bluuebunny/crossref_metadata_2025 huggingface dataset:
Formation of extremely low resistance Ti/Pt/Au ohmic contacts to p-GaAs
Authors: Stareev G.
Published: July 2002

Nonalloyed Ti/Pt/Au contacts to heavily doped p-GaAs have been fabricated using effective cleaning of the semiconductor surface by bombardment with low energy Ar+ ions (60 eV) prior to the metal deposition. Short-time annealing cycles for 1 and 20 s were employed in order to restore the primary properties of the subsurface layer disordered during ion bombardment. Annealing at temperatures ranging from 420 to 530 °C provides formation of contacts with an extremely low resistivity of 2.8×10−8 Ω cm2. A definite correlation between electrical properties and structural modifications of the contact interface was found. Measurements of the contact resistivity at different ambient temperatures yielded a good quantitative agreement with the theoretically predicted values using the field-emission model. The results indicate that the metal-semiconductor junctions formed under optimal conditions are intimate and that tunneling is the dominant mechanism of the current flow.
Quality of AlAs-like and InSb-like interfaces in InAs/AlSb superlattices: An optical study
Authors: Spitzer J. , Fuchs H. D. , Etchegoin P. , Ilg M. , Cardona M. , Brar B. , Kroemer H.
Published: July 2002

Two short-period InAs/AlSb superlattices, grown with an AlAs-like interface and an InSb-like interface, respectively, were studied with Raman spectroscopy, x-ray diffraction, and ellipsometry. Our measurements show that the InSb-like interface grows perfectly pseudomorphically, whereas the sample with the AlAs-like interface shows indications of relaxation and As interdiffusion. This different interface quality seems to be a fundamental problem, rather than the result of the growth technique.
Environmental reactivity characteristics of copper-oxide superconductors
Authors: Zhou Ji-Ping , Riley David R. , Manthiram A. , Arendt Mark , Schmerling Michael , McDevitt John T.
Published: July 2002

Although it is known that some of the high-Tc phases react rapidly with water, CO2, CO, and acids, no systematic comparison of the relative reactivities of the various cuprate superconductors is available. In this letter, x-ray powder diffraction, scanning electron microscopy, x-ray photoemission spectroscopy, and electrochemical measurements are utilized to establish a comprehensive comparison of the intrinsic reactivity characteristics of the common copper-oxide superconductors. Consequently, the following reactivity scale has been determined: YBa2Cu3O7≳Tl2Ba2Ca2Cu3O10≳Bi2Sr2CaCu2O8 ≥La1.85Sr0.15CuO4 ≳Nd1.85Ce0.15CuO4≳Nd1.85Th0.15CuO4.
Bright blue electroluminescence from poly(N-vinylcarbazole)
Authors: Kido Junji , Hongawa Kenichi , Okuyama Katsuro , Nagai Katsutoshi
Published: July 2002

Electroluminescent devices were fabricated using poly(N-vinylcarbazole) (PVK) as a hole-transporting emitter layer and a double layer of 1,2,4-triazole derivative (TAZ) and tris(8-quinolinolato)aluminum(III) complex (Alq) as an electron transport layer. A cell structure of glass substrate/indium-tin-oxide/PVK/TAZ/Alq/Mg:Ag was employed. In this cell structure, carrier injection from the electrodes to the PVK layer and concomitant electroluminescence from PVK were observed. Blue emission peaking at 410 nm and a luminance of 700 cd/m2 were achieved at a drive voltage of 14 V.
Effective band-gap shrinkage in GaAs
Authors: Harmon E. S. , Melloch M. R. , Lundstrom M. S.
Published: July 2002

Electrical measurements of the equilibrium np product (n2ie) in heavily doped n- and p-GaAs were performed. The n2ieD product (where D is the diffusivity) was measured by fitting the collector current-voltage characteristic of a homojunction bipolar transistor to an ideal diode equation modified to account for transport in thin base transistors. The n2ie product was then extracted from n2ieD by utilizing diffusivity results obtained with the zero-field time-of-flight technique. Our results show significant effective band-gap shrinkage in heavily doped p-GaAs, and very little effective band-gap shrinkage in heavily doped n-GaAs. At extremely heavy dopings, an effective band-gap widening is observed for both n- and p-GaAs and is attributed to the effects of degeneracy.
Laser induced unzipping: A thermal route to polymer ablation
Authors: Blanchet Graciela B. , Fincher C. R.
Published: July 2002

The data presented here show that polytetrafluoroethylene (PTFE) and polymethylmethacrylate (PMMA) can be ablated by the evaporation of solid polymer targets with a pulsed ultraviolet laser. In situ measurements of the plume composition show that the products emitted under laser irradiation are primarily monomer and other species resulting from energetic collisions within the plasma. The similarities between the ablative and pyrolisis mass spectra suggest that ablation of PTFE and PMMA occur through a laser induced pyrolitic decomposition.
Vertical–cavity stimulated emission from photopumped InGaN/GaN heterojunctions at room temperature
Authors: Khan M. Asif , Krishnankutty S. , Skogman R. A. , Kuznia J. N. , Olson D. T. , George T.
Published: July 2002

We report the observation of room temperature violet (415 nm) stimulated emission in the vertical cavity mode from photopumped GaN/In0.25Ga0.75N heterojunctions. The InGaN/GaN heterojunction was deposited over sapphire substrates using low-pressure metalorganic chemical vapor deposition and was of high enough optical quality to achieve room-temperature stimulated emission. The observed emission intensity was found to be a nonlinear function of incident optical pump power density. At threshold we observe a clear line narrowing of the output optical signal from 20 to 1.5 nm full width at half-maximum.
Pressure-dependent photoluminescence study of wurtzite GaN
Authors: Shan W. , Schmidt T. J. , Hauenstein R. J. , Song J. J. , Goldenberg B.
Published: July 2002

