bluuebunny/crossref_metadata_2025 huggingface dataset:
Production of ground state OH following electron impact on H2O
Authors: Harb T. , Kedzierski W. , McConkey J. W.
Published: July 2002

Laser-induced fluorescence (LIF) measurements of OH(X 2Π) production following electron impact on water have been carried out over an energy range from threshold to 300 eV. Data have been made absolute using existing measurements of OH(X) production via dissociative attachment. The cross section reaches a maximum value of 2.1×10−16 cm2 at an energy of 75 eV. Dipole-allowed processes are shown to dominate the production of OH(X) particularly via the A(1B1) repulsive state, though significant population of higher rotational levels is demonstrated, indicating the relevance of other channels as in photodissociation.
An efficient combination of computational techniques for investigating electronic resonance states in molecules
Authors: Santra Robin , Cederbaum Lorenz S.
Published: July 2002

Calculating electronic resonance states in molecules is a serious challenge to theory, because the treatment of both the scattering and the many-electron problem is a formidable task. A very promising approach, known as CAP/CI, consists of the combination of a complex absorbing potential with the method of configuration interaction. In this paper we propose the combination of three distinct computational techniques in order to boost the performance of CAP/CI. A complex absorbing potential that can be adjusted flexibly to the geometry of the molecular scattering target is presented and its representation in a Gaussian basis set is discussed. To handle the large-scale complex symmetric eigenvalue problem arising in CAP/CI, a subspace projection method is employed and its validity is shown. We advocate the use of parallel filter diagonalization for calculating the eigenvectors required in the projection step. The proposed techniques are applied to determine the lifetime of an autoionizing, inner-valence excited state of Ne2+.
As-grown superconducting MgB2 thin films prepared by molecular beam epitaxy
Authors: Ueda Kenji , Naito Michio
Published: July 2002

Superconducting thin films of magnesium diboride (MgB2) were prepared on various substrates [SrTiO3 (001), sapphire R, sapphire C and Si (111)] by molecular beam epitaxy. The growth temperature was examined in the ambient to 650 °C. Only films formed at temperatures between 150 and 320 °C showed superconductivity. The best TConset of 36 K with a sharp transition width of ∼1 K was observed. The TC of the as-grown superconducting MgB2 thin films is close to the bulk value.
Highly conductive and transparent aluminum-doped zinc oxide thin films prepared by pulsed laser deposition in oxygen ambient
Authors: Singh A. V. , Mehra R. M. , Buthrath Nuttawuth , Wakahara Akihiro , Yoshida Akira
Published: July 2002

Highly conducting and transparent aluminum-doped zinc oxide films were prepared on quartz and corning glass 7059 substrate by ablating the sintered ZnO target containing 2 wt % Al2O3 with a XeCl excimer laser (λ=308 nm). To grow the films, a repetition rate of 5 Hz and energy density of 1.5 J/cm2 was kept. The effect of substrate temperature from room temperature to 400 °C and oxygen pressure (0.1–5 mTorr) have been investigated by analyzing the optical and electrical properties of these films. The average transmittance was found to be in the range of 86%–92%, and a variable resistivity (ρ) 3.56×10−3–7.0×10−3 Ω cm have been obtained. The lowest resistivity was found to be 1.4×10−4 Ω cm at 300 °C in 1 mTorr of oxygen pressure. Structural changes in the films were also investigated by determining the full width at half maximum of (002) x-ray diffraction peak. These results show improvement in the crystallinity of films, which support our conductivity and transmittance data. The sharp decrease in the transmittance or sharp increase in reflectance near the plasma edge in the near-infrared range has been attributed to impurity scattering, which is Al doping in our films.
Characterization of the native Cr2O3 oxide surface of CrO2
Authors: Cheng Ruihua , Xu B. , Borca C. N. , Sokolov A. , Yang C. -S. , Yuan L. , Liou S. -H. , Doudin B. , Dowben P. A.
Published: July 2002

Using photoemission and inverse photoemission, we have been able to characterize the Cr2O3 oxide surface of CrO2 thin films. The Cr2O3 surface oxide exhibits a band gap of about 3 eV, although the bulk CrO2 is conducting. The thickness of this insulating Cr2O3 layer is twice the photoelectron escape depth which is about 2 nm thick. The effective Cr2O3 surface layer Debye temperature, describing motion normal to the surface, is about 370 K. From a comparison of CrO2 films grown by different techniques, with different Cr2O3 content, evidence is provided that the CrO2 may polarize the Cr2O3.
Littrow configuration tunable external cavity diode laser with fixed direction output beam
Authors: Hawthorn C. J. , Weber K. P. , Scholten R. E.
Published: July 2002

We have developed an enhanced Littrow configuration extended cavity diode laser (ECDL) that can be tuned without changing the direction of the output beam. The output of a conventional Littrow ECDL is reflected from a plane mirror fixed parallel to the tuning diffraction grating. Using a free-space Michelson wavemeter to measure the laser wavelength, we can tune the laser over a range greater than 10 nm without any alteration of alignment.
Perovskite stabilization and electromechanical properties of polycrystalline lead zinc niobate–lead zirconate titanate
Authors: Fan Huiqing , Kim Hyoun-Ee
Published: July 2002

