bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibronic coupling in excited states of acetone
Authors: ter Steege D. H. A. , Wirtz A. C. , Buma W. J.
Published: July 2002

Photoelectron spectroscopy of Rydberg states of acetone-h6 and -d6 populated by two- or three-photon excitation has been employed to unravel the vibronic description of excited-state levels. For the 3p Rydberg states vibronic transitions have been reanalyzed, leading to various reassignments and the observation of hitherto nonreported transitions. In addition, several ionic vibrational frequencies could be determined. At higher excitation energies previously identified, and in the present study newly identified, members of two Rydberg series have been characterized. The ns Rydberg series was explored up to the 8s state, the nd series up to the 7d state. Based upon the unambiguous assignments of vibronic character that we obtain for excited-state levels, various valence-Rydberg and Rydberg–Rydberg vibronic coupling pathways come to light and are analyzed.
Influence of deposition angle on the properties of NiO spin-valves
Authors: Cartier M. , Auffret S. , Bayle-Guillemaud P. , Ernult F. , Fettar F. , Dieny B.
Published: July 2002

Bottom and top spin-valves comprising NiO as pinning layer were prepared by depositing the NiO layer either at normal or oblique incidence onto Si/SiO2 substrates. When the NiO layer is deposited at oblique incidence, a strong uniaxial anisotropy is observed in the hysteresis loop of the Co layer which is exchange coupled to this layer. The easy axis is perpendicular to the plane of incidence. In contrast, when the NiO layer is deposited at normal incidence, the hysteresis loops of the exchange coupled Co layer show an in-plane isotropy. The uniaxial anisotropy observed after oblique incidence deposition is ascribed to growth-induced uniaxial strain and magnetoelastic effects in the NiO antiferromagnetic layer. It is observed that after annealing under a magnetic field, Co/NiO bilayers in which the NiO has been deposited at oblique incidence exhibit symmetric hysteresis loops whereas similar bilayers in which the NiO has been deposited at normal incidence exhibit shifted hysteresis loops. This effect is related to a difference in grain size between these systems. Furthermore, bottom and top spin-valves in both of which the NiO has been deposited at oblique incidence are compared. In contrast to the bottom spin-valves, the top spin-valves exhibit shifted hysteresis loops after annealing from the NiO Néel temperature (520 K) under a magnetic field. It is shown that this difference is due to structural defects in the bulk of the NiO layer for bottom spin-valves which do not exist in top spin-valves. These defects tend to disrupt the exchange interactions within the NiO thickness thus reducing the pinning energy of this layer.
Magnetic, structural, and transport properties of thin film and single crystal Co2MnSi
Authors: Raphael M. P. , Ravel B. , Willard M. A. , Cheng S. F. , Das B. N. , Stroud R. M. , Bussmann K. M. , Claassen J. H. , Harris V. G.
Published: July 2002

The magnetic, structural, and transport properties of the Heusler alloy Co2MnSi are reported for sputtered thin films and a single crystal. X-ray diffraction reveals a phase pure L21 structure for all films grown between 573 and 773 K. Films grown at 773 K display a four-fold decrease in the resistivity relative to those grown at lower temperatures and a corresponding 30% increase in the residual resistivity ratio (ρ300 K/ρ5 K). We show that the higher growth temperature results in lattice constants, room temperature resistivities, and magnetic properties that are comparable to that of the bulk single crystal.
Rotationally resolved spectroscopy of Pt2
Authors: Airola Marc B. , Morse Michael D.
Published: July 2002

High-resolution laser-induced fluorescence spectroscopy has been used to investigate the 18 163 cm−1 band of Pt2 in a supersonically cooled molecular beam. Rotational constants of the ground and excited states were extracted from the rotationally resolved spectrum and determined to be B0″=0.031 754(12) cm−1 and B0′=0.028 672(11) cm−1, respectively. These values correspond to ground- and excited-state bond lengths of r0″=2.33297(44) Å and r0′=2.455 16(47) Å, respectively. Nuclear spin statistics that are observed in the spectrum demonstrate that the ground state is of either 0g+ or 0u− symmetry. Of these, 0g+ is by far the more probable. Comparisons with Au2 are made, suggesting that it may be appropriate to consider Pt2 as having a double bond.
Effects of artificial defects on the electrical transport of single-walled carbon nanotubes
Authors: Park J. W. , Kim Jinhee , Lee J.-O. , Kang K. C. , Kim J.-J. , Yoo K.-H.
Published: July 2002

We have investigated the effect of defects on the electrical transport properties of metallic single-walled carbon nanotubes (SWNTs). The defects are artificially induced by ion implantation and by deforming the SWNT with the SiO2 film deposited on it. Both defects are observed to act as gate-tunable electron scatters. With the dual gate structure, it is also shown that the scattering strength by the defect at a given back-gate voltage can be tuned by adjusting the top gate.
Small angle neutron scattering studies of a polybutadiene/polystyrene blend with small additions of ortho-dichloro-benzene for varying temperatures and pressures. I. Mean field to 3D-Ising crossover behavior
Authors: Schwahn Dietmar , Frielinghaus Henrich , Willner Lutz
Published: July 2002

