bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of inserting thin Co Layers on the magnetic and reversal properties of synthetic antiferromagnetically coupled media
Authors: Shan Z. S. , Malhotra S. S. , Stafford D. C. , Bian B. , Bertero G. , Wachenschwanz D.
Published: July 2002

It was found that for the conventional synthetic antiferromagnetically coupled (SAF) media, with Co layer thickness dCo=0 Å, the bottom layer is not well ordered ferromagnetically, and that this magnetic ordering is improved significantly for dCo of only ∼4 Å. The coupling constant J, as well as magnetic and magnetization reversal properties MS, HC, SFD, Ku, (KuV*/kT), and V* were measured as a function of dCo for both the top and bottom layers. The relationships among these parameters were analyzed.
Evolution of the charge localization process in xenon cluster ions: From tetramer to dimer cores as a function of cluster size
Authors: Laarmann T. , Kanaev A. , von Haeften K. , Wabnitz H. , von Pietrowski R. , Möller T.
Published: July 2002

The charge localization process in XeN+ cluster ions (N=40–20 000) is investigated with fluorescence spectroscopy methods. New discrete and continuous luminescence bands in the visible and near infrared spectral range are observed and are assigned to radiative transitions of ionic dimers, trimers and tetramers inside Xe clusters. The bands are related to the 5p5 2P1/2→5p5 2P3/2 transition of electronically excited atomic Xe ions and the 6p→6s transitions of electronically excited Xe neutrals. The dependence of the size of the ionic centers on the Xe cluster size is discussed. In large clusters discrete lines are due to embedded dimer emission and they are identified as 2(1/2)u→1(3/2)g transitions between different vibrational levels. Line positions are blue-shifted by 30 (±1) meV with respect to free molecular dimer ions. The energy shift is due to the interaction of the surrounding neutral Xe cluster atoms with the embedded ionic Xe molecules.
Numerical study of pair creation by ultraintense lasers
Authors: Nakashima Ken’ichi , Takabe Hideaki
Published: July 2002

Now that intensity of lasers has reached 1020 W/cm2, electron–positron pairs can be created by the irradiation of such ultraintense lasers on a thin gold foil. The energy of electrons produced by ultraintense lasers reaches more than several tens of MeV. Such high energy electrons become a source for creating electron–positron pairs via interaction with nuclei. There are a few processes that create electron–positron pairs in this situation. Two processes, call the trident process and the Bethe–Heitler process, are considered in this study. A numerical simulation code based on a relativistic Fokker–Planck equation is developed for studying the hot electron transport. The equation is solved by assuming one-dimensional real space and two-dimensional momentum space with axial symmetry. It is found that the total positron yield increases logarithmically with the increase of the laser intensity, and the resultant energy distribution of the created positron is found to have a peak near the energy of 1–2 MeV.
Vibrationally resolved threshold photoelectron–photoion coincidence spectra of ArXe
Authors: Yoshii H. , Hayaishi T. , Onuma T. , Aoto T. , Morioka Y. , Ito K.
Published: July 2002

High-resolution threshold photoelectron–photoion coincidence spectra of ArXe were measured in the photon energy region 11.9–15.9 eV using the penetrating field technique and synchrotron radiation. Six vibrational progressions were observed and ascribed to the production of the six electronic states of ArXe+ ions (X 1/2, A1 3/2, A2 1/2, C1 3/2, B 1/2, and C2 1/2) via photoionization of the ground state ArXe dimer. The lowest vibrational level ν=0 of the ground X 1/2 state and some of the other vibrational levels were observed for the first time in the present measurements. The vibrational constants were obtained from observed vibrational energy levels. The dissociation energy of the ground state (X 1/2) was obtained as D00=1432±8 cm−1.
Multidimensional discrete variable representation bases: Sinc functions and group theory
Authors: Littlejohn Robert G. , Cargo Matthew
Published: July 2002

Examples of nontrivial (that is, non-Cartesian product) multidimensional discrete variable representation (DVR) basis sets are presented that are generalizations of sinc functions in one dimension. Their use in solving quantum problems in two dimensions is illustrated. Unlike all standard (one-dimensional) examples of DVR bases, these bases cannot be created by dividing out the roots of a generating function. It is argued that the difficulty of constructing nontrivial, multidimensional DVR bases is due to the restrictive nature of the DVR conditions, which cannot be satisfied on most subspaces of wave functions. The bases considered in this paper, however, are invariant under translations on a lattice in n-dimensional space, and the properties of the Abelian group of lattice translations allow the DVR conditions to be satisfied. More generally, the question of the relation between group theory and the conditions necessary to qualify a set of basis functions as a DVR basis is considered. It is shown how to construct orthonormal states that are related by the action of some group, and, in the case of Abelian groups, the extra conditions required to qualify the basis as a DVR set are discussed.
The phase behavior of charged colloidal systems in the mean spherical approximation
Authors: Petris Simon N. , Chan Derek Y. C.
Published: July 2002

The mean spherical approximation (MSA) was used to investigate the phase behavior of charged colloidal systems with and without added salt. The competition between cohesive Coulomb interactions and stabilizing entropic and hard-core interactions controls the stability of the system and under certain circumstances, a liquid–gas-type phase transition can occur. The critical parameters and phase diagrams in the MSA obtained via the internal energy path, are compared with two-component Monte Carlo simulations and other theoretical approaches.
Lower free energy bound for hard-sphere perturbation theory
Authors: Mon K. K.
Published: July 2002

