bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature dependence of magnetic anisotropy in Ni–Mn–Ga alloys exhibiting giant field-induced strain
Authors: Heczko Oleg , Straka Ladislav , Lanska Natalyia , Ullakko Kari , Enkovaara Jussi
Published: July 2002

Temperature dependence of structure and magnetic anisotropy of single crystalline Ni48.8Mn28.6Ga22.6 alloy exhibiting giant field-induced strain or magnetic shape memory (MSM) effect was studied in the temperature range 80–420 K. Upon cooling the alloy transforms from cubic austenite at 307 K to the martensite which exhibits five-layered (modulated) tetragonal structure (5M) with a=0.595 nm and c=0.559 nm. Reverse transformation occurs at 317 K. An additional intermartensitic transition takes place at about 95 K. The basic mechanism of the MSM effect was corroborated by direct simultaneous measurements of strain and magnetization as a function of magnetic field. The magnetic anisotropy of the martensite exhibiting the giant strain was determined from the magnetization curves measured by a vibrating sample magnetometer at different temperatures. The anisotropy of the single variant 5M martensite is uniaxial with easy axis along the tetragonal c axis. The first magnetic anisotropy constant is Ku1=2.0×105 J/m3 at 283 K and increase to Ku1=2.65×105 J/m3 at 130 K. Above room temperature the anisotropy steeply decreases. The second anisotropy constant is negligible and less than 4% of the first anisotropy constant.
Current-in-plane pseudo-spin-valve device performance for giant magnetoresistive random access memory applications (invited)
Authors: Katti R. R.
Published: July 2002

Write and read characteristics of current-in-plane pseudo-spin-valve (PSV) devices are demonstrated in terms of switching, resistive, and magnetoresistive properties of individual PSV devices and ensembles of PSV devices. Characteristics of PSV devices are described that are necessary and consistent with requirements for a giant magnetoresistive random access memory (GMRAM) which is an integrated magnetics-on-semiconductor memory. An architecture for a GMRAM is described that uses PSV devices for writing, readback, and nonvolatile data retention. Write and read switching, read magnetoresistance, write and read selectivity, switching repeatability, resistance, and change-in-resistance properties have been characterized and described in PSV devices and populations of PSV devices fabricated on bulk Si and complementary metal–oxide semiconductor underlayers. PSV device switching characteristics are shown to be consistent with coupled-switching models and have trends that show similarities with micromagnetic simulation results.
Beyond the Child–Langmuir law: A review of recent results on multidimensional space-charge-limited flow
Authors: Luginsland J. W. , Lau Y. Y. , Umstattd R. J. , Watrous J. J.
Published: July 2002

Space-charge-limited (SCL) flows in diodes have been an area of active research since the pioneering work of Child and Langmuir in the early part of the last century. Indeed, the scaling of current density with the voltage to the 3/2’s power is one of the best-known limits in the fields of non-neutral plasma physics, accelerator physics, sheath physics, vacuum electronics, and high power microwaves. In the past five years, there has been renewed interest in the physics and characteristics of SCL emission in physically realizable configurations. This research has focused on characterizing the current and current density enhancement possible from two- and three-dimensional geometries, such as field-emitting arrays. In 1996, computational efforts led to the development of a scaling law that described the increased current drawn due to two-dimensional effects. Recently, this scaling has been analytically derived from first principles. In parallel efforts, computational work has characterized the edge enhancement of the current density, leading to a better understanding of the physics of explosive emission cathodes. In this paper, the analytic and computational extensions to the one-dimensional Child–Langmuir law will be reviewed, the accuracy of SCL emission algorithms will be assessed, and the experimental implications of multidimensional SCL flows will be discussed.
Ion flux from vacuum arc cathode spots in the absence and presence of a magnetic field
Authors: Anders André , Yushkov George Yu.
Published: July 2002

Because plasma production at vacuum cathode spots is approximately proportional to the arc current, arc current modulation can be used to generate ion current modulation that can be detected far from the spot using a negatively biased ion collector. The drift time to the ion detector can used to determine kinetic ion energies. A very wide range of cathode materials have been used. It has been found that the kinetic ion energy is higher at the beginning of each discharge and approximately constant after 150 μs. The kinetic energy is correlated with the arc voltage and the cohesive energy of the cathode material. The ion erosion rate is in inverse relation to the cohesive energy, enhancing the effect that the power input per plasma particle correlates with the cohesive energy of the cathode material. The influence of three magnetic field configurations on the kinetic energy has been investigated. Generally, a magnetic field increases the plasma impedance, arc burning voltage, and kinetic ion energy. However, if the plasma is produced in a region of low field strength and streaming into a region of higher field strength, the velocity may decrease due to the magnetic mirror effect. A magnetic field can increase the plasma temperature but may reduce the density gradients by preventing free expansion into the vacuum. Therefore, depending on the configuration, a magnetic field may increase or decrease the kinetic energy of ions.
New solutions of relativistic wave equations in magnetic fields and longitudinal fields
Authors: Bagrov V. G. , Baldiotti M. C. , Gitman D. M. , Shirokov I. V.
Published: July 2002

