bluuebunny/crossref_metadata_2025 huggingface dataset:
Characteristics of an optically activated pulsed power GaAs(Si:Cu) switch obtained by two-dimensional modeling
Authors: Stout Phillip J. , Kushner Mark J.
Published: July 2002

Photoconductive semiconductor switches (PCSS) have high-voltage hold-off (many to tens of kilovolts) and fast current rise times (<1 ns). However, lock-on, nonuniformities in the electric field, and filamentary current flow across the device when switching at high fields (∼10 kV/cm) have been reported. These observations raise concerns about the scaling of PCSS to high currents (tens of kiloamperes). To investigate these issues a two-dimensional time dependent computer model of a GaAs PCSS with Si and Cu doping has been developed. The model solves the continuity equations, the bulk energy equation, Poisson’s equation for the electric field, and a circuit equation for external currents. Physical effects in the model include band-to-band impact ionization, trap impact ionization, photoionization, and negative differential resistance. Computed characteristics of GaAs(Si:Cu) switches will be reported. Experimentally observed electric-field distributions are explained.
Incorporation of Zn in GaAs during organometallic vapor phase epitaxy growth compared to equilibrium
Authors: Reichert W. , Chen C. Y. , Li W. M. , Shield J. E. , Cohen R. M. , Simons D. S. , Chi P. H.
Published: July 2002

The zinc concentration measured after organometallic vapor phase epitaxy (OMVPE) growth on (100)-oriented GaAs at 700 °C has been compared to the zinc concentration measured after in-diffusion under near-equilibrium conditions. During diffusion, the concentration of Zn 20 nm below the surface was found to vary with P1/2Zn, as expected for bulk solid–vapor equilibrium. During growth, the concentration of Zn varied linearly with PZn up to a maximum value which was found to correspond to the solubility limit set by second phase formation, e.g., growth of Zn3As2. Although large differences were observed between the results of the two experiments when using nominally identical ambient conditions, all of the results are consistent with a thermodynamic model in which the Fermi level at the surface is pinned approximately 200 meV below the intrinsic Fermi level. Typical OMVPE growth conditions appear to give a bulk zinc concentration which is supersaturated relative to the ambient partial pressures used, and to enhance the diffusion of Zn into the substrate.
Formation of amorphous Zr-Al-Cu-Ni with a large supercooled liquid region by mechanical alloying
Authors: Seidel M. , Eckert J. , Schultz L.
Published: July 2002

Amorphous Zr65.0Al7.5Cu17.5Ni10.0 powder has been prepared by mechanical alloying from crystalline elemental powders. The powders are characterized by x-ray diffraction and differential scanning calorimetry (DSC). The amorphous phase forms after about 50 h of milling, and exhibits a wide supercooled liquid region of up to 87 K before crystallization. The transformation to the crystalline state and the melting behavior of the crystalline alloy have been followed by isothermal and constant-rate heating DSC. The results are compared with data for melt-spun and water-quenched amorphous material.
Current–voltage characteristics of GaAs/AlAs double-barrier resonant tunneling diodes with a Si-planar-doped barrier
Authors: Fukuyama Hiroyuki , Waho Takao , Mizutani Takashi
Published: July 2002

Current–Voltage (I–V) characteristics of GaAs/AlAs double-barrier resonant tunneling diodes (RTDs) with a Si-planar-doped barrier are studied. At 4.2 K the peak-to-valley current ratio decreased when Si was introduced to the barrier next to the electrode from which electrons are emitted (emitter barrier). On the other hand, the ratio did not decrease considerably when Si was introduced to the other barrier, i.e., next to the electrode into which electrons flow (collector barrier). To investigate the origin of the decrease in the peak-to-valley ratio, we evaluated the transmission coefficient experimentally, and found that the resonance transmission widths for the double-barrier RTDs are almost the same. This means that the reduction in the peak-to-valley current ratio is not caused by the broadening of resonance transmission width. To explain why the peak-to-valley current ratio is degraded, we then introduced a new model in which an excess current flows through donor levels in the emitter barrier. To confirm this model, we investigated the I–V characteristics of GaAs/AlAs single-barrier diodes. We found an excess current, which is probably due to a tunneling current through the donor level created in the barrier, when the barrier is planar-doped. It is, therefore, concluded that the decrease in the peak-to-valley current ratio observed for the RTD with the doped emitter barrier is due to this excess current flowing through the doped emitter barrier.
Modeling giant magnetoresistance and magnetization of Ag1−xNix−yFey heterogeneous alloy films (abstract)
Authors: Wiggins J. , Watson M. L. , Gago-Sandoval P. A. , Batlle X. , Badia F. , Labarta A.
Published: July 2002