Thermal composition fluctuations were studied in the blend of polybutadiene (PB) and polystyrene (PS) in temperature and pressure fields using the technique of small angle neutron scattering. The blends had a critical composition and were mixed with different amounts of the nonselective solvent ortho-dichloro-benzene. In this part we discuss the critical behavior as measured along the critical isochore and interpret the results in terms of a crossover function describing the transition between mean field and 3D-Ising behavior. In particular, we will discuss the Ginzburg number and the critical amplitudes of the susceptibility and correlation length. For the Ginzburg number Gi we found a much stronger increase with solvent content as predicted while the pressure fields always lead to a decrease of Gi as expected. In “pure” PB/PS blends we found a further crossover to a so-called renormalized Ising behavior; such a crossover proposes density fluctuations of the total polymer concentration near the critical point. This crossover was not observed in the blend–solvent mixtures.
High temperature expansion for a chain model
Authors: Rojas Onofre , de Souza S. M. , Thomaz M. T.
Published: July 2002

We consider an arbitrary translationally invariant chain model with nearest neighbors interaction and satisfying periodic boundary condition. The approach developed here allows a thermodynamic description of the chain model directly in terms of grand potential per site. This thermodynamic function is derived from an auxiliary function constructed only from open connected subchains. In order to exemplify its application and how this approach works we consider the Heisenberg XXZ model. We obtain the coefficients of the high temperature expansion of the free energy per site of the model up to third order.
Effects of charge, size, and shape-asymmetry on the phase behavior of model electrolytes
Authors: Yan Qiliang , de Pablo Juan J.
Published: July 2002

Monte Carlo simulations have been conducted to investigate the phase behavior of 2:1 hard-core electrolyte models. Two distinct cases have been considered: In the first, both bivalent cations and monovalent anions are spherical. In the second, bivalent cations are modeled as rigid dimers composed of two tangent hard spheres, each carrying a positive charge at the center. Critical temperatures and densities have been calculated as a function of the size asymmetry between positive and negative ions. The simulated critical temperature and critical density are strongly influenced by size asymmetry and by the shape of the ions. Changes in the critical constants are traced back to ground-state energy configurations of small ionic clusters. The trends observed in simulations for the critical temperature and density as a function of size asymmetry are shown to contradict the predictions of available theoretical formalisms.
Single-electron tunneling effects in a metallic double dot device
Authors: Junno T. , Carlsson S. -B. , Xu H. Q. , Samuelson L. , Orlov A. O. , Snider G. L.
Published: July 2002

We report on differential conductance measurements on a gold double-dot structure at 4.2 K. The two dots were connected in series by tunnel junctions formed by atomic force microscopy manipulation of nanodisks. The tunnel junctions were made strongly asymmetric. The characteristic honeycomb-shaped charging diagram separating different Coulomb blockade regions of well-defined occupancy of electrons was observed and the cells in the charging diagram were found to be skewed by the asymmetry of the tunnel junctions. In addition, a double-dot Coulomb staircase structure, with steps of varying width, was observed and was studied for varying gate voltage. The occupancy of electrons on the two dots was determined as a function of both drain source and gate voltages.
Hole trapping in ultrathin Al2O3 and ZrO2 insulators on silicon
Authors: Afanas’ev V. V. , Stesmans A.
Published: July 2002

Optical injection of electron-hole pairs in 3–5 nm thick layers of SiO2, Al2O3, ZrO2 and their stacks on (100)Si is found to result in positive oxide charging, suggesting trapping of holes. In thin layers of the high-permittivity metal oxides (Al2O3, ZrO2), the positive charge exhibits a remarkable stability to neutralization by electrons which is neither observed in thicker layers of the same oxides nor in thermal SiO2. Most of the positive charge is associated with diamagnetic centers (possibly, protons).
Controlled magnetization reversal in patterned Co nanostructures
Authors: Uhlig W. Casey , Li Hui , Han Bao Shan , Shi Jing
Published: July 2002

We have successfully demonstrated the construction of patterned magnetic nanostructures down to 50 nm dimensions using electron-beam lithography. Two types of patterned structures have been fabricated for this work: nanostructured arrays consisting of isolated identical elements and nanostructured chains of alternating elements with different widths. Small patterned arrays (∼200 μm×200 μm) have been characterized using a variable temperature (5 to 325 K) focused magneto-optic Kerr effect (MOKE) measurement system. The switching field of the isolated element arrays is found to be inversely proportional to the element width. However, as the elements of two different widths are connected to form a chain, magnetization reversal is essentially controlled by the wider constituent, indicating that the switching of chains starts in the wider elements. Domain walls in these wide elements then propagate through the narrower components, requiring a lower field than domain nucleation in the narrow elements. Magnetic force microscopy (MFM) images show that in the virgin state the narrower elements are domain wall free but complex domain patterns exist in the wider elements. Both field-dependent MFM imaging and micromagnetic simulations agree very well with the MOKE measurements.
Rapid thermal annealing of GaNxAs1−x grown by radio-frequency plasma assisted molecular beam epitaxy and its effect on photoluminescence
Authors: Loke W. K. , Yoon S. F. , Wang S. Z. , Ng T. K. , Fan W. J.
Published: July 2002

