bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural characterization of the temperature dependence of C60-thin films on mica (001) by x-ray diffraction
Authors: Henke S. , Thürer K. H. , Lindner J. K. N. , Rauschenbach B. , Stritzker B.
Published: July 2002

Thin C60 films have been deposited on mica(001) substrates by thermal evaporation at substrate temperatures between room temperature and 200 °C and at a constant deposition rate. The influence of the substrate temperature on the growth of C60-thin films has been systematically investigated by x-ray diffraction. θ–2θ measurements of the (111) peaks show a decrease of the full width at half-maximum (FWHM) with increasing substrate temperature, leading to a minimum FWHM of 0.15° for a substrate temperature of 200 °C. Oriented films with an out-of-plane mosaic spread of Δω=0.2° could be grown at a substrate temperature of 150±25 °C. It can be shown that the in-plane epitaxial arrangement C60(111)∥mica(001) is determined by the seeding conditions and is independent of the substrate temperature. An increasing substrate temperature enhances the epitaxial alignment of the C60 crystals oriented with a {111} face parallel to surface and also the azimuthal alignment of the twins which are rotated by 60° about the surface normal.
Mechanical properties of nanocomposite granular metal thin films
Authors: Scanlon M. R. , Cammarata R. C.
Published: July 2002

The mechanical properties of nanocomposite Ag-Al2O3 and Fe-SiO2 granular metal films were investigated by low-load indentation and laser ultrasonic methods. A discontinuity in the rate of change of hardness as a function of metal volume fraction was observed near the percolation threshold for the metal. However, the sign of the rate of change was different for the two systems. It is suggested that this discontinuity was the result of a change in deformation mechanism as the percolation threshold for the softer phase was crossed. The elastic moduli of the Ag-Al2O3 films as measured by a laser ultrasonic technique displayed a rule-of-mixtures-type behavior, in contrast to nanoindentation measurements that gave a peak in the apparent compliance. Indentation crack testing indicated that the toughness increased in the Ag-Al2O3 system when the metallic phase was introduced into the ceramic, but that the Fe-SiO2 system displayed unexpected and novel fracture behavior.
The role of proton nuclear magnetic resonance spin-lattice relaxation centers in the strong absorption transition at 210 nm in fused silica
Authors: Scruggs Bruce E. , Gleason Karen K.
Published: July 2002

The performance of fused silica for deep ultraviolet optical applications is adversely affected by a radiation-induced absorbance centered at 210 nm which is attributed to the formation of E′ centers. In this work, the 1H nuclear magnetic resonance spin-lattice relaxation rates, 1/T1 of various types of unirradiated fused silica were shown to be correlated to variations in the transmission at 210 nm (T210) that occurs upon irradiation. High concentrations of spin-lattice relaxation centers correlate with the ability to withstand larger numbers of 248 nm laser pulses before a sudden drop in T210, known as the strong-absorption transition (SAT), occurs. If irradiation is halted prior to the SAT, higher concentrations of these centers also correlate with faster rates of partial T210 recovery. We propose that these centers are diamagnetic defects consisting of an adjacent pair of silanol groups that release mobile hydrogen upon irradiation. This hydrogen can reversibly passivate E′ centers, thus accounting for the differences in partial recovery rates of T210 prior to SAT. We also propose the onset of the SAT corresponds to the consumption of all the irradiation susceptible silanol pair defects, after which no partial recovery of T210 is observed when laser irradiation has ceased.
Compositional variations of porous silicon layers prior to and during ion-beam analyses
Authors: Loni A. , Simons A. J. , Canham L. T. , Phillips H. J. , Earwaker L. G.
Published: July 2002

The suitability of ion-beam-analysis techniques in quantifying the composition of mesoporous silicon nanostructures has been critically examined using films of moderate porosity (55%) prepared on n+ substrates. The effects of room-temperature aging of as-etched and thermally oxidized porous silicon, the oxidation conditions chosen to render the material luminescent, have been carefully monitored, as have the effects of both ion-beam irradiation and storage of samples in vacuo. It is shown that the concentrations of the three major impurities oxygen, carbon, and hydrogen can be appreciably altered during analyses, thereby limiting the reliability of the techniques, as conventionally applied to porous silicon. The use of appropriate capping layers, which should alleviate the problem, is recommended.
Kinetics of Al grain growth, Al2Cu precipitation, and dissolution in blanket thin films and fine lines
Authors: Colgan E. G. , Rodbell K. P. , Cabral C. , Harper J. M. E.
Published: July 2002

The apparent activation energy Ea for Al grain growth, Al2Cu (Θ-phase) precipitation, and Al2Cu dissolution were determined by ramped resistance measurements for both Al(Cu) blanket films and patterned lines. The Ea’s measured for the initial stages of grain growth in 0.5-, 1-, and 2-μm-thick Al(4 wt % Cu), Al(2 wt % Cu), and Al films ranged from 1.19 to 1.46 eV. The Ea’s for grain growth were higher for 0.6–0.9-μm-wide Al(Cu) lines than for blanket Al(Cu) films 1.89–3.1 eV, and the temperature of the peak transformation rate occurred at a much higher temperature, 310–400 vs 90–155 °C. This is due to the geometric constraints in patterned lines. The Ea’s for Al2Cu precipitation in Al(4 wt % Cu) and Al(2 wt % Cu) films varied from 0.86 to 1.25 eV. For 0.6-μm-wide Al(4 wt % Cu) lines, the Ea for Al2Cu precipitation was 1.7 eV. The Ea’s for Al2Cu dissolution increased with decreasing Cu content from 1.62–1.74 eV to 2.23–2.30 eV with Al(4 wt % Cu) and Al(2 wt % Cu) films, respectively. The temperature of the peak reaction rate Tp for Al2Cu dissolution increased markedly with increasing film thickness at constant ramp rates. These results demonstrate that the microstructure and Cu distribution in Al(Cu) interconnections on microelectronic chips vary as a function of feature size. This implies that blanket film data is not necessarily applicable to patterned features.
Effect of the degree of disorder on electronic and optical properties in random superlattices
Authors: Wang E. G. , Su W. P. , Ting C. S.
Published: July 2002