We have investigated the effects of the particle size distribution on the giant magnetoresistance (GMR) and magnetization of Ag1−xNix−yFey heterogeneous alloy films both experimentally and through computer simulation. The samples were prepared by sputtering from a mosaic target onto glass substrates at the system ambient temperature. They have thicknesses in the range of 100–300 nm and were rapidly thermally annealed at up to 750 °C to promote phase segregation. The resistivity and magnetoresistivity have been measured in the temperature range 4–300 K in fields of up to 12 kOe and the magnetization by vibrating sample magnetometer at fields of up to 6 T. We have modeled the magnetization and GMR of the system using an ensemble of superparamagnetic particles which have a log normal distribution of diameters. We obtain an excellent fit to the experimental magnetization data at room temperature. This is true for all annealing strategies used. Conversely, we have found a marked difference between the modeled and experimental GMR data for higher temperature annealing strategies. It can be shown that this model accurately fits the magnetoresistance if only particles under a critical diameter are considered. For Ag1−xNix−yFey heterogeneous alloy films this critical diameter has been calculated to be 6 nm. Larger particles will contribute only to the magnetization and not the magnetoresistivity. To optimize the magnetoresistance an annealing strategy that favors the creation of particles under this critical diameter is required.
Cross-barrier recombination in a GaAs/AlGaAs double-barrier resonant tunneling structure
Authors: Cockburn J. W. , Buckle P. D. , Skolnick M. S. , Birkett M. J. , Teissier R. , Smith G. W.
Published: July 2002

We report the observation by photoluminescence (PL) spectroscopy of cross-barrier recombination between spatially separated two-dimensional electron and hole gases confined respectively in the quantum well (QW) and collector accumulation layer of a GaAs/AlGaAs double-barrier resonant tunneling structure. At the onset of the n=3(E3) resonance in the current–voltage characteristic, the energy of the cross-barrier transition Ecr is found to coincide with that of the PL peak arising from recombination of electrons from the E3 confined level in the QW with n=1 confined hole states (E3lh recombination). Similarly, at the onset of the E4 resonance, Ecr≊E4lh. We show that this behavior arises as a consequence of the symmetrical potential distribution within the structure at the onsets of the resonances.
Characterization of Ar/Cu electron-cyclotron-resonance plasmas using optical emission spectroscopy
Authors: Rhoades R. L. , Gorbatkin S. M.
Published: July 2002

Optical emission spectroscopy is used to investigate trends with changes in processing parameters for Ar/Cu plasmas in an electron-cyclotron-resonance (ECR) plasma deposition system. The primary motivation for this work is to monitor trends in ionization fractions for copper deposition plasmas using a noninterfering diagnostic tool. The system, which consists of a solid copper sputter target coupled to a permanent magnet ECR microwave plasma system, is operated in the range of 1–6 mTorr argon with net microwave input power of 500–1500 W. Emission from the following excited states is monitored: Ar neutrals (696.5 nm); Ar ions (488 nm); Cu neutrals (521.8 and 216.5 nm); and Cu ions (213.6 nm). Cu ion emission and Cu neutral emission monotonically increase with net microwave input power but at slightly different rates for different pressures, while argon-ion emission as a function of pressure shows a broad peak around 4 mTorr. The ratio of Cu ion emission to Cu neutral emission is used as an indicator of the relative ionization efficiency for Cu and peaks near 5 mTorr. Spectroscopic estimates of electron temperature differences between pure Ar and Ar/Cu plasmas are also presented.
Optical study on the size effects in BaTiO3 thin films
Authors: Zhu J. S. , Lu X. M. , Jiang W. , Tian W. , Zhu M. , Zhang M. S. , Chen X. B. , Liu X. , Wang Y. N.
Published: July 2002

To study the size effects in ferroelectric thin film, we measured the optical transmittance and Raman spectra in BaTiO3 thin films deposited by the rf-magnetron sputtering technique on fused quartz and (111) Si substrates. A variation in the energy gap and Raman peaks with film thickness and grain size was observed and the possible origin was analyzed.
Transport simulation of bulk AlxGa1−xN and the two-dimensional electron gas at the AlxGa1−xN/GaN interface
Authors: Krishnan Mahesh S. , Goldsman Neil , Christou Aris
Published: July 2002

In this work, a hybrid Monte Carlo simulation with the inclusion of degenerate statistics has been performed to analyze electron transport in bulk AlxGa1−xN, and the two-dimensional electron gas at the AlxGa1−xN/GaN interface. The results of the steady-state drift velocity, average electron energy, and distribution functions for bulk AlxGa1−xN are presented. A study of the change in transport properties with compositional variations has been made and presented here. Degeneracy has been found to affect electron transport in both the low- and the high-field regions. The inclusion of degeneracy caused a pronounced negative differential mobility. Simulation of the two-dimensional electron gas takes into account three subbands at the AlxGa1−xN/GaN interface. A self-consistent solution of the Schrödinger and Poisson equations at the heterointerface is obtained through a Rayleigh–Ritz method that should result in more accurate electron wave functions, and hence, more accurate scattering rates for the two-dimensional electron gas. The inclusion of degenerate statistics caused an increase in the interband occupancy in addition to an increase in the electron kinetic energy in the subbands.
Effect of wet oxidized AlxGa1−xAs layer on the interdiffusion of InGaAs/GaAs quantum wells
Authors: Choe Joong-Seon , Ryu Sang-Wan , Choe Byung-Doo , Lim H.
Published: July 2002

