bluuebunny/crossref_metadata_2025 huggingface dataset:
Calibration of a Fenn-type nozzle beam source
Authors: Weaver Bradley D. , Frankl D. R.
Published: July 2002

Calibration of a Fenn-type nozzle beam source and the limitations due to background attenuation, skimmer interference, and condensation are discussed. The nozzle flow rate Nn is calculated, and the peaking factor κ is determined from both radial pressure surveys and effusive-to-supersonic transition measurements. Stage pressure measurements verify both Nn and κ. These quantities specify the ideal beam flux in the absence of attenuation, interference, or condensation. Background attentuation depends on the effective scattering cross section, which can be quite large for finely collimated beams. Serious skimmer interference occurs below a critical value of the skimmer Knudsen number and depends on individual skimmer details. Condensation is observed and found to be predictable according to the known scaling laws. A calculation of absolute beam fluxes is presented. Nonideal behavior of the speed ratio and average particle velocity are also examined. Data are given for the gases H2, He, Ne, and Ar.
Compact pulsed molecular beam system for real-time reactive scattering from solid surfaces
Authors: Eldridge Benjamin N. , Yu Ming L.
Published: July 2002

We describe a compact system providing the capability of striking a sample surface with a short pulse of reactive gas in an UHV environment and monitoring the evolution of ensuing volatile reaction products. A 300-Hz rotating disk chopper synchronized with a pulsed valve produces molecular beam pulses of 12-μs FWHM at the chopper position. Only one stage of differential pumping separates the pulsed valve chamber from the target chamber with the latter under UHV conditions. The source of the molecular beam, comprised of the chambers containing the pulsed valve and the rotating disk chopper, occupies less than one square meter of floor space. Beam characterization is accomplished by the use of two UHV compatible fast ion gauges located in the target chamber. Reaction products are monitored with microsecond resolution via a differentially pumped quadrupole mass spectrometer located at right angles to the beam.
Characteristics of a thermal gas flowmeter
Authors: Komiya K. , Higuchi F. , Ohtani K.
Published: July 2002

The characteristics of a thermo tube type thermal gas flowmeter were analyzed and experimental results were described. The output of this flowmeter is temperature difference between two points along an electrically heated capillary. An analytical expression of its outputs was derived assuming a simple flow condition. An experimental prototype was constructed. The temperature distribution was measured and the outputs were obtained for three kinds of gases: nitrogen, carbon dioxide, and argon. Those outputs were compared with theoretical ones, and those agree well in the case of small flow rate. The relationship among temperature difference, mass flow rate, and specific heat of gas was discussed.
Temperature anomalies observed in liquid 4He columns in magnetic fields with field–field-gradient products >21 T2/cm
Authors: McNiff E. J. , Brandt B. L. , Foner S. , Rubin L. G. , Weggel R. J.
Published: July 2002

Previously reported temperature increases above 4.2 K in an immersion geometry in liquid 4He at fields >19 T have been verified and examined. Magnetic forces (B dB/dx) above a threshold level of ∼21 T2/cm were found to trap 4He bubbles near the center of the field. Significant heat input to the region (from conduction, radiation, etc.) can then raise the temperature by as much as several kelvins. Thus, measurements at very high B dB/dx should not depend on vapor pressure thermometry. To assure temperature accuracy and stability, the temperature should be measured directly at the sample position.
Hot wall epitaxy of C60 thin films on mica
Authors: Stifter D. , Sitter H.
Published: July 2002

Using high resolution x-ray diffraction, it is shown that hot wall epitaxy is an appropriate growth technique in order to obtain perfect monocrystalline C60 thin films with a thickness up to 150 nm. The full width at half-maximum of rocking curves of the C60 (111) reflex measured on such films is about 210 arcsec. Rocking curves of thicker films exhibit a complex shape, which is interpreted as a result of a change in the growth mode of C60 films exceeding a critical thickness.
Modulated-bath calorimetry
Authors: Graebner J. E.
Published: July 2002

A new high-resolution ac calorimetric technique is described for measuring the absolute heat capacity of extremely small samples. The method uses a thermocouple as the weak link to the bath and modulates the temperature of the bath sinusoidally in time. This eliminates the need for a separate thermometer and heater on the sample while retaining the ability to make absolute measurements with minimal addenda. Results for a 2.9 μg sample of the superconductor Ba0.6K0.4BiO3 in a magnetic field of 60 kOe show a jump at Tc=21 K, which is above the noise level of ∼2 parts in 104.
Direct extraction of negative lithium ions from a lithium plasma
Authors: Wada Motoi , Tsuda Hiroshi , Sasao Mamiko
Published: July 2002

Negative lithium ions (Li−) were directly extracted from a lithium plasma in a multiline cusp plasma container. A pair of permanent magnets mounted near the extractor electrode created the filter magnetic field that separated the extraction region plasma from the main discharge plasma. The plasma electrode facing the extraction region plasma was biased with respect to the other parts of the chamber wall, which acted as discharge anodes. The larger filter magnetic field resulted larger Li− current. When the bias to the plasma electrode was several volts positive against the anode potential, extracted Li− current took the maximum for a fixed strength of the filter field. These dependences of Li− upon the filter magnetic field and the plasma electrode bias are similar to the ones of negative hydrogen ions.
Magnetic levitation of condensed hydrogen
Authors: Paine C. G. , Seidel G. M.
Published: July 2002

