bluuebunny/crossref_metadata_2025 huggingface dataset:
Quasi-optical cw mm-wave electron spin resonance spectrometer
Authors: Smith G. M. , Lesurf J. C. G. , Mitchell R. H. , Riedi P. C.
Published: July 2002

We describe a novel cw millimeter-wave electron spin resonance (ESR) spectrometer designed to operate in the frequency range of 80–200 GHz and in the temperature range of 2.5–300 K, which may be easily scaled to higher frequencies. The spectrometer uses a bimodal reflection cavity coupled to a circular corrugated guide and uses Gaussian quasi-optics for most of the front-end signal processing. This technique has very low insertion loss and allows a number of sophisticated measurement techniques to be employed including induction operation, which significantly reduces the effect of microphonics and stray reflections. A number of examples are given illustrating the sensitivity of the instrument and the advantages of using ESR at high fields.
Studies of plasma formation from exploding wires and multiwire arrays using x-ray backlighting
Authors: Shelkovenko T. A. , Pikuz S. A. , Mingaleev A. R. , Hammer D. A.
Published: July 2002

Plasma formation from exploding individual wires and multiwire arrays, powered by a 450 kA, 100 ns pulsed power generator has been studied with x-ray direct backlighting using one or more X pinches as a source of radiation. Experiments have been carried out with many different wire materials ranging in Z from Al to Au. A point source of x rays is used to cast a shadow image of the object plasma directly on film (“point-projection” backlighting). Spatial resolution in the range 1–5 μm, and temporal resolution of 0.7–2 ns, has been achieved using 2–4 keV x rays from Mo wire X pinches. Different schemes for multiframe imaging are described. We also present observations of micron scale structure on residual dense wire cores 40–70 ns after the start of the current pulse.
Laser scanning microscope for low temperature single molecule and microscale spectroscopy based on gradient index optics
Authors: Vácha Martin , Yokoyama Hiroshi , Tokizaki Takashi , Furuki Makoto , Tani Toshiro
Published: July 2002

A scanning optical microscope for low temperature imaging and spectroscopy with a gradient index rod-shaped microlens as an objective lens is presented. The solid immersion effect enhances the resolution to 310 nm of the full-width at half-maximum at the wavelength of 545 nm. Laser scanning mechanism located outside an optical cryostat enables to achieve large scanning ranges independent of temperature. The performance is demonstrated at 1.6 K on single molecules of terrylene in a dodecane crystal and on molecular J aggregates in thin polymer films.
Heteroepitaxial growth of highly conductive metal oxide RuO2 thin films by pulsed laser deposition
Authors: Jia Q. X. , Wu X. D. , Foltyn S. R. , Findikoglu A. T. , Tiwari P. , Zheng J. P. , Jow T. R.
Published: July 2002

Highly conductive ruthenium oxide (RuO2) has been epitaxially grown on LaAlO3 substrates by pulsed laser deposition. The RuO2 film is (h00) oriented normal to the substrate surface. The heteroepitaxial growth of RuO2 on LaAlO3 is demonstrated by the strong in-plane orientation of thin films with respect to the major axes of the substrate. High crystallinity of RuO2 thin films is also determined from Rutherford backscattering channeling measurements. Electrical measurements on the RuO2 thin films demonstrate a quite low room-temperature resistivity of 35±2 μΩ cm at deposition temperatures of above 500 °C.
Quasi-one-dimensional CaF2 islands formed on Si(001) by molecular beam epitaxy
Authors: Loretto D. , Ross F. M. , Lucas C. A.
Published: July 2002

Quasi-one-dimensional CaF2 islands, 5–10 nm in both width and height and several μm in length, have been grown on Si(001) by molecular beam epitaxy. Using conventional and high resolution transmission electron microscopy we show that the islands grow in two symmetry-equivalent, {011} orientations and are bounded by {111} facets. The unusual island morphology is attributed to a low density of nucleation sites, the small lattice mismatch, and the anisotropic CaF2 surface energy.
Theory of improved resolution in depth profiling with sample rotation
Authors: Bradley R. Mark , Cirlin Eun-Hee
Published: July 2002

We advance a theory that explains why sample rotation during depth profiling leads to a dramatic improvement in depth resolution. When the sample is rotated, the smoothing effects of viscous flow and surface self-diffusion can prevail over the roughening effect of the curvature-dependent sputter yield and generate a smooth surface. If the sample is not rotated initially and the depth resolution declines, we predict that subsequent rotation leads to improved resolution. This phenomenon has already been observed experimentally.
The second harmonic component in the Bessel beam
Authors: Ding Desheng , Lu Zuhong
Published: July 2002

The analysis is based on the Khokhlov–Zabolotskaya–Kuznetsov wave equation. Analytical and approximate solutions are derived for the second harmonic component in the Bessel nondiffracting beam. The theoretical results indicate that the second harmonic beam is nearly nondiffracting in the radial direction and the pressure amplitude is proportional to the square root of propagation distance in the quasilinear approximation, and that the beamwidth of the second harmonic is just 1/2 times that of the fundamental component in the Bessel field, not as 1/√2 times in the fields radiated by other sources.
Birefringence, Pockels, and Kerr effects in photorefractive polymers
Authors: Kippelen B. , Sandalphon  , Meerholz K. , Peyghambarian N.
Published: July 2002

A simple frequency-dependent ellipsometric technique for measuring the birefringence, the Pockels and Kerr properties of low glass-transition temperature photorefractive polymers is proposed. The technique is applied to the characterization of highly efficient DMNPAA:PVK:ECZ:TNF photorefractive polymers and the determination of the microscopic properties of the DMNPAA molecule.
Transient photocurrent induced in gallium nitride by two-photon absorption
Authors: Miragliotta J. , Wickenden D. K.
Published: July 2002