The perovskite structure of Pb(Zn1/3Nb2/3)O3 (PZN) ceramics was stabilized by adding a Ti-rich side tetragonal Pb(Zr0.47Ti0.53)O3 (PZT) across the morphotropic phase boundary (MPB). The 0.5 PZN–0.5 PZT specimen was found to have a two-phase zone, having a high piezoelectric coefficient, d33 (430 pC/N), an electromechanical coupling factor, kp (0.67), a field-induced strain, S (0.24% at 2 kV/mm), and a remnant polarization, Pr (27 μC/cm2). A phase transition from a rhombohedral to tetragonal phase occurred when a relatively low electrical field was applied to the specimen. This transformation occurred only when the rhombohedral phase coexisted with an equal amount of the tetragonal phase of the perovskite structure. Small rhombohedral domains, which were present around the relatively large tetragonal domains, transformed as a result of the applied field. The excellent electromechanical properties at the MPB composition are attributed to this phase transition in the PZN based ceramics.
The vibrational spectrum of the hydrated proton: Comparison of experiment, simulation, and normal mode analysis
Authors: Kim Jeongho , Schmitt Udo W. , Gruetzmacher Julie A. , Voth Gregory A. , Scherer Norbert E.
Published: July 2002

The vibrational properties of the hydrated proton and deuteron in bulk phase water and deuterated water are investigated spectroscopically and computationally. Mid-infrared spectra of aqueous acid solutions are measured by attenuated total reflectance-Fourier transform IR spectroscopy and compared with pure water and salt/counterion spectra to extract high-quality hydrated proton spectra at a series of concentrations. Multistate empirical valence bond simulations of the excess proton in bulk phase water are also performed, allowing the autocorrelation function of the time derivative of the dipole moment, and hence the power spectrum of the hydrated proton, to be evaluated. The experimental and theoretical spectra are found to be in very good agreement. Normal mode analysis of the bulk phase simulation data allows definitive assignment of the spectrum. The associated motions are found to be represented by both Eigen and Zundel forms of the hydrated proton.
Computations of diffusivities in ice and CO2 clathrate hydrates via molecular dynamics and Monte Carlo simulations
Authors: Demurov Alexander , Radhakrishnan Ravi , Trout Bernhardt L.
Published: July 2002

We report molecular simulation studies of the diffusion processes in ice and CO2 clathrate hydrates performed using classical potential models of water (SPC/E) and carbon dioxide (EPM2). The diffusivity of H2O in ice is calculated to be 1.3×10−18 m2 s−1 at 200 K using molecular dynamics simulations, a result in good agreement with experimental data. We also computed the diffusivity of H2O and CO2 in clathrate hydrates using both molecular dynamics and Monte Carlo simulations together with the Landau free energy method. We calculated free energy barriers for the CO2 hopping between clathrate cages with and without a water defect present. We determined that a water vacancy was necessary for diffusion of CO2, and we estimated the diffusion coefficient of CO2 molecules in clathrate hydrates to be 1×10−12 m2 s−1 at 273 K and H2O in the clathrate hydrates to be 1×10−23 m2 s−1 at 200 K.
Single-wall carbon nanotube/conjugated polymer photovoltaic devices
Authors: Kymakis E. , Amaratunga G. A. J.
Published: July 2002

We report the optoelectronic properties occurring in single-walled carbon nanotubes (SWNTs)—conjugated polymer, poly(3-octylthiophene) composites. Composite films were drop or spin cast from a solution on indium–tin oxide (ITO) and quartz substrates and studied using absorption spectroscopy and electrical characterization methods. Diodes (Al/polymer-nanotube composite/ITO) with a low nanotube concentration (<1%) show photovoltaic behavior, with an open circuit voltage of 0.7–0.9 V. The short circuit current is increased by two orders of magnitude compared with the pristine polymer diodes and the fill factor also increases from 0.3 to 0.4 for the nanotube/polymer cells. It is proposed that the main reason for this increase is the photoinduced electron transfer at the polymer/nanotube interface. The results show that the conjugated polymer-SWNTs composite represents an alternative class of organic semiconducting material that is promising for organic photovoltaic cells with improved performance.
Size-controlled highly luminescent silicon nanocrystals: A SiO/SiO2 superlattice approach
Authors: Zacharias M. , Heitmann J. , Scholz R. , Kahler U. , Schmidt M. , Bläsing J.
Published: July 2002

Phase separation and thermal crystallization of SiO/SiO2 superlattices results in ordered arranged silicon nanocrystals. The preparation method which is fully compatible with Si technologies enables independent control of particle size as well as of particle density and spatial position by using a constant stoichiometry of the layers. Transmission electron microscopy investigations confirm the size control in samples with an upper limit of the nanocrystal sizes of 3.8, 2.5, and 2.0 nm without decreasing the silicon nanocrystal density for smaller sizes. The nanocrystals show a strong luminescence intensity in the visible and near-infrared region. A size-dependent blueshift of the luminescence and a luminescence intensity comparable to porous Si are observed. Nearly size independent luminescence intensity without bleaching effects gives an indirect proof of the accomplishment of the independent control of crystal size and number.
Indentation-induced damage in GaN epilayers
Authors: Bradby J. E. , Kucheyev S. O. , Williams J. S. , Wong-Leung J. , Swain M. V. , Munroe P. , Li G. , Phillips M. R.
Published: July 2002