A three-dimensional tight-binding calculation is developed and used to study disorder effects in a realistic random superlattice. With increasing disorder, a tendency of possible indirect–direct band-gap transition is suggested. Direct evidence of mobility edges between localized and extended states in three-dimensional random systems is given. As system disorder increases, the optical absorption intensities increase dramatically from five to forty-five times stronger than the ordered (GaAs)1/(AlAs)1 superlattice. It is believed that the degree of disorder significantly affects electronic and optical properties of GaAs/AlAs random superlattices.
Preparation of barium strontium titanate thin film capacitors on silicon by metallorganic decomposition
Authors: Catalan Antonio B. , Mantese Joseph V. , Micheli Adolph L. , Schubring Norman W. , Poisson Roger J.
Published: July 2002

Barium strontium titanate films were deposited on 100-mm-diam silicon wafers by metallorganic decomposition. Highly uniform films ranging up to 250 nm in thickness were obtained. Scanning electron beam micrographs revealed dense, crack-free, low porosity films having grain sizes in the submicron range. Dissipations (tan δ) of less than 0.01 and capacitance densities up to 204 nF/cm2 were measured for 190-nm-thick films. These films had dc leakage currents of 0.2 μA/cm2 or less for bias voltages up to 10 V. They also exhibited relatively small temperature coefficients of capacitance. A patterning process was developed that permitted feature resolution down to 5 μm.
Transport property of Si/Si1−xGex/Si p-type modulation doped double heterostructure
Authors: Jiang R. L. , Liu J. L. , Zheng Y. D. , Li H. F. , Zheng H. Z.
Published: July 2002

High mobility Si/Si1−xGex/Si p-type modulation-doped double heterostructures with Ge fractions of 0.2, 0.25, 0.3 have been grown by rapid thermal process/very low pressure-chemical vapor deposition. Hole Hall mobilities as high as ∼300 cm2/V s (at 293 K and sheet carrier concentration of ∼2.6×1013 cm−2) and ∼8400 cm2/V s (at 77 K and sheet carrier concentration of ∼1.2×1013 cm−2) have been obtained for heterostructures with x=0.3. The variation of hole mobility with temperature and the influence of the Ge fraction on hole mobility were investigated.
Elastic constants of a material with orthorhombic symmetry: An alternative measurement approach
Authors: Lei Ming , Ledbetter Hassel , Xie Yuanfu
Published: July 2002

An alternative approach to measuring the complete anisotropic elastic constants cij(sij) of an orthorhombic-symmetry material is proposed. This approach requires measurements along only principal directions: 〈100〉. In this approach, a pulse-echo (megahertz-frequency) method was used to measure c11, c22, c33, c44, c55, and c66. A resonance (kilohertz-frequency) method was used to measure three elastic compliances: s11, s22, and s33. Combining the cii with the sii, the complete sets of cij and sij were obtained. This approach avoids the troublesome, error-prone determination of off-diagonal cij(sij) by the usual measurements along nonprincipal directions. The approach was applied to a boron-aluminum composite where the off-diagonal terms remain uncertain. In principle, the approach applies to any orthorhombic-symmetry material.
Dependence of valence-subband dispersion relations on heterointerface boundary conditions in InxGa1−xAsyP1−y/InP narrow quantum wells
Authors: Yamanaka Takayuki , Kamada Hidehiko , Yoshikuni Yuzo , Lui Wayne W. , Seki Shunji , Yokoyama Kiyoyuki
Published: July 2002

In the framework of envelope function approximation, the dependence on heterointerface boundary conditions of valence-band dispersion relations in InP-based quantum-well structures is investigated. Under conventional boundary conditions, the root-finding calculation for the eigenvalue problem of quantum well structures is carried out while taking into account a large difference in the effective-mass parameters (the Luttinger parameters) among the constituting bulk materials. In spite of a decreasing interband mixing between heavy- and light-hole states due to strain and the quantum size effect, hole subband structure of a compressive-strained quantum well with a narrow well width is found to form an anomalous dispersion curve. It is clarified that the requirement for envelope continuity at interfaces is responsible for anomaly in the dispersion curves through off-diagonal terms with an additional δ-function-like interface potential in the valence-band effective-mass Hamiltonian. We present the results of dispersion relations and envelope functions related to a newly derived connection rule based on the tight-binding bond-orbital description, under which the envelopes are free from continuity restrictions at the interfaces regardless of any difference in the effective-mass parameters.
Structure and magnetic properties of Co/Cu multilayer films
Authors: Kim J. D. , Petford-Long A. K. , Jakubovics J. P. , Evetts J. E. , Somekh R.
Published: July 2002