We have studied the subband gap induced, transient photocurrent in an epitaxial GaN film immersed in an electrolyte solution. For photon energies near the midgap position, one- and two-photon (TP) contributoins were observed in the photocurrent. The one-photon term exhibited a sublinear intensity dependence and was attributed to carrier generation from traps in the gap. The TP current was negligible for energies below Egap/2. Above this energy, the dispersion was consistent with previous calculations of the TP absorption coefficient [β(ω)] in direct gap semiconductors. A relationship between the TP photocurrent and β(ω) determined a value for the latter of ∼1.5 cm/GW at photon energies above Egap/2.
Gallium arsenide piezotransistor for dynamic pressure measurements at high temperature
Authors: Aller I. , Lang C. , Schweeger G. , Hartnagel H. L. , Dolt R. , Hohenberg G.
Published: July 2002

A piezotransistor concept for measuring transient pressures in combustion engines at high temperatures using the transversal piezoelectric effect in (100) GaAs is presented. The sensor was used without cooling to measure the pressure in a combustion chamber for a motored and fired engine. The results which are presented here are in good agreement with the signal from a commercial quartz sensor, which needs an additional preamplifier. Such an active GaAs sensor is therefore a very attractive alternative to the commonly used quartz sensors for dynamic pressure measurements at high temperatures. Compared to quartz sensors the GaAs sensor has the advantage of a possible integration of electronics on the sensor chip.
Room-temperature continuous-wave operation of InGaN multi-quantum-well-structure laser diodes with a long lifetime
Authors: Nakamura Shuji , Senoh Masayuki , Nagahama Shin-ichi , Iwasa Naruhito , Yamada Takao , Matsushita Toshio , Sugimoto Yasunobu , Kiyoku Hiroyuki
Published: July 2002

The continuous-wave operation of InGaN multi-quantum-well-structure laser diodes (LDs) was demonstrated at room temperature with a lifetime of 24–40 min. The threshold current and the voltage of the LDs were 80 mA and 6.5 V, respectively. The laser emission was a fundamental single-mode emission with a peak wavelength of 400.23 nm and a full width at half-maximum of 0.002 nm. The beam full width at half-power values for the parallel and the perpendicular near-field patterns were 1.6 and 0.8 μm, respectively. Those of the far-field patterns were 6.8° and 33.6°, respectively. The carrier lifetime and the threshold carrier density were estimated to be 5 ns and 1×1020/cm3, respectively.
Observation of a hexagonal BN surface layer on the cubic BN film grown by dual ion beam sputter deposition
Authors: Park K. S. , Lee D. Y. , Kim K. J. , Moon D. W.
Published: July 2002

Cubic-boron nitride (c-BN) thin films were grown by dual ion beam sputter deposition and the growth mechanism was studied using angle resolved in-situ XPS (x-ray photoelectron spectroscopy) analysis. The π bond shake-up satellite of the B 1s peak, which is observed only in a hexagonal boron nitride (h-BN) phase, appeared in the XPS spectrum obtained for the surface layer of the c-BN film. This can be a clear evidence that the c-BN phase grows via the transformation of an initially formed h-BN phase and the transformation is induced by the compressive stress accumulated in the subsurface region during ion bombardment.
Theory of mode-locked semiconductor lasers with finite absorber relaxation times
Authors: Dubbeldam J. L. A. , Leegwater J. A. , Lenstra D.
Published: July 2002

We investigate the influence of a finite absorber relaxation time on passively mode-locked semiconductor lasers. We find that the mode-locking mechanism of Haus is surprisingly susceptible to small perturbations and small changes in the parameters. Even when the absorber relaxation time is much larger than the pulse duration, it typically is still short enough to be a crucial part of the physics of mode-locking. Allowing for a finite absorber relaxation time, we find that stable operation of a mode-locked semiconductor is possible over a wide range of parameters. We argue that the pulse duration is inversely proportional to the square root of the pulse energy.
Schottky barrier detectors on GaN for visible–blind ultraviolet detection
Authors: Chen Q. , Yang J. W. , Osinsky A. , Gangopadhyay S. , Lim B. , Anwar M. Z. , Asif Khan M. , Kuksenkov D. , Temkin H.
Published: July 2002

We report on the fabrication and characterization of vertical geometry transparent Schottky barrier ultraviolet detectors based on n−/n+-GaN structures grown over sapphire substrates. Spectral responsivity measurements were made using illumination through the UV transparent Schottky barrier metal. A responsitivity as high as 0.18 A/W was measured for wavelengths shorter than the absorption edge of GaN. The detector speed was RC limited and the fall time was 118 ns. The 1/f noise is identified to be the main noise contribution. At 300 Hz, we measure the noise equivalent power at less than 4×10−9 W.
Titanium oxide/aluminum oxide multilayer reflectors for “water-window” wavelengths
Authors: Kumagai Hiroshi , Toyoda Koichi , Kobayashi Katsutaro , Obara Minoru , Iimura Yasuhiro
Published: July 2002

Twenty bilayers of titanium oxide and aluminum oxide with the layer-pair thickness of 4.43 nm on a silicon substrate were fabricated for novel multilayer reflectors at water-window wavelengths by an atomic layer deposition method of controlled growth with sequential surface chemical reactions. The high reflectance of over 30% at a wavelength of 2.734 nm and an incident angle of 71.8° from the normal incidence was demonstrated experimentally. The full width at half-maximum of the reflectances at 2.734 nm was 0.0381 nm, which corresponds to Δλ/λ=1.4%.
Atomic layer graphoepitaxy for single crystal heterostructures
Authors: Wallis D. J. , Browning N. D. , Sivananthan S. , Nellist P. D. , Pennycook S. J.
Published: July 2002