The effect of rapid thermal annealing (RTA) on GaNAs films grown on GaAs (100) substrates by radio frequency plasma-assisted solid source molecular beam epitaxy was investigated by low-temperature photoluminescence (PL) and high-resolution x-ray diffraction (HR-XRD) measurements. GaNAs samples with N content of 1.3% and 2.2% determined by experimental curve fitting of x-ray rocking curve with the dynamical diffraction theory, exhibit an overall blueshift in energy of 67.7 meV and an intermediate redshift of 42.2 meV in the PL spectra when subjected to RTA at 525–850 °C for 10 min. The results suggest that the GaNAs layer may have undergone an intermediate substitutional–interstitial diffusion in addition to purely outdiffusion of nitrogen atoms. Samples annealed at 700–750 °C showed 1.7–2.1 times improvement in integrated PL intensity and 1.6–1.8 times reduction in PL full width half maximum as compared to the as-grown sample. The HR-XRD results show no significant changes in GaNAs lattice parameter between the as-grown and annealed samples. This indicates the samples are thermally stable, with low overall nitrogen outdiffusion. The results have significant implication on the growth and postgrowth treatment of GaNAs material for high performance optoelectronic device application.
The role of sound propagation in concentrated colloidal suspensions
Authors: Bakker A. F. , Lowe C. P.
Published: July 2002

In a suspension, the hydrodynamic interactions between particles can propagate by two mechanisms: relatively slowly, by the diffusion of transverse momentum, or relatively rapidly, by the propagation of sound waves. Here we describe computer simulation results for the collective and single particle dynamics of colloidal particles with the aim of clarifying the role of sound. We find that for single particle motion the effect is rather trivial. As for an isolated particle, compressibility modifies the decay of velocity fluctuations only at very short times. For collective correlations this is not true. Our results show that the multiple scattering of sound waves between particles can induce correlated collective motions on time scales comparable with the diffusion of transverse momentum. The effects of compressibility are no longer restricted to very short times and manifest themselves as rapid oscillations in the time dependence of the collective diffusion coefficient. We suggest that these oscillations can largely be explained in terms of “effective” incompressible hydrodynamic theory, the suspension bulk viscosity, kinematic viscosity, and speed of sound becoming the relevant parameters. The oscillations are furthermore centered on the (hypothetical) incompressible result. Thus, while the effects of sound propagation may extend to surprisingly long times, the net effect remains limited to very short times. We discuss where these sound-induced oscillations will be relevant experimentally.
Quasiclassical and quantum mechanical modeling of the breakdown of the axial recoil approximation observed in the near threshold photolysis of IBr and Br2
Authors: Wrede Eckart , Wouters Eloy R. , Beckert Marco , Dixon Richard N. , Ashfold Michael N. R.
Published: July 2002

The photodissociations of jet-cooled IBr and Br2 molecules have been investigated using high resolution ion imaging methods, at excitation energies just above the thresholds for forming, respectively, I(2P3/2o)+Br(2P3/2o) and Br(2P3/2o)+Br*(2P1/2o) products from parent molecules in their v″=0 levels. For such molecules, we observe in both cases, that fragments with larger recoil velocities have markedly reduced angular anisotropy, whereas those from photolysis of IBr molecules with v″=1 show an essentially constant, limiting anisotropy. Given the monochromaticity of the photolysis radiation, increased recoil velocity of fragments resulting from photolysis of v″=0 molecules can only be derived from increased parent internal (rotational) energy. The measurements thus provide a particularly clear and direct observation of the breakdown of the axial recoil approximation as applied to the photodissociation of a diatomic molecule, and have been modeled, quantitatively, using both quantum and semiclassical methods together with the best available potential energy curves for the relevant excited states of IBr and Br2.
Effect of annealing temperature on physical and electrical properties of Bi3.25La0.75Ti3O12 thin films on Al2O3-buffered Si
Authors: Sun Chia-Liang , Chen San-Yuan , Chen Shi-Bai , Chin Albert
Published: July 2002

The effect of annealing temperature, especially at high temperatures, on the physical and electrical properties of Bi3.25La0.75Ti3O12 (BLT) thin films on Al2O3 (10 nm)/Si has been investigated. The width of memory window in capacitance–voltage curves for BLT/Al2O3/Si capacitors annealed at temperature range of 700 °C–950 °C increases with increasing annealing temperature. At the highest annealing temperature of 950 °C, a large ferroelectric memory window of 13 V is obtained under ±15 V sweep voltage, and this large ferroelectric memory window should be related to the reduced leakage current. Owing to the excellent electrical properties, the high-temperature stable BLT/Al2O3/Si capacitor is compatible with current very large scale integrated technology process.
Evaluation of the Barenblatt–Chorin–Prostokishin power law for turbulent boundary layers
Authors: Panton Ronald L.
Published: July 2002

For pipe flow Barenblatt, Chorin, and Prostokishin have advanced a specific power law equation for velocity profiles. Recently they have extended the work to allow the law to be used for zero-pressure-gradient boundary layers. The extension uses a newly defined boundary layer thickness. This Brief Communication evaluates the proposal and finds that the method is very sensitive and can produce profiles that do not closely match the data.
Ferroelectric and optical properties of Ba0.8Sr0.2TiO3 thin film
Authors: Pontes F. M. , Leite E. R. , Pontes D. S. L. , Longo E. , Santos E. M. S. , Mergulhão S. , Pizani P. S. , Lanciotti F. , Boschi T. M. , Varela J. A.
Published: July 2002