We demonstrate how one can describe explicitly the present arbitrariness in solutions of relativistic wave equations in external electromagnetic fields of special form. This arbitrariness is connected to the existence of a transformation, which effectively reduces the number of variables in the initial equations. Then we use the corresponding representations to construct new sets of exact solutions, which may have a physical interest. Namely, we present new sets of stationary and nonstationary solutions in magnetic field and in some superpositions of electric and magnetic fields.
State-to-state dynamics of H+HD→H2+D at 0.5 eV: A combined theoretical and experimental study
Authors: Harich Steven A. , Dai Dongxu , Yang Xueming , Chao Sheng Der , Skodje Rex T.
Published: July 2002

The state-to-state dynamics of the H+HD→H2+D reaction at a collisional energy of 0.5 eV was studied experimentally using a crossed molecular beam apparatus using the H atom Rydberg “tagging” time-of-flight detection scheme, and modeled theoretically using a converged quantum scattering calculation on the ground potential energy surface. The experimental results agreed very well with the theoretical calculations without considering the geometrical phase at this collisional energy. The results indicate that the H+HD reaction at this energy is dominated by a simple rebound mechanism along a collinear reaction path. Both experimental and theoretical results show that the rotational state distribution of the H2 product is strongly influenced by nuclear spin statistics.
Degradation and failure of organic light-emitting devices
Authors: Ke Lin , Chua Soo-Jin , Zhang Keran , Yakovlev Nikolai
Published: July 2002

The degradation and failure of organic light-emitting device are observed via optical microscopy. The “degraded area” has been identified to be made up of three regions: (1) a dark spot at the center, (2) a nonemitting area forming the core, and (3) a weakly emitting area surrounding the core. It is found that due to metal migration, as evidenced from the secondary ion mass spectrometry profiles, the indium tin oxide/polymer interface roughens during operation. The intense local current at sharp points degrades the polymer causing the formation of the dark center. Further current stress caused the central core to carbonize which may lead to short and/or open circuits accompanied by fluctuations in the device current.
Comment on “Explicit-solvent molecular dynamics simulation at constant pH: Methodology and application to small amines” [J. Chem. Phys. 114, 9706 (2001)]
Authors: Baptista António M.
Published: July 2002

A theoretical method has recently been proposed to simulate a system weakly coupled to a proton bath [J. Chem. Phys. 114, 9706 (2001)]. However, as discussed here, the method suffers from several theoretical problems and, in the absence of a more sound justification, should probably be regarded as an empirical approach.
Experimental study of inertioelastic Couette–Taylor instability modes in dilute and semidilute polymer solutions
Authors: Crumeyrolle Olivier , Mutabazi Innocent , Grisel Michel
Published: July 2002

Instabilities appearing in the circular Couette flow with a dilute or semidilute solution of high molecular weight polyethyleneoxide in water have been investigated when the outer cylinder is kept at rest. The shear-thinning behavior of solutions was determined through low shear viscosity measurements. We have found that, depending on concentration, different flow structures can appear at the onset of instability. For dilute concentrations, the critical mode is the stationary and axisymmetric Taylor vortex flow, which bifurcates to time periodic wavy vortex flow for a higher shear rate. The oscillation amplitude of wavy vortex flow decreased with the increasing shear rate. For sufficiently semidilute solutions, the critical mode occurs in the form of standing waves, the frequency of which decreases with the shear rate. The critical Taylor number increases for solutions without a shear-thinning effect and decreases for solutions exhibiting shear thinning.
The photoisomerization of aqueous ICN studied by subpicosecond transient absorption spectroscopy
Authors: Larsen Jane , Madsen Dorte , Poulsen Jens-Aage , Poulsen Tina D. , Keiding Søren R. , Thøgersen Jan
Published: July 2002

The photolysis of aqueous ICN is studied by transient absorption spectroscopy covering the spectral range from 227 to 714 nm with 0.5 ps time resolution. The experimental data show that when ICN(aq) is photolyzed at 266 nm, it dissociates into I and CN and both the I(2P3/2) and I(2P1/2) channels are populated. Approximately half the fragments escape the solvent cage while the remainder recombines within the solvent cage during the first picosecond. The majority of the recombinations form ICN while only a minor fraction produces the metastable INC isomer. INC and ICN relax to the vibrational ground state within 1 ps in good agreement with theoretical estimates based on the golden rule formalism as well as molecular dynamics simulations. Diffusive recombination involving fragments that have escaped the solvent cage further reduces the quantum yield of I and CN to 10% during the following 100 ps. This recombination produces exclusively ICN.
Table-top neutron source for characterization and calibration of dark matter detectors
Authors: Beg F. N. , Krushelnick K. , Gower C. , Torn S. , Dangor A. E. , Howard A. , Sumner T. , Bewick A. , Lebedenko V. , Dawson J. , Davidge D. , Joshi M. , Gillespie J. R.
Published: July 2002

A table-top plasma focus device is shown to be an ideal neutron source for the calibration and characterization of dark matter detectors and has been optimized to produce a maximum yield of 2.0×107 neutrons per shot. The interaction of energetic neutrons is similar to that expected from weakly interacting massive particles (WIMPs)—a favored candidate for the dominant component of dark matter in the universe. The weak interaction of a neutron with liquid xenon gas was measured in a prototype xenon two-phase detector. We have developed a detector system in which both the primary scintillation and ionization from the initial interaction can be detected. Both measurements are critical for identifying WIMP’s.
Reliability assessment of Ti/TaSi2/Pt ohmic contacts on SiC after 1000 h at 600 °C
Authors: Okojie Robert S. , Lukco Dorothy , Chen Yuanliang L. , Spry David J.
Published: July 2002