Here we report a strategy for the growth of single crystal heterostructures that dramatically reduces the nucleation of defects at the film–substrate interface. The substrate surface is patterned through miscut and passivated to enable a single domain template layer to be grown. This template is incommensurate with, and weakly bonded to, the substrate. A single domain film can then be nucleated preferentially at step edges and grown on the template. This mechanism is demonstrated for CdTe on Si(100) and should be applicable to many other systems.
Evidence of heteroepitaxial growth of copper on beta-tantalum
Authors: Kwon Kee-Won , Ryu Changsup , Sinclair Robert , Wong S. Simon
Published: July 2002

Crystallographic orientations between thin-sputtered Cu film and β-Ta adhesion layer have been studied using high resolution electron microscopy and electron diffraction. Tetragonal β-Ta deposited on SiO2 has a strong texture with its closest packed plane (002) parallel to the film surface. On (002) β-Ta, the growth of (111) Cu is preferred. Even though more than 100 β-Ta grains are found under a single Cu grain, the Ta grains under a Cu grain have long range in-plane texture with [330] direction aligned parallel to the [220] direction of Cu. This orientational coincidence is explained by the heteroepitaxial relationship between the hexagonal close-packed atomic array in Cu (111) plane and the pseudohexagonal configuration of β-Ta atoms in (002) plane with a misfit strain of 7.6%.
Study of shear force between glass microprobe and mica surface under controlled humidity
Authors: Okajima Takaharu , Hirotsu Shunsuke
Published: July 2002

Resonance characteristics of a glass micropipette near a mica surface have been investigated as functions of the tip–surface distance, D, and of the ambient humidity, H. With decreasing D, the resonance frequency of the tip increases, while its resonance amplitude decreases. The resonance curve is almost symmetric except in the close vicinity of D=0, at which the oscillation is damped completely. The effective length, D0, of the tip–surface interaction is independent of the dither amplitude. With increasing H, D0 decreases gradually, exhibiting a rather sharp drop around H=40%. These results indicate that, for large D where the direct contact does not occur, some noncontact force between the tip and the surface is operative, and that this force is sensitive to the presence of a water layer on the surface.
Electron heating effects in diffusive metal wires
Authors: Henny M. , Birk H. , Huber R. , Strunk C. , Bachtold A. , Krüger M. , Schönenberger C.
Published: July 2002

We have investigated the electron heating in metallic diffusive wires of varying length at liquid-helium temperature by measuring the electric noise. The local increase of the electron temperature can be essential already for small currents and is well described by a heat-diffusion equation for the electrons. Depending on the electron thermal conductance and the electron–phonon coupling in the wire, different length regimes are identified. The quantitative knowledge of the electron temperature is important for analysis of nonequilibrium effects involving current heating in mesoscopic wires.
Electromigration-induced stress in aluminum conductor lines measured by x-ray microdiffraction
Authors: Wang P.-C. , Cargill G. S. , Noyan I. C. , Hu C.-K.
Published: July 2002

Electromigration-induced stress distributions in 200 μm long, 10 μm wide aluminum conductor lines in 1.5 μm SiO2 passivation layers have been investigated in real time using synchrotron-based white-beam x-ray microdiffraction. The results show that a steady-state linear stress gradient along the length of the line developed within the first few hours of electromigration and that the stress gradient could be manipulated by controlling the magnitude and the direction of the current flow. From the current density dependence of the steady-state stress gradient, the effective valence Z* was determined to be 1.6 at 260 °C. From the time dependence of the transient-state stress gradient, the effective grain boundary diffusion coefficient Deff was estimated to be 8.2×10−11 cm2/s at 260 °C using Korhonen’s stress evolution model [M. A. Korhonen, P. Bo/rgesen, K. N. Tu, and C.-Y. Li, J. Appl. Phys. 73, 3790 (1993)]. Both Z* and Deff values are in good agreement with the previously reported values.
Homogeneous alignment of nematic liquid crystal induced by polyimide exposed to linearly polarized light
Authors: Wang Yinghan , Kanazawa Akihiko , Shiono Takeshi , Ikeda Tomiki , Matsuki Yasuo , Takeuchi Yasumasa
Published: July 2002

Homogeneous alignment of a nematic liquid crystal (LC) was induced by a polyimide (PI) film containing diphenyl ether component exposed to linearly polarized light at 366 nm. Ultraviolet-visible absorption spectra showed that the change in the absorption of the PI was very small after exposure; the preliminary result indicated that the homogeneous alignment of a LC could be induced without significant change in the PI structure.
A new buffer layer for high quality GaN growth by metalorganic vapor phase epitaxy
Authors: Kachi Tetsu , Tomita Kazuyoshi , Itoh Kenji , Tadano Hiroshi
Published: July 2002