The effect of wet oxidized AlAs cap layer and AlGaAs interlayer on the thermal stability of In0.2Ga0.8As/GaAs quantum well (QW) is studied. The QW interdiffusion rate is observed to increase with the Al composition of the AlxGa1−xAs interlayer until x reaches about 0.5 and then saturate for x⩾0.5. When the oxidation is performed at 380 °C for 15 min, the threshold value of x for the enhancement of QW interdiffusion is found to be 0.3. It is also confirmed that the QW interdiffusion can only be explained when the strain effect in InGaAs is taken into account.
Thick Fe3B/Nd2Fe14B nanocomposite permanent magnet flakes prepared by slow quenching
Authors: Kanekiyo H. , Hirosawa S.
Published: July 2002

Thick permanent magnet flakes of Fe3B/Nd2Fe14B nanocomposites have been prepared directly from molten alloys by means of the low surface-velocity melt-spinning technique under a vacuum. Dependence of microstructure and magnetic properties of Nd4Fe77.5B18.5 rapidly solidified alloys have been studied as functions of pressure of a melt-spinner chamber and the surface velocity of a Cu roll. An as-spun alloy of Nd3.5Dy1Fe73Co3Ga1B18.5 obtained at a Vs=3.3 m/s under a 1.3 kPa Ar atmosphere has a thickness of 240 μm and the room temperature magnetic properties of (BH)max=131.6 kJ/m3, Br=1.15 T, and HcJ=400 kA/m.
Magnetic ordering in Co films on stepped Cu(100) surfaces
Authors: Coyle S. T. , Scheinfein M. R.
Published: July 2002

Ultrathin films of Co were grown on Cu(100) and characterized by nanometer resolution secondary electron microscopy, Auger electron spectroscopy, and the surface magneto-optic Kerr effect. An unexpected out-of-plane remanence was detected in many films. The anisotropy of atoms near defects along the Co/vacuum interface calculated via the Néel model indicates that atoms at the bottom corner of a step edge are canted out-of-plane. Full three-dimensional micromagnetics simulations which incorporate site specific anisotropy (including step edges, kinks, and voids) have been performed. Simulations with unidirectional arrays of [11_0] steps, such as vicinal surfaces, do not exhibit out-of-plane remanence. Simulations with facets consisting of connected [110] and [11_0] steps exhibit out-of-plane remanence of 0.03. This is lower than the experimental value of 0.11.
Surface plasmons and breakdown in thin silicon dioxide films on silicon
Authors: Kim Jong-Hyun , Sanchez Julian J. , DeMassa Thomas A. , Quddus Mohammed T. , Smith David , Shaapur Farhad , Weiss Karl , Liu Chuan H.
Published: July 2002

The anode hole injection model is based on a surface plasmon model in which the positive charge is generated by hole injection from the anode, where it is generated via a surface plasmon mechanism resulting finally in oxide breakdown. Attempts to detect the surface plasmons can rely only on indirect observations, such as electron-energy loss, the radiative decay of the surface plasmons, or d2I/dV2 measurements. These measurements show that the emission of surface plasmons is both a strong energy-loss mechanism and an electron-hole pair generation mechanism, particularly in poly-Si/SiO2 or poly-Si/vacuum interfaces. Calculation of the surface plasmon excitation threshold energy is shown to decrease with increasing temperature and is also confirmed by experiments. Thus, the positive charge density increases and the charge to breakdown decreases with increasing temperature. We have also measured and observed the surface plasmon excitation threshold energy at the poly-Si/SiO2 interface from the electron energy loss spectrum for the first time. The surface plasmon mechanism explains the oxide thickness and gate thickness dependence of the positive charge density and temperature dependence of the charge to breakdown. The calculated electron threshold energy to generate a positive oxide charge by the surface plasmon mechanism is EC–Si+2.24 eV. Also, the origin of substrate hole current can be explained by this proposed mechanism. Therefore, the anode hole injection model based upon surface plasmons is a reasonable thin oxide breakdown model that explains measured observations.
The phototransformation of C60 thin films on GaAs(100) studied by in situ Raman spectroscopy
Authors: Park S. , Han H. , Kaiser R. , Werninghaus T. , Schneider A. , Drews D. , Zahn D. R. T.
Published: July 2002

Raman spectra in the region of the pentagonal pinch mode Ag(2) of C60 were taken in situ during the deposition of C60 on the GaAs(100) surface at different temperatures. For very low coverages, only the feature corresponding to the pentagonal pinch mode of pristine C60 is visible. The onset of polymerization under laser irradiation occurred at thicknesses of about 15 nm which is attributed to a suppressive effect on the polymerization process due to the interaction of C60 with the substrate surface. The line shape for the feature due to photopolymerized C60 was different at each temperature indicating distinct polymeric states at different temperatures. These different states are discussed in comparison to recent theoretical calculations. Additionally, the photopolymerization due to irradiation after growth was investigated in situ.
Numerical decomposition of a complex thermostimulated depolarization current spectrum in single time relaxation peaks
Authors: Faubert F. , Sanchez M.
Published: July 2002

