bluuebunny/crossref_metadata_2025 huggingface dataset:
High-brilliance Beamline at the European Synchrotron Radiation Facilitya)
Authors: Bösecke Peter , Diat Olivier , Rasmussen Bjarne
Published: July 2002

The High-brilliance Beamline at the European Synchrotron Radiation Facility is under commissioning. First official user experiments are scheduled for September 1994. During the last months the beamline optics has been installed and extensively tested. Currently, the two end stations, (a) for macromolecular crystallography and (b) for time-resolved ultrasmall-angle x-ray scattering are under construction. This article presents the final optical setup of the beamline and describes the end stations. The actual performance of the undulator, the cryogenically cooled Si-(111) channel-cut monochromator, and the toroidal mirror are discussed. All results are in very good agreement with previously published theoretical calculations. Now, at the sample position a focused monochromatic beam is available between 0.15 nm (8 keV) and 0.077 nm wavelength (16 keV). At 0.1 nm wavelength the beam size is horizontally 0.38 mm and vertically 0.75 mm (full width at half-maximum) with a photon flux density of 2.5×107 photons/s/μm2 (±30%).
HIBP diagnostics on T-10
Authors: Melnikov A. V. , Bondarenko I. S. , Efremov S. L. , Kharchev N. K. , Khrebotv S. M. , Krupnik L. I. , Nedzelskij I. S. , Zimeleva L. G. , Trofimenko Yu. V.
Published: July 2002

A heavy ion beam probe diagnostic was installed on T-10 tokamak. Cs+ and Tl+ beams with the energy 100–280 keV and intensity about a few dozen μA were used. The diagnostic is able to make two-dimensional measurements of plasma parameters along the detector line. The detector grid covered the first quadrant of the plasma cross section. The measurements are possible with r/a=0.3–1, and ω=0°–90° for B=1.5 T and Cs+ ions. High-voltage stabilizers keep the analyzer and accelerator voltage drift on the level ΔU/U∼10−5. An absolute in situ calibration of the energy analyzer, using He target will allow to make a more accurate absolute referencing of the potential. The time dependence of the local plasma parameters was obtained. A scan along the detector line by variation of injection angle and obtaining the whole profile during one pulse is possible. The diagnostic is also sensitive to poloidal magnetic field.
Scanning nonlinear dielectric microscope
Authors: Cho Yasuo , Kirihara Akio , Saeki Takahiro
Published: July 2002

This article describes a new scanning technique for imaging the state of spontaneous polarization of a ferroelectric material by measuring the microscopic point-to-point variation of its nonlinear dielectric constants. First, the theory for detecting polarization is described. Second, the technique for measuring the nonlinear dielectric response is described. Finally, using this new microscope, area scans are obtained of the polarization of poled lead zirconate titanate ceramics, a lithium niobate single crystal, and of piezoelectric thin films of the copolymer of vinylidene fluoride and trifluoroethylene.
Calibration of the heavy ion beam probe parallel plate analyzer using the gas target and reference beam
Authors: Melnikov A. V. , Kharchev N. K. , Eliseev L. G. , Bondarenko I. S. , Krupnik L. I. , Khrebtov S. M. , Nedzelskij I. S. , Trofimenko Yu. V.
Published: July 2002

The most important and difficult problem in heavy ion beam probe diagnostics is the absolute measurement of the plasma potential. The analyzer that measures the energy of the secondary ions must be perfectly calibrated. The accuracy necessary should be at least 10−4. We used two methods of calibration: in situ using the gas target, and in vitro using the reference beam. Calibration allows one to obtain the gain G and dynamic coefficient F of the analyzer. We developed a procedure of fast calibration that uses the dynamic curve Ub/Ua(δi), where Ub and Ua correspond to the accelerating voltage of the beam and the voltage on the analyzer, and δi is a normalized difference of currents on the plates. The analyzer used on the T-10 tokamak was calibrated by both methods. Results of both calibrations are in a reasonable agreement at the moderate values of the entrance angle. The nonideal effects caused by ionization of the gas target and asymmetric secondary electron exchange are considered.
Simple technique for the simultaneous measurements of the four-probe resistivity and the thermoelectric power
Authors: Kim G. T. , Park J. G. , Lee J. Y. , Yu H. Y. , Choi E. S. , Suh D. S. , Ha Y. S. , Park Y. W.
Published: July 2002

We propose simple configurations to measure the thermoelectric power and the four-probe resistivity simultaneously for different types of samples.
Optimized heavy ion beam probing for International Thermonuclear Experimental Reactor
Authors: Melnikov A. V. , Eliseev L. G.
Published: July 2002

The international workgroup developed the conceptual design of a heavy ion beam probe (HIBP) diagnostics for International Thermonuclear Experimental Reactor (ITER), which is intended for measurements of the plasma potential profile in a gradient area. Now we optimized it by the accurate analysis of the probing trajectories and variation of positions of the injection and detection points. Optimization allows us to reduce the energy of Tl+ beam from 5.6 to 3.4 MeV for standard ITER regime. The detector line starting at the plasma edge towards the center can get an outer part of the horizontal radial potential profile by variation of the energy. The observed radial interval is slightly increased up to 0.76<ρ<1 with respect to initial version 0.8<ρ<1, that allows to cover the region of the density gradient more reliably. Almost double reduction of the beam energy is a critical point. Thus we can significantly decrease the sizes of the accelerator and energy analyzer, the cost of the equipment, and impact of the diagnostics to the machine. Therefore the optimized HIBP design can be realized in ITER.
Characterization of a high-density large scale pulsed gas jet for laser–gas interaction experiments
Authors: Auguste T. , Bougeard M. , Caprin E. , D’Oliveira P. , Monot P.
Published: July 2002