Liquid and solid molecular hydrogen has been levitated using a pair of small superconducting solenoids. The hydrogen samples, up to 3 mm in dimension, were trapped in a magnetic potential having either a discrete minimum or a minimum in the form of a ring 1 cm in diameter. The hydrogen could be moved about in the magnetic trap by applying an electric field.
An imaging plate system for high-pressure powder diffraction: The data processing side
Authors: Piltz R. O. , McMahon M. I. , Crain J. , Hatton P. D. , Nelmes R. J. , Cernik R. J. , Bushnell-Wye G.
Published: July 2002

The use of an imaging plate as a two-dimensional (2-D) detector removes many of the difficulties that arise in performing angle-dispersive powder diffraction at high pressures in a diamond-anvil cell. Due to the 2-D nature of the imaging plate, a substantial part of each Debye Scherrer ring is intercepted and recorded. The averaging of the intensities around a ring so as to create a conventional one-dimensional (1-D) powder pattern results in a significant improvement in counting statistics and powder averaging, both severe problems in high-pressure diffraction due to the very small sample volumes involved. For an accurately known plate geometry the 2-D to 1-D conversion is straightforward; however, considerable complications arise when inaccuracies in plate to sample distance, plate orientations, poor powder averaging/preferred orientation, and the presence of diamond Bragg spots are considered. The current status of the software used to analyze the imaging plate data is presented along with test data to illustrate its use.
Far-infrared emission spectroscopy of hot two-dimensional plasmons in Al0.3Ga0.7As/GaAs heterojunctions
Authors: Hirakawa K. , Yamanaka K. , Grayson M. , Tsui D. C.
Published: July 2002

We have investigated the radiative decay of hot two-dimensional (2D) plasmons in Al0.3Ga0.7As/ GaAs heterostructures by far-infrared emission spectroscopy and determined the spectral line shape of the radiation. Narrowband plasmon emission lines are obtained in the terahertz regime. The experimentally observed energies of plasmon emission are in good agreement with the results of a recently developed full grating theory. The plasmon emission intensity is found to increase with increasing input electrical power to the electron system and follows the Bose–Einstein distribution law characterized by the electron temperature of the hot 2D electron system. This fact indicates that 2D plasmons are thermally excited in the present experimental regime.
Photoluminescence and microstructure of self-ordered grown SiGe/Si quantum wires
Authors: Hartmann A. , Dieker C. , Loo R. , Vescan L. , Lüth H. , Bangert U.
Published: July 2002

Employing self-ordered growth in convex corners of nonplanar Si substrates, SiGe quantum wires of approximately 30 nm lateral dimension were fabricated. Photoluminescence spectra of these structures are dominated by transitions originating from the quantum wires at measurement temperatures above 20 K. The energetic positions of the quantum wire transitions are in good agreement with Ge concentrations measured by spatially resolved energy dispersive x-ray spectroscopy using a scanning transmission electron microscope. We find that the Ge concentration inside the wire is considerably lower than the nominal value for growth on planar parts of the substrate. In addition we find a pronounced gradient in the Ge concentration of the wire.
A laser diode system stabilized on the Caesium D2 line
Authors: Rovera G. D. , Santarelli G. , Clairon A.
Published: July 2002

We describe a complete diode laser system stabilized at 852 nm on the D2 line of Cs with a linewidth narrower than 100 kHz. This system is intended to be employed for optical pumping, cooling, and detection of Cs atoms in atomic frequency standards. The square root of the Allan variance, evaluated by measuring the beat-note of two similar systems, is better than 1×10−12 for τ up to 1000 s. A month of continuous locked operation has been observed.
A high-temperature Kelvin probe for flow reactor studies
Authors: Lundgren S. , Kasemo B.
Published: July 2002

A vibrating capacitor device (Kelvin probe) has been developed for measurements of work-function changes ΔΦ during ongoing catalytic reactions up to high temperatures (750 K) and atmospheric pressure. The construction is based on the piezoelectric actuator principle, and utilizes a lever arrangement that transfers the vibrating motion of the actuator to the reference electrode. The lever arrangement eliminates a static load on the actuator, which therefore allows a large distance between the actuator and the vibrating reference electrode. This makes possible much higher temperatures in the reaction/sample zone than the piezoelectric actuator itself permits. The large actuator–electrode distance also reduces the risk of electromagnetic pickup in the ΔΦ measurements, from the driving circuit for the piezoelectric actuator. The reference electrode is a gold disk with 3 mm diameter and a thickness of 0.1 mm. The practical ΔΦ resolution is presently about 5 meV. The device has been combined with on-line mass spectrometric detection of reaction rates and its performance is illustrated by results from the H2/O2 and CO/O2 reactions on Pt.
Monochromators for the undulator U49 at the BESSY II storage ring
Authors: Petersen H. , Senf F. , Schäfers F. , Bahrdt J.
Published: July 2002

Two types of monochromators will be built for use on the planned soft x-ray undulator U49 of the BESSY II storage ring: A plane grating monochromator using the plane grating focusing condition and a spherical grating monochromator using the spherical grating focusing condition. The optical layout of the instruments and calculated performance data, i.e., spectral resolution, grating efficiencies, and available photon flux are presented for the photon energy range available from the U49, 137–1516 eV in the first, third, and fifth harmonic.
Simultaneous measurements of small angle x-ray scattering, wide angle x-ray scattering, and light scattering during phase transitions in polymers (invited)
Authors: Wutz C. , Bark M. , Cronauer J. , Döhrmann R. , Zachmann H. G.
Published: July 2002

