bluuebunny/crossref_metadata_2025 huggingface dataset:
Genetic Variation for Morphological and Allozyme Variation in Relation to Population Size in Clarkia dudleyana, an Endemic Annual
Authors: Podolsky Robert H.
Published: July 2002

Abstract: The maintenance of genetic variation within populations is expected to allow species to respond to evolutionary challenges such as selection and environmental stress. Larger populations are generally expected to maintain larger amounts of genetic variation. Although several studies have found a positive relationship between population size and levels of genetic variation for molecular markers such as allozymes, few comparisons have been made between molecular measures of variation and genetic variation that is likely to be ecologically important. Most ecologically important traits require quantitative genetic analyses. I examined the relationship between levels of genetic variation and population size for both allozymes and morphological traits in a California endemic annual plant, Clarkia dudleyana. Levels of genetic variation for allozymes did not show a significant positive relationship with population size. The level of genetic variance for all of the 18 morphological traits exhibited no significant relationship with population size. Further, allozyme heterozygosities were not related to levels of quantitative genetic variation. These results indicate that levels of allozyme variability do not predict levels of genetic variation for morphological traits in C. dudleyana, suggesting that molecular measures of variation, in general, differ from quantitative genetic measures. These results imply that conservation genetic studies should generally focus on aspects other than measuring levels of genetic variation found within populations.
Growth of InSb and InAs1−xSbx on GaAs by molecular beam epitaxy
Authors: Chyi J.-I. , Kalem S. , Kumar N. S. , Litton C. W. , Morkoç H.
Published: July 2002

InSb and InAs1−xSbx epitaxial layers have been successfully grown on (100)GaAs substrates by molecular beam epitaxy. Remarkably good morphologies were obtained despite the large lattice mismatch (14%) between InSb and GaAs. Room-temperature electron mobilities as high as 57 000 cm2/V s were measured in InSb layers of about 5 μm thick with ND−NA∼1.6×1016 cm−3. The substrate temperature and Sb/In flux ratio were found to critically influence the quality of InSb epilayers. By employing dimeric instead of tetrameric sources, the composition of the InAs1−xSbx films was observed to be relatively independent of substrate temperature. Electron mobilities of 20 000 and 8800 cm2/V s, at 300 and 77 K, respectively, were obtained for a 1.6-μm-thick InAs1−xSbx (x=0.67) layer.
Arsenic-doped CdTe epilayers grown by photoassisted molecular beam epitaxy
Authors: Harper R. L. , Hwang S. , Giles N. C. , Schetzina J. F. , Dreifus D. L. , Myers T. H.
Published: July 2002

We report the successful p-type doping of CdTe films with arsenic using the photoassisted molecular beam epitaxy growth technique. These doped epilayers were grown at substrate temperatures as low as 180 °C. The room-temperature hole concentrations in the CdTe:As layers ranged from 7×1015 to 6.2×1018 cm−3 as determined by van der Pauw–Hall effect measurements. We propose a doping mechanism responsible for the high p-type doping levels observed in the films. The arsenic acceptor ionization energy was found to ∼58–60 meV using low-temperature photoluminescence measurements.
Quantum-switched heterojunction bistable bipolar transistor by chemical beam epitaxy
Authors: Wu Ming C. , Yang Long , Tsang W. T.
Published: July 2002

We proposed and demonstrated a novel bistable transistor−the quantum-switched heterojunction bistable bipolar transistor. The transistor has two current states. With increasing base-emitter voltage, the collector current is switched from high to low, while the base current is switched from low to high. Bistability is observed for a certain range of base voltage. This device has potential applications in implementing high-speed single bipolar transistor memories, gain quenching in light-emitting devices, and optoelectronic switching.
New molecular design approach for noncentrosymmetric crystal structures: Lambda (Λ)-shaped molecules for frequency doubling
Authors: Yamamoto H. , Katogi S. , Watanabe T. , Sato H. , Miyata S. , Hosomi T.
Published: July 2002

Second-order optical nonlinearity exists in a noncentrosymmetric crystal. We investigated a new molecular concept for noncentrosymmetric crystal structure. The Λ-shaped molecules have been found to easily form noncentrosymmetric crystals by stacking along one direction. The configuration of Λ-shaped molecules is close to an optimum condition for crystal phase-matched nonlinearity. We demonstrate that methanediamine derivatives forming Λ-shaped conformation are promising as efficient χ(2) materials.
Performance of multilayers in intense synchrotron x-ray beams
Authors: Ziegler E. , Lepêtre Y. , Joksch S. , Saile V. , Mourikis S. , Viccaro P. J. , Rolland G. , Laugier F.
Published: July 2002