We present the characteristics of a high-density pulsed gas jet developed to study the interaction of a short-pulse, high-intensity laser with a large scale underdense plasma. The jet was characterized by laser interferometry. Measurements of density profile were performed for a 5 mm diam cylindrical nozzle and for a 20×1 mm slit. In this latter case, a systematic study of the output density as a function of the backing pressure and of the valve opening duration is presented. At 1 mm from its outlet, the cylindrical nozzle provides densities up to 6.9×1019 cm−3 for an 8 bar backing pressure with a 3 mm full width at half maximum Gaussian-shaped profile. Under the same conditions, the slit produces a 20 mm long flat-top profile with a maximum density of 2.1×1019 cm−3.
How many wavelength channels do we need in Thomson scattering diagnostics?
Authors: Naito O. , Yoshida H. , Kitamura S. , Sakuma T. , Onose Y.
Published: July 2002

The necessary number of wavelength channels is investigated in incoherent Thomson scattering diagnostics. The measurement errors (σ) are evaluated for a realistic case and their lower limits (σ̃) are given in explicit forms. For a given number of wavelength channels, optimal channel allocation in the wavelength space is sought by minimizing the average relative errors. As a scale of the goodness of measurement, we introduce the ratio 〈σ̃/σ〉 averaged over the electron temperature of interest. This quantity ranges from 0 to 1 and tends to 1 as the lowest achievable error is approached. If we conclude that 〈σ̃/σ〉⩾0.9 is sufficient, then for a typical ruby laser system measuring 400–700 nm, seven channels is an adequate number for covering the electron temperature of 0.01–10 keV and five channels for 0.1–10 keV.
Self-induced phase gratings due to the inhomogeneous structure of acrylamide photopolymer systems used as holographic recording materials
Authors: Beléndez A. , Fimia A. , Carretero L. , Mateos F.
Published: July 2002

We report the observation of self-induced gratings or noise gratings in an acrylamide photopolymer for use in real time holography. The possibilities of this noise source as an optimization technique for this type of material are pointed out. Noise gratings in these polymer films were created upon exposure to a He–Ne laser collimated beam at 633 nm without any subsequent processing step. The influence of intensity on recording noise gratings and angular selectivity are reported showing its influence on the recording of this type of noise source in real time holographic materials.
Deep level defects in n-type GaN compensated with Mg
Authors: Yi Gyu-Chul , Wessels Bruce W.
Published: July 2002

Deep level defects in n-type epitaxial GaN compensated with Mg were measured using photocapacitance spectroscopy on Schottky barrier diodes. The doped GaN was prepared by atmospheric pressure metalorganic vapor phase epitaxy using bis(cyclopentadienyl)magnesium as the dopant source. The Mg-doped GaN films were n type as determined by Hall-effect measurements. Addition of magnesium resulted in the formation of a series of deep centers with optical threshold energies of 1.0, 1.2, 1.8, and 3.1 eV. Upon annealing the Mg compensated GaN in nitrogen at 850 °C the midgap levels disappeared and only the trapping level at 3.1 eV remained. The midgap levels are ascribed to Mg dopant complexes which may be responsible for low doping efficiency of Mg in the as-grown, doped GaN as well as its semi-insulating behavior.
Electroluminescence from erbium and oxygen coimplanted GaN
Authors: Torvik J. T. , Feuerstein R. J. , Pankove J. I. , Qiu C. H. , Namavar F.
Published: July 2002

Room temperature operation of erbium and oxygen coimplanted GaN m-i-n (metal–insulator–n-type) diodes is demonstrated. Erbium related electroluminescence at λ=1.54 μm was detected under reverse bias after a postimplant anneal at 800°C for 45 min in flowing NH3. The integrated light emission intensity showed a linear dependence on applied reverse drive current.
Localized micromagnetic perturbation of domain walls in magnetite using a magnetic force microscope
Authors: Foss Sheryl , Proksch Roger , Dahlberg E. Dan , Moskowitz Bruce , Walsh Brian
Published: July 2002

Magnetic force microscope (MFM) profiles of domain walls (DWs) in magnetite were measured using commercially available MFM tips. Opposite polarity profiles of a single DW segment were obtained by magnetizing the MFM tip in opposite directions perpendicular to the sample surface. During a measurement, the field of the tip locally magnetized the DW, resulting in a more attractive tip-sample interaction. The difference between opposite polarity DW profiles provided a qualitative measurement of the reversible changes in DW structure due to the localized field of the MFM tip.
Composites with extremal thermal expansion coefficients
Authors: Sigmund O. , Torquato S.
Published: July 2002

We design three-phase composites having maximum thermal expansion, zero thermal expansion, or negative thermal expansion using a numerical topology optimization method. It is shown that composites with effective negative thermal expansion can be obtained by mixing two phases of positive thermal expansions with a void phase. We also show that there is no mechanistic relationship between negative thermal expansion and negative Poisson’s ratio.
The Meyer–Neldel rule in chalcogenide glasses
Authors: Shimakawa K. , Abdel-Wahab F.
Published: July 2002

The electrical conductivity in chalcogenide glasses is found to obey the Meyer–Neldel rule,σ=σ00eΔE/EMNe−ΔE/kT. The Meyer–Neldel characteristic energy, EMN, lies in the range 25–60 meV with σ00 in the range 10−5–10−15 S cm-1 which seems to be too small for band transport. A surprising correlation between σ00and EMN is also found.
Raman analysis of the configurational disorder in AlxGa1−xN films
Authors: Bergman Leah , Bremser Michael D. , Perry William G. , Davis Robert F. , Dutta Mitra , Nemanich Robert J.
Published: July 2002