The structure, magnetic properties and magnetoresistance of Co/Cu multilayer films grown by sputtering on Si (100) wafers, with Co layer thicknesses between 1.9 nm and 2.0 nm and Cu layer thicknesses between 0.5 nm and 1.3 nm, have been studied. X-ray diffraction, transmission electron microscopy, and optical diffractogram analysis show layered structures and a columnar face-centred-cubic (111) crystallographic texture extending through several layers in the films. The magnetic domain structure was studied by Lorentz microscopy, and the domain structure and image contrast were found to depend strongly on the Cu layer thickness and magnetoresistance. Hysteresis curves explain the trends of magnetic domain contrast and magnetic coupling in the films. Annealed samples show a more regular domain structure, and lower saturating field and magnetoresistance than as-sputtered samples.
Dislocation-free zone model of fracture: The effect of transgranular microcrack nucleated from a grain-boundary ledge
Authors: Shiue Sham-Tsong
Published: July 2002

The effects of transgranular microcrack nucleated from a grain-boundary ledge on a dislocation-free zone model of fracture are investigated by a continuous dislocation modeling method. The dislocation distribution functions to simulate the crack and plastic zone, the number of dislocations in the crack and plastic zone, the stress field, and stress intensity factor at the crack tip are obtained. If the Burgers vector of dislocations composing the grain-boundary ledge has the same sign as that of the plastic zone dislocations, all the dislocation distributions that simulate the crack and plastic zone, the number of dislocations in the crack and plastic zone, the stress field in the dislocation-free zone, the stress intensity factor at the crack tip, the dislocation-free zone size, and the plastic zone size increase with an increasing number of grain-boundary ledge dislocations, but the applied stress σ to accumulate the plastic zone dislocations decreases with an increasing number of grain-boundary ledge dislocations. The effects of transgranular microcrack nucleated from a grain-boundary ledge on a fracture without a dislocation-free zone are also considered. When there is no dislocation-free zone in front of the crack tip, the stress intensity factor is zero, regardless of whether the grain-boundary ledge dislocations exist.
Josephson transmission lines with multilayer Josephson junctions
Authors: Lomatch S. , Rippert E. D.
Published: July 2002

We present a new type of Josephson transmission line (JTL) in which discrete multilayer Josephson junctions are employed instead of the standard single tri-layer junction technology. It is shown via numerical simulation that such a structure supports the propagation of multiple flux quantum solitons, which are represented in this system by a 2πn kink in the phase difference across a multilayer junction of n superconductor-insulator-superconductor layers. As a practical aspect, parallel and series connected multilayer junctions in a JTL can significantly reduce the need for microstrip line inductance, thereby providing the optimal conditions for large scale integration of JTL-based logic and memory circuitry. In this article we investigate in detail through numerical simulation the properties of JTL designs with multilayer junctions, incorporating expected variations in junction characteristics. A brief description of the current fabrication of multilayer junctions for flux quantum circuitry will be provided.
Theoretical analysis of dopant redistribution in silicon after implantation through an SiO2 mask
Authors: Pelleg J. , Sinder M.
Published: July 2002

The redistribution after annealing in silicon of dopant implanted through a silicon oxide mask is discussed. The conditions which are essential to analyze diffusion profiles by simple Gaussian function are presented. The finite thickness of the oxide mask is taken into consideration. Experimental data of implanted P are compared with the exact and approximated solutions and it is shown that the use of the Gaussian approximation is within of ∼12% accuracy of the exact solution.
Field-induced tuning of the optical properties of nonlinear composites near resonance
Authors: Levy Ohad , Yagil Yoad , Bergman David J.
Published: July 2002

The bulk effective dielectric function of metal-dielectric composites has a series of poles in the vicinity of which it is extremely sensitive to small changes in the properties of the components. It is possible to induce such a change in the nonlinear component of a composite, containing at least one nonlinear component, by applying an external electric field at a similar frequency. This causes a tuning of the effective dielectric function and a continuous alteration of the optical properties of the medium. We propose a general framework for calculating this effect in two component composites and also calculate it in a few three component microgeometries. It is found that the optical transmittance of a thin film of this kind can be changed from zero up to 80% using moderate electric fields.
Electron correlations in coupled quantum dots
Authors: Ugajin R. , Suzuki T. , Nomoto K. , Hase I.
Published: July 2002

The quantum states of two electrons in a pair of coupled quantum dots confined by heterostructures of compound semiconductors were calculated. As the physical parameters were changed various quantum states of electrons were obtained. The correlation between electrons in these quantum states is analyzed using the mean distance between electrons and its variance. When the quantum states include quadruple degenerate ground states, electrons are localized on each quantum dot and all kinetic movement except spin is frozen. This suggests that a Mott insulator phase will appear in a bulk array of coupled quantum dots. Because coupled quantum dot arrays can be assembled to include both metallic regions and insulating regions without changing the density of electrons, they have great potential in applications for future electron devices.
Temperature effects on the resistivity of polycrystalline silicon titanium salicide
Authors: Naem A. A. , Deen J. , Chee L. Y.
Published: July 2002