A new buffer layer to grow high-quality GaN films was proposed. The new buffer layer consisted of a thin (20–30 nm) InN layer deposited at low temperature (∼600 °C). GaN films were grown on (112̄0)-oriented (A-face) sapphire substrates using a conventional GaN buffer layer and an InN buffer layer by atmospheric pressure metalorganic vapor phase epitaxy. Dislocations in the GaN films were observed by cross-sectional transmission electron microscopy (TEM). The dislocation densities were measured from the TEM observation and were ∼4×109 and ∼6×108 cm−2 for epilayers with the GaN and the InN buffer, respectively. The low dislocation density by the InN buffer was attributed to relaxation of the stress in the GaN epilayers due to the low melting point of InN. GaN epilayers using the InN buffer also showed good electrical properties.
Second-harmonic generation from needlelike ferroelectric domains in Sr0.6Ba0.4Nd2O6 single crystals
Authors: Kawai Satoru , Ogawa Tomoya , Lee Howard S. , DeMattei Robert C. , Feigelson Robert S.
Published: July 2002

Since Sr0.6Ba0.4Nb2O6 (SBN) is ferroelectric with a tungsten–bronze structure, numerous needlelike ferroelectric domains appear in a SBN single crystal when it is cooled through its transition temperature in the absence of a poling field. If the domains were illuminated by 1.06 μm radiation from a Nd:YAG laser, second-harmonic diffuse light was generated from these domains. This interesting effect, which can clearly be seen with the naked eye, will be used to generate all the primary colors using suitable lasers.
High gain GaN/AlGaN heterojunction phototransistor
Authors: Yang Wei , Nohava Thomas , Krishnankutty Subash , Torreano Robert , McPherson Scott , Marsh Holly
Published: July 2002

A GaN/AlGaN heterojunction bipolar phototransistor with gain in excess of 105 was demonstrated. From 360 to 400 nm, an eight orders of magnitude drop in responsivity was achieved. The phototransistor features a rapid electrical quenching of persistent photoconductivity, and exhibits high dark impedance and no dc drift. By changing the frequency of the quenching cycles, the detection speed of the phototransistor can be adjusted to accommodate specific applications. These results represent an internal gain UV detector with significantly improved performance over GaN-based photoconductors.
Optical activation of Be implanted into GaN
Authors: Ronning C. , Carlson E. P. , Thomson D. B. , Davis R. F.
Published: July 2002

Single crystalline (0001) gallium nitride layers were implanted with beryllium. Photoluminescence (PL) measurements were subsequently performed as a function of implantation dose and annealing temperature. One new line in the PL spectra at 3.35 eV provided strong evidence for the presence of optically active Be acceptors and has been assigned to band–acceptor (eA) recombinations. The determined ionization energy of 150±10 meV confirmed that isolated Be has the most shallow acceptor level in GaN. Co-implantation of nitrogen did not enhance the activation of the Be acceptors.
Generation of picosecond hard x rays by tera watt laser focusing on a copper target
Authors: Yoshida Masatake , Fujimoto Yasushi , Hironaka Yoichiro , Nakamura Kazutaka G. , Kondo Ken-ichi , Ohtani Masayuki , Tsunemi Hiroshi
Published: July 2002

Hard x-ray generation from tera-watt laser irradiation (4 TW, 42 fs at 780 nm) on a solid copper target has been studied in a range of the laser-power density between 3×1016 and 2×1017 W/cm2. Short-pulsed x rays are generated within 6.4 ps and include Cu Kα and Kβ line emissions and a continuum between 3 and 6 keV. The maximum intensity of Cu Kα radiation has been estimated to be 6.5×1010 photons (4π sr pulse)−1. The mechanism of hard x-ray generation has been discussed.
Structural and optical properties of epitaxially overgrown third-order gratings for InGaN/GaN-based distributed feedback lasers
Authors: Romano Linda T. , Hofstetter Daniel , McCluskey Matthew D. , Bour David P. , Kneissl Michael
Published: July 2002

Laser-diode heterostructures of InGaAlN containing a third-order diffraction grating for distributed optical feedback have been examined with transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The grating was defined holographically and etched by chemically assisted ion-beam etching into the upper GaN confinement layer of the laser structure. After the etch step, it was overgrown with an Al0.08Ga0.92N upper cladding layer. Threading dislocations were present that initiated at the sapphire substrate, but no new dislocations were observed at the grating/Al0.08Ga0.92N interface. A comparison of TEM and SEM micrographs reveals that there is a compositional gradient in the AlGaN upper cladding layer; however, calculations show that it did not reduce the optical coupling coefficient of the grating.
Rhombohedral superlattice structure and relaxor ferroelectric behavior of (Pb0.70Ba0.30)ZrO3 ceramics
Authors: Pokharel Bhadra P. , Ranjan Rajeev , Pandey Dhananjai , Siruguri V. , Paranjpe S. K.
Published: July 2002

Superlattice reflections observed in the neutron powder-diffraction patterns of (Pb0.80Ba0.20)ZrO3 (PBZ20) and (Pb0.70Ba0.30)ZrO3 (PBZ30) are explained in terms of doubled rhombohedral cell (R3c). A Rietveld analysis of the neutron data reveals that the displacements of Pb2+/Ba2+ and Zr4+ decrease with increasing Ba2+ content while the thermal parameter for Pb2+/Ba2+ increases and becomes similar to ferroelectric relaxors. Dielectric studies indeed confirm relaxor behavior in PBZ30 with Vogel–Fulcher type relaxational freezing.
Ln
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Thermoelectrics with low thermal conductivity
Authors: Young D. , Mastronardi K. , Khalifah P. , Wang C.-C. , Cava R. J. , Ramirez A. P.
Published: July 2002