It is known that variational perturbation theory for fluids with both upper and lower bounds may provide a more accurate estimate of the free energy for dense model fluid. This does not apply in general to the widely used hard-sphere perturbation theory because standard method produces a trivial lower bound of −∞. In this article, we propose a simple solution and demonstrate the method for the repulsive soft sphere model of r−n with n=4, 6, 9, 12, near freezing. The upper and lower bounds accurately bracket the free energy from computer simulations. The arithmetic means of the bounds agrees with the simulations within 0.1 kBT.
Binding to gold(0): Accurate computational methods with application to AuNH3
Authors: Lambropoulos Nicholas A. , Reimers Jeffrey R. , Hush Noel S.
Published: July 2002

The nature of the bonding of molecules to neutral gold atoms or surfaces is of wide interest, particularly with regard to recent molecular electronics experiments involving molecules linked to gold electrodes and nanoclusters. Here, the fundamental problem of accurate calculation of gold atom–ligand interactions is addressed, and a best-possible estimate for the binding energy of AuNH3 is obtained via coupled-cluster and density-functional calculations using series of Gaussian, Slater, and plane-wave basis sets. Poor convergence of both coupled-cluster and density-functional calculations toward the infinite basis-set limit is obtained from the Gaussian basis sets; using Slater basis sets, convergence is more rapid while plane-wave basis sets easily reached convergence. A total of 24 Gaussian basis sets are examined, and a method is introduced for determining if a particular basis set is sufficiently balanced in its treatment of the metal and its ligand. For balanced basis sets, better estimates of the binding energy are obtained neglecting corrections for basis-set superposition error. Various treatment of relativistic effects are examined including the use of relativistic effective core potentials (RECPs), ultrasoft pseudopotentials, and all electron scalar and full spin–orbit zero-order regular approximation calculations. While the use of RECPs has minimal affect, use of ultrasoft pseudopotentials and neglect of spin–orbit coupling both result in underestimation of the binding energy by 2–3 kcal mol−1 (15%–20%), as does the neglect of triples excitations in coupled-cluster theory. The PW91, B3LYP, BLYP, and LDA density functionals were investigated and of these only PW91 predicted binding energies and geometries in qualitative agreement with the coupled-cluster results. The AuNH3 complex is found to be a realistic model for the bonding of NH3 to a gold (111) surface, the primary differences being the prediction of charge transfer within the complex and associated significantly stronger binding. This may have profound implications for molecular electronics applications in which small gold clusters are used to represent macroscopic electrodes.
Artificial nanocluster crystal: Lattice of identical Al clusters
Authors: Jia Jinfeng , Wang Jun-Zhong , Liu Xi , Xue Qi-Kun , Li Zhi-Qiang , Kawazoe Y. , Zhang S. B.
Published: July 2002

A two-dimensional artificial crystal, which is made up of artificial atoms—identical Al clusters with nanometer size and spacing, was fabricated by taking advantage of surface-mediated clustering on a growth template. In situ scanning tunneling microscopy analysis and first-principles total energy calculations were used to determine the atomic structure of the Al nanoclusters. The Al clusters exhibit more remarkable thermal stability than the In clusters we reported previously. Based on our systematic observations and calculations, the formation mechanism and the high stability of these magic clusters are discussed.
Exchange torque and spin transfer between spin polarized current and ferromagnetic layers
Authors: Wegrowe J.-E. , Fábián A. , Guittienne Ph. , Hoffer X. , Kelly D. , Ansermet J.-Ph. , Olive E.
Published: July 2002

Magnetization reversal triggered by spin injection is measured in electrodeposited Co/Cu/Co pillars (diameter about 60 nm). Two protocols are used. (i) switching of magnetization after a current pulse is monitored as a function of applied field. The maximum offset from the switching field at which irreversible switching occurs is a measure of the strength of the effect; and (ii) irreversible and reversible magnetization changes are observed while the current is ramped at fixed applied field. (i) and (ii) show that irreversible transitions occur only from antiparallel to parallel magnetic configurations and for electrons flow from the polarizer to the analyzer.
Experimental and theoretical study of benzene (acetonitrile)n clusters, n=1–4
Authors: El-Shall M. Samy , Daly George M. , Wright Douglas
Published: July 2002

Well-resolved spectra of benzene–acetonitrile binary clusters BAn, with n=1–4 have been obtained by the (one-color) resonant two-photon ionization technique using the benzene’s B2u←A1g 000 and 601 resonances. The spectra reveal a rapid increase in complexity with the number of acetonitrile molecules in the cluster, associated with van der Waal modes and isomeric forms. While only single cluster origins are found for the benzene–acetonitrile (BA) and the BA2 clusters, two and four distinct isomers are identified for the BA3 and BA4 clusters, respectively. The origins of the BA and BA2 clusters are blueshifted with respect to the free benzene molecule by 38 cm−1 and 26 cm−1, respectively. Monte Carlo (MC) simulations reveal two types of isomeric structures of the BAn clusters. The clusters containing an even number of the acetonitrile molecules (BA2, BA4, and BA6) are dominated by acetonitrile anti-parallel paired dimers. The BA3 cluster consists of a cyclic acetonitrile trimer parallel to the benzene ring. In the BA5 clusters, the acetonitrile molecules are assembled in a cyclic trimer + a paired dimer configuration or in two paired dimers + a single monomer structure. The R2PI spectra, in conjunction with the MC structural models and simple energetic arguments, provide a reasonably compelling picture of the spectroscopic and dynamical phenomena associated with dipole pairing molecular cluster systems.
Time domain approach to semiclassical dynamics: Breaking the log time barrier
Authors: Heller Eric J. , Tomsovic Steven , Sepúlveda Miguel A.
Published: July 2002