Raman analysis of the E2 mode of AlxGa1−xN in the composition range 0⩽x⩽1 is presented. The line shape was observed to exhibit a significant asymmetry and broadening toward the high energy range. The spatial correlation model is discussed, and is shown to account for the line shape. The model calculations also indicate the lack of a long-range order in the chemical vapor deposition alloys. These results were confirmed by x-ray scattering: the relative intensity of the superlattice line was found to be negligible. The line broadening of the E2 mode was found to exhibit a maximum at a composition x≅0.5 indicative of a random disordered alloy system.
Low-field magnetoresistive properties of polycrystalline and epitaxial perovskite manganite films
Authors: Li X. W. , Gupta A. , Xiao Gang , Gong G. Q.
Published: July 2002

The low-field magnetoresistance (MR) properties of polycrystalline La0.67Sr0.33MnO3 and La0.67CaO33MnO3 thin films with different grain sizes have been investigated and compared with epitaxial films. MR as high as 15% has been observed in the polycrystalline films at a field of 1500 Oe at low temperatures, whereas the MR of the epitaxial films is less than 0.3% in the same field range. Based on the magnetization dependence of the MR, the current-voltage characteristics, and the temperature dependence of the resistivity, we attribute the low-field MR to spin-dependent scattering of polarized electrons at the grain boundaries which serve as pinning centers for the magnetic domain walls.
Stimulated emission and lasing in dye-doped organic thin films with Forster transfer
Authors: Berggren M. , Dodabalapur A. , Slusher R. E.
Published: July 2002

Optically pumped stimulated emission and lasing in thin films of an absorbing host 8-hydroxyquinolinato aluminum(Alq) doped with small amounts of the laser dye DCM II is observed. Forster transfer of the excitation from the Alq molecules to the DCM II molecules results in a high absorption coefficient at pump wavelength (337 nm) as well as low absorption loss at the emission wavelengths (610–650 nm).
Leakage current models of thin film silicon-on-insulator devices
Authors: Shin Hank , Hong Stella , Wetteroth Tom , Wilson Syd R. , Schroder Dieter K.
Published: July 2002

Thin film silicon-on-insulator (SOI) devices have an advantage of excellent isolation due to the buried oxide layer leading to reduced capacitance coupling and no latchup in complementary metal-oxide-silicon circuits compared with bulk silicon devices. Reduced junction area should lead to lower leakage for a given device. However, because of the buried oxide, stress is built up in the Si island during isolation processes, especially near the island edges, inducing new kinds of leakage currents, which are not observed in bulk silicon devices. This letter proposes five leakage current models of the partially depleted SOI devices, identifies their origins, and suggests methods to prevent each type.
Formation of self-assembled GaAs/AlGaAs quantum dots by low-temperature epitaxy
Authors: Lee Chae-Deok , Park Chanro , Lee Hwack Joo , Noh S. K. , Lee Kyu-Seok , Park Seong-Ju
Published: July 2002

We report the direct formation of self-assembled GaAs/AlGaAs quantum dots by low-temperature molecular beam epitaxy. To drive a three dimensional growth mode, the (1×1) AlGaAs surface was exposed alternately to the Ga and As sources. The resulting GaAs nanocrystals having {111} facets were clearly identified by high-resolution transmission electron microscopy. The emission spectra also confirmed the formation of dots. The transition to a three-dimensional growth mode is attributed to the limited surface migration of Ga adatoms on the AlGaAs surface, which has excess As at low substrate temperature.
Proton bombardment-induced electron traps in epitaxially grown n-GaN
Authors: Auret F. D. , Goodman S. A. , Koschnick F. K. , Spaeth J.-M. , Beaumont B. , Gibart P.
Published: July 2002

Using deep-level transient spectroscopy, we have studied the electrical properties of defects introduced in epitaxially grown n-GaN during 2-MeV proton bombardment. The main defects detected, ER2 and ER3, are introduced at rates of 400±150 and 600±100 cm−1, respectively, and have energy levels at 0.16±0.03 and 0.20±0.01 eV, respectively, below the conduction band. A less prominent defect, ER1, with an energy level at 0.13±0.01 eV below the conduction band, is introduced at a rate of 30±10 cm−1. The small capture cross section of ER3 [(8±4)×10−18 cm2] implies that it is in a neutral or negative state when above the Fermi level.
Photoluminescence study of CdTe/ZnTe self-assembled quantum dots
Authors: Karczewski G. , Maćkowski S. , Kutrowski M. , Wojtowicz T. , Kossut J.
Published: July 2002

We report on optical properties of CdTe self-assembled quantum dots (SADs) grown by molecular beam epitaxy on ZnTe. Formation of SADs was achieved by deposition of 1.5–2.5 monolayers of CdTe at a substrate temperature of 420 °C and by applying growth interrupts for few seconds in Cd flux. The resulting dots have a typical diameter of 2 nm and a sheet density of 1012 cm−2. At T=2 K the photoluminescence (PL) spectra consist of two emission lines. The high-energy line originates from excitonic recombination in a wetting layer while the low-energy emission PL band is assigned to recombination in SADs. The increase in temperature up to 70 K does not affect the SADs-related emission intensity. It shifts, however, the PL peak energy towards low energies and causes a significant narrowing of the PL linewidth, from 80 meV at 1.9 K to 50 meV at 130 K. The activation energy of the thermal quenching of SADs-related PL emission was found to be equal to 47 meV. This value is three times greater than the one observed in CdTe/ZnTe quantum wells, that is, 12–17 meV.
High-sensitivity quantitative Kelvin probe microscopy by noncontact ultra-high-vacuum atomic force microscopy
Authors: Sommerhalter Ch. , Matthes Th. W. , Glatzel Th. , Jäger-Waldau A. , Lux-Steiner M. Ch.
Published: July 2002