The mechanical deformation of wurtzite GaN epilayers grown on sapphire substrates is studied by spherical indentation, cross-sectional transmission electron microscopy (XTEM), and scanning cathodoluminescence (CL) monochromatic imaging. CL imaging of indents which exhibit plastic deformation (based on indentation data) shows an observable “footprint” of deformation-produced defects that result in a strong reduction in the intensity of CL emission. Multiple discontinuities are observed during loading when the maximum load is above the elastic-plastic threshold, and such a behavior can be correlated with multiple slip bands revealed by XTEM. No evidence of pressure-induced phase transformations is found from within the mechanically damaged regions using selected-area diffraction patterns. The main deformation mechanism appears to be the nucleation of slip on the basal planes, with dislocations being nucleated on additional planes on further loading. XTEM reveals no cracking or delamination in any of the samples studied for loads of up to 250 mN.
Synthetic ferrimagnet free layer tunnel junction for magnetic random access memories
Authors: Sousa R. C. , Zhang Zhongzhi , Freitas P. P.
Published: July 2002

In this work a synthetic ferrimagnet structure as a spin dependent tunnel junction free layer is proposed as a magnetic random access memories cell. The CoFe/Ru/CoFe and CoFe/NiFe/Ru/NiFe/CoFe systems show antiferromagnetically coupling range >1000 Oe and rotate rigidly anti-aligned with coercive fields below 30–40 Oe. Tunnel junctions were patterned to 4 μm×2 μm rectangles and ellipses using standard CoFe/Ni and synthetic ferrimagnet free layers. Identical signal levels are measured (17.8% and 16.3% as-deposited). Switching fields are dominated by material anisotropy for the synthetic ferrimagnet structure. Both free layer types show reversal by domain wall motion. Less switching dispersion is found for synthetic ferrimagnet free layer (≈10 Oe), with magnetization reversal occurring mostly in sharp single steps.
Thermoelastic loss in microscale oscillators
Authors: Houston B. H. , Photiadis D. M. , Marcus M. H. , Bucaro J. A. , Liu Xiao , Vignola J. F.
Published: July 2002

A simple model of thermoelastic dissipation is proposed for general, free standing microelectromechanical (MEMS) and nanoelectromechanical (NEMS) oscillators. The theory defines a flexural modal participation factor, the fraction of potential energy stored in flexure, and approximates the internal friction by assuming the energy loss to occur solely via classical thermoelastic dissipation of this component of the motion. The theory is compared to the measured internal friction of a high Q mode of a single-crystal silicon double paddle oscillator. The loss at high temperature (above 150 K) is found to be in good agreement with the theoretical prediction. The importance of this dissipation mechanism as a function of scale is briefly discussed. We find that the relative importance of this mechanism scales with the size of the structure, and that for nanoscale structures it is less important than intrinsic phonon–phonon scattering.
Aqueous dimethyl sulfoxide solutions: Inter- and intra-molecular dynamics
Authors: Wiewiór Piotr P. , Shirota Hideaki , Castner Edward W.
Published: July 2002

The inter- and intra-molecular dynamics of aqueous dimethyl sulfoxide (DMSO) solutions have been measured using optical heterodyne-detected Raman-induced Kerr effect spectroscopy. Solutions were studied over the entire range of composition at 294 K. The Kerr transients characterize both the underdamped inter- and intra-molecular vibrational motions, as well as the overdamped, diffusive orientational motions. The longer diffusive relaxation time constant τ2 is assigned to DMSO reorientation, and varies strongly with mole fraction of DMSO. The shorter time constant τ1 is assigned to water reorientation, and the value of 1.0 ps is nearly invariant across the range of solution composition. The solutions deviate substantially from hydrodynamic scaling behavior, since the ratio of DMSO reorientation time constant normalized by shear viscosity τ2/η is not a linear function of mole fraction. The peak frequencies for three of five low frequency intramolecular vibrations decrease with increasing water content. Both anisotropic and isotropic Kerr transients are recorded. The isotropic Kerr transient is dominated by the partially depolarized ν10 symmetric C–S–C stretching vibration at 670 cm−1. Electronic structure calculations for isolated DMSO, perdeuterated DMSO (d6-DMSO), and water are carried out using semi-empirical (PM3), density functional (B3LYP), and MP2 methods. Basis sets up to 6-311++G(d,p) are used and vibrational frequencies are calculated within the harmonic approximation. An unusual hemispherical shape is obtained for the electrostatic potential (ESP) about the DMSO oxygen. This hemispherical ESP was consistently observed for all levels of electronic structure theory and all basis sets that were used. Calculated polarizabilities permit us to estimate the anisotropic Kerr signal for an isolated DMSO molecule to be 79 times greater than for an isolated water molecule.
Minimizing 1/f noise in magnetic sensors using a microelectromechanical system flux concentrator
Authors: Edelstein Alan S. , Fischer Gregory A.
Published: July 2002

A device, a microelectromechanical system flux concentrator, is described that can minimize 1/f noise in magnetic sensors by modulating the magnetic field at the position of the sensor. This has the effect of shifting the operating frequency to higher frequencies where the 1/f noise can be 1 or 2 orders of magnitude smaller. Magnetic and mechanical modeling results on a design that will operate at 29 kHz are presented.
Spin-polarization in half-metals (invited)
Authors: Fang C. M. , de Wijs G. A. , de Groot R. A.
Published: July 2002

