing at 400 °C for 20 h in an oxygen atmosphere after the sintering at 875 °C for 100 to 470 h in air had little effect on the improvement of volume fraction of the high Tc phase. As for the specimen to which Pb is added, however, the post annealing at 400 °C for 20 h increased the volume fraction of the high Tc phase. Scanning electron microscopy showed that the Bi1Sr1Ca1Cu2Ox (nominal composition) specimens sintered at 875 °C for 100 to 470 h consist mostly of well-grown thin layers, while the specimen sintered at 875 °C for 14 h consists mostly of fine pieces of thin layers piling up at random. Electron probe microanalysis revealed that microstructure of the specimen sintered at 875 °C consists of Bi-poor regions and the matrix surrounding them, and that these Bi-poor regions are classified into Cu-rich regions and Ca-rich regions. The superconducting phases of this material are considered to exist in the matrix region. We identified the Cu-rich regions and Ca-rich regions as (Sr0.45, Ca0.55 )3 Cu5 Ox and (Sr0.08, Ca0.92 )2 CuOx, respectively. The average size of these regions increased with increasing time of sintering, although total volume fraction of these regions did not increase.
Microstructure effects on electronic properties of Nb/Al2O3/Nb tunnel junctions
Authors: Blamire M. G. , Somekh R. E. , Barber Z. H. , Morris G. W. , Evetts J. E.
Published: July 2002

We report results obtained from several hundred Nb/Al2O3/Nb tunnel junctions fabricated by the ‘‘whole-wafer’’ process. These results indicate that one of the factors determining resistance and quality variations in such devices is the microstructure of the underlying base layer of niobium. We have further developed the technique of anodization spectroscopy to provide a quantitative measure of the variation in the thickness of the thin aluminum layers which contain the tunnel barrier and have related this to the temperature at which the niobium base layer is deposited. We show that at least two mechanisms are responsible for normal-state resistance variations in such devices and that device quality is closely related to the integrity of the aluminum layer underlying the tunnel barrier.
Theoretical analysis of the intrasubband drift velocity of hot electrons in n-GaAs quantum wells
Authors: Kiener C. , Vass E.
Published: July 2002

The drift velocity of hot electrons in an n-GaAs quantum well is calculated with the exact distribution function deduced from the Boltzmann integral equation. The interaction of the charge carriers with surface and bulk polar optical phonons and ionized impurities is considered in the calculation. The influence of intercarrier collisions as well as the quantum-well width on the distribution function and on the drift velocity is analyzed systematically. The theoretical results are compared with recent experimental data deduced from pulsed Hall measurements.
Dynamic characteristics of level and two-axis attitude detector using magnetic fluid
Authors: Kanamaru Y. , Ogasawara Y. , Hayashi K.
Published: July 2002

Level and attitude detectors are required for the detection of level and angles, for instance, in controlling the attitude of underwater robots, to keep solar cell panels directed constantly toward the sun, and the parabola antenna constantly toward the manmade satellite in communication systems. The usual angle detector can detect angle in one axis only, but the detector described in this paper is able to detect the angles of orientation with the horizontal and the vertical planes with a single unit. In addition, the new detector uses levels, such as water levels, by itself. This detector has three tubes with the magnetic fluid placed inside and with three coils. Its structure is very simple, small, and robust. It is also a light unit. The output signals are sine waves. The static operating characteristics of this detector were shown previously. The dynamic characteristics are presented in this paper.
Effect of induced anisotropy on saturating pulse performance
Authors: Sheard S. M. , Gibbs M. R. J. , Avery R. K.
Published: July 2002

In this paper a clear correlation between the controlled drive ac and dc magnetic characteristics and the subsequent pulse performance of thin tape-wound amorphous cores is demonstrated. The effects of introducing longitudinal and transverse anisotropies by field annealing are discussed. ac and dc characterization of the toroids are used to indicate the effect of the annealing treatments, and the pulse results are then analyzed using this information. Finally, on the basis of the results obtained in these three regimes, further possible treatments which may improve the pulse behavior of these materials are suggested.
Magnetic ordering and superconducting in Fe-doped 1-2-3 compounds
Authors: Qiu Z. Q. , Du Y. W. , Tang H. , Walker J. C. , Morgan J. S. , Bryden W. A.
Published: July 2002