In order to obtain detailed information on structural changes in polymers during crystallization and melting, it is necessary to perform real-time measurements of the change in small angle x-ray scattering simultaneously with measurements of the change in wide angle x-ray scattering, light scattering and enthalpy (by differential scanning calorimetry). Different experimental setups to perform such experiments are described. Some examples demonstrate the variety of information that may be obtained by means of these setups.
Dual CO2 laser interferometer with a wavelength combination of 10.6 and 9.27 μm for electron density measurement on large tokamaks
Authors: Kawano Yasunori , Nagashima Akira , Hatae Takaki , Gunji Soichi
Published: July 2002

A new CO2 laser interferometer has been developed to measure the electron density of tokamak plasmas. Two different wavelength oscillators of 10.6 and 9.27 μm are utilized for simultaneous measurement of the density component and the optical path length change. A new technique using a common frequency shifter for two color lasers improves the stability of the system by a complete matching of both beat frequencies. The system provides advantages for practical problems of large tokmaks related to window darkening and large mechanical vibrations of reflection mirrors and offers improved laser beam monitoring and a simplified optical layout by using closer wavelengths. The electron density of JT-60U is successfully measured for a plasma current of up to 3 MA. The density behavior during a fast major disruption is also diagnosed without a fringe loss. An effective density resolution is observed to be 2×1019 m−2, which corresponds to about 1/10 of a fringe. A very high resolution (1/104 fringe) phase comparator has been developed to improve the density resolution of the dual CO2 system. The performance achieved shows the feasibility of the dual CO2 laser interferometer for future large devices such as the International Thermonuclear Experimental Reactor.
Hydrogen-radical-assisted radio-frequency plasma-enhanced chemical vapor deposition system for diamond formation
Authors: Hiramatsu Mineo , Inayoshi Muneto , Yamada Koji , Mizuno Etsuko , Nawata Masahito , Ikeda Masanobu , Hori Masaru , Goto Toshio
Published: July 2002

Diamond was successfully synthesized using an improved radio-frequency (rf) plasma-enhanced chemical vapor deposition system. In this system, conventional capacitively coupled parallel-plate rf (13.56 MHz) discharge plasma was assisted by a compact microwave (2.45 GHz) H2 plasma as a remote hydrogen radical source, and substrate heating was carried out using CO2 laser irradiation. Plasma control in rf discharge region for diamond formation was performed using the hydrogen radical source in this system. This was discussed with optical emission spectroscopy. The hydrogen radical source was improved. When water vapor was mixed to the microwave H2 plasma, namely, using water-vapor-enhanced hydrogen radical source, diamond films were grown at a low substrate temperature of 450 °C.
High-speed repeating hydrogen pellet injector for long-pulse magnetic confinement fusion experiments
Authors: Frattolillo A. , Migliori S. , Scaramuzzi F. , Combs S. K. , Baylor L. R. , Foust C. R. , Gouge M. J. , Milora S. L.
Published: July 2002

The projected fueling requirements of future magnetic confinement fusion devices [e.g., the International Thermonuclear Experimental Reactor (ITER)] indicate the need for a flexible plasma fueling capability, including both gas puffing and low- and high-speed pellet injection. Conventional injectors, based on single-stage pneumatic guns or centrifuges, can reliably provide frozen pellets (1- to 6-mm-diam sizes) at speeds up to 1.3 km/s and at suitable repetition rates (1 to 10 Hz or greater). Injectors based on two-stage pneumatic guns and ‘‘in situ’’ condensation of hydrogen pellets can reliably achieve velocities over 3 km/s; however, they are not suitable for long-pulse repetitive operations. An experiment in collaboration between Oak Ridge National Laboratory (ORNL) and ENEA Frascati is under way to demonstrate the feasibility of a high-speed (≳2 km/s) repeating (∼1 Hz) pneumatic pellet injector for long-pulse operation. A test facility has been assembled at ORNL, combining a Frascati repeating two-stage light-gas gun and an ORNL deuterium extruder, equipped with a pellet chambering mechanism/gun barrel assembly. The main issues to be investigated were the strength of extruded deuterium ice as opposed to that produced by in situ condensation in pipe guns (hence the highest acceleration which can be given to the pellet without fracturing it), and the maximum repetition rate at which the system can operate without degradation in performance. Pellet velocities of up to 2.55 km/s have been achieved in joint experiments at ORNL. A new pressure tailoring valve was developed by the Frascati group for this application and proved to be a crucial component for good performance. Tests carried out in repeating mode, at frequencies of 0.2–0.5 Hz and speeds up to 2.2 km/s, indicate no significant degradation in performance with increasing repetition rate. Some preliminary tests using 3.7 mm pellets gave very encouraging results. The equipment and the experimental results are described in this article.
Fast field-cycling nuclear magnetic resonance spectrometer
Authors: Job Constantin , Zajicek Jaroslav , Brown Michael F.
Published: July 2002