We report the result of the development and analysis of Ti/TaSi2/Pt high temperature ohmic contact metallizations on n-type 4H– and 6H–SiC that can successfully withstand 1000 h of 600 °C thermal treatment in air. Understanding the reaction kinetics and dominant failure mechanisms enabled metal thicknesses in the multilayer stack to be optimized, thereby providing stable specific contact resistivity in the range of 1–6×10−5 Ω cm2 on the n-type 4H–SiC and 6H–SiC epilayers throughout the duration of heat treatment in air. The deleterious effects of platinum in a platinum-rich silicide and the benefits in a platinum Si-rich silicide were identified within the multilayer system. A high temperature ohmic contact figure of merit is proposed as a reliability benchmark and calculated for the contacts to 4H– and 6H–SiC.
Theoretical investigation of the X Σ+2, A Π2, and B Σ+2 states of LiAr and LiKr
Authors: Kerkines Ioannis S. K. , Mavridis Aristides
Published: July 2002

The X 2Σ+, A 2Π, and B 2Σ+ states of the LiAr and LiKr molecules have been examined theoretically employing the coupled cluster method combined with augmented correlation consistent basis sets of double through sextuple zeta quality. After constructing basis set superposition error-free potential energy curves for the above states, dissociation energies (De), bond distances (re), and common spectroscopic parameters are extracted through the numerical solution of the one-dimensional rovibrational Schrödinger equation. For the “bound” A 2Π states of LiAr and LiKr, the De values can be considered in harmony with experimental values: De=890.4 (957±30) and 1220.0 (1200) cm−1 (experimental values in parentheses), respectively. Corresponding bond lengths, re=2.545 (2.50±0.08) and 2.673 (3.27) Å indicate that the experimental bond distance of the LiKr A 2Π state is rather too large.
Spectroscopic properties of lead hexamer and its ions (Pb6, Pb6+, Pb6−)
Authors: Zhao Cunyuan , Balasubramanian K.
Published: July 2002

We have computed the optimized geometries and energy separations of low-lying electronic states of the lead hexamer (Pb6) and its positive and negative ions. Our techniques have included high level relativistic electron correlation techniques such as complete active space multiconfiguration self-consistent field (CAS-MCSCF) method followed by large scale multireference singles plus doubles configuration interaction (MRSDCI) and relativistic configuration interaction (RCI) computations that included up to 16 million configurations. Our computed results have facilitated the assignment of the anion photodetachment spectra of Pb6− and also in the prediction of the properties of yet to be observed electronic states. A  1A1g tetragonal bipyramid structure (D4h symmetry) is found as the ground state for Pb6. The excitation energy, atomization energies, ionization potentials, and vertical and adiabatic electron affinities are computed and compared with the experimental results. We have assigned the observed X, A, B, C, D, and E states of the anion photoelectron spectra of Pb6−, and discuss spin–orbit versus Jahn-Teller effects.
Advanced InAs/GaSb superlattice photovoltaic detectors for very long wavelength infrared applications
Authors: Wei Y. , Gin A. , Razeghi M. , Brown G. J.
Published: July 2002

We report on the temperature dependence of the photoresponse of very long wavelength infrared type-II InAs/GaSb superlattice based photovoltaic detectors grown by molecular-beam epitaxy. The detectors had a 50% cutoff wavelength of 18.8 μm and a peak current responsivity of 4 A/W at 80 K. A peak detectivity of 4.5×1010 cm Hz1/2/W was achieved at 80 K at a reverse bias of 110 mV. The generation–recombination lifetime was 0.4 ns at 80 K. The cutoff wavelength increased very slowly with increasing temperature with a net shift from 20 to 80 K of only 1.2 μm.
Chaotic mixing induced transitions in reaction–diffusion systems
Authors: Neufeld Zoltán , Haynes Peter H. , Tél Tamás
Published: July 2002

We study the evolution of a localized perturbation in a chemical system with multiple homogeneous steady states, in the presence of stirring by a fluid flow. Two distinct regimes are found as the rate of stirring is varied relative to the rate of the chemical reaction. When the stirring is fast localized perturbations decay towards a spatially homogeneous state. When the stirring is slow (or fast reaction) localized perturbations propagate by advection in form of a filament with a roughly constant width and exponentially increasing length. The width of the filament depends on the stirring rate and reaction rate but is independent of the initial perturbation. We investigate this problem numerically in both closed and open flow systems and explain the results using a one-dimensional “mean-strain” model for the transverse profile of the filament that captures the interplay between the propagation of the reaction–diffusion front and the stretching due to chaotic advection.
Monolithic integration of near-infrared Ge photodetectors with Si complementary metal–oxide–semiconductor readout electronics
Authors: Masini Gianlorenzo , Cencelli Valentino , Colace Lorenzo , De Notaristefani Francesco , Assanto Gaetano
Published: July 2002