The heating effects on the resistance of titanium salicide films formed on doped polycrystalline silicon materials have been characterized at temperatures ranging from 15 to 195 °C. The resistance was also evaluated at various current densities in order to study the self-heating effects due to the supplied power. The polycrystalline silicon material was implanted with arsenic, boron, or phosphorus and then salicided. The resistivity of the salicide material was found to increase linearly with temperature at low current densities and deviates from linearity at higher operating current densities. An empirical model which relates the resistivity to temperature and current density has been developed for the above materials. The model can accurately predict the resistivity of salicided polycrystalline silicon material at a certain operating temperature and current density. A new expression for the temperature coefficient of resistance has also been developed, and experimentally verified under different operating conditions for n+ gate, n+ emitter, and p+ emitter salicided polycrystalline silicon materials.
Drain current reduction by source electrode overlap in hydrogenated amorphous silicon thin-film transistors
Authors: Uchikoga Shuichi , Akiyama Masahiko , Koizumi Takashi , Tada Masahiro , Ikeda Mitsushi , Wada Tetsunori , Suzuki Kouji
Published: July 2002

The characteristics of hydrogenated amorphous silicon (a-Si:H) thin-film transistors are investigated as a function of the overlap of the source electrode Lovs on top of the passivation layer. The drain current is reduced by Lovs and the effect is dependent on drain voltage and a-Si:H layer thickness. The effect is enhanced at high drain voltage and at thin a-Si:H layer thickness. At high drain voltage, field-effect mobility and threshold voltage decrease as Lovs increases. Numerical calculation of carrier concentration in a-Si:H layer showed that the carrier is depleted under Lovs and that the effective channel length is shortened by the size of Lovs. The nonlinear resistance formed by Lovs is discussed in terms of drain voltage and a-Si:H layer thickness.
Hydrogen effect on the optical activity of dislocations in silicon introduced at room temperature
Authors: Sekiguchi T. , Kveder V. V. , Sumino K.
Published: July 2002

Dislocations were introduced into silicon crystals by mechanical scratching at room temperature. The optical property of such dislocations was investigated by means of cathodoluminescence. Neither deformation-induced luminescence nor the exciton luminescence was detected in the scratched region, showing that a high density of nonradiative recombination centers is induced on or around dislocations. Hydrogen plasma treatment of a scratched crystal led to the appearance of the so-called D1–D4 luminescence lines along the scratch. Deep-level transient spectroscopy revealed that deep traps were induced by scratching and diminished drastically due to subsequent hydrogen plasma treatment. Thus, it was concluded that hydrogen passivated nonradiative recombination centers but not the luminescence centers. The characteristics in spatial distribution of D1 and D2 lines and those of D3 and D4 lines showed that these two groups of luminescence lines were of different origins. Since the specimens used were thought to be free from metal contamination, the luminescence was not related to metallic impurities incorporated in the dislocation core. The nonradiative recombination centers induced by scratching were annihilated due to annealing at temperatures higher than 500 °C. The release of hydrogen from the nonradiative centers in a hydrogenated specimen did not take place before they were annihilated by annealing.
Built-in biaxial strain dependence of Γ-X transport in GaAs/InxAl1−xAs/GaAs pseudomorphic heterojunction barriers (x=0, 0.03, and 0.06)
Authors: Yang K. , East J. R. , Haddad G. I. , Drummond T. J. , Brennan T. M. , Hammons B. E.
Published: July 2002

The effects of built-in biaxial strain on Γ-X transport in n-GaAs/i-InxAl1−xAs/n-GaAs pseudomorphic single-barrier structures (x=0, 0.03, and 0.06) are studied by measuring temperature-dependent I-V characteristics. For the accurate characterization of electron transport across each barrier, a self-consistent numerical model is used to analyze the experimental results. For each structure, the four barrier parameters defined from the thermionic-field-emission theory, the effective Richardson constant A*, the conduction-band offsets ΔEc1,2, and a tunneling mass mn* are extracted by calculating the theoretical I-V characteristics and fitting them to the experimental I-V-T data. The experimentally obtained X-point conduction-band shifts with the addition of indium are compared with the theoretical results calculated based on the model-solid theory. The results indicate that the addition of indium not only splits the degenerate X minima of the InxAl1−xAs barrier, but also shifts the relative barrier heights of both longitudinal and transverse X valleys due to the alloy-dependent band-structure modification. The comparison between the experimental and theoretical results illustrates that the transverse X valleys are the main conduction channel for the Γ-X transport across InxAl1−xAs pseudomorphic barriers.
Real-structure effects in the dynamical theory of grazing incidence x-ray diffraction
Authors: Stepanov S. A. , Köhler R.
Published: July 2002

A study is presented on the grazing incidence x-ray diffraction in multilayers, with theoretical considerations on the effects of large-scale and small-scale surface and interface roughness and on the effects of interface transition layers. Based on the dynamical theory of grazing incidence x-ray diffraction, the study provides a matrix form of solution for an arbitrary sequence of multilayers. The computations obtained with a developed computer program show the differences that can be observed by the effects of large-scale and small-scale roughness. The effects of small-scale roughness and those of transition layers are similar and thus cannot be separated from each other. But the thickness of surface and interface transition layers or the rms-values of roughness heights are determinable from the shape of diffraction curves by using two different methods.
Bloch wall energy in (Tb,Nd)-(Fe,Co) thin films
Authors: Raasch D. , Hansen P. , Mergel D.
Published: July 2002