Barium strontium titanate (Ba0.8Sr0.2TiO3) thin films have been prepared on Pt/Ti/SiO2/Si substrates using a soft solution processing. X-ray diffraction and also micro-Raman spectroscopy showed that the Ba0.8Sr0.2TiO3 thin films exhibited a tetragonal structure at room temperature. The presence of Raman active modes was clearly shown at the 299 and 725 cm−1 peaks. The tetragonal-to-cubic phase transition in the Ba0.8Sr0.2TiO3 thin films is broadened, and suppressed at about 35 °C, with a maximum dielectric constant of 948 (100 kHz). Electrical measurements for the prepared Ba0.8Sr0.2TiO3 thin films showed a remnant polarization (Pr) of 6.5 μC/cm2, a coercive field (Ec) of 41 kV/cm, and good insulating properties. The dispersion of the refractive index is interpreted in terms of a single electronic oscillator at 6.97 eV. The direct band gap energy (Eg) and the refractive index (n) are estimated to be 3.3 eV and n = 2.27–2.10, respectively.
Observation of current staircase due to large quantum level spacing in a silicon single-electron transistor with low parasitic series resistance
Authors: Saitoh Masumi , Hiramoto Toshiro
Published: July 2002

We have fabricated a silicon point-contact channel single-electron transistor (SET) with an ultrasmall dot. By narrowing only the point-contact region and suppressing the parasitic series resistance, a peak conductance as large as 8.8 μS and single-electron addition energy as large as 128 meV are simultaneously obtained. A current staircase due to the large quantum level spacing is clearly observed at low temperatures. From numerical calculations, it is found that the staircase feature due to discrete quantum levels stands out even at room temperature in future silicon SETs with an ultrasmall dot.
Optical and luminescence characteristics of thermally evaporated pentacene films on Si
Authors: Park S. P. , Kim S. S. , Kim J. H. , Whang C. N. , Im S.
Published: July 2002

We report on the optical and luminescence properties of pentacene films deposited on n-Si by thermal evaporation at room temperature, 40, 60, 80, and 120 °C. Ellipsometric spectra of the films deposited at room temperature and 60 °C exhibited a main absorption peak at 1.82 eV and additional weak features at higher energy levels. Photoluminescence spectra taken at room temperature revealed band-to-exciton and band-to-acceptor transitions at 1.7 and 1.76 eV, respectively. An electroluminescence band was also observed near 1.65 eV under a forward bias from p-pentacene/n-Si diodes which show strong rectifying behavior.
Wall stabilization and the Mathieu–Hill equations
Authors: Tasso H. , Throumoulopoulos G. N.
Published: July 2002

In a recent publication [H. Tasso and G. N. Throumoulopoulos, Phys. Lett. A 271, 413 (2000)] on Lyapunov stability of general mechanical systems, it is shown that “parametric excitations” can be stabilized by dissipation for positive potential energies. Specializing on the damped Mathieu equation permits one to establish its full stability chart. It is then seen that dissipation broadens the regions of stability to the extent that not only the response to parametric excitations is damped, but even “negative-energy” modes are stabilized by the combined action of the parametric excitation and the damping coefficient. The extension of this analysis to the “two-step” Hill’s equation shows that the stability regions become many times larger than those of the Mathieu equation. By analogy, these findings are a strong indication that the “resistive wall mode” could be stabilized by the joint action of a properly tailored time-dependent wall resistivity and a sufficient viscous dissipation in the plasma. Note that within this scheme neither the wall nor the plasma need to be in motion. An extension of this work to include more realistic models is in progress.
A highly efficient algorithm for electron repulsion integrals over relativistic four-component Gaussian-type spinors
Authors: Yanai Takeshi , Nakajima Takahito , Ishikawa Yasuyuki , Hirao Kimihiko
Published: July 2002

In the previous studies, a highly efficient computational scheme has been proposed for the Dirac–Hartree–Fock and the Dirac–Kohn–Sham solutions using the generally contracted kinetically balanced Gaussian-type spinors (GTSs). Nevertheless, the calculations based on the full Dirac Hamiltonian are limited to small systems if they contain heavy elements. The bottleneck is the calculation of the two-electron repulsions over the four-component GTSs. The present paper presents an improved algorithm for evaluation of the four-component relativistic integrals. The new algorithm fully exploits the transfer relation of Head-Gordon and Pople (HGP) and the accompanying coordinate expansion (ACE) formulas of Ishida. The HGP transfer relation can reduce the four-component integrals into several common two-center integrals (p0|q0), which can be computed rapidly using the ACE method. The algorithm is implemented into the four-component program system REL4D. Benchmark calculations demonstrate that a good performance is achieved, particularly for the calculation of the (SS|SS) integrals.
Strained-Si/SiGe-on-insulator inversion layers: The role of strained-Si layer thickness on electron mobility
Authors: Gámiz F. , Roldán J. B. , Godoy A.
Published: July 2002

We show by Monte Carlo simulation that electron mobility is greater when strained-silicon inversion layers are grown on SiGe-on-insulator substrates than when unstrained-silicon-on-insulator devices are employed (as experimentally observed). However, the electron mobility in strained-Si/SiGe-on-insulator inversion layers is strongly dependent on the strained-silicon layer thickness, TSi, due to an increase of the phonon scattering, which partially counteracts the increase in the mobility achieved by the strain. This effect is less important as the germanium mole fraction, x, is reduced, and as the value of TSi increases.
Diffusion-induced instability in chemically reacting systems: Steady-state multiplicity, oscillation, and chaos
Authors: Lengyel István , Epstein Irving R.
Published: July 2002