The presence of chaos in the classical dynamics is not necessarily the destructive element it was thought to be for semiclassical approximations in the time domain. The method of calculating the semiclassical propagation of initial states and correlation functions for nonlinear and chaotic dynamics is shown, and the excellent accuracy is noted for rather long times. The breakdown timescale is much longer than the infamous ‘‘log time’’ for the cases investigated here.
Quantum-chaotic scattering effects in semiconductor microstructures
Authors: Baranger Harold U. , Jalabert Rodolfo A. , Stone A. Douglas
Published: July 2002

We show that classical chaotic scattering has experimentally measurable consequences for the quantum conductance of semiconductor microstructures. These include the existence of conductance fluctuations—a sensitivity of the conductance to either Fermi energy or magnetic field—and weak-localization—a change in the average conductance upon applying a magnetic field. We develop a semiclassical theory and present numerical results for these two effects in which we model the microstructures by billiards attached to leads. We find that the difference between chaotic and regular classical scattering produces a qualitative difference in the fluctuation spectrum and weak-localization lineshape of chaotic and nonchaotic structures. While the semiclassical theory within the diagonal approximation accounts well for the weak-localization lineshape and for the spectrum of the fluctuations, we uncover a surprising failure of the semiclassical diagonal-approximation theory in describing the magnitude of these quantum transport effects.
Statistical properties of 2-D generalized hyperbolic attractors
Authors: Afraimovich V. S. , Chernov N. I. , Sataev E. A.
Published: July 2002

Recently Pesin introduced a large class of hyperbolic attractors, and for those attractors he established the Smale spectral decomposition. In this paper our main results are a stretched exponential bound on the decay of correlations and the central limit theorem. Also we will obtain conditions under which two well known attractors—those of Belykh and Lozi—are subject to our main results.
High-resolution turbulent simulations using the Connection Machine-2
Authors: Chen Shiyi , Shan Xiaowen
Published: July 2002

The spectral method provides an efficient algorithm for solving the three dimensional incompressible Navier–Stokes equations in periodic boundaries. Most people, so far, have used vectorized machines, such as the CRAY-2, to implement fast Fourier transformations and time integrations in the spectral calculations. In this paper, new results are presented using the spectral calculations on the Connection Machine-2 with a parallel algorithm. The large memory of the Connection Machine-2 and the parallel algorithm allows one, for the first time, to implement a 5123 mesh resolution for high Reynolds number flows. The computational speed of the present code is about 30% faster than the fastest CRAY-2 simulations with four processors. Parallel machines, such as the Connection Machine-2, will possibly provide new computational power for understanding the intermittency and cascade mechanism in fluid turbulence.
Cylindrical hollow-anode discharge
Authors: Miljević Vujo I.
Published: July 2002

Hollow-anode discharge represents a new type of discharge which is realized in a diode consisting of a cylindrical cathode and special type of hollow anode whose inner surface only is conductive. The cylindrical cathode focuses electrons toward the hollow anode placed in the center of curvature, increasing the density of high-energy electrons and efficiency of excitation and ionization in the hollow aperture of the anode. Optical spectrum consists of intense ion lines of the operating gas without anode or cathode material lines.
Angular variation of the coercivity in magnetic recording thin films
Authors: Gau J. S. , Brucker C. F.
Published: July 2002

Coercivity of vacuum-deposited magnetic thin films of CoCr and CoNi is dependent on deposition geometries and other deposition conditions. The angular variation of coercivity shows three types of behavior: bell-, M-, and shifted M-type curves. The displacement-controlled nucleation and growth of reverse domains dominates the coercivity mechanism. The shifted M-type angular-dependent coercivity in the oblique-deposited thin film is related to the out-of-plane magnetic anisotropy, and the shift angle is correlated to the angle of the easy axis, which is out of the film plane but not coincident with the axis of the canted columnar structure.
Optimal geometrical conditions of photoacoustic signal detection with a cylindrical direct coupling cell for liquids
Authors: Kitamori Takehiko , Fujii Masaaki , Sawada Tsuguo , Gohshi Yohichi
Published: July 2002

The general description of photoacoustic (PA) signal generation in liquids developed in our earlier paper [J. Appl. Phys. 55, 4005 (1984)] was extended to analyze PA signal detection. The dependence of detected PA signal magnitude on the excitation beam profile and cell geometry of the cylindrical direct coupling cell was obtained theoretically. The thermal lens effect was clarified to be one of the dominant factors for determination of an acoustic source profile in liquids. These theoretical results were confirmed experimentally. Theoretical, optimized detection conditions for a highly sensitive cell (under a near-field condition) and a wide dynamic range cell (under a far-field condition) were proposed.
Rapid thermal annealing of dopants implanted into preamorphized silicon
Authors: Seidel T. E. , Knoell R. , Poli G. , Schwartz B. , Stevie F. A. , Chu P.
Published: July 2002