Low-temperature photoluminescence (PL) in single-crystal GaN films grown on sapphire substrates by metalorganic chemical vapor deposition has been studied as a function of applied hydrostatic pressure using the diamond-anvil-cell technique. The PL spectra of the GaN at atmospheric pressure were dominated by two sharp, strong, near-band-edge exciton luminescence lines and a broad emission band in the yellow spectral region. The exciton emission lines were found to shift almost linearly toward higher energy with increasing pressure. While the yellow emission band showed a similar blue shift behavior under applied pressure, a relatively strong sublinear pressure dependence was observed. By examining the pressure dependence of the exciton emission structures, the pressure coefficient of the direct Γ band gap in the wurtzite GaN was determined. The value of the hydrostatic deformation potential of the band gap has also been deduced from the experimental results.
Data acquisition and processing: A systems approach
Authors: Sternin Edward
Published: July 2002

A new nuclear magnetic resonance (NMR) data acquisition and processing system is described. Designed at a fraction of the cost of most commercially available systems, it in many ways exceeds their capabilities. The use of state-of-the-art components in an open-architecture system where each component is relatively loosely coupled to the rest of the system and thus can be chosen, upgraded, or replaced without major changes to other components, provides for a flexible and expandable system. The principle of incorporating sufficient intelligence into each of the system components allows the use of serial protocols for all noncritical intrasystem communications, simplifying the task of system integration and reducing cost. Such ‘‘unbundling’’ of the hardware also results in a superior adherence to industry standards at the level of individual components. While the technical details are directed primarily at the NMR researchers, a number of general considerations and the description of the principles and criteria used in designing a laboratory data system are of general interest.
Oscillating pendant drop: A method for the measurement of dynamic surface and interface tension
Authors: Badran Ali A. , Marschall Ekkehard
Published: July 2002

A method is described for measuring dynamic surface and interface tension. The technique is essentially a variation of the pendant drop method in which the drop is allowed to oscillate after sudden formation at the tip of a syringe. Immediately after the oscillation stops but before the drop detaches, there is an instant of rest. At this moment, the profile of the drop is obtained using high-speed photography. The boundary tension is then calculated from the profile using established methods. The technique is demonstrated on systems consisting of aqueous solutions of sodium stearate or oleate on one hand and mineral oil or air on the other hand. Surface or interface tensions may be obtained within 0.25 to 5 s after surface formation.
Quantitative assessment of a parallel detection system for energy-loss spectrometry
Authors: Johnson Dale E. , Connick Megan
Published: July 2002

The concept of a spatial frequency-dependent, detective quantum efficiency [=DQE( f )] is applied to the performance assessment of a prototype parallel detection system for electron energy-loss spectrometry. The various factors which determine DQE( f ) in such a system are discussed and comparisons are made with a serial detection system. Also, the effects of the electron source energy spread on the overall spectrometer DQE( f ) are discussed. In these examples, the clear importance of including the spatial frequency dependence of DQE is demonstrated.
Present status of compact synchrotron light source ‘‘AURORA’’
Authors: Yamada H. ,   SHI Accelerator Research Group
Published: July 2002

The construction program of AURORA, the compact synchrotron light source, was started in the fall of 1986, and is now at the final assembly stage. We have observed a good uniform field from the superconducting magnet. In this article the construction status of the superconducting electron storage ring, injection microtron, beam monitoring, and control system is briefly reported.
rf driven multicusp H− ion source
Authors: Leung K. N. , DeVries G. J. , DiVergilio W. F. , Hamm R. W. , Hauck C. A. , Kunkel W. B. , McDonald D. S. , Williams M. D.
Published: July 2002

An rf driven multicusp source capable of generating 1-ms H− beam pulses with a repetition rate as high as 150 Hz has been developed. This source can be operated with a filament or other types of starter. There is almost no lifetime limitation and a clean plasma can be maintained for a long period of operation. It is demonstrated that rf power as high as 25 kW could be coupled inductively to the plasma via a glass-coated copper-coil antenna. The extracted H− current density achieved is about 200 mA/cm2.
High-pressure pulse radiolysis. Modification of an optical cell for 2-MeV electron pulse radiolysis at pressures up to 200 MPa
Authors: Wishart James F. , van Eldik Rudi
Published: July 2002

The construction of a high-pressure cell for pulse-radiolysis experiments with 2-MeV electrons at pressures up to 200 MPa using optical UV-VIS detection is described. Details on the window construction are given, and typical results for the thiocyanate test system are presented. The cell enables systematic studies of the effect of pressure on pulse radiolytically induced reactions using lower energy accelerators than previously possible.
Spectrophotometric stopped-flow apparatus suitable for high-pressure experiments to 200 MPa
Authors: van Eldik R. , Gaede W. , Wieland S. , Kraft J. , Spitzer M. , Palmer D. A.
Published: July 2002