The dynamical behavior of two coupled cells or reactors is described. The cells are coupled by diffusion, e.g., through a semipermeable membrane, and the chemical reactions and initial or feed concentrations of all species are the same in the two cells. Each cell has only a single stable steady state in the absence of coupling, and the coupled system may exhibit multiple steady states, periodic oscillation, or chaos. The attractors of the coupled system may be either homogeneous (the two cells have equal concentrations) or inhomogeneous. Three two-variable kinetic models are examined: the Brusselator, a model of the chlorine dioxide–iodine–malonic acid reaction, and the Degn–Harrison model. The dynamical behavior of the coupled system is determined by the nonlinearities in the uncoupled subsystems and by two ratios, that of the diffusion constants of the two species and that of the area of the membrane to the product of the membrane thickness and the volume of a cell.
Jamming and static stress transmission in granular materials
Authors: Cates M. E. , Wittmer J. P. , Bouchaud J.-P. , Claudin P.
Published: July 2002

We have recently developed some simple continuum models of static granular media which display “fragile” behavior: They predict that the medium is unable to support certain types of infinitesimal load (which we call “incompatible” loads) without plastic rearrangement. We argue that a fragile description may be appropriate when the mechanical integrity of the medium arises adaptively, in response to a load, through an internal jamming process. We hypothesize that a network of force chains (or “granular skeleton”) evolves until it can just support the applied load, at which point it comes to rest; it then remains intact so long as no incompatible load is applied. Our fragile models exhibits unusual mechanical responses involving hyperbolic equations for stress propagation along fixed characteristics through the material. These characteristics represent force chains; their arrangement expressly depends on the construction history. Thus, for example, we predict a large difference in the stress pattern beneath two conical piles of sand, one poured from a point source and one created by sieving.
Impedance scaling of Applied-B ion diodes
Authors: Miller P. A.
Published: July 2002

Applied-B ion diodes are used in inertial confinement fusion research to generate multiterrawatt ion beams. Impedance characteristics of these ion diodes deviate strongly from simple space-charge-limited flow, particularly in the higher power experiments. An examination of recent data has yielded an empirical voltage-current relationship with surprising implications, including a ‘‘critical voltage’’ limit.
Magnetic characteristics of R2Fe14B systems prepared with high purity rare earths (R=Ce, Pr, Dy, and Er)
Authors: Boltich E. B. , Oswald E. , Huang M. Q. , Hirosawa S. , Wallace W. E. , Burzo E.
Published: July 2002

Commercial rare earth metals contain several atomic % oxygen, and intermetallics synthesized using these ingredients deviate markedly from the intended compositions. In this work, high purity rare earths (<20 ppm oxygen by weight) were used. Results for R2Fe14B systems with R = Y, Nd, Sm, or Gd have been previously reported. The earlier work has now been extended to include systems containing Ce, Pr, Dy and Er. All were ordered magnetically at room temperature, with Curie temperatures ranging from 437 K (Ce) to 592 K (Dy). Results suggest that Ce in this system exists in the quadripositive state. Saturation magnetizations (77 K) are 29.4, 34.8, 12.1, and 14.7 μB/formula unit, respectively, for R = Ce, Pr, Dy, and Er. These may be contrasted with 30.4 μB/f.u. for Y2Fe14B, which is the Fe magnetization alone. These results imply ferromagnetic coupling for Pr2Fe14B and antiferromagnetic coupling for the Dy and Er compounds. Room temperature anisotropy fields ranged from 37 kOe (R = Ce) to 158 kOe (R = Dy). Er2Fe14B exhibits a spin reorientation near room temperature. Pr2Fe14B, unlike Nd2Fe14B, exhibits no spin reorientation.
Effects of impurity transitions on electroreflectance in thin epitaxial GaAs and Ga1−xAlxAs/GaAs layers
Authors: Glembocki O. J. , Bottka N. , Furneaux J. E.
Published: July 2002

A detailed electroreflectance (ER) and photoluminescence study has been performed of impurity-related, below-gap electroabsorption (EA) features in bulk Sn- and Si-doped GaAs as well as in thin epitaxial layers of GaAs and Ga1−xAlxAs/GaAs, with and without a known carbon impurity. A correlation is observed between the below-gap electroreflectance structure and impurity related emission from the conduction band and donor levels (when present) to acceptor states. In all cases in which impurities are present, the dc bias dependence of the EA below-gap structure and the current through the sample are found to be similar suggesting impact ionization as the mechanism which allows observation of the impurity transitions in ER. It is also shown that regardless of the carrier type of the material, upward band bending at the sample surface is required to prevent rapid recombination and maintain empty impurity levels whose population can be modulated by impact ionization. The photoresponse of the samples verifies these observations. For thin epitaxial films, criteria are presented for reducing the effects of below-gap structure arising from the substrate material.
Electron transport studies of gas mixtures for use in e-beam controlled diffuse discharge switches
Authors: Hunter S. R. , Carter J. G. , Christophorou L. G.
Published: July 2002