Using a low-temperature technology for polycrystalline Ge deposition, we report on the monolithic integration of an array of near-infrared Ge photodiodes on Si complementary metal–oxide–semiconductor (CMOS) electronics. The integrated microsystem consists of a linear array of 120×120 μm2 pixels, an analog CMOS multiplexer and a transimpedance amplifier. The resulting photoresponse covers the near-infrared up to 1.6 μm.
Approximate entropy (ApEn) as a complexity measure
Authors: Pincus Steve
Published: July 2002

Approximate entropy (ApEn) is a recently developed statistic quantifying regularity and complexity, which appears to have potential application to a wide variety of relatively short (greater than 100 points) and noisy time-series data. The development of ApEn was motivated by data length constraints commonly encountered, e.g., in heart rate, EEG, and endocrine hormone secretion data sets. We describe ApEn implementation and interpretation, indicating its utility to distinguish correlated stochastic processes, and composite deterministic/ stochastic models. We discuss the key technical idea that motivates ApEn, that one need not fully reconstruct an attractor to discriminate in a statistically valid manner—marginal probability distributions often suffice for this purpose. Finally, we discuss why algorithms to compute, e.g., correlation dimension and the Kolmogorov–Sinai (KS) entropy, often work well for true dynamical systems, yet sometimes operationally confound for general models, with the aid of visual representations of reconstructed dynamics for two contrasting processes.
Fractional differentiability of nowhere differentiable functions and dimensions
Authors: Kolwankar Kiran M. , Gangal Anil D.
Published: July 2002

Weierstrass’s everywhere continuous but nowhere differentiable function is shown to be locally continuously fractionally differentiable everywhere for all orders below the ‘‘critical order’’ 2−s and not so for orders between 2−s and 1, where s, 1<s<2 is the box dimension of the graph of the function. This observation is consolidated in the general result showing a direct connection between local fractional differentiability and the box dimension/local Hölder exponent. Lévy index for one dimensional Lévy flights is shown to be the critical order of its characteristic function. Local fractional derivatives of multifractal signals (non-random functions) are shown to provide the local Hölder exponent. It is argued that Local fractional derivatives provide a powerful tool to analyze pointwise behavior of irregular signals.
Computer-generated holograms: A simplified ray-tracing approach
Authors: Stein Alan D. , Wang Zhiyue , Leigh John S.
Published: July 2002

A method that allows holograms to be generated by computer is described. The conventional ray-tracing algorithm is simplified by the use of a real-valued numerical simulation instead of making computations for the complex-valued field. Output via a laser printer and conventional photographic methods has yielded excellent fidelity in the reconstructed images of three-dimensional objects with simple geometries. Increasing computation capacity may allow routine application of this real-valued algorithm for displaying Magnetic Resonance Image data.
Padé approximant algorithm for solving nonlinear ordinary differential equation boundary value problems on an unbounded domain
Authors: Boyd John P.
Published: July 2002

We describe a four-step algorithm for solving ordinary differential equation nonlinear boundary-value problems on infinite or semi-infinite intervals. The first step is to compute high-order Taylor series expansions using an algebraic manipulation language such as Maple or Mathematica. These expansions will contain one or more unknown parameters z which will be determined by the boundary condition at infinity. The second step is to convert the Taylor expansions into diagonal Padé approximants. The boundary condition that u(x) decays to zero at infinity becomes the condition that the coefficient of the highest power of x in the numerator polynomial must be zero. The third step is to solve this equation for the free parameter z. The final step is to evaluate each of the multiple solutions of this equation for physical plausibility and convergence (as N increases). This algorithm can be implemented in as few as seven lines of Maple (sample program provided!). We illustrate the method with three examples: the Flierl–Petviashvili vortex of geophysical fluid dynamics, the quartic oscillator of quantum mechanics, and the Blasius function for the boundary layer above a semi-infinite plate in fluid mechanics. Methods for nonlinear problems are almost always iterative and need a first guess to initialize the iteration. The Padé algorithm is unusual in that it is a direct method that requires no a priori information about the solution. © 1997 American Institute of Physics.
On the numerical integration of motion for rigid polyatomics: The modified quaternion approach
Authors: Omelyan Igor P.
Published: July 2002

A revised version of the quaternion approach for numerical integration of the equations of motion for rigid polyatomic molecules is proposed. The modified approach is based on a formulation of the quaternion dynamics with constraints. This allows one to resolve the rigidity problem rigorously using constraint forces. It is shown that the procedure for preservation of molecular rigidity can be realized particularly simply within the Verlet algorithm in velocity form. We demonstrate that the method presented leads to an improved numerical stability with respect to the usual quaternion rescaling scheme and it is roughly as good as the cumbersome atomic-constraint technique. © 1998 American Institute of Physics.
Application of permanent magnets in accelerators and electron storage rings (invited)
Authors: Halbach Klaus
Published: July 2002

After an explanation of the general circumstances in which the use of permanent magnets in accelerators is desirable, a number of specific magnets will be discussed. That discussion includes magnets needed for the operation of accelerators as well as magnets that are employed for the utilization of charged particle beams, such as the production of synchrotron radiation.
Study of molybdenum-aluminum interdiffusion kinetics and contact resistance for VLSI applications
Authors: Singh R. N. , Brown D. M. , Kim M. J. , Smith G. A.
Published: July 2002