We describe here the design and construction of a modern, state-of-the-art nuclear magnetic resonance (NMR) field-cycling instrument. Fourier transform NMR spectra of both liquid and solid samples can be measured, and spin-lattice relaxation times (T1Z) investigated over a broad range of magnetic field strengths ranging from 0 to 2 T. The instrument is based upon an existing personal computer-based NMR spectrometer [C. Job, R. M. Pearson, and M. F. Brown, Rev. Sci. Instrum. 65, 3354 (1994)] which has been expanded into a fully computer-controlled field-cycling instrument. The magnetic field cycling is accomplished electronically by utilizing fast switching thyristors and a storage capacitor based on the Redfield energy storage concept. Unique aspects of the design include the field-cycling magnet, which can reach fields as high as 2 T; the personal computer-based NMR spectrometer and associated waveform electronics; and the use of a commercially available pulse width modulation switching current amplifier, having low internal power dissipation and a fast current settling time. Using this new technology T1Z relaxation times as short as 1 ms can be readily measured.
Apparatus for the study of Rayleigh–Bénard convection in gases under pressure
Authors: de Bruyn John R. , Bodenschatz Eberhard , Morris Stephen W. , Trainoff Steven P. , Hu Yuchou , Cannell David S. , Ahlers Guenter
Published: July 2002

We review the history of experimental work on Rayleigh–Bénard convection in gases, and then describe a modern apparatus that has been used in our experiments on gas convection. This system allows for the study of patterns in a cell with an aspect ratio (cell radius/fluid layer depth) as large as 100, with the cell thickness uniform to a fraction of a μm, and with the pressure controlled at the level of one part in 105. This level of control can yield a stability of the critical temperature difference for the convective onset of better than one part in 104. The convection patterns are visualized and the temperature field can be inferred using the shadowgraph technique. We describe the flow visualization and image processing necessary for this. Some interesting results obtained with the system are briefly summarized.
Neutral beam diagnostics for Alcator C-Mod
Authors: Rowan William L. , Bengtson Roger D. , Bravenec R. V. , He H. , Jagger J. , Patterson D. M. , Ross D. W. , Valanju P. M. , Wootton A. J. , Marmar E. S. , Irby J. H. , Snipes J. A. , Terry J. L.
Published: July 2002

A diagnostic neutral beam will be installed on Alcator C-Mod for measurement of ñi, nz, Tz, vθ, vφ, and j. The beam will be used primarily for the study of thermal transport, Er in H-mode discharges, and to provide critical profile and turbulence data for tests of theoretically based models of turbulence and transport. The beam selected for this work was used on Texas Experimental Tokamak and is rated at 50 kV and 6 A of extracted current in hydrogen. It can produce beams of deuterium or helium as well. It is currently being reconfigured for installation on C-Mod. Capabilities for the intended measurements are described using simulations based on measured C-Mod profiles.
Generalized hard x-ray detection model for microchannel plate detectors
Authors: Shikhaliev P. M.
Published: July 2002

A generalized hard x-ray detection model for microchannel plate (MCP) detectors has been developed. This model includes all geometrical and compositional parameters of an MCP and the energy of a detected x-ray. An analytical investigation and numerical calculation of the detection efficiency were carried out; data obtained were compared with experimental results. A square and hexagonal channel MCP were considered in addition to a traditional circular channel MCP. Moreover, it is shown that the first ones have advantages as x-ray detectors.
Paper I: Design and construction of a Knudsen-cell reactor for the study of heterogeneous reactions over the temperature range 130–750 K: Performances and limitations
Authors: Caloz François , Fenter Frederick F. , Tabor Kevin D. , Rossi Michel J.
Published: July 2002

A new low-pressure flow reactor operated as a Knudsen cell and intended for chemical kinetic studies is described. The reactor is specifically designed to study the kinetics of heterogeneous reactions. Gas-phase species are detected either by mass-spectrometric sampling or by in situ optical techniques, e.g., laser-induced fluorescence, resonantly enhanced multiphoton ionization. A feature of the reactor is its modular design, allowing full interchangeability of several sample holders at minimal effort, allowing the measurement of uptake coefficients ranging from 10−7 to 1.0. Sample supports operating at low and high temperatures have been developed which cover the stated temperature range. Several experimental examples of the utility of the reactor are detailed. The reliability and error bars of the kinetic results due to the errors and uncertainties associated with the experimental procedures are discussed, in particular for fast heterogeneous processes. It is found that even in the molecular flow regime, for fast reaction, the effects of diffusion limitations within the cell must be taken into account. This fact has been shown here from an experimental point of view. In a companion article the phenomena are studied using Monte Carlo simulation of the gas dynamics under molecular flow conditions.
A new method for the determination of thermal conductivity and thermal diffusivity from linear heat source measurements
Authors: Banaszkiewicz M. , Seiferlin K. , Spohn T. , Kargl G. , Kömle N.
Published: July 2002

The new algorithm of the thermal conductivity and thermal diffusivity determination from the transient hot-wire method has been applied to measurements performed in several solid materials. The algorithm makes use of the exact formula for the temperature variations, instead of its simple, asymptotic form that has been employed earlier. In the process of the least-square optimization of the residual function three parameters are obtained; thermal conductivity, thermal diffusivity, and the initial temperature. Two different variants of the method are presented: the classical one with the power kept constant during the measurements and the newly introduced constant current technique. The latter one has an advantage of requiring simpler conditioning electronics, and can therefore be recommended in space applications. The results of data processing show that thermal conductivity can be reliably determined even from the nonasymptotic part of the temperature measurements. The determination of thermal diffusivity is more difficult and requires high quality temperature data from the whole measurement interval.
Microwave-submillimeter wave double-resonance spectrometer for the investigation of van der Waals complexes
Authors: Markov Vladimir N. , Xu Yunjie , Jäger Wolfgang
Published: July 2002