Gas mixtures for possible use in diffuse discharge switching applications require both high dielectric strength and specific electron transport properties in the conducting and the opening stages of the operation of the switch. In the conducting stage, the electron drift velocity must be large, and the electron loss processes (e.g., due to electron attachment and recombination) must be low so as to maximize the current in the discharge while maintaining low-discharge impedance. In the opening stage, strong electron attachment along with high dielectric strength is required of the gas mixtures in order to extinguish the discharge as quickly as possible (and, thus, achieve a fast opening time) and to prevent arcing occurring between the switch electrodes due to the high voltages induced across the switch in the opening phase. Measurements of the electron drift velocity, attachment, ionization and transverse diffusion coefficients, and high-voltage dielectric strengths of several gas mixtures proposed as candidates for use in diffuse discharge switching applications have been made and are reported and discussed.
Magnetoresistive thin-film sensor with permanent magnet biasing film
Authors: Kitada Masahiro , Kamo Yoshihisa , Tanabe Hideo , Tsuchiya Hiroshi , Momata Kazuhiro
Published: July 2002

Magnetic characteristics have been investigated for a magnetoresistive thin-film sensor biased using a permanent magnet thin film. Here, Co-Pt thin film is applied to the device as a bias film. Such microlithographed Co-Pt thin films show the same magnetic properties as as-sputtered films. Biasing strength depends on the permanent magnet film thickness, its composition or magnetic properties, and the biasing angle. The output of the sensor is very high.
Pyroelectric conversion cycles
Authors: Olsen Randall B. , Bruno David A. , Briscoe J. Merv
Published: July 2002

The effect of the type of power cycle upon the amount of output electrical work for a pyroelectric converter has been measured. Output electrical energy densities are reported for ceramic lead zirconate modified with Sn4+ and Ti4+ in the execution of a variety of thermal-electrical cycles. The effect upon the energy density due to changes in the voltage cycle limits and changes in the load resistance were also studied. A conversion cycle which is an electric analog of the Ericsson cycle is shown to yield the largest output energy density (100 mJ/cm3 for a 12.6 K temperature excursion and a 28-kV/cm electric field excursion).
Thermal conductivity of coated filler composites
Authors: Hatta Hiroshi , Taya Minoru
Published: July 2002

The temperature field and thermal conductivity of a coated-fiber composite were studied analytically. The analytical model is based on the classical separation of variables and equivalent-inclusion method. Closed-form solutions are given for the thermal conductivity of a coated-fiber composite. The present results on the temperature gradient in a thin coating were compared with those based on the thin-coating model by Walpole, resulting in a complete agreement when the thickness of the coating approaches zero. Then, a parametric study was conducted to study the effect of coating properties and other parameters on the thermal conductivity of the composite. It was found in this parametric study that the use of highly conductive (or resistive) coating, even if its thickness is small, is quite effective in enhancing the overall thermal conductivity (or resistivity) of the composite.
Effect of thermal and solution history on the Curie point of VF2-TrFE random copolymers
Authors: Green Jennifer S. , Farmer Barry L. , Rabolt John F.
Published: July 2002

The Curie transition in a VF2-TrFE random copolymer of fixed chemical composition (60/40 mol %) has been investigated as a function of processing history. Both solution cast and melt crystallized materials were investigated using differential scanning calorimetry and wide angle x-ray diffraction. Results indicate that due to the higher solubility of TrFE units in dimethylformamide, dimethylacetamide, and cyclohexanone, solution cast films of VF2-TrFE yielded crystalline regions deficient of TrFE units. Upon annealing, a redistribution and equalization of the TrFE comonomer units within the crystalline regions was observed to occur resulting in a lowering of the Curie point. Melt crystallization, on the other hand, provided a more uniform distribution of TrFE units within the crystalline and amorphous regions, each having nearly the same proportion as in the overall composition. Subsequent annealing caused negligible changes in the Curie temperature.
External-circuit effects on Pierce-diode stability behavior
Authors: Kuhn S. , Hörhager M.
Published: July 2002

The effects of external-circuit resistance, inductance, and capacitance on the linear complex eigenfrequencies of the Pierce diode [J. Appl. Phys. 15, 721 (1944)] are separately investigated in the parameter range 0<α<3π, where α=ωpd/v̄. The results are compared to the ‘‘classical’’ (i.e., short-circuit) case. External-circuit resistance does not alter the stability behavior fundamentally for 0<π<2π, but can lead to complete stabilization in the domain 2π<α<3π, most of which is classically unstable. External-circuit inductance induces a new mode, which may go unstable even in the classically stable domain 0<α<π, thus providing a new possibility of generating HF oscillations in the Pierce diode. Finite external-circuit capacitance tends to stabilize the classically unstable parts of the domain π<α<3π, with the exception of a small, classically stable α domain near 3π, which is destabilized. Our results indicate that the effect of external-circuit elements on Pierce-diode stability behavior is significantly stronger than predicted by Raadu and Silevitch [J. Appl. Phys. 54, 7192 (1983)].
Piezoresistivity of heterogeneous solids
Authors: Carmona F. , Canet R. , Delhaes P.
Published: July 2002