We present quantitative measurements of the work function of semiconductor and metal surfaces prepared in ultrahigh vacuum (UHV) using a combination of UHV noncontact atomic force microscopy and Kelvin probe force microscopy. High energetic and lateral resolution is achieved by using the second resonance frequency of the cantilever to measure the electrostatic forces, while the first resonance frequency is used to simultaneously obtain topographic images by the frequency modulation technique. Spatially resolved work-function measurements reveal a reduced work function in the vicinity of steps on highly oriented pyrolytic graphite. On the GaAs(110) surface it could be demonstrated that defect states in the forbidden band gap cause a local pinning of the Fermi level along monolayer steps. On p-WSe2(0001) work-function variations due to the Coulomb potential of single dopant sites were resolved.
The mechanism of micropipe nucleation at inclusions in silicon carbide
Authors: Dudley M. , Huang X. R. , Huang W. , Powell A. , Wang S. , Neudeck P. , Skowronski M.
Published: July 2002

A model is presented for a possible mechanism of screw dislocation (including micropipe) nucleation in silicon carbide. The model is based on the observation of micropipe nucleation at the sites of foreign material inclusions using synchrotron white beam x-ray topography and transmission optical microscopy. It is shown that incorporation of the inclusion into the growing crystal can lead to deformation of the protruding ledge which constitutes the overgrowing layer. Accommodation of this deformation into the crystal lattice leads to the production of pairs of opposite sign screw dislocations which then propagate with the growing crystal. Evidence for the existence of such pairs of dislocations is presented.
Determination of single-pass optical gain and internal loss using a multisection device
Authors: Thomson J. D. , Summers H. D. , Hulyer P. J. , Smowton P. M. , Blood P.
Published: July 2002

We describe a technique for the measurement of optical gain and loss in semiconductor lasers using a single, multisection device. The method provides a complete description of the gain spectrum in absolute units and over a wide current range. Comparison of the transverse electric and transverse magnetic polarized spectra also provides the quasi-Fermi-level energy separation. Measurements on AlGaInP quantum well laser structures with emission wavelengths close to 670 nm show an internal loss of 10 cm−1 and peak gain values up to 4000 cm−1 for current densities up to 4 kA cm−2.
Optical resonance modes in InGaN/GaN multiple-quantum-well microring cavities
Authors: Zeng K. C. , Dai L. , Lin J. Y. , Jiang H. X.
Published: July 2002

Microrings of varying sizes have been fabricated from InxGa1−xN/GaN (x∼0.15) multiple quantum wells (MQWs). Photolithography and dry etching techniques including both ion-beam and inductively coupled plasma etching were employed to pattern the III–nitride MQW microrings. Individual microrings were optically pumped and optical resonance modes were observed. The observed mode spacings were consistent with those expected for whispering-gallery (WG) modes within a resonant cavity of cylindrical symmetry, refractive index, and dimensions of the rings under investigation. The results obtained from the microring cavities were compared with those of the III–nitride MQW microdisk cavities. Our results have indicated that resonance modes corresponding to the radial and the WG modes are simultaneously present in microdisk cavities, but only WG modes are available from the microring cavities. Implications of our results on future GaN-based microcavity light emitters have been discussed.
Intra- versus intergranular low-field magnetoresistance of Sr2FeMoO6 thin films
Authors: Yin H. Q. , Zhou J.-S. , Zhou J.-P. , Dass R. , McDevitt J. T. , Goodenough John B.
Published: July 2002

Thin films of (001)-oriented Sr2FeMoO6 have been epitaxially deposited on LaAlO3 and SrTiO3 (001) substrates. Comparison of their transport and magnetic properties with those of polycrystalline ceramic samples shows a metallic versus semiconductor temperature dependence and a saturation magnetization Ms at 10 K of 3.2 μB/f.u. in the film as against 3.0 for a tetragonal polycrystalline sample. However, the Curie temperature TC≈389 K is reduced from 415 K found for the tetragonal ceramic, which lowers Ms at 300 K in the thin films to 2.0 μB/f.u. compared to 2.2 μB/f.u. in the ceramics. A Wheatstone bridge arrangement straddling a bicrystal boundary has been used to verify that spin-dependent electron transfer through a grain boundary is responsible for the low-field magnetoresistance found in polycrystalline samples below TC.
Nanoscale investigation of domain retention in preferentially oriented PbZr0.53Ti0.47O3 thin films on Pt and on LaNiO3
Authors: Hong J. W. , Jo W. , Kim D. C. , Cho S. M. , Nam H. J. , Lee H. M. , Bu J. U.
Published: July 2002

We report results on domain retention in preferentially oriented PbZr0.53Ti0.47O3 (PZT) thin films on Pt and on LaNiO3 (LNO) electrodes. Domain images are obtained by detecting an electrostatic force exerted on the biased conductive probe. We demonstrate that polarization loss of PZT domains on LNO electrodes occurs less under no external field rather than that of PZT on Pt. The time dependence of the remnant polarization is found to follow a stretched exponential decay.
Influence of crystal structure and oxygen content on exchange-coupling properties of IrMn/CoFe spin-valve films
Authors: Fuke Hiromi N. , Saito Kazuhiro , Yoshikawa Masatoshi , Iwasaki Hitoshi , Sahashi Masashi
Published: July 2002