We have proposed a numerical method for the decomposition of an experimental thermally stimulated depolarization current (TSDC) curve into a sum of elementary spectra obeying the Debye law (a single relaxation time). This method allows one to solve the case of two close overlapping relaxations as well as the case of a distributed relaxation. It consists of fitting the rightmost part of an experimental TSDC curve with a single time relaxation theoretical curve. A new spectrum is obtained by subtracting the theoretical curve from the experimental one. This operation is repeated until the points ordinates of the resulting spectrum are smaller than a fixed limit. The numerical treatment has to always begin with the subtraction of a part, or the whole, of the high temperature relaxation on the decreasing right side of the curve. The probability of an overlap with another relaxation is smaller than elsewhere. We explain how to choose the best fitting curve among all the theoretical curves found. In the particular case of distributed relaxation the decomposition is made by a step method. Our numerical treatment is confronted with experimental results. In the case of the distributed relaxation of an amorphous polymer at glass transition, the decomposition leads to a compensation law. The results are in accordance with those found by fractional polarizations method.
Positron transmission into Kapton: The effective mass absorption coefficient of relevance to positron lifetime experiments
Authors: Saoucha A.
Published: July 2002

The penetration of positrons from a Na22 source into Kapton foils sandwiched between different backscattering materials (Al, Cu, and Pt) has been studied for the geometry characteristic of positron annihilation techniques. Kapton has served as absorber and each of the selected metals as detector. The method, based on measuring the positron lifetimes, makes use of the difference in values of the fractions of positrons annihilating inside and outside the bulk matrix of the detector. A model on positron transmission and backscattering into 2π solid angle was constructed for the purpose. A correction term was added to the classical exponential form of the transmitted intensity, for absorber thickness z smaller than a critical thickness zc above which the transmitted intensity function falls exponentially. Numerical analysis of the data based on the model function has revealed the existence of two effective mass absorption coefficients pertaining, respectively, to the regions below and above zc. The coefficient αs+ for z<zc of relevance to positron lifetime experiments was found to be considerably smaller (by a factor 3) than the coefficient αl+ extracted by exponential regression measurements for sufficiently large absorber thicknesses. The need for new data on the coefficient for backscattering into 2π solid angle led to measuring the latter for the three used metals, yielding in the end the result αs+=14.12 cm2 g−1. The observed gap between αs+ and αl+ is qualitatively discussed.
Photoluminescence study on the effects of the surface of CdTe by surface passivation
Authors: Kim Young-Hun , An Se-Young , Lee Ju-Young , Kim InJae , Oh Ki-Nam , Kim Sun-Ung , Park Mann-Jang , Lee Tae-Seok
Published: July 2002

The etching effects on the CdTe surface treated by the different chemical etchants, such as 2% Br-methanol (BM) and 2% Br-20% latic acid in ethylene glycol (BLE) solutions are studied by both photoluminescence (PL) and atomic force microscopy. After etching, the dielectric materials ZnS and CdZnTe are used for the passivation of the CdTe surface. The PL studies of the CdTe surfaces passivated by the dielectrics are carried out, and the results are correlated with the value of the lifetime of the CdTe. The results show that the etching of CdTe surface by BLE solution yields the better surface condition for CdTe. The surface trap states of CdTe are reduced by the CdZnTe and ZnS passivant which results in the increase in the surface lifetime of CdTe.
Optimization of an endoreversible chemical engine based on osmosis
Authors: Delgado E. J.
Published: July 2002

Chemical engines, also known as chemical converters, generate work from differences in chemical potential. Chemical potential and mass transfer play roles analogous to temperature and entropy in thermal engines. In this article, an isothermal endoreversible chemical engine based on the osmosis phenomenon is reported. The engine is driven by differences in the solvent chemical potential. The engine is assumed to operate endoreversibly, i.e., the finite-rate mass transfer is the sole irreversibility considered in the model. The optimal initial conditions of the working fluid in the engine, which maximizes the work output, are determined.
Surface discharge and tracking phenomena induced on acrylonitrile-butadiene-styrene polymer dielectric material by acid rain
Authors: Wang X. , Yoshimura N.
Published: July 2002

The discharge and tracking phenomena induced on the polymer dielectric materials by acid rain are investigated by the accelerated aging of acrylonitrile-butadiene-styrene copolymer in artificial rainwater in this article. Based on the investigation of acid rain, the artificial rainwater is chosen to agree well with the actual ingredients of precipitation. The influence of hydrophobicity degradation on the surface discharge and tracking is studied. The relations among the surface discharge, tracking, hydrophobicity, and microchemical structure and physical morphology of material are furthermore discussed. Experimental results show that the polymer dielectric materials suffer a large attack and degradation from acid rain. The dielectric surface degrades and becomes rough, and the hydrophobicity decreases so that the surface discharge and tracking may occur on them.
Thermal desorption spectra of SiO2 films deposited on Si and on thermal SiO2 by tetraethylorthosilicate/O3 atmospheric-pressure chemical vapor deposition
Authors: Cheng Degang , Tsukamoto Koji , Komiyama Hiroshi , Nishimoto Yuko , Tokumasu Noboru , Maeda Kazuo
Published: July 2002