The Bloch wall energy has been determined for (Tb,Nd)-(Fe,Co) magneto-optic disks with varying Nd content. The procedure consists of applying a magnetic field to thermomagnetically written cylindrical domains and measuring the magnetic fields for which the domains expand and collapse. The wall energy can then be calculated in terms of a bubble model where a regular wall motion is assumed. The measured Bloch wall energy decreases continuously with increasing Nd content, reaching about one third of its initial value for more than 15 at. % Nd. This effect cannot be explained by a corresponding decrease in the exchange stiffness constant A or the anisotropy constant Ku. We therefore conclude that the addition of Nd, with its tendency to a sperimagnetic coupling, disturbs the magnetic ground state to such an extent that the conventional formula for the Bloch wall energy is no longer valid.
Local ac magnetic response in type-II superconductors
Authors: Prozorov R. , Shaulov A. , Wolfus Y. , Yeshurun Y.
Published: July 2002

We present an analysis of the local ac magnetic response in type-II superconductors as measured by a miniature Hall probe. The results show that the response is a function of the position of the probe even in a homogeneous sample. The local response in an inhomogeneous sample is further influenced by local variations of superconducting properties across the sample. Applications of this analysis in characterizing and mapping of inhomogeneities in a sample are illustrated.
GaAs heteroepitaxial growth on vicinal Si(110) substrates by molecular beam epitaxy
Authors: Yodo T. , Tamura M. , López M. , Kajikawa Y.
Published: July 2002

We report the growth mechanism of GaAs on vicinal Si(110) substrates by molecular beam epitaxy, and investigate the effects of off-angle and -direction on the crystalline quality of grown films. The quality was improved with the increase of the off-angle and strongly influenced by the off-direction. Off-direction toward the [001] was better than that toward the [11̄0] for obtaining high-quality films. The cause is related to the structure and density of the steps. We infer from reflection high-energy electron diffraction results that GaAs films on vicinal Si(110) with off-angles above 2° toward the [001] direction grew up to a thickness of 3 nm in step-flow-like mode. The spotty reflection high-energy electron diffraction patterns characteristic of three-dimensional growth in GaAs/Si(100) were not observed at a thickness above 3 nm.
Electron transport in InGaAs/AlInAs heterostructures and its impact on transistor performance
Authors: Zahurak John K. , Iliadis Agis A. , Rishton Stephen A. , Masselink W. Ted
Published: July 2002

We have studied electron transport in a variety of doped and modulation-doped InGaAs/AlInAs quantum wells within the context of field-effect transistor performance. Both quantum-well width and doping profile were varied with all layers lattice matched to the InP substrate. Electron transport properties in the structures were characterized using Hall, geometric magnetoresistance, and microwave velocity-field measurements, and transistor performance in terms of transconductance and channel current in 1.8- and 0.5-μm gate-length devices. Transconductances in 0.5-μm gate-length transistors were as high as 590 mS/mm and were more directly correlated to the peak electron velocity than to the low-field mobility.
Optical pulse mixing measurement of carrier lifetime and absorption recovery time in reverse-biased GaAs/AlGaAs single quantum well laser structures
Authors: Brovelli L. R. , Hugi J. , Jäckel H. , Melchior H.
Published: July 2002

We describe an optical pulse mixing experiment with a two-segment GaAs/AlGaAs single quantum well, graded-index separate confinement heterostructure laser to determine the effective lifetime of photogenerated carriers in a short reverse-biased segment with subpicosecond resolution. This lifetime is of importance if such a segment is used as a fast photodetector or as a saturable absorber in a monolithic mode-locked laser structure. We found that the lifetime depends not only on the applied bias but also on the excitation pulse energy. Lifetimes shorter than 5 ps have been observed. The strong dependence on excitation pulse energy is attributed to screening effects of the escaped carriers. A simple model based on an exact solution of the one-dimensional Schrödinger equation for a particle in a quantum well in an electric field together with an electrical equivalent circuit verifies this assumption.
The effects of strain on intrasubband scattering rates in InP-based strained-layer quantum-well lasers
Authors: Seki Shunji , Yokoyama Kiyoyuki , Sotirelis Paul
Published: July 2002

We analyze the strain effects (both compressive and tensile) on the intrasubband scattering rates for electrons and holes in InP-based strained-layer quantum-well (SL-QW) lasers. Carrier-carrier and carrier-LO-phonon interactions are taken into account on an equal basis within the fully dynamic random phase approximation for multi-subband structures at finite temperatures. The principal influences of the strain-induced changes in the valence band structures on the scattering rates are discussed in terms of the changes in the valence-band density of states and the phonon coupling with holes. We show that the hole-hole interaction plays a dominant role in determining the hole scattering rates regardless of the carrier energy. On the other hand, for electron scattering rates, the dominant scattering mechanism switches from electron-hole interaction to electron-electron interaction as the initial energy of electron increases. We also demonstrate that it is essential to take into account the mutual interactions among the individual scattering processes, such as carrier-carrier scattering and carrier-LO-phonon scattering, when evaluating the intrasubband scattering rates in SL-QW lasers.
Effect of high hydrostatic pressure on phase composition and microstructure in Au-28 at. % Ge alloy/GaAs contacts
Authors: Zhao Y.-C. , Wu Z.-Q.
Published: July 2002