Half-metals are defined by an electronic structure, which shows conduction by charge carriers of one spin direction exclusively. Consequently, the spin polarization of the conduction electrons should be 100%. In reality this complete spin polarization is not always observed. Since the experimental search for half-metals is tedious and the verification of the expected spin polarization is involved, electronic structure calculations have played an important role in this area. So, an important question is, how the approximations in such calculations influence the resulting spin polarization of the conduction. Another aspect is the well-known fact that bulk properties can be very different from surface and interface properties. Indeed, measurements of the spin polarization in the bulk for, e.g., NiMnSb, show results different from surface sensitive measurements. In this respect it is important to realize that the origin of half-metallic behavior is not unique. Consequently, the deviations from the bulk behavior at the surface/interface can be important. Three different categories of half-metals can be distinguished and their expected surface properties will be discussed. Finally, ways will be described to control the properties at interfaces.
Fluorescence and absorption spectra of oligophenylenevinylenes: Vibronic coupling, band shapes, and solvatochromism
Authors: Gierschner Johannes , Mack Hans-Georg , Lüer Larry , Oelkrug Dieter
Published: July 2002

Fluorescence emission and excitation spectra of para-phenylene vinylenes nPV with n=1–4 styryl units are investigated experimentally and theoretically as a function of the temperature and the polarizability of the solvent. At low temperatures, the vibronic structures of the S0↔S1 emission and excitation bands are mirror symmetrical with negligible 0–0 energy gaps. The frequencies of the prominent vibrational modes are assigned to the second longitudinal acoustic phonon modes of the entire molecules and to localized carbon–carbon stretching vibrations. The complete vibronic structures of the spectra are calculated at the ab initio Hartree–Fock (HF/6-311G*) and restricted configuration interaction singles (RCIS/6-311G*) levels of theory assuming planar C2h molecular symmetry. The theoretically predicted spectra are in good agreement with the experiments. At room temperature, a 0–0 energy gap between the first band maxima opens, and the mirror symmetry between absorption and emission is lost. The vibronic band shapes and 0–0 band gaps are successfully simulated with a combination of Gaussian and exponential broadening of the low temperature spectra. The exponential term reflects the differences in thermal population of the phenyl-vinyl torsional modes in the S0 and S1 electronic states. Spectral shifts upon changes in temperature and solvents are quantitatively explained by changes in the refractive index of the environment. From extrapolation of the experimental data the vertical and adiabatic transition energies of the oligomers in vacuo are obtained and compared to RCIS and semiempirical quantum chemical calculations, respectively.
Derivation of an improved simple point charge model for liquid water: SPC/A and SPC/L
Authors: Glättli Alice , Daura Xavier , van Gunsteren Wilfred F.
Published: July 2002

Different approaches to improve the simple point charge model for liquid water (SPC) were investigated. This led to a whole series of new water models with additional van der Waals interaction sites at the hydrogen atoms, modified partial charges and modified geometries. The properties of these models are analyzed and discussed. Particular emphasis has been put on the study of the dependence and sensitivity of water properties on the model parameters. We found that a simultaneous improvement of the dielectric permittivity and the diffusion coefficient is difficult to attain for a rigid, nonpolarizable three interaction site model. Nevertheless, two of the models presented here, SPC/A and SPC/L, show good agreement with experimental data on water and have been characterized in more detail. We conclude that SPC/L represents the overall properties of water better than SPC. Especially, it shows excellent dielectric properties, an improved shear viscosity and a slightly lower diffusion coefficient.
Room-temperature ferromagnetism in Cr-doped GaN single crystals
Authors: Park Sang Eon , Lee Hyeon-Jun , Cho Yong Chan , Jeong Se-Young , Cho Chae Ryong , Cho Sunglae
Published: July 2002

We report on the discovery of a room-temperature ferromagnetism in Cr-doped GaN single crystals with a Tc=280 K. The addition of Cr into GaN single crystals grown by the flux method induces the lattice constant increase due to the larger Cr atomic radius. In x-ray photoelectron spectroscopy measurement, Cr 2p3/2 core-level exhibited spectra near 575.7 eV. This binding energy is similar to the reported value of CrN. The coercive field by magnetization–magnetic field (M–H) hysteresis curve at 250 K was 54 Oe. We verified the presence of ferromagnetic transition in the temperature dependence of the electrical resistance measurements. We discuss the ferromagnetic ordering in Cr-doped GaN bulk single crystals excluding the contribution of the substrate crystal structure.
Overview of coupled map lattices
Authors: Kaneko Kunihiko
Published: July 2002

Studies in coupled map lattices are briefly surveyed in connection with the papers in the present focus issue.
Lateral diffusion of In and formation of AuIn2 in Au-In thin films
Authors: Hasumi Yuji
Published: July 2002

Lateral diffusion of In in 20–100-nm-thick Au-In films has been studied in the 60–150 °C temperature range. Indium diffusion due to the chemical potential gradient occurs from In to Au-In with an AuIn2 phase being formed in the diffusion zone. The growth of the diffusion zone obeys a parabolic rate law with the growth rate strongly dependent on In concentration of the films. In order to understand these phenomena, the kinetics of the AuIn2 phase formation is analyzed considering In diffusion due to the chemical potential gradient with subsequent formation of AuIn2 at the diffusion front. As a result of this analysis, the diffusion coefficient of In in AuIn2 films is obtained as DIn=1.2×10−3 exp(−0.52 eV/kT) cm2/s. Grain boundary diffusion is thought to be the dominant mechanism. Additionally, a combined diffusion parameter δDg (δ is the grain boundary width) of 1.3×10−16 cm3/s (60 °C) is obtained.
The statistical self-similarity hypothesis in grain growth and particle coarsening
Authors: Mullins W. W.
Published: July 2002