Investigations were made into the relationship between the exchange-coupling properties and the IrMn crystal grain size for CoFe/IrMn spin valves. The IrMn crystal grain size increased with increasing thickness and decreasing oxygen content of the IrMn layer. The increase in the IrMn grain size resulted in a higher blocking temperature (TB) but a lower unidirectional anisotropy field (HUA). This opposite dependence of TB and HUA on the IrMn grain size is different from those of other antiferromagnetic films such as FeMn, NiMn, and CrMnPt. An HUA of 400 Oe at room temperature and a TB of 300 °C are obtained for CoFe (2 nm)/IrMn (10 nm) spin valves by controlling the oxygen content of the IrMn layer.
Nanoscopic switching behavior of epitaxial SrBi2Ta2O9 films deposited by pulsed laser deposition
Authors: Gruverman A. , Pignolet A. , Satyalakshmi K. M. , Alexe M. , Zakharov N. D. , Hesse D.
Published: July 2002

We report results on scanning force microscopy (SFM) studies of epitaxial SrBi2Ta2O9 films which, in conjunction with complementary x-ray diffraction, scanning and transmission electron microscopy data, allow us to establish direct correlation between the crystallographic structure at the submicrometer range and the nano- and macroscopic switching behavior of the films. SFM topographic analysis of the films revealed a high degree of inhomogeneity at the submicrometer level: a number of rectangular and spherical grains protruding out of the flat surface. It has been found that the ferroelectric behavior of the films is primarily due to the (110) and (100)-oriented grains, while a flat background is c oriented and therefore is not switchable. Remanent polarization values obtained using SFM data were consistent with the results of the macroscopic hysteresis loop measurements.
Control of domain structures and magnetotransport properties in patterned ferromagnetic wires
Authors: Taniyama T. , Nakatani I. , Yakabe T. , Yamazaki Y.
Published: July 2002

Observations of controlled domain structures in zigzag patterned cobalt wires are demonstrated. Distinct domain structures are accessible by changing the orientation of magnetic field using the zigzag geometry, which provides a prospective potential to design the desired magnetic structures. Utilizing the subtle technique, we further throw light on the issue of magnetoresistance induced by the different domain structures. It is found that a negative contribution to the resistance is due to the spin configuration around the corner of the zigzag wires and shows an anomalous maximum around 100 K.
High-efficiency organic polymer light-emitting heterostructure devices on flexible plastic substrates
Authors: He Yi , Kanicki Jerzy
Published: July 2002

In this letter, we describe a high-efficiency organic polymer light-emitting heterostructure device with aluminum cathode fabricated on a thin, flexible plastic substrate. The device consists of a hole transporting (amine-fluorene) and an emissive (benzothiadiazole-fluorene) conjugated organic polymer layers. The best heterostructure device has a green light emission and a maximum luminance higher than 2000 cd/m2. Device shows a maximum emission of ∼56.2 cd/A and, accordingly, a maximum luminous and external quantum efficiency of ∼9.0 lm/W and ∼15%, respectively. This organic light-emitting diode performance is acceptable for flat panel display applications.
Propagation of femtosecond optical pulses through uncoated and metal-coated near-field fiber probes
Authors: Müller Roland , Lienau Christoph
Published: July 2002

The spatiotemporal evolution of a 10-femtosecond light pulse (λ=805 nm) propagating through uncoated and metal-coated near-field fiber probes is analyzed theoretically within a two-dimensional model for s and p polarization of the incident field. Internal reflection inside uncoated fiber probes (cone angle of 28°) results in an efficient guiding towards the fiber tip and a diffraction-limited spatial resolution of about 260 nm≈λ/3 in case of s polarization. While the transmission through uncoated fiber probes has negligible effects on the temporal and spectral pulse profile, strong modifications are observed for metal-coated aperture probes. The wavelength-dependent aperture transmission gives rise to a pronounced blueshift and spectral narrowing of the transmitted pulses.
Acoustic stop-bands in periodically microtapered optical fibers
Authors: Diez A. , Kakarantzas G. , Birks T. A. , Russell P. St. J.
Published: July 2002

A one-dimensional grating microstructure was fabricated on a tapered optical fiber and its acoustic properties experimentally studied. The structure consisted of a number of concatenated and equally spaced microtapers, fabricated using a focused CO2 laser. Acoustic characterization of the device showed a strong frequency stop-band for flexural waves. Such structures are suitable as frequency-selective acoustic mirrors in acousto-optic fiber devices.
Thermal activation energies of Mg in GaN:Mg measured by the Hall effect and admittance spectroscopy
Authors: Kim D. J. , Ryu D. Y. , Bojarczuk N. A. , Karasinski J. , Guha S. , Lee S. H. , Lee J. H.
Published: July 2002

A series of GaN:Mg structures were grown in molecular beam epitaxy, using either one or two rf nitrogen sources, and in metalorganic chemical vapor deposition systems with varying Mg flux. Acceptor energies were measured using the Hall effect and admittance spectroscopy techniques. The acceptor energies were found to be different for the two methods, i.e., 135–155 meV for the Hall effect measurement and 80–115 meV for the admittance spectroscopy measurement. The apparently small acceptor energies from the admittance spectroscopy measurement were explained, through a simulation process, by the combined effects of (1) high Mg acceptor concentration with no other free carrier sources, and (2) the Mg emission kinetics assisted by the Frenkel–Poole field effect in the GaN:Mg structures.
Collective diffusion, self-diffusion and freezing criteria of colloidal suspensions
Authors: Banchio Adolfo J. , Nägele Gerhard , Bergenholtz Johan
Published: July 2002