Effects of external hydrostatic pressure up to 7 kbar on the Au-Ge binary phase diagram and on the arsenic sublimation are evaluated by thermodynamic calculations. Experimental observations in the eutectic Au-28 at. % Ge/GaAs contacts which were annealed at vacuum and in a 7 kbar argon gas, respectively, are taken as illustrative examples for the comparison with the calculated results. It is concluded that pressures up to 7 kbar shift only slightly the phase boundaries in the Au-Ge phase diagram that were obtained in standard conditions (1 atm) and a pure argon gas with a pressure of 7 kbar acts as a good capsulation to prevent species As from sublimation. The measurements indicate that the film/GaAs interface and the film surface in the contacts annealed at 7 kbar are rich in Ge and are morphologically smooth.
The effects of frustrated biquadratic interactions on the phase diagrams and criticality of the Blume–Emery–Griffiths model (abstract)
Authors: Snowman Daniel P. , McKay Susan R.
Published: July 2002

The Blume–Emery–Griffiths model, a spin-1 Ising model with bilinear (J), biquadratic (K), and the crystal field (Δ) terms, provides a general system for the study of both density and magnetic fluctuations. Previous work has shown that a rich variety of phase diagrams occurs in cases of positive or negative biquadratic coupling. In this study, we consider an exactly solvable system in which frustration is present due to competing biquadratic interactions. Thus, this calculation models a dilute ferromagnetic material with two types of nearest neighbor site pairs, distinguished by whether or not simultaneous occupation is energetically favored. To determine the effects of this competition, we have constructed exactly solvable frustrated hierarchical models similar to those introduced to study spin glasses. The resulting phase diagrams show that, when frustration is present, the dense disordered and dilute disordered phases can become separated by a phase with chaotic rescaling of K and Δ. In the J=0 plane, this chaotic rescaling is directly linked to that previously reported in spin glasses via the Griffiths symmetry.
Na-doping effect on the magnetic properties of the YBCO ceramics
Authors: Nurgaliev T. , Miteva S. , Nedkov I. , Veneva A. , Taslakov M.
Published: July 2002

The ac magnetization and the microwave surface resistance (at 12 GHz) were measured on a series of Na-doped YBCO ceramic samples at 77 K. Their magnetic behavior was explained on the basis of the modified critical state model by taking into account the existence of a field-dependent component Jc1 (due to weakly linked grains) and a field-independent component Jc0 (due to perfectly linked grains) in the bulk critical current density of the samples. The Na and Cu remaining after the heat treatment of the samples changed the intergranular medium parameters and impeded the correlation between the grains. As a result, an increase of the Na concentration led to a decrease of the specimens critical current density and an increase of their surface resistance. At small Na concentrations, a certain increase of Jc1 was observed, which can be explained by taking into account the possibility of partial pinning of Josephson vortices in the ‘‘weakly seeded’’ places in the intergranular media.
Annealing temperature and O2 partial pressure dependence of T
c in HgBa2CuO4+δ
Authors: Xiong Q. , Cao Y. , Chen F. , Xue Y. Y. , Chu C. W.
Published: July 2002

Samples of HgBa2CuO4+δ (Hg-1201) were annealed under various conditions. After carefully controlling annealing time, annealing temperature (Ta), and O2 partial pressure (P0), we were able to find the reversible annealing conditions for Hg-1201. Under 1 atm O2 at 260 °C≤Ta≤400 °C, the obtained Tc is nearly the same (∼97 K). However, it decreases quickly with Ta≳300 °C in high vacuum (P0∼10−8 atm), and reaches zero at Ta=400 °C. On the other hand, Tc decreases with the decrease of Ta in high-pressure O2 (∼500 atm) and reaches ∼20 K at about 240 °C. In the entire annealing region, the oxygen surplus varies significantly from 0.03 to 0.4, and a wide range of Tc variation (0→97 K→20 K) was obtained with anion doping alone.
Superconducting YBa2Cu3O7−x/Y4Ba3O9 multilayers: Field independent critical current and dimensional crossover (abstract)
Authors: Xu Jun-Hao , Grishin A. M. , Rao K. V.
Published: July 2002

A new class of c-axis-oriented YBa2Cu3O7−x/Y4Ba3O9 (YBCO/YBO) multilayer films with YBCO and YBO individual thicknesses ranging from 24 to 156 Å have been fabricated by a single target on-axis biased rf magnetron sputtering technique on LaAlO3 (001) substrates. SQUID dc magnetic measurements reveal a striking feature of magnetic field independent critical current jc(T) from 5 K to almost near superconducting transition Tc. For thin YBCO layer thickness the magnitude of jc is found to decrease as (1−T/Tc)2, and exponentially with the thickness of the isolating spacer YBO. It is distinctly shown that jc(T) in the (a,b) plane is determined by interlayer Josephson current. All the SQUID data for the temperature dependencies of magnetization for a few families of multilayers with thin YBCO layers are shown to scale and collapse into a single universal behavior consistent with a model of ‘‘slipped out pancake vortices in Josephson coupled heterostructures.’’ On approaching Tc, the characteristics of critical current jc change gradually from the field independent behavior to another dependence similar to an ordinary H−1/2 dependence typical for collective pinning of the vortex lattice in the YBCO single layer film. This suggests that a dimensional crossover takes place when the quantum coherence between individual YBCO layers sets. The crossover temperature is found to depend on the multilayer period and individual thicknesses of superconducting and isolating layers while a linear temperature dependence of jc appears for multilayers with thick YBCO layers.
Spin reversal in Co/Au(111)/Co trilayers
Authors: Grolier V. , Ferré J. , Galtier M. , Mulloy M.
Published: July 2002