The use of multilayer reflectors under intense synchroton x-ray beams requires to develop a new generation of multilayered materials that can withstand a high-power load in excess of 100 W/mm2. Multilayers with the high-Z layer consisting either of a pure element or of compounds such as carbides, nitrides, or silicides have been produced. Because the fabrication conditions are not yet optimized, thin films with satisfactory layer were not obtained leading to poor reflectivities. Such multilayers have been both thermally annealed in a furnace and exposed to a synchrotron beam with a power density of about 1 W/mm2. The resulting damage ranges from the total destruction of the layering to a reduction of the reflectivity by typically 40%–60%. In some cases an only 1%–15% loss in reflectivity has been observed.
New radiation detector of plastic scintillation fiber
Authors: Imai Shun-ichi , Soramoto Seiki , Mochiki Ko-ichi , Iguchi Tetsuo , Nakazawa Masaharu
Published: July 2002

Fundamental characteristics of a new plastic scintillation fiber (PSF) has been studied for four kinds of radiations, and the possibility of using the PSF as a radiation detector has been discussed. Irradiation experiments of scintillation output characteristics have been made for γ rays, x rays, fast neutrons, and α particles. Various scintillation pulse height spectra have been obtained, depending on the variations of radiation types, energies, intensities, and scintillating positions on the PSF. Position detection experiments have been also made for γ rays and fast neutrons, and a new position sensitive detector of the PSF has been proposed.
Combined elevated pressure reactor and ultrahigh vacuum surface analysis system
Authors: Szanyi János , Goodman D. W.
Published: July 2002

A combined elevated pressure reactor/ultrahigh vacuum (UHV) surface analytical system has been designed for studies of heterogeneous catalytic reactions. The reaction cell is separated from the surface analysis chamber by a differentially pumped sliding seal. This feature allows simple in situ sample transfer capabilities from the surface analysis region to the reactor cell and also facilitates the introduction and repair of the sample and/or holder with no significant perturbation of the UHV analysis chamber vacuum. The pumping arrangements provide for rapid evacuation of the reaction cell as well as maximum sensitivity with respect to the detection of reaction products.
Upconversion luminescence of Er3+ in CdX2 system glasses (X=Cl, Br, I)
Authors: Shojiya Masanori , Takahashi Masahide , Kanno Ryoji , Kawamoto Yoji , Kadono Kohei
Published: July 2002

Near-infrared to visible upconversion characteristics of Er3+ in CdX2-based halide glasses (X=Cl, Br, or I) has been investigated. When 4I9/2 state is pumped by 800 nm radiation from an AlGaAs laser diode, the upconverted green emissions (520–560 nm) of 2H11/2+4S3/2→I15/2 transition are clearly observed in all the glasses. The linewidth of green emission in these glasses becomes narrow in the order of chloride, bromide, and iodide. The CdCl2 and CdBr2 system glasses give blue upconversion luminescence (around 410 nm) due to the 2H9/2→I15/2 transition, but this emission is not observed in the CdI2 system glass because of the absorption edge at around 450 nm. The fluorescence spectra of CdX2 system glasses suggest that the multiphonon relaxation rate depends on choice of the X species.
Improved compositional abruptness at the InGaAs on GaAs interface by presaturation with In during molecular-beam epitaxy
Authors: Kaspi R. , Evans K. R.
Published: July 2002

Surface segregation of In atoms during molecular beam epitaxy of InGaAs layers greatly influences the composition profile in the vicinity of both the normal and the inverted Ga(Al)As/InGaAs interface, inherently limiting compositional abruptness. We find, for example, that the intended alloy composition in In0.22Ga0.78As is not reached until nearly 35 Å from the InGaAs on GaAs interface for growth at 500 °C. We propose and demonstrate how the compositionally graded region in InGaAs can be eliminated by preadsorbing a fixed amount of In onto the GaAs surface to match the surface segregated layer during steady state, before depositing the InGaAs layer.
Estimation of scattering losses in dielectrically apertured vertical cavity lasers
Authors: Hegblom E. R. , Babic D. I. , Thibeault B. J. , Coldren L. A.
Published: July 2002