A double-resonance spectrometer, suitable for the investigation of low lying intermolecular vibrational modes and of tunneling transitions of van der Waals complexes, is described. The instrument utilizes a pulsed molecular beam Fourier transform microwave spectrometer (frequency range 4–26 GHz) for signal detection, and a high frequency, phase stabilized, backward wave oscillator (258–375 GHz), as well as a millimeter wave synthesizer (78–118 GHz), as pump radiation sources. A brief description of the Fourier transform microwave spectrometer and of the phase stabilization system of the backward wave oscillator is given. The experimental arrangement is such that the molecular ensemble travels through the submillimeter radiation field prior to the pulsed excitation microwave experiment, in which the resulting population changes are detected. The sensitivity and the resolution capabilities of the new technique are illustrated with double resonance spectra of the van der Waals complexes Ar–CO and CO–N2.
Two-dimensional sampling-image x-ray streak camera for ultrafast imaging of inertial confinement fusion plasmas
Authors: Shiraga H. , Nakasuji M. , Heya M. , Miyanaga N.
Published: July 2002

Ultrafast two-dimensional (2D) x-ray imaging with a temporal resolution better than 10 ps is of great importance in diagnosing the final stages of the imploded core plasmas of inertial confinement fusion (ICF) targets. The multi-imaging x-ray streak camera (MIXS) has been one of such imaging techniques. Recently, we have proposed another scheme, a 2D sampling-image x-ray streak camera method (2D-SIXS). In this scheme, a 2D image is sampled two dimensionally with a set of sampling points distributed regularly over the whole image on a cathode plate of an x-ray streak camera. The sampled image is streaked, and then, reconstructed to form the time-resolved 2D images like movie pictures. In this article, we report results of our proof-of-principle experiments of 2D-SIXS scheme performed at Gekko-XII glass laser system. A gold-coated spherical target was irradiated by three beams (0.53 μm) of Gekko-XII laser. Streaked data of 2D-SIXS were obtained and a series of time-resolved 2D x-ray images were successfully reconstructed. 2D-SIXS is suitable for very fast, short-lived, and small x-ray sources such as a hot spark in the ICF plasma as well as short-pulse-laser-produced plasmas.
Digital quadrature detection in nuclear magnetic resonance spectroscopy
Authors: Gengying Li , Haibin Xie
Published: July 2002

A digital quadrature detection technique for the acquisition of the nuclear magnetic resonance signal is proposed. It is demonstrated with experimental results that the suggested method is superior to the conventional quadrature detection (QD) technique. The superiority arises mainly from the fact that only one channel of analog circuit is utilized and no spectral distortion due to unbalance between two channels used by the QD occurs. As a result, no phase cycling for canceling the distortion mentioned above is necessary.
Ultralow-angle dynamic light scattering with a charge coupled device camera based multispeckle, multitau correlator
Authors: Cipelletti Luca , Weitz D. A.
Published: July 2002

We use a charge coupled device (CCD) camera and a multi-tau software correlator to measure dynamic light scattering (DLS) at many angles simultaneously, from 0.07° to 5.1°. Real-time autocorrelation functions are calculated by averaging both over time and over CCD pixels, each corresponding to a different coherence area. In order to cover the wide spectrum of decay times associated with the large range of accessible angles, we adopt the multitau scheme, where the correlator channel spacing is quasilogarithmic rather than linear. A detailed analysis is presented of the effects of dark noise, stray light, and finite pixel area, and methods to correct the data for these effects are developed, making a CCD camera a viable alternative for a DLS detector. We test the apparatus on a dilute suspension of colloidal particles. Very good agreement is found between the particle radius derived from the CCD data, and that obtained with a conventional DLS setup.
Ray optics model and numerical computations for the radiation pressure micromotor
Authors: Gauthier Robert C.
Published: July 2002

In this letter, the spinning of a micron sized radiation pressure motor is analyzed using geometrical optics to model for interactions of a highly focused laser beam with the medium discontinuities. The motor’s direction of rotation and dependence on focused beam parameters, predicted from the computations, are in agreement with previously published experimental results. The motor can be made to rotate in the clockwise or counterclockwise direction by changing the ambient medium only.
Pulsed laser ablation synthesis and characterization of layered Pt/SrBi2Ta2O9/Pt ferroelectric capacitors with practically no polarization fatigue
Authors: Dat R. , Lee J. K. , Auciello O. , Kingon A. I.
Published: July 2002

Pulsed laser ablation deposition was used to synthesize polycrystalline SrBi2Ta2O9 layered ferroelectric thin films on platinized silicon substrates. Top electrodes were produced by dc magnetron sputter deposition to fabricate capacitors for electrical tests. The polarization electric field hysteresis loops showed saturation in the 2–4 V range with a coercive field of 25 kV/cm. The capacitors showed practically no polarization fatigue up to 1011 switching cycles. The resistivity of the SrBi2Ta2O9 for a coercive field of 100 kV/cm was approximately 2×1011 Ω cm. Retention and imprint characteristics of these capacitors showed no degradation as a function of cumulative waiting times.
Temperature dependent pohotoluminescence investigation of AlGaAs/GaAs quantum wires grown by flow rate modulation epitaxy
Authors: Wang Xue-Lun , Ogura Mutsuo , Matsuhata Hirofumi
Published: July 2002