We have doped Fe into the ground YBa2Cu3O7 and have discovered that the Fe goes primarily into one of the two Cu sites. At least 95% of the Fe substitutes into the Cu(1) site in the Cu-O layers between Ba layers. A high valence state of iron, Fe4+, is found to exist inside this compound. For even small amounts of Fe we see a spin-glass type of magnetic ordering of the Fe spins which we feel is associated with a tendency for antiferromagnetic couplings of the Cu(1) ions. We conclude that the Cu(2)-O2 plane is more important than the Cu(1)-O chain for the superconductivity in the 1-2-3 compounds.
High coercivities in as-cast Nd-Fe and Nd-Fe-Ti alloys
Authors: Tsoukatos A. , Strzeszewski J. , Hadjipanayis G. C.
Published: July 2002

Bulk Nd73Fe27 and Nd66.8Fe24.7Ti8.5 alloys exhibit relatively high coercivities with a room-temperature value of about 5 kOe. The origin of the high coercivities has been investigated with magnetic and structural studies. Thermomagnetic data show the presence of a magnetic phase with a Curie temperature of about 270 °C which is close to that of NdFe2. This phase is metastable changing to a magnetic phase with an ordering temperature of 65 °C after heating to 700 °C. The lower Curie temperature phase is related to Nd2Fe17. The coercivity of the sample decreased to negligible values after the heat treatment. It is believed that the high coercivities are due either to the anisotropic NdFe2 phase or possibly to an oxygen-stabilized Nd-Fe-O phase which is probably present in the form of fine particles.
Neutron depolarization effects in a high-T
c superconductor (abstract)
Authors: Nunes A. C. , Pickart S. J. , Crow L. , Goyette R. , McGuire T. R. , Shinde S. , Shaw T. M.
Published: July 2002

Using the polarized beam small-angle neutron scattering spectrometer at the Rhode Island Nuclear Science Center Reactor, we have observed significant depolarization of a neutron beam by passage through polycrystalline high-Tc superconductors, specifically 123 Y-Ba-Cu-O prepared and characterized at the IBM Watson Research Center. We believe that this technique will prove useful in studying aspects of these materials, such as the penetration depth of shielding currents, the presence and structure of trapped flux vortices, and grain size effects on the supercurrent distribution in polycrystalline samples. The two samples showed sharp transitions at 87 and 89 K, and have been studied at temperatures of 77 K; the second sample has also been studied at 4 K. The transition to the superconducting state was monitored by the shift in resonant frequency of a coil surrounding the sample. No measurable depolarization was observed in either sample at 77 K in both the field-cooled and zero-field-cooled states, using applied fields of 0 (nominal), 54, and 1400 Oe. This negative result may be connected with the fact that the material is still in the reversible region as indicated by susceptibility measurements, but it allows an estimate of the upper bound of possible inhomogeneous internal fields, assuming a distance scale for the superconducting regions. For the 10-μm grain size suggested by photomicrographs, this upper bound for the field turns out to be 1.2 kOe, which seems reasonable. At 4 K a significant depolarization was observed when the sample was cooled in low fields and a field of 1400 Oe was subsequently applied. This result suggests that flux lines are penetrating the sample. Further investigations are being carried out to determine the field and temperature dependence of the depolarization, and attempts will be made to model it quantitatively in terms of possible internal field distributions. We are also searching for possible diffraction effects from ordered vortex arrays and plan to extend the measurements to Bi and Tl compositions. These results will be reported in detail elsewhere.
Measurement of the effective linewidth in the millimeter waves range
Authors: Labeyrie M. , Mage J. C. , Ganne J. P.
Published: July 2002

An experimental setup is presented to measure the effective linewidth in the millimeter waves range. It uses the existence of magnetic modes just above the top of the manifold (versus field). This new method is used in order to characterize strontium and barium hexaferrite at 94 GHz between 77 and 300 K.
Magnetic studies of fine iron and iron-oxide particles
Authors: Nafis S. , Tang Z. X. , Dale B. , Sorensen C. M. , Hadjipanayis G. C. , Klabunde K. J.
Published: July 2002

The magnetic and structural properties of fine iron and iron-oxide particles have been studied. The samples of iron oxide were prepared by the aerosol method and those of pure iron were prepared by the vapor deposition technique. Electron micrographs showed the size of aerosol particles to be 200 nm and that of vapor deposition particles to be 10–20 nm. The maximum coercivity at 10 K obtained for aerosol particles was approximately 700 Oe, and for vapor deposited particles it was 1540 Oe.
Some magnetic properties of submicronic Fe3O4 particles
Authors: Arajs Sigurds , Amin Nurul , Anderson Elmer E.
Published: July 2002