Dielectric apertures, formed by oxidation or wet-etching of high Al content AlGaAs layers in vertical cavity lasers, have recently been used for improved device performance. In this letter an iterative numerical analysis is used to estimate the excess optical losses as the device diameter is reduced. For smaller sizes, where single-mode devices are possible, the optical scattering losses become significant unless the aperture is tapered to approximate a perfect lens or is thin to approximate a weaker waveguide.
Effective generation-recombination parameters in high-energy proton irradiated silicon diodes
Authors: Simoen E. , Vanhellemont J. , Claeys C.
Published: July 2002

The degradation of the generation and the recombination lifetime of silicon junction diodes by 10 MeV proton irradiation is compared with the introduction of the radiation-induced deep levels. It is shown that for the fluence range studied, both the reverse current and the reciprocal lifetime increase linearly with 10 MeV H+ fluence. From this study it follows that n-type Si is more prone to high-energy proton irradiation damage than p-type material. From the electrical diode characteristics, it is derived that the dominant generation center in both n- and p-type Float-Zone Si is approximately 0.12 eV from the midgap position. This strongly suggests that in both types of material, the divacancy level at Ec−0.42 eV is the dominant generation center.
Plastic microring lasers on fibers and wires
Authors: Frolov S. V. , Vardeny Z. V. , Yoshino K.
Published: July 2002

Photopumped, pulsed, narrow line laser emission is demonstrated using cylindrical microcavities formed by π-conjugated polymer thin films wrapped around thin glass optical fibers and metal wires with various diameters D. A variety of cavity-dependent resonant laser mode structures were observed, which for D<10 μm contain a single resonant spectral line of less than 1 Å in width. The microring lasers are also characterized by a well-defined, very low threshold excitation intensity, at which beam directionality and polarization degree dramatically increase. These findings open up the fields of lasers and fiber optics to organic materials.
Magnetoresistance of artificial La0.7Sr0.3MnO3 grain boundaries as a function of misorientation angle
Authors: Isaac S. P. , Mathur N. D. , Evetts J. E. , Blamire M. G.
Published: July 2002

The resistance of polycrystalline doped LaMnO3 materials has been shown to be highly sensitive to low magnetic fields. To enable direct study of the properties of isolated grain boundaries we have grown epitaxial La0.7Sr0.3MnO3 films on a series of bicrystal substrates with different misorientation angles and patterned the films into a Wheatstone bridge geometry. We show that the grain boundary resistance and its magnetic field dependence vary strongly with the misorientation angle. The temperature dependence of the grain boundary resistance is also presented. We have obtained resistance changes of over 3% in fields of 2 mT at 300 K.
Enhanced critical currents in melt textured YBa2Cu3O7 by cold isostatic pressing
Authors: Martı́nez B. , Sandiumenge F. , Puig T. , Obradors X. , Richard L. , Rabier J.
Published: July 2002

An enhancement of the critical currents in the H‖c configuration, reaching 100% at 77 K, has been obtained in melt textured YBa2Cu3O7/Y2BaCuO5 composites after a cold isostatic pressing process (300 °C, PAr=200 MPa). A transmission electron microscopy analysis of the microstructure demonstrates that a high density of nanometric stacking fault loops, with anisotropic growth within the (001) plane, is generated. The improvement of critical currents is attributed to the flux pinning enhancement due to the increased length of the 1/6〈031〉 partial dislocations surrounding nanometric stacking faults. We stress that this technique has high potential for the large scale processing of high critical currents superconducting ceramic composites.
Experimental demonstration of photonic crystal based waveguides
Authors: Temelkuran B. , Ozbay E.
Published: July 2002

We report the experimental demonstration of waveguides built around layer-by-layer photonic crystals. An air gap introduced between two photonic crystal walls was used as the waveguide. We observed full (100%) transmission of the electromagnetic (EM) waves through these planar waveguide structures within the frequency range of the photonic band gap. The dispersion relations obtained from the experiment were in good agreement with the predictions of our waveguide model. We also observed 35% transmission for the EM waves traveling through a sharp bend in an L-shaped waveguide carved inside the photonic crystal.
Diagnostics of laser ablated plasmas using fast photography
Authors: Misra A. , Mitra A. , Thareja R. K.
Published: July 2002