A modified high-pressure, stopped-flow apparatus has been developed to enable kinetic experiments to be monitored spectrophotometrically to 200 MPa, with efficient mixing of the reactant solutions. This apparatus is compact, relatively inexpensive to construct, and the solutions are only exposed to inert materials, such as Kel-F, Teflon, quartz (low-pressure windows), and glass or Viton. The drive-syringe pistons are propelled by a step motor housed on top of the stopped-flow unit inside the pressure vessel, resulting in a dead time for the system of ∼10 ms at 25 °C.
Measurement of small forces using an optical trap
Authors: Ghislain Lucien P. , Switz Neil A. , Webb Watt W.
Published: July 2002

A simple and sensitive method for detecting small forces applied to an optically trapped microsphere utilizes the transmitted beam of the trapping laser to monitor microsphere deflections within the potential well of the trap. The rms dynamic displacement detection sensitivity measured in the frequency range from 1 Hz to 10 kHz is ∼1 nm in the radial direction and ∼10 nm along the z (optic) axis. Radiation trapping force constants were calibrated against viscous drag on microspheres in the range between the Rayleigh (r<0.2λ) and Mie (r≫λ) size regimes. For a 1-μm-diam polystyrene sphere trapped with a 60 mW beam the rms spontaneous thermal motion limits the force sensitivity to better than 10−12 N and lateral spatial resolution to ∼10 nm in a frequency range from 1 Hz to the viscous rolloff frequency ∼1 kHz. The measured maximum trapping efficiencies are compared with the theoretical predictions of the ray-optics approximation.
Quartz crystal microbalance setup for frequency and Q-factor measurements in gaseous and liquid environments
Authors: Rodahl Michael , Höök Fredrik , Krozer Anatol , Brzezinski Peter , Kasemo Bengt
Published: July 2002

An experimental setup has been constructed for simultaneous measurements of the frequency, the absolute Q factor, and the amplitude of oscillation of a quartz crystal microbalance (QCM). The technical solution allows operation in vacuum, air, or liquid. The crystal is driven at its resonant frequency by an oscillator that can be intermittently disconnected causing the crystal oscillation amplitude to decay exponentially. From the recorded decay curve the absolute Q factor (calculated from the decay time constant), the frequency of the freely oscillating crystal, and the amplitude of oscillation are obtained. All measurements are fully automated. One electrode of the QCM in our setup was connected to true ground which makes possible simultaneous electrochemistry. The performance is illustrated by experiments in fluids of varying viscosity (gas and liquid) and by protein adsorption in situ. We found, in addition to the above results, that the amplitude of oscillation is not always directly proportional to the Q factor, as the commonly used theory states. This puts limitations on the customary use of the amplitude of oscillation as a measure of the Q factor.
Specific conditions for Ni catalyzed carbon nanotube growth by chemical vapor deposition
Authors: Yudasaka Masako , Kikuchi Rie , Matsui Takeo , Ohki Yoshimasa , Yoshimura Susumu , Ota Etsuro
Published: July 2002

Chemical vapor deposition using 2-methyl-1,2′-naphthyl ketone as a starting material has been done between 1000 and 600 °C on Ni particles with diameters ranging from 10 to 500 nm. The Ni particles were prepared by annealing Ni thin film deposited on quartz glass substrates. The size of the Ni particle was controlled by the thickness of the Ni film. Carbon nanotubes were obtained at 700 °C when the diameter of the Ni particles was about 20–30 nm.
2D tomography with bolometry in DIII-D (abstract)a)
Authors: Leonard A. W. , Meyer W. H. , Geer B. , Behne D. M. , Hill D. N.
Published: July 2002

A 48-channel platinum-foil bolometer system on DIII-D was installed to achieve better spatial and temporal resolution of the radiated power in diverted discharges. Two 24-channel arrays provide complete plasma coverage with optimized views of the divertor. The divertor radiation profile was measured for a series of radiative divertor and power balance experiments. A significant change in the magnitude and distribution of divertor radiation with heavy gas puffing was observed. Unfolding the radiation profile with only two views requires one to treat the core and divertor radiation separately. The core radiation is fitted to a function of magnetic flux and is then subtracted from the divertor viewing chords. The divertor profile is then fit to a 2D spline as a function of magnetic flux and distance from divertor floor.
Density perturbation studies by means of pulsed radar reflectometry
Authors: Heijnen S. H. , de Baar M. , Donné A. J. H. , van de Pol M. J. , Hugenholtz C. A. J. ,   the RTP team
Published: July 2002

A four-channel pulsed radar reflectometry system has been developed for the Rijnhuizen Tokamak Project. The system combines a high spatial and temporal resolution with limited access demands to the tokamak plasma. Steady-state density profiles as measured with the pulsed radar system match very well to the profiles as measured by Thomson scattering and by interferometry. As will be illustrated in this paper, the diagnostic is able to follow fast phenomena as pellet injection and plasma disruptions as well as MHD oscillations.
Measurement of first hyperpolarizabilities by hyper-Rayleigh scattering
Authors: Morrison I. D. , Denning R. G. , Laidlaw W. M. , Stammers M. A.
Published: July 2002

The origin of spurious and irreproducible signals in the measurement of the hyper-Rayleigh scattering (HRS) of solutions is investigated. A new cell design, coupled with continuous solution filtration through a 20 nm filter, is shown to improve the reproducibility of the measurement to ±5%. A transverse cell dimension of ∼2 mm reduces absorption of the second harmonic. A simple monochromaticity test is used to identify luminescence, and we recommend that it is made routinely. Scattering from the [Ru(bipy)3]2+ cation, attributed to HRS by Zyss et al. [Chem. Phys. Lett. 206, 409 (1993)], appears to be largely due to two-photon excited luminescence. The reliability of calibration procedures is examined, and some revised hyperpolarizabilities are reported.
Focusing x-rays to a 1 μm spot using elastically bent, graded multilayer-coated mirrors
Authors: Underwood J. H. , Thompson A. C. , Kortright J. B. , Chapman K. C. , Lunt D.
Published: July 2002