The time dependence of particle size (e.g., mean volume) in normal grain growth, bubble growth, and late-stage coarsening is deduced from a statistical self-similarity (SSS) hypothesis, according to which consecutive configurations of the system in the self-similar mode are geometrically similar in a statistical sense, and from the scaling characteristic of v̇, the rate of change of the volume of a given particle. It is shown that if v̇ scales as v̄α under uniform magnification, where v̄ is the mean particle volume and α a constant depending on the controlling kinetics and geometry of the system, then v̄1−α is a linear function of the time. Values of α are obtained for a number of cases. The treatment is free of many of the approximations and geometrical simplifications used in most theories. Evidence for the SSS hypothesis is discussed. Special new results are obtained for two-dimensional bubble and idealized grain growth that should be testable by computer simulation or by observation of a well-behaved soap-bubble array.
A fractal model of dielectric breakdown and prebreakdown in solid dielectrics
Authors: Wiesmann H. J. , Zeller H. R.
Published: July 2002

We introduce and discuss a fractal model for dielectric breakdown which exhibits a breakdown voltage and a region of stable prebreakdown structures. The model provides a unifying picture covering homogeneous space charge injection, treelike structures, and filamentary breakdown. A simple qualitative relation between the global form of the pattern and two simple physical parameters is found. The model illustrates the intricate relationship between local stochastic and global deterministic aspects of dielectric instablilities.
Dynamics of UV laser ablation of organic polymer surfaces
Authors: Sutcliffe E. , Srinivasan R.
Published: July 2002

A model based on a time-dependent treatment of the ablation of organic polymer surfaces by UV laser radiation is proposed. It relates the dynamics of the etching process to the experimental parameters such as the fluence, wavelength, and the width of the laser pulse. The model is applied to poly(methyl methacrylate) and to polyimide in order to predict the etch characteristics quantitatively and to compare them systematically to the experimental results. This model accounts for a wide variety of observations such as fluence thresholds, wavelength-dependent etch rates, ‘‘incubation’’ pulses, filtering by photofragments, fast intrapulse etching, thermal contributions, and even the influence of pulse compression so that a consistent picture of the mechanisms underlying the ablation process becomes apparent.
Elastic interaction and stability of misfitting cuboidal inhomogeneities
Authors: Johnson William C. , Voorhees P. W.
Published: July 2002

The elastic interaction energy between several rectangular parallelepipeds and the elastic self-energy of a cuboid are calculated to first order in the difference in elastic constants between the precipitate and matrix. The system is assumed to be elastically isotropic, the precipitates are coherent with the matrix and possess a dilatational misfit. The functionality and magnitude of the interaction energy per precipitate are extremely sensitive to the precipitate morphology and number of precipitates considered. The interaction energy between three or more precipitates cannot be accurately estimated by summing the pairwise interactions. The influence of the interaction energy on the stability of precipitate arrays is discussed.
Optical, mechanical, and thermal properties of barium borate
Authors: Eimerl D. , Davis L. , Velsko S. , Graham E. K. , Zalkin A.
Published: July 2002

We report measurements of all the material constants necessary to fully characterize barium borate as a nonlinear optical material. All data was taken on crystals supplied by Professor Chuangtien Chen, Fuzhou, People’s Republic of China. We have determined the crystal structure, the optical absorption, the refractive indices from the UV to the near IR, the thermo-optic coefficients, the nonlinear optical or coefficients, the resistance to laser damage, the elastic constants, the thermal expansion, thermal conductivity and dielectric constants, and the fracture toughness. This data is used to evaluate barium borate for a variety of applications. We find that, in general, barium borate has a low acceptance angle, and that despite its higher optical nonlinearity, it is therefore not significantly more efficient than other commonly available materials, except in the UV below 250 nm. On the other hand, it has a high damage threshold, it is physically robust, it has good UV and IR transparency, and it has excellent average power capability. It permits deep UV generation, and has great potential for generating tunable visible and IR light as an optical parametric amplifier.
Magnetic anisotropy in metallic ultrathin films and related experiments on cobalt films (invited)
Authors: Chappert C. , Bruno P.
Published: July 2002

We present experimental and theoretical investigations on the magnetic anisotropy of Co ultrathin films sandwiched by Au. Ferromagnetic resonance experiments revealed the presence of a large perpendicular surface anisotropy that makes the easy magnetization direction become perpendicular to the film plane for Co thicknesses lower than 11 Å, as is observed in magnetization measurements. In order to explain this surface anisotropy, we propose various models, taking into account the imperfections of the films. For thicknesses below 11 Å, there is a large increase of the coercive field with decreasing thickness. This effect is tentatively interpreted in a model of propagating Bloch walls, where the interfacial roughness plays an important role.
Nozzleless droplet formation with focused acoustic beams
Authors: Elrod S. A. , Hadimioglu B. , Khuri-Yakub B. T. , Rawson E. G. , Richley E. , Quate C. F. , Mansour N. N. , Lundgren T. S.
Published: July 2002