The temperature dependence of photoluminescence (PL) properties of AlGaAs/GaAs quantum wire (QWR) grown on V-grooved substrates by flow rate modulation epitaxy is investigated. PL from a 7.1 nm thick QWR is easily observed even at room temperature. The full width at half-maximum (FWHM) of the QWR emission peak increases linearly with increasing temperature at low temperatures and becomes almost independent of temperature at high temperatures, while that of a quantum well layer (QWL) sample increases with increasing temperature up to room temperature. The FWHM of QWR is found to be considerably narrower than that of the QWL sample at high temperatures, which is expected theoretically from the sharp one-dimensional density of states of QWR but has not been clearly observed experimentally.
Elimination of SiC/SiO2 interface states by preoxidation ultraviolet-ozone cleaning
Authors: Afanas’ev V. V. , Stesmans A. , Bassler M. , Pensl G. , Schulz M. J. , Harris C. I.
Published: July 2002

The preoxidation cleaning of silicon carbide surfaces (3C, 4H, 6H polytypes) by exposing them to ultraviolet radiation and oxygen is shown to produce a significant improvement in the electronic properties of SiC/SiO2 interfaces. It is found that this treatment results in a removal of defect species, otherwise present at the SiC surface after thermal oxidation of SiC. Carbon clusters are proposed as the attacked species responsible for a substantial part of the SiC/SiO2 interface states.
Improvement of carrier capture efficiency of short-period GaAs/AlGaAs quantum wire array by a new lithography method
Authors: Kim Tae Geun , Kim Eun Kyu , Min Suk-Ki , Park Jung-Ho
Published: July 2002

Short-period GaAs/AlGaAs quantum wire array (QWA) was fabricated by metalorganic chemical vapor deposition on the GaAs substrate with submicron gratings. A strong photoluminescence signal derived from highly dense QWA was detected in the as-grown sample. To identify the signal more clearly, all the layers except QWR regions should be removed. However, the small dimension of the sample made it difficult to do that with conventional lithography techniques. We have developed a novel lithography technique which can be applied to nonplanar structures. We could completely remove the (100) and the (111)A quantum well (QW) layers using the technique. As a result, we could clearly observe the optical properties of short-period QWA by improving the carrier capture efficiency of QWR regions.
Far-infrared emission from parabolically graded quantum wells
Authors: Maranowski K. D. , Gossard A. C. , Unterrainer K. , Gornik E.
Published: July 2002

We have observed grating coupled far infrared (FIR) emission from parabolically graded quantum wells (PQWs) by the application of an in-plane electric field. The peak emission frequency from different wells matches the designed harmonic oscillator frequency for each well, as determined by the curvature of the PQWs. This is a confirmation that the generalized Kohn theorem applies for emission of FIR radiation.
Optical gain and crystal symmetry in III–V nitride lasers
Authors: Suzuki Masakatsu , Uenoyama Takeshi
Published: July 2002

Optical gains of bulk GaN and GaN/AlGaN quantum wells (QWs) are calculated in wurtzite (WZ) and zinc-blende (ZB) structures. The physical parameters are extracted by reproducing the band-edge electronic structures from the first-principles band calculations. It is found that the lower crystal symmetry, that is the WZ structure, is preferable for the lower transparent current density in bulk GaN. Although the introduction of QW structures leads to symmetry lowering only in the ZB structure, we cannot find a significant benefit of the ZB QW structure, owing to the weak spin-orbit coupling of the nitrogen atom.
Microstructure and magnetostriction of (Dy0.7Tb0.3)1−xPrxFe1.85 and (Dy0.7Tb0.3)0.7Pr0.3Fey alloys
Authors: Wang B. W. , Tang S. L. , Jin X. M. , Cheng L. Z. , He K. Y.
Published: July 2002

The structure, Curie temperature, and magnetostriction of R1−xPrxFe1.85 and R0.7Pr0.3Fey (R=Dy0.7Tb0.3, x≤0.5, 1.55≤y≤1.85) alloys were investigated. The matrix of R1−xPrxFe1.85 alloys is the MgCu2-type cubic (Dy,Tb,Pr) Fe2 and the second phase was found to be (Dy,Tb,Pr) Fe3 when x≤0.3. When x>0.4, (Dy,Tb,Pr)Fe3 is the main phase with the PuNi3-type structure and (Dy,Tb,Pr) Fe2 becomes the minority phase. In the range of 0.31.55, which increases with increasing y. When y=1.55, the alloy is essentially single phase with the MgCu2-type cubic structure. The lattice parameter of (Dy,Tb,Pr)Fe2 compound for R1−xPrxFe1.85 alloys increases slowly with increasing x when x≤0.3, and sharply increases when x>0.3. The Curie temperature of the alloys decreases steadily with increasing Pr content. The magnetostrictions of R1−xPrxFe1.85 and R0.7Pr0.3Fey alloys decrease with increasing Pr content and Fe content, respectively. The largest magnetostriction at room temperature was found in the alloy R0.7Pr0.3Fe1.55 (1480×10−6 at H=796 kA/m).
Simultaneous field emission and photoemission from diamond
Authors: Bandis C. , Pate B. B.
Published: July 2002

The electron field emission properties of the (111)1×1:H surface of natural semiconducting (p-type) diamond have been examined with simultaneous field emission and photoemission measurements. We find that the origin of the field emission is due to the electron tunneling from the valence band and show that the shape of the field emission energy distributions can be described by the theory of semiconductor field emission. Analysis of our results demonstrate that the combination of field emission and photoemission is a powerful technique for the study of the electron emission properties of materials.
Role of self-formed InGaN quantum dots for exciton localization in the purple laser diode emitting at 420 nm
Authors: Narukawa Yukio , Kawakami Yoichi , Funato Mitsuru , Fujita Shizuo , Fujita Shigeo , Nakamura Shuji
Published: July 2002