SiO 2 films deposited on Si and on thermal SiO2 by tetraethylorthosilicate [TEOS, Si(OC2H5)4]/O3 atmospheric-pressure chemical vapor deposition (APCVD) were analyzed by thermal desorption spectra (TDS). The TDS results show that more silanols were incorporated during deposition and more water was absorbed during and after deposition in films deposited on Si substrates than on thermal oxide substrates. The latter result indicates that the elimination of water by-products is not the limiting step in TEOS/O3 APCVD. Based on the former result, a silanol model was proposed for the surface processes. On surfaces with a uniform and high density of silanol sites, or on which silanols readily form under TEOS/O3 APCVD conditions, the active silanol groups in the gas phase contribute to film formation and replenish silanol sites, resulting in continuous, high growth rates. On surfaces with few silanol sites, it is difficult to form silanol sites and the nonsilanol-containing polysiloxanes contribute to film formation, resulting in continuous, low growth rate. This model explains well both the surface dependence and the memory effect of TEOS/O3 APCVD.
Modification of Al–oxide tunnel barriers with organic self-assembled monolayers
Authors: Okur Salih , Zasadzinski John F.
Published: July 2002

Al–oxide tunneling barriers were modified by exposure to a vapor of n-octadecyltrimethoxysilane which forms self-assembled monolayers. The dynamic conductance dI/dV of the modified Al–oxide barrier between Al and Pb electrodes was measured at 4.2 K. Quasilinear conductance backgrounds are observed up to 200 mV with a strength that increases with increasing exposure time from 10 to 60 min. A saturation effect is observed around 200 mV. Beyond 200 mV the dynamic conductance shows a parabolic behavior indicative of elastic tunneling from an asymmetric barrier. The linear background is attributed to inelastic tunneling from a continuum of excitations.
Effect of chloride substitution in the fast ion conductor Ag3SBr
Authors: Beeken R. B. , Wright T. J. , Sakuma T.
Published: July 2002

Substitution of the larger iodide anion for bromide in the solid electrolyte Ag3SBr has previously been shown to enhance ionic conductivity in the superionic phase and to reduce the superionic–covalent phase transition temperature. In this investigation, we find it is possible to substitute appreciable quantities of the smaller chloride anion for bromide in Ag3SBr. As composition parameter x increases in nominally prepared Ag3SBr1−xClx samples, greater concentrations of silver halide are removed from the silver sulfide halide lattice. However, x-ray diffraction measurements confirm that the silver sulfide halide lattice persists up to the x=0.50 composition. In contrast to the case of iodide substitution, the effect of replacing bromide with chloride is to raise the transition temperature separating the superionic and covalent phases of the silver sulfide halide component. The overall magnitude of ionic conductivity does not change appreciably as a function of composition near the superionic–covalent phase transition temperature. But the presence of increasing quantities of low conductivity silver halide in these samples suggests that the ionic conductivity of the silver sulfide halide component is enhanced by chloride substitution. With increasing composition parameter, x-ray diffraction and specific heat measurements also confirm the presence of free Ag2S which leads to an electronic contribution to the conductivity above room temperature.
Role of surface instability and anisotropy in strain relaxation of epitaxial SiGe on Si (110)
Authors: Deng X. , Krishnamurthy M.
Published: July 2002

We report on the morphological evolution of strained, low-mismatch Si0.67Ge0.33 and Si0.75Ge0.25 films on Si (110), covering both the elastic and early stages of plastic relaxation during growth. It is seen that three-dimensional island formation is suppressed at 650 and 700 °C on the Si (110) surface, unlike on the Si (100) surface under the same growth conditions. Instead, a high density of ledges oriented along an elastically hard 〈111〉 direction, and a very low density of nanowires oriented along the single in-plane 〈110〉 azimuth are observed. Continued growth leads to the formation of misfit defects oriented along the 〈110〉 direction and disappearance of the nanowires.
Inhomogeneous and thickness-dependent chemical reactivity at GaSe–Si interfaces
Authors: Almeida J. , Zacchigna M. , Berger H. , Margaritondo G.
Published: July 2002

Scanning photoelectron spectromicroscopy experiments on the edge of patterned Si overlayers on GaSe revealed the presence of an inhomogeneous reaction with a metallic-like Ga phase separation. The Si–Se chemically shifted components show lateral variations and a thickness-related SiSe2/SiSex (x=0.5, 1, and 1.5) ratio on a micrometer scale. The dependence of the peak intensities on the overlayer thickness suggests an initial layer-by-layer coverage until ≈2 Å, followed by a clustering growth mode.
Interface polariton modes in GaAs/AlAs superlattices with randomly distributed layer thickness
Authors: Chen Xiaoshuang , Liu Xingquan , Lu Wei , Shen S. C. , Sasaki Akio
Published: July 2002

We study here the dispersion relation and propagation of the interface polariton modes in specially constructed disordered GaAs/AlAs superlattices by using the transfer matrix method. The calculation of dispersion relation shows that the polariton modes exist only in two frequency regions, occur in some minibands and some isolated modes in each polariton band due to disordered effect and exhibit only small shift by varying the degree of thickness randomness, which are qualitatively in agreement with the experimental conclusions. We also find that some modes of interface polaritons with special frequencies are completely unscattered by randomness and can propagate through the structure, in contrast to the localized theory of one-dimensional (1D) disordered systems, while the other modes rapidly decay and are completely scattered. These results can be used to obtain the more realistic photonic band gaps and locations of the resonant photonic states in the 1D systems. This provides a possibility of building a high-quality optical filter.
Temperature dependence of the properties of heteroepitaxial Y2O3 films grown on Si by ion assisted evaporation
Authors: Cho M.-H. , Ko D.-H. , Jeong K. , Lyo I. W. , Whangbo S. W. , Kim H. B. , Choi S. C. , Song J. H. , Cho S. J. , Whang C. N.
Published: July 2002