The rapid thermal annealing behavior of BF+2 and As+ +BF+2 implanted into crystalline and preamorphized silicon is studied. After solid phase epitaxy nearly complete electrical activity is obtained without channeling tails (for the preamorphized silicon) or significant thermal diffusion. Dislocation loops always appear near the amorphous-crystalline (α/c) interface of the preamorphized layer after solid phase epitaxy annealing (called ‘‘deep disorder’’). For preamorphization using Si+ damage into room-temperature silicon targets, dislocations also span between the deep disorder and the surface, called ‘‘spanning dislocations.’’ The spanning dislocations are eliminated by preamorphization using Ge+ implanted into room-temperature silicon targets. Transmission electron microscopy studies show the spanning dislocations move to the surface under thermal treatment, while the deep disorder remains to act as a getter region. The deep disorder is shown to getter F, or Au when Au is intentionally diffused from the wafer backside. The same kind of disorder correlates the limited diffusion behavior of B for BF+2 implants into crystalline Si. Otherwise, for the cases studied, the secondary ion mass spectrometry and Rutherford backscattering spectrometry profiles show nearly normal diffusive behavior for B or As dopants for 10-sec rapid thermal anneal. General physical interpretations are noted.
Comparison of magnetostatic surface wave propagation characteristics at 77 and 299 K
Authors: Halchin D. J. , Stancil D. D.
Published: July 2002

The propagation of magnetostatic surface waves in yttrium iron garnet at 77 K is reported. The bandwidth was increased by about 60%, the dispersion was reduced, and the required bias field for a given frequency of operation was decreased by over 20% as compared to room-temperature results. These results can be explained by the increase in the magnetization of yttrium iron garnet at 77 K. In addition, the minimum delay line insertion loss was reduced by 1–2 dB at the lower temperature.
Water vapor-enhanced electron-avalanche growth in SF6 for nonuniform fields
Authors: Van Brunt R. J.
Published: July 2002

When water vapor content is increased from 10 to 100 ppmv in SF6 at pressures from 200 to 300 kPa, a dramatic enhancement occurs in the mean size of electron avalanches formed near a positive-point electrode. Although this effect can be attributed to a change in gas composition, it is not due to a change in the ionization rate for the gas. It is proposed that the avalanche enhancement is due primarily to an increase in the probability for initiating electron release from minor negative ions associated with water vapor that collisionally detach more readily at a given field strength than the predominant negative ions associated with SF6. The profiles of the electron avalanche size distributions exhibit a monotonic decrease of probability with increasing number of electrons for avalanches with fewer than 107 electrons, but pronounced peaks appear in the distributions as the mean electron number exceeds 107. These peaks are not consistent with the behavior expected from a stochastic model of electron-avalanche growth in nonuniform electric fields which neglects the influence of space charge.
Production and diagnosis of a lithium anode plasma source for intense ion beam diodes
Authors: Dreike P. L. , Tisone G. C.
Published: July 2002

A scheme for the production of a thin layer of dense lithium plasma is outlined, and the results of a proof-of-principle experiment are presented. Our objective is to produce a thin layer of lithium plasma for the anode plasma of an intense ion beam diode, such as the one being designed for inertial confinement fusion experiments with the Particle Beam Fusion Accelerator II. Minimum requirements for the lithium anode plasma density, thickness, and purity are developed. Production of a lithium vapor layer by pulsed evaporation, and plasma formation by laser ionization based on resonant saturation (LIBORS) of the vapor are described. Lithium was evaporated, by ohmic heating with a two step capacitor discharge, from a thin layer of lithium silver alloy deposited on a tantalum foil. A two-photon pumped fluoresence technique was developed and used to characterize the density and thickness of the vapor layer. A peak vapor density near 1×1016 cm−3 with a thickness of a few millimeters was observed. The electron density of the final plasma was inferred from broadening of the 460.3 nm lithium line, and agreed with the lithium neutral density within experimental uncertainty, for several different lithium densities.
Material properties of high-quality GaAs epitaxial layers grown on Si substrates
Authors: Fischer R. , Morkoç H. , Neumann D. A. , Zabel H. , Choi C. , Otsuka N. , Longerbone M. , Erickson L. P.
Published: July 2002

We report an investigation of the materials properties of GaAs on Si epitaxial layers. By using properly oriented substrates, we have found that a substantial reduction in the density of threading dislocations can be achieved. In the presence of steps, dislocations with their Burgers vectors in the (100) substrate plane are preferentially generated, which are more effective in accommodating lattice mismatch and do not thread into the epitaxial layer. We have also found that the density of threading dislocations can be reduced significantly by the use of GaAs/InGaAs pseudomorphic superlattices. Using these techniques, dislocation densities of as low as 103–104 cm−2 have been achieved in 2-μm-thick GaAs on Si epitaxial layers. In growth on nominal (100) orientations, where it is known that single atomic steps dominate, we have found no evidence of antiphase domains by transmission electron microscopy or chemical etching. This result suggests that it may not be energetically favorable for antiphase domains to form in these samples. Alternatively antiphase domains may propagate along the {111} directions and annihilate one another. For GaAs/(Al,Ga)As double heterojunction (DH) laser structures on Si substrates, the dislocation control techniques have made possible electroluminescence intensity (spontaneous emission) within a factor of 2 of state-of-the-art DH lasers on GaAs (which for reference lased at a current threshold density of 600 A/cm2). These results compare to within experimental determination. Electroluminescence intensities were also found to increase with increasing initial growth temperature. Stripe geometry lasers with room-temperature pulsed threshold currents as low as 170 mA for 10×240 μm2 stripe have been obtained using these techniques.
Gas-phase silicon atoms in silane chemical vapor deposition: Laser-excited fluorescence measurements and comparisons with model predictions
Authors: Breiland William G. , Ho Pauline , Coltrin Michael E.
Published: July 2002