Several series of heterogeneous materials composed of conducting carbon particles randomly dispersed inside a polymeric matrix have been prepared. The particles are a carbon black and short graphite fibers. The polymers are epoxy resins and a silicon elastomer. In each series of materials an insulator-to-conductor transition is evidenced in agreement with percolation theory as the volume-particle concentration is varied. The piezoresistive effects of the materials are investigated under hydrostatic and uniaxial pressures. It is shown that the piezoresistance depends on materials composition, pressure, and carbon concentration. In each series, the piezoresistance increases rapidly as the concentration decreases towards the conductivity threshold. The range of pressure values over which the piezoresistance varies faster depends on the elastic properties of the matrix and on whether the applied pressure is hydrostatic or uniaxial. All these behaviors are accounted for by an extension of percolation theory involving volume-particle concentration changes under pressure. It is then demonstrated that the piezoresistance arises mostly from the heterogeneity of the materials, and to a lesser extent to geometrical changes of the samples under pressure. In conclusion, we examine the practical interest of designing composite materials for making strain or pressure gauges with desired sensitivity, resistivity, and useful pressure range given appropriate choices of the polymeric matrix and of the conducting particles concentration.
Radiation defect distribution in proton-irradiated silicon
Authors: Wondrak W. , Bethge K. , Silber D.
Published: July 2002

Defect concentration profiles covering a range of ∼100 μm in high-energy proton-irradiated silicon (3 MeV) have been investigated using deep-level transient spectroscopy. Radiation damage is produced in a layer between the surface and the proton end of range. The maximum defect concentrations are found at about the projected proton range. In this region, the relative concentration of centers with favorable energy levels for power device applications is remarkably large.
On the application of the Rutherford-scattering diagnostics to ion temperature measurements
Authors: Donné A. J. H. , Barbian E. P. , van der Ven H. W.
Published: July 2002

The feasibility for application of small-angle Rutherford scattering of fast neutrals by plasma ions as a possible ion-temperature diagnostic is investigated for a medium-sized tokamak. The analysis is based on elastic scattering of helium atoms from a 20–40 keV and 10 A m−2 probing beam, which are subsequently recorded by a time-of-flight spectrometer with high-energy resolution. Calculations show that the ion temperature can be measured with a 10% accuracy within 1 ms and with a spatial resolution of about 0.1 m. The possible influence of highly charged impurity atoms on the accuracy of the determined temperature value has been closely examined and appears to be negligible at the chosen set of parameters. The good spatial and temporal resolution make the method very suitable to determine the energy deposition profiles in case localized heating is applied to the plasma.
Study of the rapid thermal nitridation and silicidation of Ti using elastic recoil detection. I. Ti on Si
Authors: Krooshof G. J. P. , Habraken F. H. P. M. , van der Weg W. F. , Van den hove L. , Maex K. , De Keersmaecker R. F.
Published: July 2002

This paper describes a study of the rapid thermal processing of Ti thin films on Si in Ar and N2 in the temperature range 400–1100 °C. The resulting layer structures were analyzed by means of Rutherford backscattering spectrometry and elastic recoil detection. The latter technique yields quantitative information with a low detection limit about the depth distribution of O and N. Rapid thermal annealing in both Ar and N2 results in the uptake of nitrogen in the Ti surface region. Very little oxygen, if any, is incorporated. This nitridation leads to a shift of the originally present oxygen from the surface region into deeper layers, notably at the higher temperatures. For rapid thermal processing (RTP) in both Ar and N2 the evolution of the layer structures is governed by a competition between silicide (TiSi2) formation at the interface and nitridation at the outer surface. For the case of RTP in N2, the nitride to silicide thickness ratio is considerably larger than for RTP in Ar. The originally present oxygen is found between the silicide and the Ti nitride. In the case of RTP in Ar, Si appears at the surface at temperatures above 600 °C via diffusion through a thin titanium (oxy) nitride layer. This results in a buried titanium nitride layer via lateral overgrowth of Si on a nanoscale. This silicon surface layer effectively stops further nitridation.
Study of reactive-ion-etch-induced lattice damage in silicon by Ar, CF4, NF3, and CHF3 plasmas
Authors: Connick I-Wen H. , Bhattacharyya Anjan , Ritz Kenneth N. , Smith W. Lee
Published: July 2002

Reactive-ion-etch-induced damage in silicon has been investigated using transmission electron microscopy (TEM), Rutherford backscattering (RBS) ion channeling, and laser-induced thermal waves (TW). A correlation has been found between lattice damage in silicon due to reactive ion etching and leakage current properties of thermal oxide films subsequently grown on the damaged silicon. The silicon wafers were plasma etched using Ar, CF4, NF3, and CHF3 etch gases at dc bias voltages ranging from 150 V to 450 V. Lattice damage at the silicon surface, as determined by TEM and RBS, was found to depend on both the dc bias voltage and the etch chemistry. Subsequent leakage current measurements of the silicon oxides show that the samples with more silicon substrate lattice damage prior to oxidation also have correspondingly higher leakage. The thermal wave technique also indicates a damage dependence on dc bias and on etch chemistry; however, the thermal wave measurements indicate a damage dependence on etch chemistry different from TEM and RBS measurements. The source of this difference is not yet understood.
Impulse coupling to targets in vacuum by KrF, HF, and CO2 single-pulse lasers
Authors: Phipps C. R. , Turner T. P. , Harrison R. F. , York G. W. , Osborne W. Z. , Anderson G. K. , Corlis X. F. , Haynes L. C. , Steele H. S. , Spicochi K. C. , King T. R.
Published: July 2002