In this paper, we examine collective and self-diffusion properties of dispersions of spherically shaped colloidal particles at intermediate and long times. Our analysis is based on a fully self-consistent (rescaled) mode coupling theory (MCT) adjusted to describe the overdamped dynamics in concentrated suspensions of neutral and charged colloidal particles. The dynamical quantities studied in dependence on various experimentally controllable system parameters are the particle mean-squared displacement, long-time collective and self-diffusion coefficients, dynamic structure factors, nonexponentiallity factors and collective and self-memory functions. The results of our theoretical treatment are compared with Brownian dynamics computer simulation data, experiment and other existing theories. It is shown that the rescaled MCT can be successfully applied to a wide range of dynamical properties. Our calculations reveal in particular an exponential long-time mode of the dynamic structure factor for a limited range of wave numbers and at sufficiently high concentrations. A dynamic scaling behavior of the dynamic structure factor and self-intermediate scattering function is predicted for the important case of salt-free charge-stabilized suspensions. As a consequence of the dynamic scaling, the static freezing criterion for colloids by Hansen and Verlet [Phys. Rev. 184, 151 (1969)] is shown to be equivalent with the dynamic criterion by Löwen et al. [Phys. Rev. Lett. 70, 1557 (1993)] related to long-time self-diffusion.
Time step truncation error in direct simulation Monte Carlo
Authors: Garcia Alejandro L. , Wagner Wolfgang
Published: July 2002

The time step truncation error in direct simulation Monte Carlo calculations is found to be O(Δt2) for a variety of simple flows, both transient and steady state. The measured errors in the transport coefficients (viscosity, thermal conductivity, and self-diffusion) are in good agreement with predictions from Green-Kubo analysis [N. Hadjiconstantinou, Phys. Fluids 12, 2634 (2000)].
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unification model building II. Clebsch–Gordan coefficients of 78⊗78
Authors: Anderson Gregory W. , Blažek Tomáš
Published: July 2002

We have computed the Clebsch–Gordan coefficients for the product (000 001)⊗(000 001), where (000 001) is the adjoint 78-dimensional representation of E6. The results are presented for the dominant weights of the irreducible representations in this product. As a simple application we express the singlet operator in 27⊗78⊗27¯ in terms of multiplets of the Standard Model gauge group.
Helium-3 transport experiments in the scrape-off layer with the Alcator C-Mod omegatron ion mass spectrometer
Authors: Nachtrieb R. , LaBombard B.
Published: July 2002

He 3 gas was puffed from the wall into Ohmic low confinement-mode discharges of the Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1551 (1994)] tokamak and the charged states were measured near the wall with the omegatron ion mass spectrometer. Analysis of the data shows that the concentrations of singly- and doubly-ionized helium near the wall are approximately equal. The electron temperature and density at the omegatron are too low to account for ionization of helium in the local flux tube, therefore the helium is ionized in a hotter region of the edge plasma and is transported to the omegatron. A simple one-dimensional radial transport model reproduces the observed values of charge state flux and density, but only if rapid cross-field transport is included, increasing with distance from the separatrix. A constant cross-field diffusion coefficient of order 2 m2/s and an outward convection velocity profile increasing to of order 100 m/s in the far scrape-off layer is implied.
Curie–Weiss-type law for the strain and stress effects on the dielectric response of ferroelectric thin films
Authors: Pertsev N. A. , Koukhar V. G. , Waser R. , Hoffmann S.
Published: July 2002

Variations of the dielectric properties of ferroelectric thin films with the misfit strain Sm in the film/substrate system and the associated biaxial stress σ inside the film are analyzed theoretically. Calculations are performed in a mean-field approximation for the dielectric response displayed in a plate-capacitor setup by single crystalline films epitaxially grown on tensile substrates (Sm>0). It is shown that, in the absence of misfit-strain-induced phase transitions, the film dielectric susceptibility η33 obeys the Curie–Weiss-type law η33(Sm)=ε0KS/(Sm−Sm*), η33(σ)=ε0Kσ/(σ−σ*). Theoretical predictions are compared with the measured stress dependence of the dielectric response of polycrystalline Ba0.7Sr0.3TiO3 films grown on Pt/SiO2/Si. The theory explains the observed dielectric behavior and predicts the existence of in-plane polarization state in this film/substrate system.
Asymmetry of coercivity dependence on temperature in exchange-biased FeMn/Co bilayers
Authors: Wang J. , Wang W. N. , Chen X. , Zhao H. W. , Zhao J. G. , Zhan W. Sh.
Published: July 2002

The temperature dependence of the coercivity in biased FeMn/Co bilayers is investigated. An asymmetric behavior of the left and right coercivity with varying temperature is observed. The asymmetry can be understood by taking into account the variation of the spin arrangements in the antiferromagnetic layer. The calculated results are in agreement with the experimental ones qualitatively. All these results suggest that the magnetization reversal mechanisms are different for increasing and decreasing fields.
A density functional theory study of hydroxyl and the intermediate in the water formation reaction on Pt
Authors: Michaelides A. , Hu P.
Published: July 2002