Nearly spherical Fe3O4 particles of narrow size distribution with the average diameter between 0.07 and 0.60 μm have been prepared by H2 transformation of α-Fe2O3 particles produced from homogeneous precipitation of ferric salt solutions. The magnetic behavior of these particles has been investigated as a function of temperature between 300 and 900 K in magnetic fields up to 7 kOe. The coercive force Hc has been found to be proportional to Mr/N and Mns, where Mr is the remanent magnetization, Ms is the saturation magnetization, N is the effective self-demagnetization constant, and n is a power index. The implication of these results is briefly discussed.
Modification of the complex permeability of garnet and spinel ferrites by application of a static magnetic field
Authors: Bush Gary G.
Published: July 2002

The complex permeability of ferrites below 1 GHz has been investigated as a function of applied static magnetic field. Previous work by Gorter indicated that the real and imaginary parts of the permeability vary with the ratio of the magnetic ions in the structure. This paper investigates the effect of a magnetic field on the complex permeability of a spinel ferrite. The modification of the frequency dependence of the complex permeability of another spinel ferrite (magnesium-zinc) by the application of a static magnetic field was reported previously. The results of a second experiment to investigate the effect of crystalline structure on the magnetic field dependence of the complex permeability of a garnet ferrite (yttrium-aluminum) are also presented. The magnetic field effect was most pronounced in the region of domain wall rotation resonance in the VHF range (0.2–1 GHz). As in the previously reported results, permittivity did not vary significantly as a function of applied magnetic field. The wide availability of spinel and garnet ferrites with predictable properties as a function of composition should facilitate the design of devices requiring variable permeability and adjustable losses.
A parameter representing eddy current loss of soft magnetic materials and its constitutive equation
Authors: Saito Y. , Hayano S. , Sakaki Y.
Published: July 2002

Previously, Sakaki has proposed a parameter which is closely related to eddy current loss of soft magnetic materials. Also, Saito has proposed a similar parameter to represent the magnetization characteristics exhibiting hysteresis loops. In this paper, we show that these independently proposed parameters are essentially the same ones, and lead to a constitutive equation corresponding to a Chua-type magnetization model. Furthermore, a relationship between our proposed parameter and domain walls is examined.
A Mössbauer effect study of Y2Fe14B and its aluminum solid solutions
Authors: Tharp Dwayne E. , Long Gary J. , Pringle O. A. , Marasinghe G. K. , James W. J. , Grandjean F.
Published: July 2002

The Mössbauer spectra of Y2Fe14B have been measured from 85 to 296 K. Analysis of the spectra indicates that the near-neighbor rare-earth atoms directly influence the orientation of the principal axis of the electric field gradient. The hyperfine parameters are very similar to those of Nd2Fe14B, but the internal fields are somewhat smaller at room temperature in the yttrium compound. The Mösssbauer spectra of Y2(Fe1−xAlx)14B, where x equals 0.00, 0.02, 0.04, 0.06, and 0.08, have been measured at 85 K. The average internal hyperfine field in these compounds decreases linearly with increasing aluminum content.
Physical and micromagnetic structural characterizations of rf-sputtered amorphous CoZrNb thin films
Authors: Choh K. K. , Judy J. H. , Sivertsen J. M.
Published: July 2002

rf-sputtered amorphous CoZrNb (CZN) thin films have been shown to be promising materials for high-density magnetic recording heads. In this paper, we report correlations of the physical and micromagnetic structures with the magnetic properties of CZN films using transmission electron microscopy (TEM) techniques. Observations of the physical microstructures of CZN films verify that one of the apparent effects of the applied magnetic field during film deposition is to suppress crystallization. We have also observed that the number of the microcrystallites which coexist with the amorphous matrix is reduced by an applied magnetic field. We show that the coercivities of CZN films are correlated with the sizes and volume fractions of the microcrystallites. The contribution of the microcrystalline phase to magnetic anisotropies is discussed in terms of the residual anisotropy after rotational magnetic field annealing and the magnetization ripple amplitudes observed in the films. In addition, we report on investigations of the in situ crystallization processes of thin CZN films (500 Å) using a TEM hot stage.