The thermoelectric properties of intermetallic compounds with the Y3Au3Sb4 structure type, of formula Ln3Au3Sb4 for Ln=Gd, Nd, Ho, and Sm, are reported. The highest Seebeck coefficients are on the order of 100–200 μV/K, indicating that the dominant carriers are holes, and increase with increasing temperature. Variation of the Au:Sb ratio significantly affects the resistivities and Seebeck coefficients. Materials with mixtures of lanthanides on the large atom site show improved Seebeck coefficients without degradation of the electrical resistivity. The thermal conductivities are very low, even for the stoichiometric materials, and decrease in materials with mixed lanthanides. Band-structure calculations show a complex multivalley character for both valence and conduction bands.
Optical losses in plasma-etched AlGaAs microresonators using reflection spectroscopy
Authors: Rivera T. , Debray J.-P. , Gérard J. M. , Legrand B. , Manin-Ferlazzo L. , Oudar J. L.
Published: July 2002

The optical losses in dry-etched monolithic microresonators have been studied as a function of their lateral dimensions. Cylindrical microresonators with various radii have been etched from a planar GaAlAs/GaAs microcavity with a very high quality factor (Q≅11 700). Measurements of the resonance linewidth, using Ti-sapphire laser spectroscopy allowed to study the degradation of the Q factor at small radii. The Q factor is four times smaller in 1.1 μm radius microresonators, compared to the unprocessed cavity. This degradation is attributed to optical scattering from sidewalls, whose efficiency is shown to scale with the guided mode intensity at the microresonator edge.
Photoinduced alignment control of photoreactive side-chain polymer liquid crystal by linearly polarized ultraviolet light
Authors: Kawatsuki Nobuhiro , Yamamoto Tohei , Ono Hiroshi
Published: July 2002

Alignment control of mesogenic groups in a photocrosslinkable polymer liquid crystalline film was achieved by linearly polarized (LP) ultraviolet (UV) irradiation followed by heating. The alignment direction was parallel to the electric vector of the incident LPUV light and the tilt angle of the mesogenic groups could be controlled by a one-step slantwise LPUV irradiation. The photoinduced birefringence dn was 0.07 and the order parameter of the mesogenic group was 0.28.
Turbostratic boron carbonitride film and its field-emitting behavior
Authors: Yu Jie , Wang E. G.
Published: July 2002

The turbostratic structure of boron carbonitride (BCN) films with various compositions synthesized by bias-assisted hot-filament chemical-vapor deposition was studied by x-ray diffraction and high-resolution transmission electron microscopy. Boron atoms can be incorporated into the turbostratic structure with a concentration up to 70%, and the interplanar spacing 3.49 Å is independent of the film compositions in this range. Field-emission behavior of the BCN films is characterized. A low threshold electric field of 4 V/μm and maximum emission current of 0.31 mA are obtained, which suggest a new candidate as a field-emitting material.
A vertical cavity light emitting InGaN quantum well heterostructure
Authors: Song Y.-K. , Zhou H. , Diagne M. , Ozden I. , Vertikov A , Nurmikko A. V. , Carter-Coman C. , Kern R. S. , Kish F. A. , Krames M. R.
Published: July 2002

A method is described for fabricating a vertical cavity light emitting structure for nitride semiconductors. The process involves the separation of a InGaN/GaN/AlGaN quantum well heterostructure from its sapphire substrate an its enclosure by a pair of high reflectivity, low loss dielectric mirrors to define the optical resonator. We have demonstrated a cavity Q factor exceeding 600 in initial experiments, suggesting that the approach can be useful for blue and near ultraviolet resonant cavity light emitting diodes and vertical cavity lasers.
Nonlinear optical properties of a coherent array of submicron SiO2 spheres (opal) embedded with Si nanoparticles
Authors: Ajgaonkar M. , Zhang Y. , Grebel H. , White C. W.
Published: July 2002

The optical properties of a coherent array of 300 nm SiO2 spheres implanted with Si ions were shown to change nonlinearly with light intensity. These changes have been attributed to transverse confinement of the optical beam within the opal structure.
High electrostrictive strain under high mechanical stress in electron-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymer
Authors: Bharti Vivek , Cheng Z.-Y. , Gross S. , Xu T.-B. , Zhang Q. M.
Published: July 2002

In this letter, we show that the electric field induced strain in the electron irradiated poly(vinylidene fluoride-trifluoroethylene) copolymer can generate high strain even under a high mechanical stress. The observed change in strain with stress is due to the electrostrictive coupling of the local polarization with stress, and can be directly related to the change of the induced strain with temperature. The results indicate that the field induced strain observed in the films investigated is indeed from the local polarization regions in the material, and is electrostrictive in nature.
Diffuse x-ray streaks from stacking faults in Si analyzed by atomistic simulations
Authors: Nordlund K. , Beck U. , Metzger T. H. , Patel J. R.
Published: July 2002

Since extrinsic stacking faults can form during postimplantation annealing of Si, understanding their properties is important for reliable control of semiconductor manufacturing processes. We demonstrate how grazing incidence x-ray scattering methods can be used as a nondestructive means for detecting extrinsic stacking faults in Si. Atomistic analysis of diffuse intensity streaks is used to determine the size of the faults, the minimum size at which the streak pattern in the scattering will be visible, and the magnitude of atomic displacements in the center of the stacking fault.
Interface structure of ultrathin CoSi2 films epitaxially grown on Si(111)
Authors: Seubert A. , Schardt J. , Weiß W. , Starke U. , Heinz K. , Fauster Th.
Published: July 2002