In the LBNL x-ray fluorescence microprobe, a synchrotron source of x-rays is demagnified several hundred times using a pair of mirrors in the Kirkpatrick-Baez configuration. These are coated with multilayers to increase reflectivity and limit the pass band of the x-rays striking the sample. With spherical mirrors, the spot size obtained is limited by spherical aberration. This can be corrected by using an initially flat mirror elastically bent by a combination of end couples into an ellipse. By grading the multilayer coatings in d-spacing, the throughput of the focusing system is increased and the pass band narrowed. A pair of such mirrors, installed in the microprobe on a bending magnet at the Advanced Light Source (ALS), achieved focal spots of dimensions 1μm×1 μm at energies of 8.5 keV and 12 keV, with an energy pass band of 10%.
Radiation-hardened x-ray imaging for burning-plasma tokamaks
Authors: Séguin Fredrick H. , Petrasso Richard D. , Li Chi Kang
Published: July 2002

A special type of vacuum-photodiode detector is being developed for x-ray imaging of plasma in fusion-producing tokamaks such as the international thermonuclear experimental reactor (ITER), where the radiation environment will be too hostile for conventional x-ray detectors. The vacuum photodiode has modest efficiency, but it is intrinsically immune to radiation damage if built in such a manner as to expose only metal components to radiation. A design based on appropriately chosen materials (including high-Z cathodes) and geometries (including a small angle between cathode surface and incident x rays) can provide good signals from the 1–100 keV x rays that are of particular importance for imaging the plasmas in the Joint European Torus (JET) and ITER. It should also provide better rejection of signal distortion and noise due to unwanted detection of neutrons and hard gamma rays than conventional detectors. A prototype design is described, along with performance parameters predicted for JET and ITER. In addition, we show results of laboratory experiments that confirm some of the predicted behaviors of the design.
Fielding and calibration issues for diamond photoconducting detectors
Authors: Spielman Rick B. , Ruggles Lawrence E. , Pepping Richard E. , Breeze Stephen P. , McGurn John S. , Struve Kenneth W.
Published: July 2002

Diamond photoconducting detectors (PCDs) are routinely fielded as soft x-ray diagnostics on Sandia’s Saturn facility. We have developed an improved detector mount that provides a 200 ps time response, is easily cleanable, and is very rugged. In addition, we have developed a new, fast insertion unit to apply bias voltage to the detectors. Absolute calibration of the PCDs is carried out either at the Brookhaven National Synchrotron Light Source or on Sandia’s laser calibration facility. We are now fielding diamond elements that have the dimensions 1×3×0.5 mm and 1×1×0.5 mm. We are neutron damaging some of the 1×1×0.5 mm detectors to reduce their sensitivity. We can tailor PCD sensitivity by adjusting element size and neutron damage level.
Identification of Alfvén mode location via multichannel interferometer measurements (abstract)
Authors: Chang Z. , Fredrickson E. D. , Park H. , Nazikian R. , Budny R. V. , Wong K. L. , McGuire K. M. ,   TFTR Group
Published: July 2002

Quasicoherent magnetohydrodynamics modes with an Alfvén frequency scaling are seen routinely in TFTR neutral beam heated plasmas as well as some Ohmic plasmas. So far, they are only observed in external magnetic fluctuation measurement (Mirnov coils). A close correlation is observed between the changes in the mode frequency and electron density measured by the multichannel infrared interferometer. This correlation allows us to determine the location of the Alfvén modes. The result shows that they are near the plasma edge, r/a>0.85. This method is also used to identify the location of the toroidicity-induced Alfvén eigenmode driven by fast ions in a radiowave heating experiment. The result is consistent with the location determined by the reflectometer measurement.
Design of surface forces apparatus for tribology studies combined with nonlinear optical spectroscopy
Authors: Frantz P. , Wolf F. , Xiao X.-d. , Chen Y. , Bosch S. , Salmeron M.
Published: July 2002

We describe the design, calibration, and performance of surface forces apparatus with the capability of illumination of the contact interface for spectroscopic investigation using optical techniques. The apparatus can be placed in the path of a Nd-YAG laser for studies of the linear response or the second harmonic and sum-frequency generation from a material confined between the two surfaces. In addition to the standard fringes of equal chromatic order technique, which we have digitized for accurate and fast analysis, the distance of separation can be measured with a fiber-optic interferometer during spectroscopic measurements (2 Å resolution and 10 ms response time). The sample approach is accomplished through application of a motor drive, piezoelectric actuator, or electromagnetic lever deflection for variable degrees of range, sensitivity, and response time. To demonstrate the operation of the instrument, the stepwise expulsion of discrete layers of octamethylcyclotetrasiloxane from the contact is shown. Lateral forces may also be studied by using piezoelectric bimorphs to induce and direct the motion of one surface.
Key issues in ITER diagnostics: Problems and solutions (invited)
Authors: Costley A. E. ,   Members of the ITER Joint Central Team and Home Teams
Published: July 2002