Laser-excited fluorescence was used to detect gas-phase silicon atoms during chemical vapor deposition from silane. Relative Si densities were measured as a function of height above the susceptor, susceptor temperature, amount of H2 added to the He carrier gas, and total pressure. The Si atom density decreased both upon addition of H2 and as the total pressure was lowered below ∼100 Torr. Comparisons of these measurements with the model developed by Coltrin, Kee, and Miller [J. Electrochem. Soc. 131, 425 (1984)] show that the model predicts the qualitative behavior of the experimental Si atom profiles over a wide range of experimental conditions. Addition of a mechanism for particulate nucleation to the model may resolve some of the discrepancies between model and experiment.
Sputtering of negative hydrogen ions by cesium bombardment
Authors: Lopes J. L. , Greer J. A. , Seidl M.
Published: July 2002

The production of negative hydrogen ions sputtered from a low work function converter surface has been investigated. Hydrogen and cesium admitted into the vacuum chamber are chemisorbed on a polycrystalline molybdenum target. H−, Mo−, and e− are sputtered from this cathode by Cs+ ions in the energy range 150–1000 eV. Angular and parallel energy distributions of H−, Mo−, and e− are measured as a function of hydrogen gas pressure, cesium coverage, and incident ion energy. For optimum coverage, the H− ion temperature varies from 0.65% and 0.35% of the incident Cs+ bombarding energy for Cs+ ion energies of 250 and 1000 eV, respectively. The secondary electrons have a temperature of 0.04% of the bombarding energy almost independent of Cs+ energy. The spreads increase with decreasing target coverage and are independent of surface roughness. The optimum H−, Mo−, and e− yields are also measured as a function of hydrogen pressure and incident Cs+ bombarding energy. The optimum H− ion yield is 0.41 at a Cs+ ion energy of 750 eV. By extrapolating the H− ion yield at low Cs+ bombarding energy, a Cs+ ion threshold energy of 120 eV may be estimated. This indicates a binding energy of hydrogen smaller than 3.6 eV.
Temporal behavior of modulated optical reflectance in silicon
Authors: Opsal Jon , Taylor Michael W. , Smith W. Lee , Rosencwaig Allan
Published: July 2002

We report on the results of a study of the temporal behavior of the laser-induced modulated optical reflectance from the surfaces of crystalline silicon wafers, epitaxial silicon films, and ion implanted but unannealed silicon wafers. The observed temporal behavior of this signal appears to be associated with the presence and temporal evolution of electronic surface states.
Polarized light scattering from metal surfaces
Authors: Bickel William S. , Zito Richard R. , Iafelice Vince
Published: July 2002

The stretching and bending of polished and industrial-grade metallurgical surfaces changes the appearance of the specimen’s surface. The changes in surface structure can be detected in the polarized light scattered from the surface cracks and other defects, which act as scattering centers. In order to interpret light scattering signals from complex surfaces, we first examine the nature of light scattered from a perfect mirror surface, a degraded mirror, a line on a mirror, a randomly sanded surface, and, finally, the industrial grade metal surfaces damaged by stretching and bending.
Evaluation of Čerenkov converters for measuring current density distribution in large-area electron beams
Authors: Samlin G. E.
Published: July 2002

An evaluation is presented of several types of Čerenkov converters to study time-resolved current uniformity in large-area electron beams with current densities up to 4 kA/cm2. The materials used for this study were Mylar, fused silica, glass, and Lucite. The final choice for a Čerenkov converter to be used on a large-area diode was type 0080 clear lime glass, which is a commercial plate glass. The thickness used was 0.32 cm. The glass converter showed excellent time response and correlation of light output to electron current density. This converter survives in intense current beams and is economically available in large sizes.
Pyroelectric photothermal spectroscopy for thin solid films
Authors: Tanaka Keiji , Ichimura Yoshikatsu , Sindoh Kiyotaka
Published: July 2002

A pyroelectric photothermal spectroscopy has been developed for evaluating optical absorption spectra of thin films. The temperature change in a film illuminated with chopped monochromatic light is measured with transparent temperature sensors consisting of pyroelectric materials, and the output voltage is employed to calculate absorption spectra. In a practical system, the absorption coefficient down to 102 cm−1 can be determined for thin films 1 μm thick at desired temperatures.
Analysis of photoluminescence generated in a semi-infinite semiconductor slab by a time-varying focused laser beam
Authors: Wilson T. , Pester P. D.
Published: July 2002