The interface structure of ultrathin CoSi2 films grown on Si(111) was investigated by quantitative low-energy electron diffraction. Codeposition of the elements leads to a film composed of domains with two and three Si–Co–Si trilayers in CaF2 structure. As within the film, Co atoms at the interface are eightfold coordinated. The lateral unit cells of the film and substrate are mutually rotated by 60° (B-type orientation). The interfacial trilayer is substantially distorted, its distance to the substrate expanded, and its sublayer spacings considerably modified from the bulk. Also, the substrate’s top spacing is expanded. The results compare almost quantitatively with recent density-functional calculations.
A nonequilibrium fixed-parameter subgrid-scale model obeying the near-wall asymptotic constraint
Authors: Yoshizawa Akira , Kobayashi Katsutoshi , Kobayashi Toshio , Taniguchi Nobuyuki
Published: July 2002

A subgrid-scale (SGS) model is presented with two fixed nondimensional model parameters adopted, which is in the same line of the Smagorinsky model combined with Van Driest’s wall damping function. The proposed model, however, possesses two features that are typical of the dynamic SGS modeling. One is that it is not based on the equilibrium assumption about the SGS-energy production and dissipation rates. The other is that the model is not dependent on a wall-unit coordinate and obeys the correct near-wall asymptotic behavior, unlike the Smagorinsky–Van Driest model. This model is applied to channel flows, and the computed results are discussed in comparison with the direct numerical simulation and the Smagorinsky–Van Driest model. Special attention is paid to the dependence of the results on the Reynolds number, the grid resolution, and the choice of a filter function.
Atomistic simulation of diffuse x-ray scattering from defects in solids
Authors: Nordlund K. , Partyka P. , Averback R. S. , Robinson I. K. , Ehrhart P.
Published: July 2002

Diffuse x-ray scattering is a powerful means to study the structure of defects in crystalline solids. The traditional analysis of diffuse x-ray scattering experiments relies on analytical and numerical methods which are not well suited for studying complicated defect configurations. We present here an atomistic simulation method by which the diffuse x-ray scattering can be calculated for an arbitrary finite-sized defect in any material where reliable interatomic force models exist. We present results of the method for point defects, defect clusters and dislocations in semiconductors and metals, and show that surface effects on diffuse scattering, which might be important for the investigation of shallow implantation damage, will be negligible in most practical cases. We also compare the results with x-ray experiments on defects in semiconductors to demonstrate how the method can be used to understand complex damage configurations.
Coarse-grained Ginzburg–Landau free energy for Lennard-Jones systems
Authors: Gracheva M. E. , Rickman J. M. , Gunton J. D.
Published: July 2002

We discuss the application of Monte Carlo methods to the self-consistent calculation of a Ginzburg–Landau free energy functional for Lennard-Jones systems in three dimensions. Following this discussion, we demonstrate that the parameters in the coarse-grained free energy can be extracted from a multivariate distribution of energies and particle densities which, when suitably reweighted, permit one to extrapolate the results to other nearby points in the thermodynamic parameter space. For the purposes of illustration, both single-phase and liquid–gas coexistence are considered here with the aim of describing various regions of the phase diagram with a single function and, in doing so, providing a link between atomistic and mesoscopic length scales.
Role of iron in lithium-niobate crystals for the dark-storage time of holograms
Authors: Nee I. , Müller M. , Buse K. , Krätzig E.
Published: July 2002

The dark decay of holograms stored in iron-doped photorefractive lithium-niobate crystals is studied for samples containing up to 0.25 wt% Fe2O3 (iron concentration 71×1018 cm−3). The oxidation/reduction state of the crystals, i.e., the concentration ratio of Fe2+ and Fe3+ ions, is changed in a wide range by thermal annealing. The dark decay is attributed to two effects: An ionic dark conductivity arising from mobile protons and an electronic dark conductivity caused by tunneling of electrons between iron sites. The latter is proportional to the effective trap density, i.e., to the density of charge carriers which can be moved between the iron sites. The proportionality factor is the specific dark conductivity which increases exponentially with the third root of the entire iron concentration.
Permanent polarization gratings in photosensitive Langmuir–Blodgett films
Authors: Cipparrone G. , Mazzulla A. , Palto S. P. , Yudin S. G. , Blinov L. M.
Published: July 2002

An investigation was carried out on thin permanent phase polarization gratings recorded in Langmuir–Blodgett films using two orthogonal circularly polarized Ar-ion laser beams. The films are composed of amphiphilic azo-dye molecules and manifest an extremely large value of photoinduced optical anisotropy, Δn≈0.36. The experimental results are in excellent accord with the theoretical model based on the Jones matrix representation of the polarization pattern. The gratings are stable at least, for half a year.
Magnetoresistance effect and interlayer coupling of (Ga, Mn)As trilayer structures
Authors: Chiba D. , Akiba N. , Matsukura F. , Ohno Y. , Ohno H.
Published: July 2002

We have investigated the magnetic and magnetotransport properties of (Ga, Mn)As/(Al, Ga)As/(Ga, Mn)As semiconductor-based magnetic trilayer structures. We observe a weak ferromagnetic interlayer coupling between the two ferromagnetic (Ga, Mn)As layers as well as magnetoresistance effects due to spin-dependent scattering and to spin-dependent tunneling. Both the coupling strength and the magnetoresistance ratio decrease with the increase of temperature and/or the increase of Al composition of the nonmagnetic (Al, Ga)As layer.
Advanced impurity measurement for deuterium–tritium-burning plasmas using pulsed CO2 laser collective Thomson scattering
Authors: Lee S. , Kondoh T.
Published: July 2002