Interdiffusion barrier characteristics of molybdenum thin film with aluminum-1% Si is studied between 733 and 763 K via sheet and contact resistance measurements, Rutherford backscattering spectrometry, secondary ion mass spectrometry, and x-ray diffraction analysis. The results indicate that thermal annealing of Mo/Al-1% Si thin film couples leads to MoAl12 compound formation initially as a nonplanar front, but extensive annealing results in complete transformation of Al-1% Si to MoAl12 and a significant increase in contact resistance. The interdiffusion kinetics is diffusion controlled and shows parabolic time dependence, incubation periods, and extremely high activation energy value of 5.9 eV. The incubation periods and an high activation energy values are explained by the presence of silicon precipitates at the Mo/Al-1% Si interface. Implications of these observations to VLSI device characteristics are discussed and a safe time-temperature processing regime is proposed.
Optical properties of fluorine-doped SnO2 films
Authors: Abass A. K. , Mohammad M. T.
Published: July 2002

Thin layers of conducting glass (SnO2: F) 3-Ω/⧠ sheet resistance were chemically deposited on borosilicate glass slides. Conducting glass showed high transparency around solar maxima (0.5 μm) and low reflectivity compared with silicon. In an optical investigation of the conducting glass at room temperature a direct allowed transition at 4.13 eV and an indirect allowed transition at 2.69 eV together with an assisting phonon of 0.085 eV were observed.
Laser-induced etching of Si with chlorine
Authors: Baller T. , Oostra D. J. , de Vries A. E. , van Veen G. N. A.
Published: July 2002

Photo-induced dry etching of silicon with chlorine is studied by measuring mass spectra and time-of-flight (TOF) distributions of the particles desorbed from a chlorinated target during irradiation with 308- and 248-nm photons. The detected masses are Si, SiCl, SiCl2, and SiCl3. The measured TOF spectra can be fitted with Maxwell–Boltzmann-like distributions. The temperatures obtained by these fits depend on laser power and chlorine pressure. A higher laser power or gas pressure results in a higher temperature. Activation energies for desorbing Cl, SiCl, and SiCl2 are obtained. Possible mechanisms to explain the results will be discussed. Etching of rough silicon is much more efficient than the etching of polished silicon. The maximum etch rate obtained is 30 Å per laser pulse. No difference is found between p- and n-type silicon.
Electrical conduction in polyimide films
Authors: Sessler G. M. , Hahn B. , Yoon D. Y.
Published: July 2002

Electrical conduction phenomena in polyimide (Kapton) films were studied with particular attention devoted to the separation of interface and bulk phenomena. The measurements were carried out with a variety of methods in the temperature range of 50 to 270 °C at electrical fields of 104 to 6×105 V/cm and at time intervals of up to 2×104 s after voltage application. Biased, two-side metallized samples yield, after sufficiently long voltage application, interface-controlled steady-state currents described by Schottky injection, modified by space-charge layers in the vicinity of the electrodes. The effective work function for aluminum-polyimide is estimated to be 1.7 eV in the temperature range between 100 and 270 °C. A distinct dependence of these currents on electrode material is observed. Bulk phenomena were studied on one-side metallized samples subject to positive-corona charge injection. At temperatures below 200 °C, significantly larger currents than those for biased, two-side metallized samples were observed. The current-voltage characteristics are ohmic at low fields and space-charge limited at high fields. From these data, trap modulated mobilities for positive carriers of 4×10−12 cm2/V s at 50 °C and 10−9 cm2/V s at 200 °C, ohmic bulk conductivities of 10−16 (Ω cm)−1 at 100 °C and 10−14 (Ω cm)−1 at 200 °C and an intrinsic carrier density of 5×1013 cm−3 independent of temperature are obtained. Activation energies for the mobility are between 0.2 and 0.8 eV for the temperature range between 50 and 200 °C.
Single-roller rapid quenching of high melting temperature alloys using plasma-arc melting
Authors: Kubo Y. , Igarashi H.
Published: July 2002

A single-roller rapid quenching apparatus with a plasma-arc torch and a water-cooled copper hearth has been developed for the rapid quenching of high melting temperature alloys (mp>2000 °C). Details of the apparatus and a mathematical analysis of thermal conditions during the quenching process are described together with the results obtained from high melting temperature alloy experiments. Amorphous ribbons were obtained from Nb80Si20, Nb3Ge, and Ta–(Si,B) alloys. Amorphous Nb3Ge ribbon was successfully converted into an A15 single phase by heat treatment, and showed a superconducting transition temperature as high as 18.3 K. Crystallization of Ta80Si10B10 amorphous ribbon was observed at a very high temperature of 1233 K.
Optical properties of zinc phosphide
Authors: Munoz V. , Decroix D. , Chevy A. , Besson J. M.
Published: July 2002

The optical absorption edge of Zn3P2 has been measured by transmittance and photoconductivity at 300 K. The absorption coefficient in both polarizations is shown to follow the expected behavior for indirect gaps at 1.39 and 1.41 eV in parallel and perpendicular configurations. The high-absorption region has been measured in thin single crystals and the absorption coefficient is shown to agree with the optical constants derived from reflectivity for values of the absorption coefficient up to 105 cm−1. The optical properties and the behavior of photoconductivity are discussed in view of possible applications of Zn3P2 to solar energy conversion.
Varistors based on barium titanate-graphite-rubber composites
Authors: Lalkishore K. , Ramkumar K. , Satyam M.
Published: July 2002