Heteroepitaxial Y2O3 films were grown on an Si(111) substrate by ion assisted evaporation in an ultrahigh vacuum, and their properties such as crystallinity, film stress, and morphological change were investigated using the various measurement methods. The crystallinity was assessed by x-ray diffraction (XRD) and reflection high-energy electron diffraction. Interface crystallinity was also examined by Rutherford backscattering spectroscopy (RBS) channeling and transmission electron microscopy. The strain of the films was measured by RBS channeling and XRD. Surface and interface morphological characteristics were observed by atomic force microscopy and x-ray scattering method. By comparing the interface with the surface characteristics, we can conclude that many defects at the interface region were generated by interface interaction between the yttrium metal and Si substrate. Moreover, the film quality dominantly depended on the deposition temperature. The crystallinity was greatly improved and the surface roughness was drastically decreased in the temperature range 500–600 °C. On the other hand, in the temperature range 600–700 °C, the compressive stress and film density were further increased, and the island size decreased. Also, the shape of the surface islands was transformed from elliptical to triangular. The film stress was found primarily at the interface area because of the interaction between yttrium and Si substrate.
Structure and magnetic properties of multilayer Co/CN granular films
Authors: Zhou J. N. , Du J. , Khapikov A. F. , Wang S. , Barnard J. A. , Harrell J. W. , Doyle W. D.
Published: July 2002

The magnetic and structural properties of a series of granular Co/CN films prepared by annealing multilayers of Co and CN have been studied. The films contained both hcp and fcc phases, and grain sizes were in the range 18–27 nm. The coercivity varied with cobalt fraction and with bilayer thickness, and a maximum room temperature coercivity of 1230 Oe was obtained for 50% cobalt and 2.7 nm bilayer thickness. Measurements of the time and temperature dependence of the highest coercivity films suggest that their coercivity is due primarily to the hcp phase and to the grain size. All films had relatively low values of magnetic viscosity (2%–3%/decade).
Standing spin waves in granular Fe–SiO2 thin films
Authors: Butera A. , Zhou J. N. , Barnard J. A.
Published: July 2002

Magnetically heterogeneous Fe–SiO2 composite thin films (20 nm thick) prepared using standard rf sputtering techniques have been investigated by means of ferromagnetic resonance (FMR). Besides a single FMR absorption line, corresponding to the uniform precession mode, several nearly equally spaced absorption lines could be observed close to the percolation threshold of the Fe granules fp if the applied field H is parallel to the film normal. These extra absorptions have been identified as standing spin waves. A constant separation between lines suggests that volume inhomogeneities are important (as expected in a granular film). We have determined the following characteristic lengths of the standing spin waves for three Fe volume concentrations: L=370 nm, 680 nm, and 270 nm for f=0.63, 0.66, and 0.69, respectively. In continuous thin films L is normally coincident with the film thickness but in this case L is more than an order of magnitude larger than the film thickness. This discrepancy and the absence of additional lines for f≠fp suggests that L is not related to the film thickness but to the correlation length of the percolating network, eventually modulated by the sample magnetization M. The temperature dependence of the average separation between lines supports this picture.
Grain size analysis of longitudinal thin film media
Authors: Park Dong-Won , Sinclair Robert , Lal Brij B. , Malhotra Sudhir S. , Russak Michael A.
Published: July 2002

Magnetic layer grain size plays an important role in determining the materials properties of longitudinal magnetic thin film media. Therefore, accurate and appropriate microstructural analysis is crucial to understanding the effects of changes in sputtering parameters. In this work we introduce a cumulative percentage frequency plot that is superior in representing and comparing grain size distributions. The advantages of the cumulative method are demonstrated by discussing the outcome of applying a CoCr interlayer between the magnetic layer and the underlayer. Cumulative percentage frequency plots show that a log-normal distribution fits the grain size data best. In addition, applying an interlayer was found to result in improved magnetic properties.
A study on pinned layer magnetization processes in different antiferromagnetic coupling systems of spin-valves
Authors: Yang Zhijun , Han Dehua , Mao Sining , Wang Hong , Al-Jumaily Ghanim , Ryan Pat J. , Crozier Peter A. , Tsen Shu-Chen Y. , McCartney Molly , Scheinfein Mike
Published: July 2002

In this paper we report on magnetization reversal processes of pinned layers in different ferromagnetic and anti-ferromagnetic bilayer and spin-valve structures by observing hysteresis, switching field distribution (SFD), and time-dependent effects. The fact that time-dependent coercivity Hc tendency is more pronounced in ordered AF materials than that of disordered AF materials implies a spin reversal of some AF grains with locally low pinning field and low blocking temperature. We propose a simple qualitative model to explain our results in terms of a distribution of the pinning reversal field and temperature in AF layer, which may be due to a grain size distribution in the AF layer and an incomplete and inhomogeneous phase transformation in some ordered AF materials. High resolution electron microscopy (HREM) results reveal that lattice spacing does change from place to place in AF region of some ordered AF materials. The evidence indicates an incomplete and inhomogeneous phase transformation in the ordered AF systems, supporting the results of the magnetization reversal process study.
Magnetization processes and magnetoresistance in Co/Cu multilayers as a function of the Cu layer thickness.
Authors: Aitchison P. R. , Chapman J. N. , Holloway H. , Kubinski D. J. , Parsons M.
Published: July 2002