We report the use of focused acoustic beams to eject discrete droplets of controlled diameter and velocity from a free-liquid surface. No nozzles are involved. Droplet formation has been experimentally demonstrated over the frequency range of 5–300 MHz, with corresponding droplet diameters from 300 to 5 μm. The physics of droplet formation is essentially unchanged over this frequency range. For acoustic focusing elements having similar geometries, droplet diameter has been found to scale inversely with the acoustic frequency. A simple model is used to obtain analytical expressions for the key parameters of droplet formation and their scaling with acoustic frequency. Also reported is a more detailed theory which includes the linear propagation of the focused acoustic wave, the coupling of the acoustic fields to the initial surface velocity potential, and the subsequent dynamics of droplet formation. This latter phase is modeled numerically as an incompressible, irrotational process using a boundary integral vortex method. For simulations at 5 MHz, this numerical model is very successful in predicting the key features of droplet formation.
Transient generation by a wide thermoelastic source at a solid surface
Authors: Bresse L. F. , Hutchins D. A.
Published: July 2002

A method for computing the radiation from a thermal expansion source, generated at a solid surface by a laser pulse of finite width, is evaluated for two radial beam energy distributions. A theoretical expression for the impulse response is obtained for two components of the displacement at any point within the solid, and may be used to compute the field for an arbitrary time excitation. Numerical results are presented, and are compared to experimental results for generation by a pulsed laser.
The electric field dependence of the mobility in molecularly doped polymers
Authors: Schein L. B. , Peled A. , Glatz D.
Published: July 2002

One of the most puzzling aspects of the hole mobility in molecularly doped polymers, its electric field dependence, is studied. New data over the widest electric field range yet reported have been obtained and are compared with various functions proposed in the literature. Comprehensive linear and nonlinear statistical analysis reveals that only the function exp(E)1/2 is consistent with the data. The theoretical implications of this result are discussed.
Kim model for magnetization of type-II superconductors
Authors: Chen D.-X. , Goldfarb R. B.
Published: July 2002

We have calculated the initial magnetization curves and complete hysteresis loops for hard type-II superconductors. The critical-current density Jc is assumed to be a function of the internal magnetic field Hi according to Kim’s model, Jc(Hi)=k/(H0+‖Hi‖), where k and H0 are constants. As is the case for other critical-state models, additional assumptions are that bulk supercurrent densities are equal to Jc, and that the lower critical field is zero. Our analytic solution is for an infinite orthorhombic specimen with finite rectangular cross section, 2a×2b (a≤b), in which a uniform field H is applied parallel to the infinite axis. Assuming equal flux penetration from the sides, we reduced the two-dimensional problem to a one-dimensional calculation. The calculated curves are functions of b/a, a dimensionless parameter p=(2ka)1/2/H0, and the maximum applied field Hm. The field for full penetration is Hp=H0[(1+p2)1/2−1]. A related parameter is H*m=H0[(1+2p2)1/2−1]. Hysteresis loops were calculated for the different ranges of Hm : Hm<Hp, Hp<Hm<H*m, and H*m<Hm. The equations for an infinite cylindrical specimen of radius a are the same as those for a specimen with square cross section, a=b. In the limit p≪1 and a=b, our results reduce to those of the Bean model (Jc independent of Hi) for cylindrical geometry. Similarly, in the limit p≪1 and b→∞, the results are the same as those for a slab in the Bean model. For H>1.5 Hp, or H>0 when p≪1, the width of the hysteresis loop ΔM may be used to deduce Jc as a function of H: Jc(H)=ΔM(H)/[a(1−a/3b)].
Relations between elastic constants C
i
j and texture parameters for hexagonal materials
Authors: Li Yan , Thompson R. Bruce
Published: July 2002

Ultrasonic techniques have recently been applied to the texture characterization in polycrystalline aggregates of hexagonal crystals. The basis of this application lies in the relations between the elastic constants C̄ij of the aggregates, which can be inferred from ultrasonic wave velocity measurements, and the orientation distribution coefficients. This communication presents such relations for aggregates which possess orthotropic material symmetry and hexagonal crystal symmetry for Voigt, Reuss, and Hill averaging methods in a unified and concise representation.
Temperature effect on ion-irradiation-induced grain growth in Cu thin films
Authors: Liu Joyce C. , Li Jian , Mayer J. W.
Published: July 2002

The average grain size in Ar+-irradiated Cu films at room temperature increases with ion dose, following a relationship of d̄3.3−d̄3.30 =Kφ. For Ar+ and Xe++ irradiations, the grain growth kinetics are independent of temperature at T≤−60 °C and increase with temperature in the range from −60 to 102 °C. The activation energy of the temperature-dependent contribution to grain boundary migration is about 0.14 eV, and the growth rate is independent of ion flux, suggesting that the ion-irradiation-induced grain growth is associated with the thermal spike diffusion.
Formalism for the calculation of local elastic constants at grain boundaries by means of atomistic simulation
Authors: Kluge M. D. , Wolf D. , Lutsko J. F. , Phillpot S. R.
Published: July 2002