In Co/Au(111)/Co trilayers exhibiting oscillatory interlayer coupling, we show how the nature and the strength of this interaction modify the magnetization reversal. In the case of antiferromagnetic (AF) coupling the magnetic domain patterns observed by Faraday microscopy differ drastically according to the amplitude of the coupling. This effect can be understood on the basis of the fluctuations of the effective field acting on a layer produced by the roughness of the Au interlayer. Among the ferromagnetically (F) coupled layers slighter differences are visible that can be accounted by changes of the anisotropy energy.
Influence of Cr growth on exchange coupling in Fe/Cr/Fe(100) (invited) (abstract)
Authors: Stroscio Joseph A. , Pierce D. T. , Unguris J. , Celotta R. J.
Published: July 2002

The giant magnetoresistance and the interlayer exchange coupling between magnetic layers separated by nonmagnetic spacer layers are sensitive to the roughness at the interfaces in multilayer magnetic structures. We present scanning tunneling microscopy (STM) measurements of the thickness fluctuations in Cr films grown at different temperatures on near-perfectly flat Fe(100) whiskers and correlate these results with our scanning electron microscopy with polarization analysis (SEMPA) measurements of the oscillations of the exchange coupling in Fe/Cr/Fe(100) structures grown at similar temperatures. Layer-by-layer growth was observed by STM for Cr deposition on an Fe substrate at deposition temperatures greater than 300 °C. The SEMPA measurements of the Fe overlayer magnetization as a function of Cr spacer layer thickness for Cr growth at this temperature could be simulated well by oscillatory coupling with periods 2.105±0.005d and 12±1d, where d is the layer spacing. Rougher Cr growth, limited by diffusion kinetics, is observed at lower temperatures, giving a distribution of thicknesses in the growth front. We modeled the Fe magnetization for lower temperature Cr growth by assuming that the exchange coupling at each discrete Cr thickness is the same as found for layer-by-layer growth. The total coupling at each average Cr spacer layer thickness was determined by adding the weighted contribution to the coupling from each Cr layer thickness contributing to the average thickness. Very good agreement was obtained with the SEMPA measurement of the Fe overlayer magnetization for Cr growth at lower temperatures without including other consequences of roughness at the interface, such as the breakdown of translational invariance.
Cluster model studies on the electronic and magnetic properties of LaCo13 and La(FexAl1−x)13 alloys
Authors: Zhang G. W. , Gong X. G. , Zheng Q. Q. , Zhao J. G.
Published: July 2002

We present studies on both the electronic and magnetic properties of LaCo13 and La(FexAl1−x)13 intermetallic compounds by cluster model calculations within the scheme of density-functional theory. The equilibrium structures of the isolated and embedded clusters, obtained by minimizing the total binding energy, are in good agreement with available experimental data and other theoretical results. The relative stability and formation of LaCo13 and La(FexAl1−x)13 phases have been successfully explained. Interesting changes of magnetic properties in an Al doped Fe13 cluster have been obtained. The possible effect of Al on the magnetic phase diagram of La(FexAl1−x)13 is discussed.
On vector generalization of the critical state model for superconducting hysteresis
Authors: Mayergoyz I. D.
Published: July 2002

The critical state model for superconducting hysteresis is based on an analytical study of the penetration of electromagnetic fields into hard superconductors. In the literature, this study has been carried out for linear and circular polarizations of electromagnetic fields. In the paper, the attempt is made to extend this study to arbitrary electromagnetic field polarizations, which can be treated as perturbations of circular polarizations. Another distinct feature of our work is that this extension is carried out for gradual resistive transitions described by the ‘‘power law.’’
Effects of additives on magnetic properties of sheet Sr-Ba ferrite magnets
Authors: Oh Young Jei , Shim In Bo , Jung Hyung Jin , Park Jae Yun , Park Seung Iel , Um Young Rang , Lee Young Jong , Lee Seung Wha , Kim Chul Sung
Published: July 2002

Sr0.75Ba0.25Fe12O19 hexagonal ferrite has attracted much attention due to its large (BH)MAX values and workability. We have prepared sheet magnets by the Dr. Blade method. To examine the effects of additives, such as SiO2, TiO2, Al2O3, and Cr2O3, on magnetic properties of sheet magnets, we used VSM magnetometer, x-ray diffraction, and Mössbauer spectrometer. The crystal structure is found to be a magnetoplumbite of typical M-type hexagonal ferrite, but the α-Fe2O3 phase develops with increasing additives concentration. Using our refined computer program, we have analyzed the Mössbauer spectra in the temperature range from 13 to 800 K. The Mössbauer spectra indicate that the line intensity for the 12k site is reduced with increasing SiO2 concentration, which is different from the reports of Fe-substituted Ba ferrite. This suggests that the developing α-Fe2O3 phase is related to 12k sites. The isomer shifts show the charge states of Fe ions is ferric. When the additives concentrations increase, the Curie temperatures, Tc go down. One sextet for α-Fe2O3 phase still persists above Tc, so it suggests that the high-Tc values do not result from α-Fe2O3. While Al2O3 and Cr2O3 additives increase coercive force Hc, the other additives reduce it.
Neutron scattering studies of the vortex lattice in niobium and R123 superconductors (invited)
Authors: Rosov N. , Lynn J. W. , Grigereit T. E.
Published: July 2002