This paper describes the experimental and theoretical studies carried out on particulate composites consisting of BaTiO3, graphite, and rubber. It is shown that such composites exhibit a positive voltage coefficient of resistance beyond a certain voltage. A theoretical model developed to explain the observed V-R characteristics and their dependence on parameters of the composite like composition and grain size of the particles is also described. These composites seem to be useful as varistors with positive voltage coefficient of resistance and may find applications as voltage-regulating devices.
Theoretical and experimental study of rotation in a vacuum-arc centrifuge
Authors: Prasad Rahul R. , Krishnan Mahadevan
Published: July 2002

Measurements of rotation frequency, plasma potential, ion temperature, and density in a vacuum-arc centrifuge are described. The vacuum-arc centrifuge is a magnetized plasma column 1 m long, 5 cm in diameter, with ne ∼1014 cm−3, and Ti ∼3 eV. The source of this plasma column is a vacuum-arc discharge between a negatively biased cathode and a grounded-mesh anode 6 cm downstream of it. This source plasma region is at one end of a 2-m-long vacuum vessel. An externally applied axial magnetic field collimates the plasma, which streams through the anode mesh and induces rotation. Rigid–rotor frequencies ∼105 rad s−1 lead to radial centrifugal separation between isotopes. A piezoelectrically scanned Fabry–Perot interferometer is used to measure ion temperature and rotation frequency. Langmuir probes are also used to corroborate these rotation measurements, and to measure the plasma potential and ion-density profiles. These measurements lead to scaling laws for the rotation. The scaling laws are compared with the predictions of a simple fluid model of such centrifuges.
Vacuum emission of hot electrons from silicon dioxide at low temperatures
Authors: DiMaria D. J. , Fischetti M. V.
Published: July 2002

Low temperature (to 83 K) vacuum emission of hot electrons from silicon dioxide films is reported. This technique is specifically used to study the temperature dependence of the electronic distributions emerging into vacuum from very thin (50–60 Å) oxide layers where a significant number of the electrons have traveled through the insulator ballistically. The measured energy distributions of the emerging carriers are shown to reflect the temperature-dependence of the distribution of the electron source in the silicon substrate at the abrupt interface with the silicon dioxide layer, particularly the Fermi tail, and possibly quantized levels in the silicon accumulation layer. The other features in the electron distributions are shown to be due to single phonon scattering of ballistic electrons in the silicon dioxide layer. Additionally, it is shown that as the oxide thickness is increased, the distribution broadens into its steady-state characteristic, showing very little temperature dependence. All data are shown to be in good agreement with a temperature-dependent, Monte Carlo simulation that includes the details of the electron source function at the interface of the silicon substrate and the oxide layer.
Shapiro steps on current-voltage curves of dc SQUIDs
Authors: Vanneste C. , Chi C. C. , Gallagher W. J. , Kleinsasser A. W. , Raider S. I. , Sandstrom R. L.
Published: July 2002

We have studied in detail the current-voltage (I-V) curves of dc SQUIDs in the presence of rf fields in the frequency range 2–18 GHz. Strong voltage locking at half-integral as well as the usual integral Shapiro step spacing is observed when the applied dc-magnetic flux is close to half-integral numbers of flux quanta. Numerical simulations are in excellent agreement with the experimental I-V curves. The half-integral steps are found to be associated with the flip-flop between two adjacent fluxoid states of the SQUID synchronized to the rf field.
Stress and elastic constants in anisotropic solids: Molecular dynamics techniques
Authors: Lutsko J. F.
Published: July 2002

The local stress tensor commonly used in statistical mechanics is cast in a form useful for molecular dynamics (MD) simulations. It is then used to derive fluctuation formulas for the local elastic constants of a system. The formulas are used in an MD measurement of the local elastic constants of an ideal crystal which are found to be in good agreement with the bulk elastic constants of the same system.
Domain wall excitations and their contributions to the weak-signal response of doped lead zirconate titanate ceramics
Authors: Zhang Q. M. , Pan W. Y. , Jang S. J. , Cross L. E.
Published: July 2002

The ac field dependence of the polarization and strain responses of three different compositions of doped lead zirconate titanate ceramics were measured for samples in both the poled and the depoled states. The results indicate that a reversible domain wall excitation exists which contributes to the weak-signal response of these materials. Irreversible domain wall motion can be excited within the frequency range measured by applying a field above a threshold field, which is much smaller than the coercive field. Therefore, the weak-signal linear dielectric, piezoelectric, and elastic coefficients of these materials may not provide a suitable description of their behavior when the external applied field exceeds this threshold field. All the observed phenomena can be explained by thermally activated domain wall fluctuations and nucleations.
The use of hydrogen in the production and characterization of NdFeB magnets
Authors: McGuiness P. J. , Harris I. R.
Published: July 2002