A new formalism for use in atomistic simulations to calculate the full local elastic-constant tensor in terms of local stresses and strains is presented. Results of simulations on a high-angle (001) twist grain boundary are illustrated, using both a Lennard–Jones potential for Cu and an embedded-atom potential for Au. The two conceptionally rather different potentials show similar anomalies in all elastic constants, confined to within a few lattice planes of the grain boundary, with an especially dramatic reduction in the resistance to shear parallel to the grain-boundary plane. It is found that the primary cause of the anomalies is the atomic disorder near the grain boundary, as evidenced by the slice-by-slice radial distribution functions for the inhomogeneous interface system.
Recursion method in quantum spin dynamics: The art of terminating a continued fraction
Authors: Viswanath V. S. , Müller Gerhard
Published: July 2002

The results obtained from applications of the recursion method to quantum many-body dynamics can be greatly improved if an appropriate termination function is employed in the continued-fraction representation of the corresponding relaxation function. We present a general recipe for the construction and use of such termination functions along with two applications in spin dynamics. The method can be adapted to any other problem in quantum many-body dynamics.
Systematics of secondary-ion-mass spectrometry relative sensitivity factors versus electron affinity and ionization potential for a variety of matrices determined from implanted standards of more than 70 elements
Authors: Wilson R. G. , Novak S. W.
Published: July 2002

We have measured and plotted the dependence of secondary-ion-mass spectrometry (SIMS) relative sensitivity factor (RSF) on electron affinity and ionization potential of up to 74 elements implanted into 23 materials, measured using oxygen-ion bombardment and positive SIMS, and cesium-ion bombardment and negative SIMS. Reproducibility of ±60% in RSF has been demonstrated for many of these implanted standards over a period of 8 years in several Cameca magnetic sector instruments, especially in Si, GaAs, and HgCdTe matrices. More recently, reproducibilities of ±20%–30% have been achieved using more controlled experimental techniques. Similarities among plots of logRSF versus ionization potential or electron affinity have been found for a variety of semiconductor, metal, and insulator matrices. Universal patterns and trends are observed for oxygen bombardment and positive SIMS, and for cesium bombardment and negative SIMS. Four major regions are seen in the dependence of logRSF on ionization potential, one with possibly three branches with slightly differing slopes, that are characterized by elements from various groups of the periodic table. There are two major regions in the plots of RSF versus electron affinity, one of which is a region of constant RSF versus electron affinity, which includes the more electronegative elements, and meaning that the RSF is a constant for most of the elements commonly analyzed using negative (cesium) SIMS. This constant RSF has been measured for 20 matrices. We report the values of slope of logRSF versus ionization potential for the matrices studied and discuss the possible extension to other matrices. We tabulate the values of RSF for 74 elements in Si and for 69 elements in GaAs, as examples. Tentative explanations of the features of these dependencies are consistent with existing theories and models. We give generalized patterns of RSF (ion yield) versus ionization potential or electron affinity and an equation from which RSFs can be calculated for the matrices studied, and possibly others.
X-ray photoelectron spectroscopic studies on yttria, zirconia, and yttria-stabilized zirconia
Authors: Majumdar Debasis , Chatterjee Dilip
Published: July 2002

Surfaces of yttria, zirconia, and yttria-stabilized zirconia were studied using x-ray photoelectron spectroscopy. An almost threefold increase in the surface yttrium concentration was observed in the yttria-stabilized zirconia samples. The core level binding energies of yttrium, zirconium, and oxygen ions in yttria-stabilized zirconia showed chemical shifts. Valence bands and Auger parameters were monitored for the monoclinic and the tetragonal phases of zirconia. Characteristic differences were observed for the two phases due to their different oxygen coordination. The results were used to identify surface phase transitions which were difficult to detect by x-ray diffraction.
Electron transport of inhomogeneous Schottky barriers: A numerical study
Authors: Sullivan J. P. , Tung R. T. , Pinto M. R. , Graham W. R.
Published: July 2002

Numerical simulations are presented of the potential distribution and current transport associated with metal-semiconductor (MS) contacts in which the Schottky barrier height (SBH) varies spatially. It is shown that the current across the MS contact may be greatly influenced by the existence of SBH inhomogeneity. Numerical simulations indicate that regions of low SBH are often pinched-off when the size of these regions is less than the average depletion width. Saddle points in the potential contours in close proximity to the low-SBH regions, which are shown to vary with the dimension and magnitude of the inhomogeneity as well as with bias, essentially determine the electron transport across the low-SBH regions. It is these dependences of the saddle point which give rise to various abnormal behaviors frequently observed from SBH experiments, such as ideality factors greater than unity, various temperature dependences of the ideality factor, including the T0 anomaly, and reverse characteristics which are strongly bias-dependent. The results of these numerical simulations are shown to support the predictions of a recently developed analytic theory of SBH inhomogeneity.
Semiconductive polymer-based Schottky diode
Authors: Turut Abdulmecit , Köleli Fatih
Published: July 2002

Polythiophene is easily obtained electrochemically on a Au surface in an acetonitrile/0.25 M LiClO4 solution. On the basis of this knowledge, we prepared a metal-semiconductor-metal Schottky diode, in which Au and Al were used as metals and freshly prepared polythiophene as a semiconductor. Current-voltage and capacity-voltage characteristics of this diode have been obtained under atmospheric conditions.
Thermal stress relaxation behavior in thin films under transient laser-pulse heating
Authors: Tang D. W. , Zhou B. L. , Cao H. , He G. H.
Published: July 2002