The key problems associated with designing diagnostic systems for the International Thermonuclear Experimental Reactor (ITER) are identified and representative solutions are described. The plans for dealing with some outstanding issues are briefly presented. The detailed work is specific to the ITER, but much of what has been learned will be applicable to any reactor grade tokamak plasma.
Analysis of neutron cameras for ITER
Authors: Johnson L. C. , Barnes Cris W. , Batistoni P. , Fiore C. , Janeschitz G. , Khripunov V. , Krasilnikov A. , Marcus F. B. , Nishitani T. , Sadler G. , Sasao M. , Zaveriaev V. ,   ITER Joint Central Team,   Home Teams
Published: July 2002

Neutron cameras with horizontal and vertical views have been designed for ITER, based on systems used on JET and TFTR. The cameras consist of fan-shaped arrays of collimated flight tubes, with suitably chosen detectors situated outside the biological shield. The sight lines view the ITER plasma through slots in the shield blanket and penetrate the vacuum vessel, cryostat, and biological shield through stainless steel windows. This article analyzes the expected performance of several neutron camera arrangements for ITER. In addition to the reference designs, we examine proposed compact cameras, in which neutron fluxes are inferred from N16 decay gammas in dedicated flowing water loops, and conventional cameras with fewer sight lines and more limited fields of view than in the reference designs. It is shown that the spatial sampling provided by the reference designs is sufficient to satisfy target measurement requirements and that some reduction in field of view may be permissible. The accuracy of measurements with N16-based compact cameras is not yet established, and they fail to satisfy requirements for parameter range and time resolution by large margins.
Irradiation effects on potential diagnostic materials and components (abstract)
Authors: Costley A. , Dekock L. , Yamamoto S. , Belyakov V. , Bender S. , Ebisawa K. , Farnum E. , Hodgson E. , Kasai S. , Krasilnikov A. , Nishitani T. , Orlinski D. , Shikama T. , Snider R. , Stott P. , Young, and the K. ,   ITER Joint Central Teams and Home Teams
Published: July 2002

Some diagnostic components in ITER will be subject to high levels of radiation (neutrons and gammas) and changes in their physical properties can result. During the ITER EDA an extensive range of tests on relevant materials, e.g., window materials, metals, and insulators, have been carried out and the changes in their physical properties have been measured. The effects examined include radiation induced electrical damage (RIED) and radiation induced conductivity (RIC) in potential insulators; radiation induced absorption and radio luminescence in potential optical materials; and changes to the reflectivity and surface properties of bulk metal mirrors. The results give a database of information which is of use to the designers of ITER diagnostic systems. Recent and planned work is concentrating on the testing of diagnostic prototypes in relevant radiation fields. The results obtained thus far will be summarized and the planned work outlined.
Overview of Korea Superconducting Tokamak Advanced Research diagnostics
Authors: Lee S. G. , Hwang S. M. , Kim B. C. , Lee H. G. , Na H. K. , Yang H. L. , Yoo S. J. ,   the KSTAR Project Team, Medley S. S. , Park H. K. , Young K. M.
Published: July 2002

The Korea Superconducting Tokamak Advanced Research (KSTAR) project is the Korean national fusion program for constructing a superconducting tokamak, that is capable of a steady-state operation. Currently, the KSTAR diagnostic group is concentrating on conceptual design activities to provide measurements of the plasma behavior that is necessary to satisfy the KSTAR missions. A diagnostic overview for the KSTAR device is presented.
Novel flow apparatus for investigating shear-enhanced crystallization and structure development in semicrystalline polymers
Authors: Kumaraswamy Guruswamy , Verma Ravi K. , Kornfield Julia A.
Published: July 2002

An instrument to study the effects of shearing on the crystallization process in semicrystalline polymers is described. It can impose transient stresses similar to those encountered in polymer processing and provides in situ monitoring of microstructure development during and after cessation of flow. Box-like wall shear stress profiles (rise and fall times under 50 ms with maximum wall shear stress on the order of 0.1 MPa) can be applied for controlled durations. A unique feature of our device is that it accommodates a wide variety of real-time probes of structure such as visible and infrared polarimetry and light and x-ray scattering measurements. The design also allows us to retrieve the sample for ex situ optical and electron microscopy. Data are acquired with millisecond resolution enabling us to record the extent of shear deformation of the polymer melt during the pressure pulse. Our device works with small sample quantities (as little as 5 g; each experiment takes ∼500 mg) as opposed to the kilogram quantities required by previous instruments capable of imposing comparable deformations. This orders-of-magnitude reduction in the sample size allows us to study model polymers and new developmental resins, both of which are typically available only in gram-scale quantities. The compact design of the shear cell makes it possible to transport it to synchrotron light sources for in situ x-ray scattering studies of the evolution of the crystalline structure. Thus, our device is a valuable new tool that can be used to evaluate the crystallization characteristics of resins with experimental compositions or molecular architectures when subjected to processing-like flow conditions. We demonstrate some of the features of this device by presenting selected results on isotactic polypropylenes.
Development of ECR ion sources in China (invited)
Authors: Zhao H. W. , Wei B. W. , Liu Z. W. , Wang Y. F. , Zhao W. J.
Published: July 2002