A combination of hydrogen decrepitation (HD) and jet milling (JM) has been used to produce powder for the processing of permanent magnets. The procedure has proved to be very successful for both NdFeB (Neomax) alloys and the NdDyFeNbB high coercivity alloys. The magnets produced by the HD/JM process showed excellent coercivities when sintered between 980 and 1040 °C. At higher temperatures, excessive grain growth reduced the coercivity significantly. The hydrogen absorption/desorption (HAD) behaviors of alloys based on Nd16Fe76B8 and the stoichiometric phase Nd2Fe14B have been examined in an attempt to relate these effects to the microstructures of these alloys. The HAD processes were studied by microbalance, differential thermal analysis (DTA), differential scanning calorimetry (DSC), and mass spectroscopy, and these techniques showed that the absorption process in the Nd16Fe76B8 alloy are multistage and occur readily at room temperature. The Nd2Fe14B alloy, on the other hand, could not be activated at room temperature and hydrogen absorption could only be achieved at elevated temperatures. The mass spectroscopy/vacuum degassing studies showed a two-stage desorption process whereby hydrogen was first desorbed from the matrix (Nd2Fe14B) phase and then from the phase(s) at the grain boundaries.
Picosecond time-resolved measurements of crystallization in noble metals
Authors: MacDonald C. A. , Malvezzi A. M. , Spaepen F.
Published: July 2002

Crystallization rates in pure metals have been measured by monitoring changes in reflectivity on a picosecond time scale during pulsed laser quenching. Multilayered films were used to establish that the transient decreases in reflectivity correspond to melting. Depth profiling with Auger electron spectroscopy on laser-irradiated areas showed complete mixing of the layers, which could only have occurred by melting. Crystallization velocities as high as 100 m/s were measured. These growth speeds are too large to be the result of a crystallization process governed by liquid state diffusion. The results of computer simulations based on a collision-limited growth model for crystallization were consistent with the measurements.
Soft x-ray emission from double-pulse laser-produced plasma
Authors: Weber R. , Balmer J. E.
Published: July 2002

A frequency-doubled 2-GW Nd:glass laser with a small satellite pulse (1% of the main pulse) was used for investigations of the soft x-ray conversion efficiency in double-pulse irradiation of planar Cu targets. The separation of the two pulses was varied between ±1.3 ns, and the pulse duration between 45 and 800 ps. For the case of a prepulse, an enhancement of the soft x-ray emission by a factor of 2–3 was observed at pulse durations between 45 and 200 ps. A smaller increase was achieved for positive delays (‘‘post-pulse’’), and no improvement for 800-ps pulses.
Plasma source ion implantation dose uniformity of a 2×2 array of spherical targets
Authors: Conrad J. R. , Baumann S. , Fleming R. , Meeker G. P.
Published: July 2002

We have employed Rutherford backscattering spectroscopy and secondary ion mass spectrometry to characterize the ion implantation dose uniformity which can be achieved with plasma source ion implantation (PSII), a non-line-of-sight technique for ion implantation of nonplanar targets in nonsemiconductor applications. In order to characterize the dose uniformity achievable with PSII, four spherical Ti-6Al-4V targets were PSII implanted simultaneously as a 2×2 square array in a nitrogen plasma with density 3×109 cm−3 at an energy of 50 keV to a nominal dose of 3×1017 atoms/cm2. The measured root-mean-square variation of both the retained dose and the mean range was found to be less than 15%, which is well within the acceptable tolerance range for nonsemiconductor applications of ion implantation. Our results demonstrate that PSII can achieve acceptable dose uniformity on nonplanar targets without target manipulation, and that such uniformity can be achieved in a batch processing mode.
Theoretical models of the electrical discharge machining process. II. The anode erosion model
Authors: Patel Mukund R. , Barrufet Maria A. , Eubank Philip T. , DiBitonto Daryl D.
Published: July 2002

As a second in a series of theoretical models for the electrical discharge machining (EDM) process, an erosion model for the anode material is presented. As with our point heat-source model in the previous article, the present model also accepts power rather than temperature as the boundary condition at the plasma/anode interface. A constant fraction of the total power supplied to the gap is transferred to the anode. The power supplied is assumed to produce a Gaussian-distributed heat flux on the surface of the anode material. Furthermore, the area upon which the flux is incident is assumed to grow with time. The model is capable of showing, via the determined migrating melt fronts, the rapid melting of the anodic material as well as the subsequent resolidification of the material foation from plasma dynamics modeling could improve substantially our results.
Semiconductor biased superlattice tunable electron interference filter/emitter
Authors: Glytsis E. N. , Gaylord T. K. , Brennan K. F.
Published: July 2002

It is shown that a voltage-biased semiconductor superlattice structure can serve simultaneously as a tunable electron-wave interference filter and electron emitter. A systematic design procedure for selecting the quantum well and barrier widths to be alternately high and low electron refractive indices and a quarter (or a half) of an electron wavelength in thickness is developed. A practical narrow-band filter/emitter consisting of layers of Ga1−xAlxAs and designed to emit 0.20-eV electrons is presented and analyzed. Such a structure would serve well as a tunable hot-electron emitter in ballistic transistors, and in future guided electron-wave integrated circuits.
Mechanism of texture formation by hot deformation in rapidly quenched FeNdB
Authors: Li Lin , Graham C. D.
Published: July 2002