We derive general expressions for the photoluminescence generated within a semi-infinite semiconductor slab by excitation with a focused convergent light beam. A simple Fourier–Bessel transform technique is used to solve the continuity equation. We specialize to the specific cases of excitation pulses which can be described by delta functions and step functions, and consider both finite duration pulses and sinusoidally modulated beams. Conditions are also discussed under which beam focusing and finite beam absorption may be ignored and accurate results still obtained. The results apply to the case of linear recombination mechanisms.
Evaluation of the electron energy distribution in a low-pressure Hg-Ar discharge with a two-temperature approximation
Authors: Wani Koichi
Published: July 2002

The low-pressure mercury-argon discharge was analyzed employing a two-temperature representation of the electron energy distribution. The electron energy distribution function was approximated with two Maxwellian portions, which represented a low-energy part of the electrons (bulk) characterized by higher temperature and a high-energy part of the electrons (tail) characterized by lower temperature. As a parameter representing non-Maxwellian distribution, the ratio of the tail temperature to the bulk temperature was introduced. The ratio was determined experimentally by adjusting the calculated field strength of the positive column to the observed one. The plasma parameters under the two-temperature approximation were shown to be obtainable by solving the rate equations for the particle densities and the energy balance equation for the whole electron group. The model was applied to the positive column of a 24-mm-diam tube. The rate coefficient of Hg 61S0 to Hg 61P1 transition was found to be 0.64 times that obtained under the Maxwell approximation.
The TM dependence of the magneto-optic signal in GdTb-TM thin films
Authors: Hairston D. K. , Kryder M. H.
Published: July 2002

The magneto-optic polar Kerr rotation for various GdTb-TM thin films was measured at room temperature as a function of TM composition and was found to correlate with the Slater–Pauling curve. In spite of the fact that the temperature dependence of magnetization is a complicating factor, the data clearly establish the above-mentioned correlation. The TM compositions ranged from Mn through Fe and Co to Ni. The Fe-based films had Curie temperatures covering a range from 100 to 300 °C in contrast to the Co-based films whose Curie temperatures were clearly much greater than 300 °C. Neither the Mn- nor the Ni-based films exhibited desirable magneto-optic activity when fabricated under the same conditions as the Fe- and/or Co-based films. When the Fe constituent of the GdTbFe films was gradually substituted with Mn or Co the room-temperature magneto-optic signal was found to decrease and increase, respectively. In a similar manner, when the Co constituent of GdTbCo films was gradually substituted with Fe or Ni the room-temperature magneto-optic effect was found to increase and decrease, respectively. However, when the Co constituent of GdTbCo films was gradually substituted with Mn the room-temperature magneto-optic effect also decreased, clearly establishing the correlation to the Slater–Pauling curve. These results are further evidence that the magneto-optic effect of traditional RE-TM thin films is dominated by the TM composition and explain the fact that the magneto-optic signal is largest in GdTbFeCo thin films. New data supporting the correlation between magneto-optic effect and perpendicular anisotropy was also obtained.
Ultraviolet laser-induced oxidation of silicon: The effect of oxygen photodissociation upon oxide growth kinetics
Authors: Orlowski T. E. , Mantell D. A.
Published: July 2002

New results on ultraviolet laser-induced formation of high-quality patterned silicon dioxide (SiO2) layers on silicon substrates at room temperature are presented. Comparison studies of oxide growth using pulsed excimer lasers at wavelengths of 193 nm (ArF) and 248 nm (KrF) show a dramatic rate enhancement at 193 nm which is attributed to the greater reactivity of O atoms produced by the photodissociation of O2. This growth enhancement depends exponentially upon laser pulse intensity over the range 0.05–0.40 J/cm2. The initial oxide growth rate at 193 nm varies linearly with laser exposure and sublinearly with oxygen pressure indicating that the observed rate is primarily the reaction of Si with O atoms at the Si-SiO2 interface. Growth kinetics and oxide stoichiometry are determined by careful analysis of the laser exposed regions following each laser shot using angle-resolved x-ray photoelectron spectroscopy with 150-μm spatial resolution. After one laser shot, a SiOx interface layer is clearly visible. Subsequent laser shots show the oxide peak shifting to higher energy as the oxide thickness increases until the characteristic ∼4.5-eV chemical shift for SiO2 (relative to the bulk Si 2p peak) is realized at a thickness of ∼2 nm. The nonstoichiometric SiOx layer is still present at this oxide thickness but is confined to the interface region (<1 nm) as observed in thermally grown oxides. Fourier transform infrared spectra show a frequency of 1070 cm−1 for the Si-O stretching mode and a bandwidth of 99 cm−1 implying that these laser-grown stoichiometric oxides are under compressive stress with a broader SiOSi bond angle distribution than thermal oxides.
Negative ion formation at a barium surface exposed to an intense positive-hydrogen ion beam
Authors: van Os C. F. A. , van Amersfoort P. W. , Los J.
Published: July 2002

A fundamental study of the formation of negative hydrogen ions via surface conversion is presented. Employed is a novel type of converter, namely a pure barium metal surface. In spite of the high work function of barium compared to more conventional cesiated converters, considerable yields of negative ions were produced. Conversion efficiencies up of 4% are obtained, which is of the same order as for cesiated converters. The high negative-ion yield is probably related to the electron density of barium, which is almost twice that of cesiuim. This is confirmed by model calculations and by UHV scattering experiments under well-defined conditions. Furthermore, calculations showed that the hydrogen coverage of the converter increases with increasing flux of positive hydrogen ions to the surface. This behavior is confirmed experimentally. Seeding the hydrogen plasma with argon has no significant effect on the conversion efficiency. This is believed to be related to the competition between the lowering of the surface hydrogen coverage and the increase of the hydrogen desorption rate, both due to the higher sputter coefficient of argon compared to hydrogen.
Interactions of dislocations in GaAs grown on Si substrates with InGaAs-GaAsP strained layered superlattices
Authors: El-Masry N. A. , Tarn J. C. , Karam N. H.
Published: July 2002