Density functional theory has been used to study the adsorption of hydroxyl at low and high coverages and also to investigate the nature of the intermediate in the H2O formation reaction on Pt(111). At low coverages [1/9 of a monolayer (ML) to 1/3 ML] OH binds preferentially at bridge and top sites with a chemisorption energy of ∼2.25 eV. At high coverages (1/2 ML to 1 ML) H bonding between adjacent hydroxyls causes: (i) an enhancement in OH chemisorption energy by about 15%; (ii) a strong preference for OH adsorption at top sites; and (iii) the formation of OH networks. The activation energy for the diffusion of isolated OH groups along close packed rows of Pt atoms is 0.1 eV. This low barrier coupled with H bonding between neighboring OH groups indicates that hydroxyls are susceptible to island formation at low coverages. Pure OH as well as coadsorbed OH and H can be ruled out as the observed low temperature intermediate in the water formation reaction. Instead we suggest that the intermediate consists of a mixed OH+H2O overlayer with a macroscopic surface coverage of 3/4 ML in a 2:1 ratio of OH and H2O.
High-pressure Raman study on nanocrystalline CeO2
Authors: Rekhi Sandeep , Saxena S. K. , Lazor Peter
Published: July 2002

To evaluate the reported change in the physical and electronic properties of nanomaterials as compared to bulk material, we performed high-pressure Raman spectroscopy on nanocrystalline CeO2 at room temperature up to a pressure of 36 gigapascals (GPa) with and without pressure medium. We observed a linear dependence of the first order Raman signal on pressure. The compression and decompression follow the same path under hydrostatic conditions but show hysteresis (∼20 R cm−1) under nonhydrostatic compression and decompression. We found that fluorite type CeO2 undergoes a phase transition to an orthorhombic PbCl2 type structure at a pressure around 26.5 GPa, which is less than that reported for bulk CeO2 (32 GPa). On decompression, we found that the high-pressure phase can exist down to a pressure of 1.8 GPa. The results obtained on compression of sample with CsCl as the pressure medium were compared to that compressed without any medium.
Ar
2
excimer emission from a laser-heated plasma in a high-pressure argon gas
Authors: Takahashi A. , Okada T. , Hiyama T. , Maeda M. , Uchino K. , Nohdomi R. , Mizoguchi H.
Published: July 2002

We investigated a pump scheme to establish a practical Ar2 excimer laser operating at 128 nm. In this scheme, electrons generated by preionization of Ar gas at high pressure were heated by intense pulsed CO2 laser radiation. The resultant high-density plasma in the high-pressure Ar gas was used to create excited Ar dimers. In the experiment, a CO2 laser beam from a transversely excited atmospheric CO2 laser was focused on a narrow line, 100 mm in length, in Ar gas at a pressure of 2 MPa. A vacuum ultraviolet (VUV) signal was observed only when the Ar was preionized, and the VUV signals were identified as the emission of Ar2 excimers from the features of the spectrum which was centered on 128 nm.
In-plane polarization states and their instabilities in polydomain epitaxial ferroelectric thin films
Authors: Koukhar V. G. , Pertsev N. A. , Waser R.
Published: July 2002

A Landau–Ginsburg–Devonshire-type thermodynamic theory is used to describe epitaxial ferroelectric films with dense laminar domain structures, where spontaneous polarizations in both domains are oriented in the film plane. For PbTiO3 films, it is found that, at positive misfit strains, the domain patterning transforms the orthorhombic aa phase, which is stable in single-domain films, into a pseudotetragonal a1/a2/a1/a2 polydomain state. In BaTiO3 films, the latter is further replaced at low temperatures by a polydomain aa1/aa2/aa1/aa2 state. With decreasing misfit strain, both a1/a2/a1/a2 and aa1/aa2/aa1/aa2 patterns become unstable with respect to the appearance of polarization component orthogonal to film surfaces. This results in the formation of polydomain states in BaTiO3 films, which do not exist in bulk crystals. The stability ranges of various states in the misfit strain-temperature phase diagram are determined.
Temperature invariant lasing and gain spectra in self-assembled GaInAs quantum wire Fabry–Perot lasers
Authors: Wohlert D. E. , Cheng K. Y. , Chou S. T.
Published: July 2002

GaInAs quantum wire (QWR) heterostructures have been grown by molecular beam epitaxy using the strain-induced lateral-layer ordering (SILO) process. Broad-area Fabry–Perot QWR lasers have been fabricated from this material. The lasing wavelength from the QWR laser shifts at a rate of 0.9 Å/°C between 77 and 300 K compared to 4.6 Å/°C for a quantum well laser control sample. Furthermore, the gain spectra of the QWR laser are derived from the amplified spontaneous emission spectra at 77 and 300 K using the Hakki–Paoli method. The gain peak is also stabilized against temperature changes indicating that temperature stable lasing behavior seen in SILO grown GaInAs QWR Fabry–Perot laser diodes is due to a temperature stable band gap.
Influence of tip modulation on image formation in scanning near-field optical microscopy
Authors: Walford J. N. , Porto J. A. , Carminati R. , Greffet J.-J. , Adam P. M. , Hudlet S. , Bijeon J.-L. , Stashkevich A. , Royer P.
Published: July 2002