The development of crystallographic texture in rapidly quenched Fe14Nd2B has been investigated by hot deformation. The method was to catch the process in a state of partial completion, and then use transmission electron microscopy to examine the structure. The degree of texture formation was determined by x-ray diffraction and by magnetic measurements, and the hardness and the anisotropy in hardness were measured up to 600 °C. It was concluded, in agreement with others but with additional evidence, that preferential growth of favorably oriented grains during plastic deformation produces the texture. The nature of the plastic deformation remains unclear, since no dislocations are observed in Fe14Nd2B. It was found that when samples are compressed at temperatures near 600 °C under low stresses for long times, they become Nd rich at the bottom, presumably because of flow of the Nd-rich liquid phase under the influence of gravity. In such samples, plastic deformation and crystallographic orientation occurs preferentially at the Nd-rich end.
Optical response of nongranular high-T
c Y1Ba2Cu3O7−x superconducting thin films
Authors: Frenkel A. , Saifi M. A. , Venkatesan T. , England P. , Wu X. D. , Inam A.
Published: July 2002

We have investigated the optical response of c-axis oriented crystalline Y1Ba2Cu3O7−x thin films (bridge and meander devices) on strontium titanate and MgO substrates. cw optical response to a He-Ne laser radiation (wavelength of 0.63 μm) was primarily bolometric. The pulsed optical response was studied with Q-switched and mode-locked Q-switched short pulses from a Nd:YAG laser at the wavelength of 1.06 μm. We identify two distinct components contributing to the pulsed optical response: a nonbolometric (as fast as 1 ns) and a bolometric component (several ns). The bolometric component is strong at temperatures in the vicinity of the transition region to the normal state. The nonbolometric component is dominant at temperatures below the transition region showing weak temperature dependence and a linear dependence on the bias current. These results are discussed using the flux motion model and also electron-phonon scattering relaxation dynamics of nonequilibrium superconductors based on the theory of Bardeen-Cooper-Schrieffer. The results suggest that with proper optimization of device parameters (geometry, critical current density, etc.) sensitive bolometers and high-speed detectors covering a broad electromagnetic spectrum (visible and infrared) may be developed.
Coupled-mode theory for chirowaveguides
Authors: Pelet Philippe , Engheta Nader
Published: July 2002

In this paper, electromagnetic wave propagation and mode coupling in a chirowaveguide are treated using the coupled-mode theory. A chirowaveguide, as defined in our previous work, is a conventional cylindrical waveguide filled with homogeneous chiral materials. A set of coupled linear differential equations is derived for various mode amplitudes in the waveguide. We then show that, in any single chirowaveguide, owing to the handed properties of chiral materials filling the waveguide, energy coupling occurs from one mode to the other. We also demonstrate that in a parallel-plate chirowaveguide a TE mode can be completely converted into a TM mode and vice versa as they propagate in the guide. Thus a chirowaveguide can indeed be used as a mode converter. Selected results are compared with those reported in the literature. Applications of such mode coupling in the design of novel microwave, millimeter-wave, and optical devices and components are mentioned.
Ion beam synthesis of heteroepitaxial Si/CoSi2/Si structures
Authors: Van Ommen A. H. , Bulle-Lieuwma C. W. T. , Ottenheim J. J. M. , Theunissen A. M. L.
Published: July 2002

The formation of buried single crystalline CoSi2 layers within a monocrystalline Si substrate by high-dose ion implantation of Co has been studied. Comparison of measured Co distributions with profiles obtained from Monte Carlo calculations has revealed the two basic phenomena that are responsible for the formation of buried layers. The enhanced stopping due to the incorporation of high concentrations of Co into Si has been identified as the dominant effect in the ion beam synthesis of buried layers. The high stopping near the top of the implanted distribution causes accumulation of Co at this point, which promotes buried layer formation. Sputtering brings the entire Co profile closer to the surface. After implantation at a temperature of 450 °C, Co is present in the form of coherent CoSi2 precipitates. Precipitates occur both in a twinned and an aligned orientation and are highly strained due to the lattice mismatch with Si. For high doses a buried monocrystalline and aligned CoSi2 layer forms within the Si lattice, during implantation. Annealing of the implanted structures results in the formation of a buried layer when, near the top of the implanted distribution, more than 50% of the Si is converted into CoSi2. These layers too are monocrystalline and have on aligned orientation. Cobalt diffusion in the Si lattice has been suggested to be the rate-determining step in the growth process of the buried layers. Finally, the electrical properties of these ion beam synthesized structures exhibit some unique features.
Recrystallization and refractive index profiles of titanium-implanted optical waveguides in LiNbO3
Authors: Bremer T. , Heiland W. , Buchal Ch. , Irmscher R. , Stritzker B.
Published: July 2002

Y-cut LiNbO3 single crystals have been implanted with 200-keV Ti+ at doses up to 4.0×1017 Ti/cm2. The annealing behavior has been studied with implantations performed at 77, 300, and 620 K. The amorphous surface layers have been annealed at 1273 K in a wet oxygen atmosphere for 2 to 8 h to achieve epitaxial regrowth. Channeling and mode spectroscopy have been applied to investigate the regrowth process. Refractive index profiles have been calculated with an inverse WKB algorithm. A maximum increase of 0.040 for the extraordinary and 0.019 for the ordinary refractive index was found at He-Ne wavelength (632.8 nm). This indicates a solubility limit of 3.3×1027/m3 for Ti in LiNbO3. The annealing time to achieve perfect regrowth depends on the ion dose and irradiation temperature. Higher implantation temperatures yield dynamical annealing; thus, a shorter post-implantation anneal is required.