The magnetic flux lattice undergoes a melting transition not only in high-Tc oxide superconductors, but also in conventional superconductors, as recently observed in superconducting niobium films. Small-angle neutron scattering was used to investigate the properties of the magnetic flux lattice in a large, high-quality single crystal of niobium. The small London penetration depth of niobium gives a large magnetic scattering signal, and the use of a high-quality single crystal eliminates other unwanted scattering (from twin boundaries, voids, etc.). The signal-to-noise ratio is therefore improved by several orders of magnitude over the best available measurements of high-Tc oxide superconductors. A sixfold hexagonal pattern of peaks is observed in the mixed state (Hc1<H<Hc2) at all temperatures. These peaks are resolution limited below the irreversibility line; above it, the width in the transverse direction increases with temperature due to the vortex dynamics. Close to Hc2, the radial widths of the peaks also broaden. The increase in broadening is a direct observation of a transition to a disordered phase. Nevertheless, the basic hexagonal pattern of peaks is maintained throughout the mixed state, indicating that a correlated flux fluid exists in the reversible regime. Some results on the vortex lattice in superconducting DyBa2Cu3O7 are presented and some of the possible exotic states resulting from the coexistence of antiferromagnetic order and superconductivity are described.
Granular giant magnetoresistive materials and their ferromagnetic resonances (abstract)
Authors: Rubinstein M. , Das B. N. , Koon N. C. , Chrisey D. B. , Horwitz J.
Published: July 2002

Ferromagnetic resonance (FMR) can reveal important information on the size and shape of the ferromagnetic particles which are dispersed in granular giant magnetoresistive (GMR) materials. We have investigated the FMR spectra of three different types of granular GMR material, each with different properties: (1) melt-spun ribbons of Fe5Co15Cu80 and Co20Cu80, (2) thin films of Co20Cu80 produced by pulsed laser deposition, and (3) a granular multilayer film of [Cu(50 Å)/Fe(10 Å)]×50. We interpret the linewidth of these materials in as simple a manner as possible, as a ‘‘powder pattern’’ of noninteracting ferromagnetic particles. The linewidth of the melt-spun ribbons is caused by a completely random distribution of crystalline anisotropy axes. The linewidth of these samples is strongly dependent upon the annealing temperature: the linewidth of the as-spun sample is 2.5 kOe (appropriate for single-domain particles) while the linewidth of a melt-spun sample annealed at 900 °C for 15 min is 3.8 kOe (appropriate for larger, multidomain particles). The linewidth of the granular multilayer is attributed to a restricted distribution of shape anisotropies, as expected from a discontinuous multilayer, and is only 0.98 kOe with the magnetic field in the plane of the film.
Magnetoresistance of granular Cu-(Co,Fe) and Cu-Co-B (abstract)
Authors: Helmolt R. v. , Wecker J. , Samwer K.
Published: July 2002

Giant magnetoresistance (GMR) is known to occur in alloys consisting of superparamagnetic precipitates in a metallic matrix. This had been demonstrated for the first time for Cu-Co prepared as co-sputtered thin films and later for rapidly quenched alloys. For Cu-Fe no GMR has been reported even in multilayer systems. In order to study the transition from Cu-Co to Cu-Fe in more detail we prepared quasibinary Cu90Co10−xFex ribbons by conventional melt spinning followed by an annealing treatment to precipitate Co-Fe clusters in the Cu matrix. The particle sizes of the ferromagnetic phase have been determined by fitting the magnetization and magnetoresistance curves with a Langevin function. The saturation magnetization increases with increasing Fe content x for x≤4 as expected from the Slater–Pauling curve. However, a strong decrease of the magnetization is observed at high Fe contents. The GMR effect continuously decreases with increasing Fe content and for x≳0.7 no GMR is observed, similar to Cu-Fe multilayers. Small B additions were added to Cu-Co in order to improve the homogeneity of the as-quenched ribbons. For these samples the magnetoresistance is suppressed even by small B additions below 3 at. %. This is different from the Au-Co system, where B additions of up to 20% enhance the GMR effect.
The use of polytetrafluoroethylene in the production of high-density bonded Nd-Fe-B magnets
Authors: Tattam C. , Williams A. J. , Hay J. N. , Harris I. R. , Tedstone S. F. , Ashraf M. M.
Published: July 2002

Rotary forging has been used to produce high-density bonded magnets from rapidly quenched Nd-Fe-B based ribbons [MQP-D, of nominal composition 28%Nd-56%Fe-15%Co-1%B (wt %)]. Polytetrafluoroethylene (PTFE), when used as an additive (5%–15% by volume) has been found to act as an effective binder and greatly enhances the forgeability of the MQI, allowing higher forging pressures to be used. Densities of up to 98% of the fully dense composite have been achieved. The forging process can be undertaken in air at room temperature. Magnetically, the compacts are comparable to conventional epoxy resin bonded MQI, with energy products of up to 84 kJ/m3. Equivalent volume fractions of MQI (∼83.5 vol %) have been achieved in the compacts with increased PTFE content due to the displacement of pores by the PTFE. The effect of PTFE content on the mechanical strength of the compacts has been assessed and it has been found that strength increases with increasing PTFE content, consistent with the reduction in porosity.
Orbital ordering and magneto-optical effects in CeSb
Authors: Antropov V. P. , Harmon B. N. , Liechtenstein A. I.
Published: July 2002

The influence of on-site Hubbard correlations on electronic structure and magneto-optical spectra of CeSb have been studied. As the new key mechanism to produce the large magneto-optical signal is linked to the interaction between the anisotropy of the Coulomb matrix and m-dependent p-f hybridization, this effect suggests a systematic search for materials with large magneto-optical effects.