Recent development of electron cyclotron resonance (ECR) ion sources in China is reviewed. Emphasis is put on high charge state ECR ion sources which have been mainly developed in China by Institute of Modern Physics (IMP). Presently two ECR ion sources built by IMP for highly charged ion beams are put into operation for cyclotrons and atomic physics research. The development of high charge state ECR ion sources at IMP has progressed with a new magnetic field configuration, better condition for extraction of highly charged ions, high mirror magnetic field, large plasma volume, and special techniques to provide extra cold electrons. These techniques greatly enhance the production of highly charged ions from IMP ECR ion sources. So far more than 185 eμA of Ar11+ and 50 eμA of Xe26+ were produced by the IMP ECR ion sources. The metallic ion beam production was tested and the first beam Ca11+40 was provided to the cyclotrons at IMP. The beam intensity of Ca11+40 could reach 130 eμA. The next part of this article will report the latest progress of 2.45 GHz ECR ion sources in China. A 2.45 GHz compact permanent magnet proton ion source was designed and constructed by IMP. A new microwave feeding system is applied on this ion source. The ion source is able to deliver 90 mA of mixed ion beam (H1++H2++H3+) after preliminary commissioning. The article also mentions a small 2.45 GHz ECR ion source which was built by Sichuan University and used for industry applications. The dependence of plasma density and electron temperature on radio frequency power, neutral gas pressure, and different microwave windows was measured by a Langmuir probe on this ion source.
Performance of YBa2Cu3O7−δ direct current SQUIDS with high-resistance step-edge junctions
Authors: Dillmann F. , Glyantsev V. N. , Siegel M.
Published: July 2002

We measured the performance of dc superconducting quantum interference devices (SQUIDs) fabricated from YBa2Cu3O7−δ films as a function of the SQUID inductance, L, and normal resistance, Rn, of step-edge Josephson junctions and compared it to theoretical expectations. A process for adjusting critical currents and obtaining high normal resistances of junctions has been developed. All measurements were performed in liquid nitrogen, i.e., at 77 K. We could obtain controllably normal resistances of 5–10 Ω per junction at critical currents of 20–50 μA. A spectral density of white flux noise of SΦ1/2=2.5μΦ0/√Hz was obtained with a dc SQUID having L=50 pH, Rn=7 Ω per junction, βL=0.5 and ∂V/∂Φ=110 μV/Φ0. The best magnetic field resolution at 1 kHz was 20 fT/√Hz, with L=150 pH.
Efficient production of O+ and O− ions in a helicon wave oxygen discharge
Authors: Mieno T. , Kamo T. , Hayashi D. , Shoji T. , Kadota K.
Published: July 2002

High density oxygen plasmas (ne=1010–1013 cm−3) are produced by a helicon wave discharge source with rf powers of 0.1–3 kW. Positive and negative ion species in the plasmas are measured by a time-of-flight mass spectrometer. The intensity ratio of O+ to O+2 increases with the electron density ne and is almost proportional to it in the region of ne=1010–1012 cm−3. When the electron density increases up to 8×1012 cm−3, the ratio becomes about 4. In a high density plasma of 1.3×1013 cm−3 obtained by mixing Ar gas, about 83% of the positive oxygen ions becomes O+. By using pulse modulation of the rf power, O− ions are mainly observed with remarkable increase in the afterglow. The maximum density of O− is about 3×1011 cm−3 at 30 μs after turning off the rf power of 0.85 kW and the decay time of O− is about 60 μs.
Room-temperature continuous-wave operation of InGaN multi-quantum-well structure laser diodes with a lifetime of 27 hours
Authors: Nakamura Shuji , Senoh Masayuki , Nagahama Shin-ichi , Iwasa Naruhito , Yamada Takao , Matsushita Toshio , Sugimoto Yasunobu , Kiyoku Hiroyuki
Published: July 2002

The continuous-wave (cw) operation of InGaN multi-quantum-well structure laser diodes (LDs) was demonstrated at room temperature (RT) with a lifetime of 27 h. The threshold current and the voltage of the LDs were 80 mA and 5.5 V, respectively. The threshold current density was 3.6 kA/cm2. Longitudinal modes with a mode separation of 0.042 nm were observed under cw operation at RT. When the temperature of the LDs was varied, large mode hopping of the emission wavelength was observed between adjacent quantum well or quantum dot subbands. The carrier lifetime and the threshold carrier density were estimated to be 10 ns and 2×1020/cm3, respectively.
Rubbing-free, vertically aligned nematic liquid crystal display controlled by in-plane field
Authors: Lee S. H. , Kim H. Y. , Park I. C. , Rho B. G. , Park J. S. , Park H. S. , Lee C. H.
Published: July 2002

We fabricated a homeotropically aligned nematic liquid crystal display with positive dielectric anisotropy, whose on and off states are controlled by in-plane field. The rubbing-free device, dark in voltage-off state, reveals bright uniformity in all directions due to the dual domainlike director configuration in the voltage-on state. The electro-optic characteristics of one prototype with excellent viewing angles are reported herein.
CoPt–C nanogranular magnetic thin films
Authors: Delaunay J.-J. , Hayashi T. , Tomita M. , Hirono S. , Umemura S.
Published: July 2002

Cobalt–platinum–carbon thin film was deposited with a chemical composition of Co50Pt15C35. The film had a nanogranular morphology with a grain size ranging from 5 to 15 nm. It consisted of cobalt–platinum grains which had a faulted hexagonal close-packed phase and were separated by graphitelike carbon boundaries. The film in-plane coercivity was 1500 Oe, compared to a few hundreds oersteds in the case of cobalt–carbon. This result establishes a way of fabricating high coercivity cobalt–carbon based materials, which have potential applications as high density magnetic recording media.
Ultrasound induced lubricity in microscopic contact
Authors: Dinelli F. , Biswas S. K. , Briggs G. A. D. , Kolosov O. V.
Published: July 2002