Modulation of the probe height in a scanning near-field optical microscope (SNOM) is a technique that is commonly used for both distance control and separation of the near-field signal from a background. Detection of higher harmonic modulated signals has also been used to obtain an improvement in resolution, the elimination of background, or artifacts in the signal. This article presents a theoretical model for the effects induced in SNOM images by modulation of the probe. It is shown that probe modulation introduces a spatial filter into the image, generally suppressing propagating field components and enhancing the strength of evanescent field components. A simple example of detection of a single evanescent field above a prism is studied in some detail, and a complicated dependence on modulation parameters and waveform is shown. Some aspects of the application of this theory in a general experimental situation are discussed. Simulated images are displayed to explicitly show the effects of varying modulation amplitude with first and second harmonic detection. Finally, we discuss the suppression of background artifacts due to propagating fields through the use of higher harmonic detection.
Turbulence intermittency and burst properties in tokamak scrape-off layer
Authors: Antar G. Y. , Devynck P. , Garbet X. , Luckhardt S. C.
Published: July 2002

Density fluctuation measured by a reciprocating Langmuir probe on the Tore Supra tokamak [Garbet et al., Nucl. Fusion 32, 2147 (1992)] is investigated. The purpose of this article is to give a rather comprehensive analysis of intermittency by using several data analyses to compare the bursts properties to that of coherent structures and avalanches. The probability distribution of the density fluctuations is found positively skewed, while a Gaussian shape for the negative values is recorded. It is shown that the fluctuation spectra possess one scaling region with a power law close to the one predicted by a Kolmogorov–Kraichnan model in the inverse cascade subrange. However, a net deviation from this law at higher moment orders is demonstrated. The deviation from the mono-fractal model is investigated by the multifractal analysis that reveals the variety of the dissipative structures similar to what is found in fully developed fluid turbulence. The spectra are found asymmetric, indicating the presence of structures not generated by a multiplicative process. Using conditional analysis, a detailed study of the intermittent bursts independently of the background is performed. The typical form of the intermittent structures is asymmetric. Furthermore, they do not conserve mass for only positive density fluctuations are recorded. Their poloidal velocity is estimated to be 70% greater than the background turbulence, suggesting that they may not result from a diffusive process.
Divergence behavior due to surface channeling in capillary optics
Authors: Cappuccio G. , Dabagov S. B. , Gramaccioni C. , Pifferi A.
Published: July 2002

Recent studies on the transmitting and focusing properties of capillary optical systems have shown that several unexpected effects take place during the experiments. One such effect is a decrease in the beam divergence behind the capillary structures. In this letter, we present results on x-ray scattering at grazing angles inside capillaries. During x-ray propagation in capillary channels, there is a strong angular redistribution of the beam, which has been explained in the framework of the wave scattering theory.
Band-gap engineering in sputter-deposited ScxGa1−xN
Authors: Little M. E. , Kordesch M. E.
Published: July 2002

Reactive sputtering was used to grow thin films of ScxGa1−xN with scandium concentration of 20%–70% on quartz substrates at temperatures of 300–675 K. X-ray diffraction (XRD) of the films showed either weak or no structure, suggesting the films are amorphous or microcrystalline. Optical absorption spectra were taken of each sample and the optical band gap was determined. The band gap varied linearly with composition between 2.0 and 3.5 eV. ScN and GaN have different crystal structures (rocksalt and wurzite, respectively), and thus may form a heterogeneous mixture as opposed to an alloy. Since the XRD data were inconclusive, bilayers of ScN/GaN were grown and optical absorption spectra taken. A fundamental difference in the spectra between the bilayer films and alloy films was seen, suggesting the films are alloys not heterogeneous mixtures.
Micromechanical properties of silicon-carbide thin films deposited using single-source chemical-vapor deposition
Authors: Stoldt C. R. , Fritz M. C. , Carraro C. , Maboudian R.
Published: July 2002

1,3-Disilabutane is used as a single-source precursor to deposit conformal silicon-carbide films on silicon atomic-force-microscopy cantilevers. By measuring the resonance frequency of the cantilever as a function of silicon-carbide film thickness and developing an appropriate model, the value of the film’s elastic modulus is determined. This value is in good agreement with those reported for silicon-carbide films deposited using conventional dual-source chemical-vapor deposition. Additionally, we comment on the feasibility of integrating this process into the fabrication technology for microelectromechanical systems.
Oxygen content dependence of the transport property of La2/3Sr1/3CoO3−δ film
Authors: Sun J. R. , Yeung H. W. , Li H. , Zhao K. , Chan H. N. , Wong H. K.
Published: July 2002

Effects of oxygen content on the resistivity of epitaxial (110)La2/3Sr1/3CoO3−δ film have been experimentally studied. The oxygen release, measured by the change of lattice parameter, was controlled by annealing the film at different temperatures in vacuum. Significant oxygen loss can take place at temperature as low as ∼200 °C. A quantitative relation between resistivity ρ and oxygen deficiency δ was established. For δ<0.07, a linear decrease of the resistive transition temperature with δ was found. The temperature dependence of the low temperature resistivity follows ρ∝T2 for δ<0.046, and ρ∝exp[(T0/T)1/2] for 0.067<δ<0.16. A transition from T1/2 variable-range hopping to T1/4 variable-range hopping takes place for δ>0.17, which may be a result of the absence of Co4+ which are the nuclei of metallic clusters. Compared to La1−xSrxCoO3 with similar Co4+ content, the resistivity of the La2/3Sr1/3CoO3−δ film is significantly low, which may be the consequence of a larger quantity of Sr in the film.
Forward current propagation beyond the virtual cathode formed by a high injection current
Authors: Uhm Han S. , Choi Eun H. , Choi Myung C.
Published: July 2002

A virtual cathode is formed in a cavity, if the injection current from the diode exceeds the critical current. The propagation properties of the forward current beyond the virtual cathode are investigated. It is shown that the forward current increases significantly as the diode gap distance LCA increases, although the injection current from the diode decreases drastically. Experimental data agree well with theoretical results obtained analytically.