We present a laser-target scaling model which permits approximate prediction of the dependence of ablation pressure, mechanical coupling coefficient, and related parameters in vacuum upon single-pulse laser intensity (I), wavelength (λ), and pulse width (τ) over extremely broad ranges. We show that existing data for vacuum mechanical coupling coefficient for metallic and endothermic nonmetallic, surface-absorbing planar targets follows this empirical trend to within a factor of 2 over 7 orders of magnitude in the product (Iλ(τ)1/2). The comparison we present is valid for intensity equal to or greater than the peak-coupling intensity Imax, where dense

plasma formation mediates laser-target coupling. Mechanical coupling coefficients studied ranged over two orders of magnitude. The data supporting this trend represent intensities from 3 MW/cm2 to 70 TW/cm2, pulse widths from 1.5 ms to 500 ps, wavelengths from 10.6 μm to 248 nm, and pulse energies from 100 mJ to 10 kJ. With few exceptions, data approximating one-dimensional or planar expansions were selected. Previously, meaningful scaling of ablation pressure parameters with I, λ, τ was not possible because existing data concentrated in a small range of these parameters. Our own data, obtained in the low- and midrange of (Iλ(τ)1/2), completes the experimental picture. Since this new data was derived from five separate experiments with specialized character and purpose, detailed accounts of this work will appear separately. In this paper, we summarize the experimental conditions and select

only those data which are relevant to the scaling issue. We find that laboratory-scale laser experiments can often give impulse coupling data which agree with results from much higher-energy experiments without much error, and at much lower cost. We review a theory of vacuum laser ablation, specialize it to a quantitative description of mechanical coupling, and show that the resulting model provides a simple physical description which comes quite close to the observed empirical trend. This is accomplished with minor elaborations of the theory as originally presented to account for the temperature dependence of plasma ionization states, while adhering to the premise that a simple and generally applicable treatment of laser impulse production should be available. The theoretical model can quantitatively predict vacuum ablation pressure for

opaque targets without adjustable parameters to the factor-of-2 accuracy in which we are interested. Other published scaling models omit one or more of the important variables, lack broad applicability, or deviate more noticeably from the observed trend.
Near-zone fields behind circular apertures in thick, perfectly conducting screens
Authors: Roberts A.
Published: July 2002

A rigorous electromagnetic theory describing diffraction by a circular aperture in a perfectly conducting screen is used to model the flux and field intensity profiles in the near field behind the screen. In particular, the case of an aperture having a diameter one-fifth that of the wavelength of the radiation being used is considered. The degree of collimation of both the Poynting vector and the electric field intensity profiles of the transmitted beam is observed and its decrease with increasing distance from the screen is investigated. The effect of increasing the thickness of the screen is observed. These results are relevant to the study of near-field microscopy where the sample under investigation is either imaged with a collimated beam or light transmitted through the object is sampled using a probe having an electrically small diameter.
Theory of thermal diffusivity determination by the ‘‘mirage’’ technique in solids
Authors: Salazar A. , Sánchez-Lavega A. , Fernández J.
Published: July 2002

A complete theoretical treatment of the ‘‘mirage’’ (optical beam deflection) technique has been performed for thermal diffusivity measurements in solids. Numerical calculations of the transverse deflection show there is a linear relation between the first noncentral zeros of the real part of this function and the inverse root frequency. The slope of this straight line is related to the thermal diffusivity through a factor (γ) which depends on the thermal and optical properties of the sample. We found γ=1 for transparent materials as well as for opaque thermally thin materials, whereas γ=1.44 for opaque thermally thick samples. We study further the experimental limits which make a sample to be considered as an opaque or transparent and as thermally thick or thin, and present a complete classification of solids according to their optical and thermal properties. The different behaviors observed may be related to the different geometry of thermal waves propagating through the media.
Charge phenomena induced by low-energy ion bombardment in SiO2
Authors: Lefebvre F. , Vigouroux J. P. , Perreau J.
Published: July 2002

In order to study the mechanisms of electrical charge creation in insulators under particle bombardment, we used different techniques: metastable atom deexcitation spectroscopy (MDS) and secondary ion mass spectroscopy (SIMS). SiO2 (quartz for MDS and a thin silica film for SIMS) was bombarded with a He* atom (MDS) and a He+ ion (SIMS). MDS involves neutralization of the incident atom, and SIMS involves a competition of two mechanisms: (i) neutralization through potential energy transfer, and (ii) a cascade of collisions through kinetic energy transfer. Under the He* metastable atom, the surface charges up positively; under the He+ ion, the initial potential on the surface is negative and slowly evolves towards a positive value, which is still under bombardment. On the basis of electron transfer models, we propose to explain the observed kinetics of charge under ions in three steps: (1) appearance of initial negative charge; (2) increase in the positive counterpart of the charge because of the neutralization of He+ with electrons trapped in the localized levels due to oxygen vacancies; (3) stabilization of the charge at a positive value because of the dominant character of the neutralization versus sputtering.
Fire ball formation and evolution in the case of low-threshold optical breakdown plasma generation in ambient gases in front of various solid samples
Authors: Ursu I. , Stoica M. , Mihailescu I. N. , Hening Al. , Prokhorov A. M. , Nikitin P. I. , Konov V. I. , Silenok A. S.
Published: July 2002

New experimental results are reported concerning both the early and the very late evolution stages of low-threshold optical breakdown in ambient gases in front of samples prepared from various materials. The coupling and transfer processes from laser to sample through the plasma—by radiation and/or thermal contact—are analyzed experimentally and theoretically.