A physical effect of ultrasound induced lubricity is reported. We studied the dynamic friction dependence on out-of-plane ultrasonic vibration of a sample using friction force microscopy and a scanning probe technique, the ultrasonic force microscope, which can probe the dynamics of the tip–sample elastic contact at a submicrosecond scale. The results show that friction vanishes when the tip–surface contact breaks for part of the out-of-plane vibration cycle. Moreover, the friction force reduces well before such a break, and this reduction does not depend on the normal load. This suggests the presence on the surface of a layer with viscoelastic behavior.
Noncollinearly phase-matched femtosecond optical parametric amplification with a 2000 cm−1 bandwidth
Authors: Shirakawa Akira , Kobayashi Takayoshi
Published: July 2002

An optical parametric amplifier generating as short as 14 fs pulses in a visible region has been constructed in a noncollinear phase-matching configuration. The group-velocity matching between the signal and idler enormously broadens the gain bandwidth up to 2000 cm−1, which is mainly limited by the chirp of the seed pulses. Pulses shorter than 20 fs are tunable from 550 to 690 nm by scanning the delay-line of the pump beam.
Independent detection of vertical and lateral forces with a sidewall-implanted dual-axis piezoresistive cantilever
Authors: Chui B. W. , Kenny T. W. , Mamin H. J. , Terris B. D. , Rugar D.
Published: July 2002

A dual-axis atomic force microscope (AFM) cantilever with independent piezoresistive sensors has been developed for simultaneous detection of vertical and lateral forces. The cantilever consists of a flat, triangular probe connected to a base by four tall, narrow ribs. The vertically compliant triangular probe and the laterally compliant ribs incorporate separate piezoresistors for vertical and lateral force sensing. In the fabrication process, a special oblique ion implant technique is used to produce electrical elements on vertical sidewalls and horizontal surfaces of the cantilever structure at the same time. The dual-axis cantilever has been used to perform microfriction measurements as well as obtain simultaneous vertical-force and lateral-force AFM images.
Temperature-dependent terahertz output from semi-insulating GaAs photoconductive switches
Authors: Markelz A. G. , Heilweil E. J.
Published: July 2002

The temperature dependence of the terahertz (THz) output power and spectra from biased photoconductive switches was measured for several antenna gap widths and applied biases. The spectrally integrated THz output had a nonmonotonic temperature dependence in all cases with the value increasing by a factor of 3 from room temperature to 150 K for low biases and 100 K at high biases. An abrupt decrease in output power occurs below 90 K, and the spectrum shifts to lower frequencies as the temperature is lowered.
Persistent photoconductivity in Cu(In,Ga)Se2 heterojunctions and thin films prepared by sequential deposition
Authors: Rau U. , Schmitt M. , Parisi J. , Riedl W. , Karg F.
Published: July 2002

A characteristic, reversible metastability is observed for Cu(In,Ga)Se2 thin films and ZnO/CdS/Cu(In,Ga)Se2 heterojunctions. Annealing at 80 °C leads to a decrease of the dark conductivity of the thin films by up to a factor of 2 at room temperature and several orders of magnitude when measured at lower temperatures. By exposure to light, the initial state can be re-established. This reenhancement of the dark conductivity can be looked at as persistent photoconductivity. Admittance measurements at Cu(In,Ga)Se2 heterojunction solar cells display a reversible shift of the activation energy of a distinct dielectric loss peak ranging from 70 to about 160 meV upon illumination or annealing at 80 °C, respectively. We propose that both phenomena as well as commonly observed light-soaking effects of Cu(In,Ga)Se2 solar cells have a common origin.
Hydrogen desorption and adsorption measurements on graphite nanofibers
Authors: Ahn C. C. , Ye Y. , Ratnakumar B. V. , Witham C. , Bowman R. C. , Fultz B.
Published: July 2002

Graphite nanofibers were synthesized and their hydrogen desorption and adsorption properties are reported for 77 and 300 K. Catalysts were made by several different methods including chemical routes, mechanical alloying, and gas condensation. The nanofibers were grown by passing ethylene and H2 gases over the catalysts at 600 °C. Hydrogen desorption and adsorption were measured using a volumetric analysis Sieverts’ apparatus, and the graphite nanofibers were characterized by transmission electron microscopy and Brunauer–Emmett–Teller surface area analysis. The absolute level of hydrogen desorption measured from these materials was typically less than the 0.01 H/C atom, comparable to other forms of carbon.
Abiotic synthesis of amino acids by x-ray irradiation of simple inorganic gases
Authors: Takahashi Jun-ichi , Hosokawa Teruo , Masuda Hitomi , Kaneko Takeo , Kobayashi Kensei , Saito Takeshi , Utsumi Yuichi
Published: July 2002

1–2 keV x-ray irradiation was carried out using a synchrotron radiation source on simulated primitive earth environment: a gas mixture of carbon monoxide, nitrogen, and water at atmospheric pressure. High-speed liquid chromatography of the hydrolyzed product solution detected amino acids. The amount of amino acids increased with the total energy absorbed by the gas molecules, and the antipodal optical isomers were generated in almost equal quantities. These imply that the precursors for amino acids were produced through x-ray-induced photolysis of inorganic molecules followed by recombination and polymerization into bio-organic compounds. These results suggest that prebiotic formation of amino acids is possible in primitive earth atmosphere by x ray as well as cosmic ray.
Self-collimating phenomena in photonic crystals
Authors: Kosaka Hideo , Kawashima Takayuki , Tomita Akihisa , Notomi Masaya , Tamamura Toshiaki , Sato Takashi , Kawakami Shojiro
Published: July 2002