Structural analysis was performed on a purple laser diode composed of In0.20Ga0.80N (3 nm)/ In0.05Ga0.95N (6 nm) multiple quantum wells, by employing transmission electron microscopy and energy-dispersive x-ray microanalysis, both of which are assessed from the cross-sectional direction. It was found that the contrast of light and shade in the well layers corresponds to the difference in In composition. The main radiative recombination was attributed to excitons localized at deep traps which probably originate from the In-rich region in the wells acting as quantum dots. Photopumped lasing was observed at the high energy side of the main spontaneous emission bands.
Cyclic crystalline–amorphous transformations of mechanically alloyed Co75Ti25
Authors: Sherif El-Eskandarany M. , Akoi K. , Sumiyama K. , Suzuki K.
Published: July 2002

We have found that a cyclic crystalline–amorphous phase transformation can occur in Co75Ti25 alloy powder when subjected to ball milling. The results have shown that a single amorphous phase of Co75Ti25 is obtained after 11 ks of mechanical alloying (MA) time. This amorphous phase transforms into a new metastable phase of bcc-Co3Ti upon milling for 86 ks. The bcc-Co3Ti is thermally stable and does not transform to any other phase(s) upon heating up to 1300 K. It however returns to the same amorphous phase of Co75Ti25 upon milling for 360 ks. Further milling leads to the formation of crystalline and/or amorphous phases depending on the MA time.
Type conversion in irradiated silicon diodes
Authors: Taylor Stephen J. , Yamaguchi Masafumi , Yang Ming-Ju , Imaizumi Mitsuru , Matsuda Sumio , Kawasaki Osamu , Hisamatsu Tadashi
Published: July 2002

We have observed conversion from p- to n-type of the base layer of n+\p\p+ silicon diodes irradiated with more than roughly 5×1016 cm−2 1 MeV electrons. Annealing for 15 min at 200 °C results in a recovery of p-type conduction in diodes in which type conversion had been induced. Solar cells which employ the same diode structure are severely degraded by irradiation with more than 1016 cm−2 1 MeV electrons and show only a weak infrared response after irradiation with 1017 cm−2 1 MeV electrons, consistent with the creation of an n+\n\p structure due to type conversion.
Cyclotron resonance and quantum Hall effect studies of the two-dimensional electron gas confined at the GaN/AlGaN interface
Authors: Knap W. , Contreras S. , Alause H. , Skierbiszewski C. , Camassel J. , Dyakonov M. , Robert J. L. , Yang J. , Chen Q. , Asif Khan M. , Sadowski M. L. , Huant S. , Yang F. H. , Goiran M. , Leotin J. , Shur M. S.
Published: July 2002

We report on high magnetic fields (up to 40 T) cyclotron resonance, quantum Hall effect and Shubnikov-de-Hass measurements in high frequency transistors based on Si-doped GaN–AlGaN heterojunctions. A simple way of precise modelling of the cyclotron absorption in these heterojunctions is presented. We clearly establish two-dimensional electrons to be the dominant conducting carriers and determine precisely their in-plane effective mass to be 0.230±0.005 of the free electron effective mass. The increase of the effective mass with an increase of two-dimensional carrier density is observed and explained by the nonparabolicity effect.
Effect of surface acoustic waves on low-temperature photoluminescence of GaAs
Authors: Zhuravlev K. S. , Petrov D. V. , Bolkhovityanov Yu. B. , Rudaja N. S.
Published: July 2002

The near-band-gap low-temperature photoluminescence (PL) in a pure film of GaAs in the presence of surface acoustic waves (SAW) has been studied experimentally. The complex behavior of the PL peak intensities with SAW power in the excitonic and acceptor spectral regions results from a charge bunching due to the piezoelectric field of SAW.
Mid-infrared electroluminescence in GaAs/AlGaAs structures
Authors: Strasser G. , Kruck P. , Helm M. , Heyman J. N. , Hvozdara L. , Gornik E.
Published: July 2002

Design, growth, and operation of an unipolar light emitting diode based on the material system GaAs/AlGaAs is reported. We present mid-infrared transmission, photocurrent, and electroluminescence measurements on a quantum cascade structure with intersubband transition energies greater than the optical phonon energy. Electroluminescence powers up to a few nanowatts at 6.9 μm have been measured.
Optical metastability in bulk GaN single crystals
Authors: Shmagin I. K. , Muth J. F. , Lee J. H. , Kolbas R. M. , Balkas C. M. , Sitar Z. , Davis R. F.
Published: July 2002

Bulk GaN single crystals were grown from cold pressed GaN powder by sublimation in flowing ammonia. Optical transmission measurements indicated that the absorption coefficient for the transparent samples is 50 cm−1 in the wavelength region from 650 to 400 nm. Optical metastability in bulk GaN crystals was studied through time dependent photoluminescence both at room and liquid–nitrogen temperatures. The observation included decreasing output intensity of the ultraviolet emission attributed to the band edge and increasing output intensity of a new emission band centered at 378 nm at room temperature. At liquid–nitrogen temperature, the photoinduced emission band consisted of at least one LO-phonon replica of the zero-phonon line centered at 378 nm. The ratio of output intensities of the photoinduced band to the band edge increased by a factor of 10 during 27 min of exposure time. The photoinduced effect is attributed to the metastable nature of traps in bulk GaN.
Optical data storage in InGaN/GaN heterostructures
Authors: Shmagin I. K. , Muth J. F. , Kolbas R. M. , Dupuis R. D. , Grudowski P. A. , Eiting C. J. , Park J. , Shelton B. S. , Lambert D. J. H.
Published: July 2002