InGaAs-GaAsP strained-layered superlattices have been used as a buffer layer to reduce the dislocation density in GaAs grown on Si substrates. These superlattices have been grown lattice matched to GaAs. Several interactions between the strain field of the strained layered superlattice [GaAs1−yPy-InxGa1−xAs (y=2x)] and the threading dislocations in GaAs/Si are observed. Mixed dislocations are strongly affected by the strain field of the superlattice, however, the interactions with the edge dislocations are less likely to occur. The stress field associated with the strained layered superlattice has a shear component that forces the 60° mixed dislocations to bend at the strained layered superlattice interfaces. Dislocations annihilation or repulsion are observed at the superlattice interfaces.
Time-dependent evolution of SiO2/Si interface traps after repeated radiation damage
Authors: Nishioka Yasushiro , da Silva Eronides F. , Ma T.-P.
Published: July 2002

The post-irradiation evolution of the interface traps in metal/SiO2/Si (MOS) capacitors after repeated x-ray irradiation has been investigated. The results indicate that, immediately after the first irradiation, a characteristic peak (peak-1) in the energy distribution of the interface traps above midgap appears, which subsequently decreases with time in storage at 75 °C. Accompanying this decrease is the formation and growth of a second peak (peak-2) below midgap. Evidence will be presented which suggests that this is due to an interfacial defect transformation process. Measurements done after repeated irradiation-storage cycles indicate that peak-2 is not generated directly by the x-ray irradiation; rather, the existence of peak-1 is a necessary condition for the formation of peak-2 through the defect transformation process. There is also evidence that only a portion (although a significant portion) of peak-1 defect undergoes the transformation process.
Dynamic consolidation of type 304 stainless-steel powders in gas gun experiments
Authors: Flinn J. E. , Williamson R. L. , Berry R. A. , Wright R. N. , Gupta Y. M. , Williams M.
Published: July 2002

Specimens of type 304 stainless-steel powder consolidated using projectile impacts from a 10.2-cm gas gun have been recovered intact and the microstructures have been characterized. Variables for the study included two types of 304 stainless-steel powder, cavity depth, and peak stresses ranging from 11.7 to 21.1 GPa. All specimens exhibited three-dimensional contractions with approximately one-half of the densification associated with thickness. Some porosity from entrapped gas as a result of not evacuating the powder-filled cavities and microcracks due to the recovery method were observed. In the regions free of these defects, full densification was observed. The 11.7-GPa experiment produced considerable work hardening. The bonding around the particle perimeters ranged from melted regions to mechanical abutment. The 21.1-GPa experiment resulted in lower hardness, reduced metastable bcc phase, and more extensive bonding around the particle perimeters. These observations suggest that very high localized, and relatively high bulk temperatures were achieved for this peak stress. A two-dimensional numerical simulation, with the powder region considered as a two-component mixture, was used to examine the overall response of the powder-filled cavities and surrounding target to the stress wave. The calculations predicted the observed density change and the observed final shape for the compacted powder. Two-dimensional numerical simulations, in which the particles were individually modeled, were also performed to assess the behavior at the particle level. The deformation of the particles necessary to fill the interstitial regions along with the corresponding temperatures around the perimeters of the particles and their interiors were calculated. The deformation varies markedly around the particle periphery and the corresponding computed temperature rise ranges from slightly above ambient to above melting in regions of maximum particle extrusion. The nonuniform temperature rise around the particles correlates with the heterogeneous nature of the bonding observed experimentally. The model also predicts higher and broader temperature zones with increasing dynamic stress, in agreement with the experimental inferences.
Internal acoustical excitation of shells: Scholte and whispering gallery-type waves
Authors: Pareige P. , Maze G. , Izbicki J. L. , Ripoche J. , Rousselot J. L.
Published: July 2002

The authors study the acoustic radiation from cylindrical elastic shells immersed in water and containing an internal acoustic source. To this end, they use the method of isolation and identification of resonances (MIIR) perfected at Le Havre in the 1980’s [G. Maze and J. Ripoche, Phys. Lett. 84A, 309 (1981); Rev. Phys. Appl. 18, 319 (1983)] to obtain the resonance spectra of targets and group the resonances in families by means of the experimental identification of the mode number. They analyze the results in light of experimental studies on the acoustic scattering from elastic targets insonified by an external transmitter and of theoretical studies (calculation of natural modes and resonance scattering theory). For the targets studied, two series are put forward in the normalized frequency range examined in this paper.
Flashboards as a plasma source for plasma opening switch applications
Authors: Renk T. J.
Published: July 2002

A plasma opening switch (POS) requires the use of a source to inject plasma, usually into the vacuum section of a transmission line of a pulsed power generator. The injection point is typically just upstream of the generator load. A flashboard, which consists of a network of flashover gaps built into a stripline geometry, is capable of providing the drifting plasma source used in the POS. This paper details a series of measurements aimed at optimizing this source plasma in order to obtain improved performance of the combination POS-load system.
Sodium-fluoride discharge for fast Z-pinch experiments
Authors: Welch B. L. , Young F. C. , Commisso R. J. , Hinshelwood D. D. , Mosher D. , Weber B. V.
Published: July 2002