We found that self-determining collimated light is generated in a photonic crystal fabricated on silicon. The divergence of the collimated beam is insensitive to that of the incident beam and much smaller than the divergence that would be generated in conventional Gaussian optics. The incident-angle dependence of the self-collimated light propagation including lens-like divergent propagation was interpreted in terms of the highly modulated dispersion surfaces with inflection points, where the curvature changes from downward to upward corresponding to respectively a concave/convex-lens case. This demonstration is an important step towards controlling beam profile in photonic crystal integrated light circuits and towards developing “photonic crystalline optics.”
Coexistence of ferroelectricity and ferromagnetism in BiFeO3–BaTiO3 thin films at room temperature
Authors: Ueda Kenji , Tabata Hitoshi , Kawai Tomoji
Published: July 2002

( Bi 0.7 Ba 0.3 ) ( Fe 0.7 Ti 0.3 )O 3 films have been constructed on Nb-doped SrTiO3 (100) by the pulsed-laser deposition technique, and their physical properties have been examined. The films exhibit both ferroelectricity and ferromagnetism (weak ferromagnetism) with Pr=2.5 μC/cm2 and Mr=0.2 emu/g at room temperature. The film-thickness dependence of their magnetic and electric properties (size effect) is also discussed simultaneously. The dielectric constants of the films decrease with reducing film thickness below 1000 Å. The magnetic Curie temperature of the films, on the other hand, does not change at all down to 250 Å.
Electron field emission from silicon nanowires
Authors: Au Frederick C. K. , Wong K. W. , Tang Y. H. , Zhang Y. F. , Bello I. , Lee S. T.
Published: July 2002

Silicon nanowires (SiNWs) were synthesized using laser ablation. A continuous SiNW film was prepared by grinding the pieces of sponge-like SiNWs to powder, then dispersing and sticking the powder onto a Si wafer. The field emission characteristics of the SiNW film were studied based on current–voltage measurements and the Fowler–Nordheim equation. The electron field emission increased with decreasing diameter of SiNWs. A hydrogen plasma treatment of the SiNW film aimed at reducing the oxide overlayer improved the emission uniformity of the film.
Molecular dynamics simulation of thermal conductivity of silicon nanowires
Authors: Volz Sebastian G. , Chen Gang
Published: July 2002

We investigate the thermal conductivity of silicon nanowires based on molecular dynamics (MD) simulations. The simulated thermal conductivities of nanowires with square cross sections are found to be about two orders of magnitude smaller than those of bulk Si crystals in a wide range of temperatures (200–500 K) for both rigid and free boundary conditions. A solution of the Boltzmann transport equation is used to explore the possibility of explaining the MD results based on boundary scattering.
Controlled Li doping of Si nanowires by electrochemical insertion method
Authors: Zhou G. W. , Li H. , Sun H. P. , Yu D. P. , Wang Y. Q. , Huang X. J. , Chen L. Q. , Zhang Z.
Published: July 2002

Si nanowires (NWs) were doped with large amounts of Li+ ions by an electrochemical insertion method at room temperature. Si NWs with different doping levels were obtained by controlling the discharging/charging of Li/Si NWs cell. The microstructures of Si NWs with different doses of Li+ ions were investigated by high-resolution electron microscopy. The crystalline structure of the Si NWs was destroyed gradually with the increasing of Li+ ion dose. When the Li+ ions were extracted from the amorphous Li-doped Si NWs by the same electrochemical method, local ordering of atoms occurred and recrystallization was observed. The photoluminescence peak and intensity of Li+-doped Si NWs are closely related to the doping dose.
Supercapacitors based on nanostructured carbon electrodes grown by cluster-beam deposition
Authors: Diederich L. , Barborini E. , Piseri P. , Podestà A. , Milani P. , Schneuwly A. , Gallay R.
Published: July 2002

Nanostructured carbon films have been grown at room temperature by supersonic cluster beam deposition. Due to a structure based on nanotube embryos and a porosity with grain sizes of a few tens of nanometers, these films have a highly accessible surface area needed for electrochemical applications such as supercapacitors. Films with a density of 1 g/cm3 show, in the dc regime, a specific capacitance per electrode of 75 F/g on a single-cell device with polycarbonate as the organic electrolyte. The resulting energy and power densities of cluster-assembled carbon electrodes are 76 Wh/kg and 506 kW/kg. The possibility of depositing nanostructured films over a large area on a variety of substrates makes cluster-beam deposition very interesting for the realization of supercapacitors.
Defect-related local-electric-field impact on p–n junction parameters
Authors: Czerwinski A.
Published: July 2002

A method is proposed to accurately determine the electric-field impact on a p–n junction. The generation current component is separated from the total leakage current, with correction for the reverse-bias-dependent diffusion current. The sources of the electric-field enhancement are considered. The type of the dominant center, its distribution and energy, the local electric field due to imperfections, and the mechanical stress are determined.
Dephasing of coherent phonons by lattice defects in bismuth films
Authors: Hase Muneaki , Ishioka Kunie , Kitajima Masahiro , Ushida Kiminori , Hishita Shunichi
Published: July 2002

We have studied the effect of point defects on coherent optical phonons in ion-implanted bismuth polycrystalline films. Ultrafast dynamics of coherent phonons and photogenerated carriers in the femtosecond time domain have been investigated by means of pump-probe reflectivity measurements. The dephasing rate of the A1g phonon increases linearly with increasing ion dose, which is explained by the additional dephasing process of the coherent phonon originated from scattering of phonons by the defects. Carrier dynamics are also found to be affected by additional scattering process mediated by point defects.