Transient laser pulse has been used to generate nonsynchronous change of temperature rise and thermal expansion in thin films. The transient process of thermal expansion is recorded by a photoelectric high-speed acquisition system. By comparison of the calculated temperature rise and thermal expansion, thermal stress relaxation processes in Al films of 20–50 μm thickness is obtained. This result shows that, for transient heating, thermal stress exists even in the case of uniform temperature distribution and free expansion. And also there is a transient high stress in thin films under high-speed heating.
Surface kinetics of adatoms in vapor phase epitaxial growth of SiC on 6H-SiC{0001} vicinal surfaces
Authors: Kimoto Tsunenobu , Matsunami Hiroyuki
Published: July 2002

Growth of SiC on off-oriented 6H-SiC{0001} substrates was quantitatively analyzed based on the theory proposed by Burton, Cabrera, and Frank (BCF theory). By measuring the critical growth conditions where the growth mode changed from step-flow to two-dimensional nucleation, the surface diffusion lengths of adatoms on 6H-SiC(0001)Si and (0001̄)C faces were calculated with the model. Although the nucleation rate was much higher on (0001̄)C faces, the surface diffusion length was longer on (0001̄)C faces. This brought about little difference in the step-flow growth condition on both faces. Growth conditions where step-flow growth takes place were predicted as a function of growth parameters such as temperature, growth rate, and off-angle of substrates. The analysis on off-angle dependence of growth rate revealed that desorption of adatoms was negligible and the condensation coefficient of supplied species onto the surface was almost unity on substrates with off-angles of more than 3°.
Boron channeling implantations in silicon: Modeling of electronic stopping and damage accumulation
Authors: Hobler G. , Simionescu A. , Palmetshofer L. , Tian C. , Stingeder G.
Published: July 2002

Channeling implantations of 20 keV boron into silicon have been performed with doses between 1013 and 1016 cm−2 in the [100], [110], and [211] direction, and parallel to a (111) plane. Simulations using an empirical electronic stopping model agree very well with the experimental results. The model has been obtained considering a large number of random and channeling implantations published in the literature. It contains a nonlocal and an impact parameter dependent part with the nonlocal fraction increasing with energy. Moreover, a computationally efficient damage accumulation model is presented which takes point defect recombination into account. It is found that due to interactions within a recoil cascade only 1/8 of the generated damage is stable, and that damage saturation takes place at a concentration of 4×1021 cm−3. Comparison of simulations and experiments indicates that displaced atoms reside on random positions rather than on tetrahedral interstitial sites in the silicon lattice.
Focused bulk ultrasonic waves generated by ring-shaped laser illumination and application to flaw detection
Authors: Wang Xiao , Littman Michael G. , McManus John B. , Tadi Mohsen , Kim Young Sik , Askar Attila , Rabitz Herschel
Published: July 2002

Focused bulk ultrasonic waves have been generated in aluminum plates by surface irradiation with ring-shaped laser light. The waves are detected by a piezoelectric transducer. Compression and shear peak amplitudes drop quickly when the detector is moved away from the epicenter. This shows that strong focusing exists at the epicenter as the result of constructive interference of the waves generated by different parts of the ring. The focusing persists when the radius of the laser light is scanned over a large range, indicating that the elastic disturbance concentrates in depth along the ring’s central axis. Numerical simulations are presented for comparison. The ‘‘pencil-like’’ acoustic wave structure is used to observe a sample plate with an artificial flaw. Strong new features including compress-shear mode conversion at the site of the flaw are observed. These features are used to locate the flaw within the sample.
Sintered alumina with low dielectric loss
Authors: Alford Neil McN. , Penn Stuart J.
Published: July 2002

Low dielectric loss materials are required for applications in radio-frequency and microwave communications. Aluminium is the second most abundant element in the Earth’s crust and aluminium oxide (alumina) is one of the commonest ceramics. Single crystals of aluminium oxide, i.e., sapphire, possess one of the lowest dielectric losses of any material. Polycrystalline alumina has a higher loss due to extrinsic factors. The dielectric loss of sintered alumina is studied in an attempt to determine the causes of extrinsic loss. Impurities are shown to play an important role, but the microstructure also is a key factor. High-purity aluminas, sintered to near theoretical density, are found to display very low loss, tan δ=2.7×10−5 at 10 GHz. Doping alumina with titanium dioxide was found to reduce the tan δ=2×10−5.
Thermal conductivity in nickel solid solutions
Authors: Terada Yoshihiro , Ohkubo Kenji , Mohri Tetsuo , Suzuki Tomoo
Published: July 2002

Thermal conductivity in nickel solid solutions with a γ single phase was comprehensively surveyed. The major findings are summarized as follows: First, alloying definitely decreases the thermal conductivity of nickel. The relationship between thermal conductivity and concentration in the dilute solid solution is characterized by the Nordheim relation. Second, for most solutes, the thermal conductivity of nickel alloys decreases significantly as the position of the solute element becomes horizontally more distant from nickel in the periodic table, which is equivalent to the Norbury–Linde rule of electrical resistivity. Third, cold deformation has no significant influence on the thermal conductivity of nickel alloys. Fourth, a monotonic increase in thermal conductivity as a function of temperature is observed in highly concentrated nickel alloys, while the thermal conductivity of dilute nickel alloys shows a minimum at the Curie temperature.