Optical storage data was realized using an InGaN/GaN single heterostructure. It was observed that exposure to a high power density ultraviolet light temporarily changes the optical properties of the InGaN epitaxial layer. The photo-induced changes can be observed under an optical microscope with low intensity ultraviolet excitation. This effect was used to create high contrast optical patterns on the sample at room temperature and 77 K. The photo-induced changes completely disappear in about four hours at room temperature. After the recorded pattern is erased, the information can be rewritten without a change in efficiency or retention time.
Persistent photoconductivity in n-type GaN
Authors: Beadie G. , Rabinovich W. S. , Wickenden A. E. , Koleske D. D. , Binari S. C. , Freitas J. A.
Published: July 2002

Persistent photoconductivity has been observed in n-type GaN:Si. The effect is seen at room temperature in both nonoptimally grown films as well as in device quality channel layers. The relaxation dynamics are found to agree with a stretched exponential model of recovery. A comparison between different samples, based upon stretched exponential parameters, Hall measurements, and photoluminescence data is made. The data suggest that the cause of persistent photoconductivity is the same among the different samples and that there is a transition in the relaxation dynamics between room temperature and 130 °C.
Evidence of compensating centers as origin of yellow luminescence in GaN
Authors: Schubert E. F. , Goepfert I. D. , Redwing J. M.
Published: July 2002

The dependence of the near-band edge and the yellow luminescence in n-type GaN grown by organometallic vapor-phase epitaxy is investigated as a function of doping concentration. The band edge and yellow luminescence intensity increase as the doping concentration is increased. However, the band-edge-to-yellow luminescence ratio does not change significantly as the doping concentration is increased by two orders of magnitude. A theoretical model based on rate equations is developed for the band-edge-to-yellow intensity ratio. Analysis of the experimental data in terms of the model reveals that the concentration of the level causing the yellow luminescence increases linearly with doping concentration. This dependence shows that the yellow luminescence is due to a compensating center.
Optical memory effect in GaN epitaxial films
Authors: Joshkin V. A. , Roberts J. C. , McIntosh F. G. , Bedair S. M. , Piner E. L. , Behbehani M. K.
Published: July 2002

We report on memory effects in the optical properties of GaN and AlN epitaxial-films grown by atmospheric pressure metal organic chemical vapor deposition. After exposing selected areas of particular samples with He–Cd laser light (3.8 eV), we observed a persistent and marked decrease in the near band edge photoluminescence (PL) intensity emitted from these areas. This effect has been observed in epitaxial films that typically have a pyramidlike hillock surface. This ability to modulate PL emission intensity at individual points in these materials can be exploited as a method for optical data storage. A means of erasing information stored using this effect has also been investigated using lower energy (∼2 eV).
Self-limiting process for the bismuth content in molecular beam epitaxial growth of Bi2Sr2CuOy thin films
Authors: Migita Shinji , Kasai Yuji , Ota Hiroyuki , Sakai Shigeki
Published: July 2002

A new technology utilizing a self-limiting mechanism for the Bi content is demonstrated for Bi2Sr2CuOy (2201) thin film growth by an atomic layer controlled molecular beam epitaxy. This technology is based on peculiar behavior of the Bi sticking coefficient that depends on the kinds of oxide thin films to be grown. When Bi atoms are supplied in excess with ozone molecular beam, only Bi atoms, being necessary for forming the structural unit (2201 half unit cell), are just incorporated in the film. Surplus Bi is reevaporated from the surface. Using this technique, high quality 2201 thin films are obtained with good reproducibility.
Lateral confinement in ZnSe/ZnCdSe quantum wells grown on patterned substrates
Authors: Heiß W. , Stifter D. , Prechtl G. , Bonanni A. , Sitter H. , Liu J. , Tóth L. , Barna Á.
Published: July 2002

ZnSe/Zn 0.75 Cd 0.25 Se quantum wells have been grown on GaAs submicron gratings. In these quantum structures, we observe an optical anisotropy induced by lateral confinement. Photoluminescence (PL) energy-shifts and the PL temperature dependence indicate a moderate modulation of the two-dimensional electron gas in the quantum well.
Observation of Si cluster formation in SiO2 films through annealing process using x-ray photoelectron spectroscopy and infrared techniques
Authors: Furukawa Katsuhiko , Liu Yichun , Nakashima Hiroshi , Gao Dawei , Uchino Kiichiro , Muraoka Katsunori , Tsuzuki Hirohisa
Published: July 2002

SiO 2 films having high breakdown characteristics were deposited by a sputtering-type electron cyclotron resonance microwave plasma at room temperature. As-deposited films were annealed in an Ar ambient at temperatures ranging from 450 to 1000 °C. Transmitted infrared (IR) absorption and x-ray photoelectron spectroscopy (XPS) were used to characterize the as-deposited and annealed films. XPS measurements indicated that the as-deposited films had an approximately stoichiometric composition containing a few intermediate SiOx(x≠2) species and Ar atoms around some dangling-bond defects. The dependence of XPS spectra on annealing temperature showed that steep diffusion of the Ar atoms occurs at annealing temperatures of 450–550 °C and the SiOx species separate into SiO2 phase and Si clusters by an annealing process of 750–950 °C. Based on the full width at half-maximum variations of Si 2p XPS spectra and Si–O stretching mode of IR spectra for the annealed films, we discuss the Si cluster formation in SiO2 films through the annealing process.