Knowledge of the dynamics and concentration of impurities in the plasma core is crucial to the development of a deuterium–tritium (DT) tokamak fusion reactor. In this article, we first describe an advanced diagnostic technique to measure the impurities, thermalized helium density, and the deuterium-to-tritium density ratio in DT-burning plasmas. The measurement is made by small-angle collective Thomson scattering using a high-power pulsed CO2 laser and heterodyne receiver system. The component of scattered laser power nearly perpendicular to the magnetic field gives rise to ion cyclotron modulation of the scattered spectrum. Scattered spectrum of expected impurities (Ar, Be, and He ash) are presented for the International Thermonuclear Experimental Reactor plasmas. The method for the determination of the helium ash density and deuterium-to-tritium ratio are also proposed. Modulation spectra as a function of frequency for different magnetic-field angles are confirmed. Experimental requirements to measure a scattered spectrum with fine details resulting from ion cyclotron oscillations are also discussed.
Dynamics of collapse of flexible polyampholytes
Authors: Lee Namkyung , Thirumalai D.
Published: July 2002

We provide a theory for the dynamics of collapse of flexible polyampholytes (PAs) using Langevin equation. Metastable pearl-necklace structures form in a time scale proportional to N4/5 (N is the number of monomers). In the late stage of collapse the pearls merge with the largest one growing at the expense of smaller ones (Lifshitz–Slyozov mechanism). The time scale for this process is τcoll∼N. Counterion-mediated collapse of strongly charged polyelectrolytes (PEs) follow a similar route to the globular state. Simulations support the proposed collapse mechanism for PAs and PEs.
A simple method for deriving kinetic energy operators
Authors: Wang Xiao-Gang , Carrington Tucker
Published: July 2002

We derive a general two-vector-embedded ro-vibrational kinetic energy operator (KEO) for a molecule with N atoms. Our KEO is valid for any molecule-fixed axis system attached to the two embedding vectors and any vibrational coordinates for the two-vector part of the KEO. It can be used to build up KEOs for larger molecules from KEOs for smaller molecules.
Spatiotemporal behaviors of excited Xe atoms in unit discharge cell of ac-type plasma display panel studied by laser spectroscopic microscopy
Authors: Tachibana Kunihide , Feng Shaojun , Sakai Tetsuo
Published: July 2002

Two-dimensional spatiotemporal behaviors of excited Xe atoms in the 1s4 resonance state and the 1s5 metastable state were measured in a unit discharge cell of an ac-type plasma display panel by a laser absorption technique combined with an optical microscope. The measured density of Xe(1s5) has two large peaks on both the temporal anode and cathode sides. The peak at the anode has a narrower spatial distribution while the peak at the cathode is distributed over the electrode area. In its temporal behavior, the anode peak rises slightly faster than the peak at the cathode and decays faster at the beginning of afterglow, but both peaks tend to have the same decay rate in the later period. The behavior of Xe(1s4) shows similar features, but the decay rate is much larger, corresponding to the effective lifetime of imprisoned resonance radiation. The maximum densities of Xe(1s5) and Xe(1s4) are 5×1013 and 2×1013 cm−3, respectively. Emission from Xe(2p) atoms was also observed, and this nearly followed the current wave form. With these results, we estimated the efficiency of vacuum ultraviolet emissions from excited Xe(1s4) atoms and Xe2* excimers formed from Xe(1s5) atoms.
High level ab initio studies on the excited states of HOCO radical
Authors: Li Yumin , Francisco Joseph S.
Published: July 2002

The HOCO radical has a ground (X 2A′) and two lowest A″2 excited states that are located using the CCSD(T) level of theory with the cc-pVDZ and cc-pVTZ basis sets. The harmonic frequencies are calculated at the CCSD(T) level of theory with the cc-pVDZ basis set. The vertical excitation energies for the 2 2A′, 3 2A′, 1 2A″, and 2 2A″ states of HOCO are obtained at the MRCI level of theory with the cc-pVTZ and aug-cc-pVTZ basis sets. The first excited state (1 2A″) is calculated to be 70.7 kcal mol−1 above the ground state for trans-HOCO. Comparisons are made between the excited states of HOCO and HCO. It is demonstrated that the HOCO states are not similar to those of HCO.
Diamagnetism of helium
Authors: Bruch L. W. , Weinhold F.
Published: July 2002

The diamagnetic susceptibility of two helium atoms is calculated as a function of atomic separation R and the spherical-average incremental susceptibility is evaluated. The Hartree–Fock approximation is used at intermediate separations and perturbation theory for the leading, 1/R6, term in the increment at large separations. The change in average atomic susceptibility in liquid helium, relative to that for isolated atoms, is estimated. It is concluded that measurements on the liquid may be used to accurately determine the isolated atom diamagnetic susceptibility.
Double-exchange ferromagnetism and magnetoresistance in LaMn1−xCuxO3 (x⩽0.3)
Authors: Sun Young , Xu Xiaojun , Tong Wei , Zhang Yuheng
Published: July 2002

We have studied the magnetic and magnetotransport properties of the perovskite LaMn1−xCuxO3 (x⩽0.3). All of these compounds show ferromagnetism and cluster glass behavior at low temperatures. Though no metallic-insulating transition associated with the ferromagnetic–paramagnetic transition appears, a large magnetoresistance was observed in the ferromagnetic phase. Due to the fact that the Cu ion in these compounds is divalent, the ferromagnetism in LaMn1−xCuxO3 is believed to be consistent with the Mn3+–O–Mn4+ double-exchange mechanism. These results suggest that double exchange and large magnetoresistance can be obtained by direct Mn-site doping.
Incorporating physical implementation concerns into closed loop quantum control experiments
Authors: Geremia J. M. , Zhu Wusheng , Rabitz Herschel
Published: July 2002