We have studied Co(15 Å)/Cu(t) multilayers with nominal Cu spacer layer thicknesses of 7, 8, 9, and 10 Å. For multilayers with identical dimensions, transport measurements showed that the introduction of oxygen during growth increased the magnetoresistance while transmission electron microscopy revealed the effect of the oxygen bleed on the microstructure was reduced grain size, suppression of the Cohcp phase, and reduced texturing. Lorentz microscopy was used to determine the angle between magnetization vectors in adjacent magnetic layers and the values so deduced were found to correlate well with the variation of magnetoresistance within the multilayer sets.
Spin dynamics in perovskites, pyrochlores, and layered manganites
Authors: Oseroff S. B. , Moreno N. O. , Pagliuso P. G. , Rettori C. , Huber D. L. , Gardner J. S. , Sarrao J. L. , Thompson J. D. , Causa M. T. , Alejandro G. , Tovar M. , Alascio B. R.
Published: July 2002

High temperature electron spin resonance (ESR) and magnetic susceptibility (χ) are analyzed for manganites related with colossal magnetoresistance (CMR). The properties of compounds with different crystalline structures: three-dimensional (3D) perovskites, pyrochlore, and La1.2Sr1.8Mn2O7, a two-dimensional layer, are compared. In the paramagnetic regime, and outside the critical regions associated with phase transitions, the temperature dependence of the ESR linewidth presents a universal behavior dominated by the variations of χ(T), ΔHpp(T)=[C/Tχ(T)]ΔHpp(∞). The high temperature limit of the linewidth, ΔHpp(∞), is related to the parameters of the Hamiltonian describing the interactions of the spin system. The role played by magnetic anisotropy, isotropic superexchange, and double exchange is revealed and discussed in the analysis of the experimental data. In CMR and non-CMR pyrochlores, ΔHpp(∞)∝ωp2/J where J is proportional to the Curie–Weiss temperature, including the hybridization mechanism producing CMR. Instead, ΔHpp(∞) of CMR perovskites seems not to be affected by the double-exchange interaction. In contrast with the 3D perovskites, the ESR linewidth and resonance field of La1.2Sr1.8Mn2O7, a bilayer compound, although isotropic at high temperatures, becomes anisotropic for Tc=125 K<T<Tp≈450 K.
Ruderman-Kittel-Kasuya-Yosida interactions between spin distributions of arbitrary shape
Authors: Skomski R. , Sabiryanov R. F. , Jaswal S. S.
Published: July 2002

The net Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between well-separated spin distributions can be mapped onto a RKKY interaction between two point-like effective moments. The effective moments, which a strongly oscillating dependence on the size, shape, and orientation of the spin distribution, are discussed for a variety of microscopic and macroscopic configurations. Particular emphasis is on the relation between RKKY interactions and band-structure Heisenberg exchange constants in itinerant magnets and on two-ion anisotropy contributions to the magnetocrystalline anisotropy of rare-earth ions.
Dust charging and levitating in cathode sheath of glow discharges with energetic electron beam
Authors: Wang Dezhen , Liu Deyong , Liu Jinyuan
Published: July 2002

The dust charging and levitating in a collisionless cathode sheath of dc glow discharges with energetic electron beam released from a plane cathode are investigated with a self-consistent theoretical model. The dust charging associated with the electron beam including secondary emission of dust can dominate the dust surface potential in the close cathode region. In this case, dusts of the same size can levitate at two different positions in the cathode sheath.
Effects of the potential barrier on vibrational levels in bent symmetric triatomic molecules
Authors: Lee Jae Shin
Published: July 2002

Full three-dimensional quantum variational calculations have been carried out using model potentials to investigate the effect of barrier height to linearity on bending vibrational levels for zero total angular momentum states (J=0) in a bent symmetric triatomic molecule AB2. Contrary to the results of a previous study of one-dimensional bending vibration only, the precise spectral pattern is found to depend on not only the height of the potential barrier, but also detailed features of the potential energy surface and various coupling terms in the kinetic energy operator. The results for some real bent triatomic molecules are analyzed and compared with the results for model potentials. Both results indicate that there is no simple formula to compute the barrier height from the analysis of the spectral pattern of the molecule. A possible explanation is presented with the focus on mode mixing in high vibrational states and it is well manifested by the isotopic substitution of hydrogen (1H) with deuterium (2H) atom in an AH2 type molecule.
Structure of the metal-electrolyte solution interface: Theoretical results for simple models
Authors: Bérard D. R. , Kinoshita M. , Ye X. , Patey G. N.
Published: July 2002

Theoretical results are given for electrolyte solutions in contact with uncharged metallic surfaces. The metal is represented by a jellium slab and the electron density distribution across the interface is obtained using density functional theory. The solution ‘‘structure’’ is found by solving the reference hypernetted-chain approximation. The two interfacial phases interact electrostatically and fully self-consistent electron density distributions and wall-particle molecular correlation functions are calculated. The electron densities, wall-induced solution structure and the electrostatic potential drop across the interface are discussed in detail and compared with the inert wall case. It is found that a highly ordered region exists near the metal surface. The dipoles are strongly ordered by the metal field and this solvent structure effectively dictates the ion distributions. The direct ion-metal interactions are found to be less important.
Exact enumeration of all conformations of a heteropolymer chain in a prescribed, non-compact volume
Authors: Mai J. , Sokolov I. M. , Blumen A.
Published: July 2002