A capillary-discharge plasma source has been developed to produce a sodium-bearing plasma for fast Z-pinch implosion experiments. Peak currents of 40–50 kA from a 0.5-kJ capacitor bank were driven through a 0.5-mm-diam, few cm long capillary drilled in packed sodium-fluoride powder to form the source. A nozzle was used to collimate plasma ejected from one end of the capillary to produce a 1–2-cm-diam, several cm long cylindrical plasma. Ions with velocities of 2.2–3.4 cm/μs and densities of up to 5×1015 cm−3 were measured with biased charge collectors located at least 5 cm from the nozzle. Measurements of visible light from neutrals near the nozzle exit gave velocities of 1.5–1.7 cm/μs. Indications of axial and radial nonuniformities of the plasma were observed in framing photographs of visible-light emission and in spatially resolved spectral measurements. Neutral-sodium and neutral-fluorine lines were identified in the spectral range from 2300 to 6700 Å. Also, impurity lines of carbon, copper, and hydrogen were identified and used to characterize the plasma. Stark broadening of the Balmer alpha line of hydrogen was used to deduce a peak electron density of 8×1016 cm−3 at the exit of a 2-cm-diam nozzle. Electron temperatures of 1.4–1.6 eV at the nozzle exit were inferred from relative intensities of the C i and C ii lines. At this density and temperature, Saha-equilibrium-model calculations indicate that the plasma consists primarily of singly ionized sodium and neutral fluorine. A total mass per unit length (sodium and fluorine) of at least 15 μg/cm is deduced from this analysis of the plasma constituents. This capillary discharge has been used to produce 50–100 GW of sodium K-shell x rays in fast Z-pinch experiments.
Thermal magnetic noise in rf SQUIDS coupled to ferromagnetic cores
Authors: Vitale S. , Prodi G. A. , Cerdonio M.
Published: July 2002

We present a simple model for the thermal magnetic noise in a rf SQUID coupled to a ferromagnetic core via a superconducting dc flux transformer. We show that, under conditions that are almost always fulfilled in practice, the coupling to the core can be described in terms of a SQUID frequency-dependent complex inductance Ls (ω). This inductance reduces to an ordinary, frequency-independent real value above a roll-off frequency ωc . To this inductance, a Nyquist flux noise generator is associated, with spectral density S′2φi(ω) =−4kBT Im[Ls(ω)]/ ω, where kB is the Boltzmann constant, T the temperature, and Im[Ls (ω)] the imaginary part of Ls (ω). If ωc ≪ωrf , with ωrf the frequency of the rf pump, then the response of the SQUID to low-frequency signals is not modified by the coupling to the core, while the above Nyquist noise acts as an additive input noise. We experimentally demonstrate the model for a SQUID coupled to a toroidal core.
A quantum chemical study on hydrogen radical reactions with methane and silane
Authors: Sato Kota , Kojima Kuniharu , Kawasaki Masashi , Matsuzaki Yoshio , Hirano Tsuneo , Nakano Masatake , Koinuma Hideomi
Published: July 2002

A quantum chemical study on the reaction of CH4 , CF4 , SiH4 , and SiF4 with a hydrogen radical is performed on the basis of an ab initio molecular orbital calculation to predict the photochemical reactivity of methane, silane, and their analogues. The transition state geometry of the reactions is determined by employing a 3-21G basis set. The total energies of reactant molecules at the initial, transition, and final states are calculated by employing a 6-31G** basis set. The exponential parts of the rate constants of these reactions determined from these energies on the basis of the transition state theory are in good agreement with the experimentally obtained relative rates of the reaction. The present calculation was consistent with the experimental results of photochemical reactions for methane and silane derivatives.
X-ray diffraction of γ-Fe at high temperatures and pressures
Authors: Boehler R. , Besson J. M. , Nicol M. , Nielsen M. , Itie J. P. , Weill G. , Johnson S. , Grey F.
Published: July 2002

Wire heating in diamond-anvil cells has been used to measure the pressure, temperature, resistance, and x-ray diffraction patterns of γ-Fe at pressures to 42 GPa and temperatures to 2340 K. The results can be described by a bulk modulus, K=127±8 GPa+2.2 (P/GPa). Extrapolation of these results to conditions of the earth’s core yields a density in good agreement with shock compression and of about 15% higher than estimates of the preliminary reference earth model.
Measured laser parameters for reactor-pumped He/Ar/Xe and Ar/Xe lasers
Authors: Alford W. J. , Hays G. N.
Published: July 2002

We have measured laser parameters for reactor pumping of He/Ar/Xe gas mixtures lasing predominantly at 2.03 μm and Ar/Xe mixtures lasing predominantly at 1.73 μm. Gains as high as ∼3%/cm have been measured in He/Ar/Xe at pump powers of ∼200 W/cm3 . Both systems exhibit small distributed losses. Intrinsic laser energy efficiencies as high as 2.4% (3.0%) have been observed for He/Ar/Xe (Ar/Xe). These efficiencies are the highest reported for reactor-pumped lasers.