In quantum control experiments, it is desirable to build features into the field that address physical concerns such as simplicity, robustness, dynamical coherence, power expenditure, etc. With a judicious choice for the cost functional, it is possible to incorporate such secondary features into the field, often without altering the experimental procedure or apparatus. Through simulated closed-loop population transfer experiments, we demonstrate the benefit of carefully designed cost functionals. As specific examples, we address two common physical concerns: removing extraneous structure from the control pulse and finding robust fields. Removing unnecessary field components is critical if information about the mechanism is to be interpreted from the structure of the optimal pulse. Robust fields produce a stable outcome despite noise in the field and, perhaps, environmental inhomogeneities in the quantum system as is typical of condensed phase experiments.
Initial-value problems for evolutionary partial differential equations and higher-order conditional symmetries
Authors: Basarab-Horwath P. , Zhdanov R. Z.
Published: July 2002

We suggest a new approach to the problem of dimensional reduction of initial/boundary value problems for evolution equations in one spatial variable. The approach is based on higher-order (generalized) conditional symmetries of the equations involved. It is shown that reducibility of an initial value problem for an evolution equation to a Cauchy problem for a system of ordinary differential equations can be fully characterized in terms of conditional symmetries which leave invariant the equation in question. We also give some examples of the solution of initial value problems for second- and third-order nonlinear differential equations by reduction by their conditional symmetries. We give a systematic classification of general second-order partial differential equations admitting second-order conditional symmetries, based on Lie’s classification of invariant second-order ordinary differential equations. This yields five classes of principally new initial value problems for nonlinear evolution equations which admit no Lie symmetries and are reducible via second-order conditional symmetries.
Chemical dynamics of d1-methyldiacetylene (CH3CCCCD; X 1A1) and d1-ethynylallene (H2CCCH(C2D); X 1A′) formation from reaction of C2D(X 2Σ+) with methylacetylene, CH3CCH(X 1A1)
Authors: Kaiser R. I. , Chiong C. C. , Asvany O. , Lee Y. T. , Stahl F. , von R. Schleyer P. , Schaefer H. F.
Published: July 2002

The crossed beam reaction of the d1-ethynyl radical C2D(X 2Σ+), with methylacetylene, CH3CCH(X 1A1), was investigated at an average collision energy of 39.8 kJ mol−1. Our experimental results were combined with electronic structure calculations. The chemical reaction dynamics are indirect, involve three distinct channels, and are initiated via a barrierless addition of C2D to the acetylenic bond through long lived cis and trans CH3CCH(C2D), 1-ethynylpropen-2-yl, intermediates. The reduced cone of acceptance of the carbon atom holding the methyl group favors a carbon–carbon σ bond formation at the carbon atom adjacent to the acetylenic hydrogen atom. A crossed beam experiment of C2D with partially deuterated methylacetylene, CD3CCH, shows explicitly that the reactive intermediates decompose to form both methyldiacetylene, CD3CCCCD (channel 1, 70%–90%), and to a minor amount ethynylallene, D2CCCH(C2D) (channel 2; 10%–30%), isomers through exit transition states located 7–15 kJ mol−1 above the products. The computed reaction energies to form both isomers are −135 and −107 kJ mol−1, respectively, with respect to the separated reactants. A minor reaction pathway involves a H shift in CH3CCH(C2D) to an 1-ethynylpropen-1-yl radical which fragments to methyldiacetylene via a barrier of 8.8 kJ mol−1 (channel 3). Neither methyl group elimination nor the formation of the CC(CH3)(C2D) carbene was observed in our experiments. The experimentally observed “sideways scattering” and ab initio investigation verify our conclusions of a predominate formation of the methyldiacetylene isomer. These electronic structure calculations depict a hydrogen atom loss in the exit transition state to methyldiacetylene almost parallel to the total angular momentum vector J as found in our center-of-mass angular distribution. Since the title reaction and the corresponding reaction of the C2H radical with CH3CCH both have no entrance barriers, are exothermic, and all the involved transition states are located well below the energy of the separated reactants, the assignment of the ethynyl versus H atom exchange suggests the formation of both isomers under single collision conditions in extraterrestrial environments such as cold, molecular clouds as well as the atmosphere of Saturn’s moon Titan.
Investigation of the gas pressure influence on patterned platinum etching characteristics using a high-density plasma
Authors: Delprat S. , Chaker M. , Margot J.
Published: July 2002

A high-density surface-wave magnetized argon plasma operated in the very low pressure regime together with a rf biased system is used to study the pure physical etching characteristics of platinum thin films. It is shown that, for a given dc self-bias voltage, the platinum etch rate strongly decreases as the operating pressure increases, which results from a decrease of the ion density at the sheath edge and from enhanced redeposition. It is found that using a high-density plasma in the very low pressure regime yields high etch rates with a good selectivity over resist. Fence-free features can also be achieved at bias voltages that, in contrast with reactive ion etching reactors, are only slightly above the platinum sputtering threshold.
Integrable Schrödinger operators with magnetic fields: Factorization method on curved surfaces
Authors: Ferapontov E. V. , Veselov A. P.
Published: July 2002

The factorization method for Schrödinger operators with magnetic fields on a two-dimensional surface M2 with nontrivial metric is investigated. This leads to the new integrable examples of such operators and brings a new look at some classical problems such as the Dirac magnetic monopole and the Landau problem. The global geometric aspects and related spectral properties of the operators from the factorization chains are discussed in detail. We also consider the Laplace transformations on a curved surface and extend the class of Schrödinger operators with two integrable levels introduced in the flat case by S. P. Novikov and one of the authors.