We study the protein structure problem in a prescribed, non-compact volume (with a hole). The choice of such a volume offers the advantage of being closer in structure to real proteins. Using an exact enumeration technique we investigate the thermodynamic properties of the chain’s configurations. These findings are compared to the results obtained for fully compact structures, such as the 3×3×3 cube. Interestingly we find here deviations from the random energy model.
New statistical mechanical treatment of systems near surfaces. III. Polydisperse linear and branched polymers in an interacting solution
Authors: Chhajer Mukesh , Gujrati P. D.
Published: July 2002

We apply a recently developed analytical but approximate method to study the behavior of polydisperse linear and branched polymers in a solution with interaction and near various kinds of surfaces. We consider equilibrium polydispersity controlled by a set of activities. The method allows us to account for polymer connectivity and excluded-volume effects and goes beyond the random mixing approximation. The method also allows us to probe the entire density range, which is not possible in a Monte Carlo simulation. The density profiles show oscillations for bulk densities φmb larger than some threshold value φmt, the amplitudes of those oscillations being a strong function of the solvent-monomer interaction parameter w. The behavior of linear and branched polymers is almost identical for the same bulk densities φmb, except when the solvent-monomer interaction is repulsive and the surface-monomer interaction is not attractive. In this case, the effect of branching is to increase the correlation length ξ describing the approach of a density profile to its bulk value. We also compute the surface thermodynamic functions (the entropy and the free energy). While the surface free energies are identical for linear and branched polymers, the surface entropies are very different.
Compaction and folding in model proteins
Authors: Chiu Ting-Lan , Goldstein Richard A.
Published: July 2002

Protein folding is modeled as diffusion on a free-energy landscape, allowing use of the diffusion equation to study the impact of energetic parameters on the folding dynamics. The free-energy landscape is characterized by two different order parameters, one representing the degree of compactness, the other a measure of the progress towards the folded state. For marginally stable proteins, fastest folding is achieved when the nonspecific interactions favoring compaction are strong, resulting in a high folding temperature. Such proteins fold by rapid collapse followed by slower accumulation of correct contacts.
Polyisoprene local dynamics in solution: Comparison between molecular dynamics simulations and optimized Rouse–Zimm local dynamics
Authors: Perico Angelo , Moe Neil E. , Ediger M. D.
Published: July 2002

The optimized Rouse–Zimm local dynamics (ORZLD) approximation to the Smoluchowski generalized diffusion approach has been used to predict the local dynamics of cis-1,4-polyisoprene in dilute solution. The equilibrium averages needed in the theory are calculated from a time average along a molecular dynamics trajectory. The first and second order orientation autocorrelation functions for C–H, C–C, and virtual bonds in the repeat unit are calculated from the theory and compared to the same quantities calculated directly from the molecular dynamics trajectory. The correlation functions from the simulation and the theory decay on similar time scales (within a factor of two) and have somewhat similar shapes. The theory predicts a universal relationship between P1 and P2 correlation functions which is obeyed to a good approximation by the correlation functions from the simulation. Variations in the local dynamics along the chain contour are qualitatively well described by the theory.
Impurity solvation in a liquid
Authors: Tehver Riina , Cole Milton W. , Maritan Amos , Koplik Joel , Banavar Jayanth R.
Published: July 2002

A set of model calculations is presented concerning the problem of impurity solvation. The methods include lattice gas, evaluated exactly as well as in the mean-field approximation, and molecular-dynamics simulation. Our results are in quantitative accord with simple energy balance arguments, but a solvation parameter suggested by Ancilotto, Lerner and Cole [J. Low Temp. Phys. 101, 1123 (1995)] is found to be nonuniversal.
Anharmonicity and cross section for absorption of radiation by water dimer
Authors: Tso H. C. W. , Geldart D. J. W. , Chýlek Petr
Published: July 2002

We calculate the absorption cross section of water dimer molecules in thermal equilibrium at temperatures typical of the lower atmosphere using quantum mechanical coupled nonlinear equations of motion. Empirical Morse-oscillator potentials are used to describe the local modes of water monomer, and the RWK2 potential is employed for the interaction between atoms of different water monomers. The strong anharmonicity is taken into account by an extension to molecular dimers of methods originally developed for the lattice dynamics of solid helium. Approximations based on exploiting the hierarchy of energy scales in the dynamics of the weakly hydrogen-bonded water dimer allow the determination of the absorption spectrum over the range of significant solar radiation, up to 20 000 cm−1, including the important contributions of overtone and combination transitions. This approach can tackle the complicated task of mixing of vibrational fundamentals and overtones. We have found that the absorption by these vibrational overtones, within the solar energy range, is quite significant due to the anharmonicity of Morse-oscillator potentials and the large vibrational amplitude of hydrogen atoms. These overtones may play a role in the solar energy absorption of the atmosphere.