We report on dynamics of the laser ablated plumes in an ambient atmosphere using fast photography. The expanding plasma is studied at different delay times with respect to the ablating pulse. Dependence of plasma parameters such as velocity, temperature, density, and pressure on time and ambient atmosphere is presented. The images of the expanding plume are used to estimate the size of the particles in the plume.
Six-bilayer periodic structures in GaN grown on GaAs(001)
Authors: Funato Mitsuru , Ishido Teruki , Fujita Shizuo , Fujita Shigeo
Published: July 2002

We have observed six-bilayer periodic structures in GaN grown on GaAs(001). The periodicity occurs along the zinc-blende(ZB)-[111]A direction, suggesting that it originates from stacking faults on close-packed planes. GaN grown on GaAs includes both ZB and wurtzite phases as a result of formation of stacking faults and the periodic structures are mostly located between these two crystalline phases. Based on this observation, possible layer stacking sequences are proposed, which are classified as 6H polytypes.
Depletion and bridging forces in polymer systems: Monte Carlo simulations at constant chemical potential
Authors: Broukhno Andrei , Jönsson Bo , Åkesson Torbjörn , Vorontsov-Velyaminov Pavel N.
Published: July 2002

A new Monte Carlo simulation method designed for polymer solutions confined to planar slits is presented. The slit is in equilibrium with a surrounding bulk solution and the method allows a variation of the slit width while maintaining the polymer chemical potential constant. This is achieved by changing the tangential pressure as a function of slit width. An analysis of chain parameters and monomer distribution within the slit has been carried out. The model system used is supposed to mimick a macromolecular solution whose stability is manipulated by addition of adsorbing and/or nonadsorbing polymers. Generally, for the nonadsorbing polymer an attractive depletion force is found. At high volume fractions the attraction is reduced and a repulsive force appears at short separations. The depletion force can also be extinguished in the case of an adsorption potential of intermediate strength, while strong adsorption gives rise to a significant attraction due to polymer bridges.
Effects of nitric-oxide-plasma treatment on the electrical properties of tetraethylorthosilicate-deposited silicon dioxides on strained-Si1−xGex layers
Authors: Senapati B. , Samanta S. K. , Maikap S. , Bera L. K. , Maiti C. K.
Published: July 2002

Gate-quality ultrathin silicon dioxide films on strained-Si0.74Ge0.26 layers have been deposited by microwave plasma-enhanced chemical vapor deposition technique using tetraethylorthosilicate. Effect of nitric-oxide (NO)-plasma treatment on the electrical properties of the deposited oxides have been studied using a metal–insulator–semiconductor structure. A significant improvement in the interface trap level density (Dit) and charge trapping behavior under Fowler–Nordheim constant current stressing is observed for NO-plasma treated deposited oxide films.
Co
2+
:LiGa
5
O
8
saturable absorber passive Q switch for 1.34 μm Nd3+:YAlO3 and 1.54 μm Er3+:glass lasers
Authors: Denisov I. A. , Demchuk M. I. , Kuleshov N. V. , Yumashev K. V.
Published: July 2002

Passive Q switching of a Nd3+:YAlO3 laser at 1.34 μm and an Er3+: glass laser at 1.54 μm was demonstrated using a Co2+:LiGa5O8 crystal as a saturable absorber. Q-switched pulses at 1.34 μm of 13 mJ energy and 90 ns duration and at 1.54 μm of 8 mJ energy and 70 ns duration were obtained. The ground state absorption cross section of the Co2+:LiGa5O8 was estimated to be (2.6±0.4)×10−19 cm2 and (2.5±0.5)×10−19 cm2 at 1.34 and 1.54 μm, respectively. The4T1(4F)→4A2 relaxation time of the Co2+ ion in the LiGa5O8 crystal was measured to be ∼130 ns.
Experimental imaging diagnosis of superconducting tunnel junction x-ray detectors by low-temperature scanning synchrotron microscope
Authors: Pressler H. , Ohkubo M. , Koike M. , Ukibe M. , Zama T. , Nakamura T. , Katagiri M.
Published: July 2002

Imaging diagnosis of superconducting tunnel junction x-ray detectors has been performed by an apparatus called the low-temperature scanning synchrotron microscope (LTSSM) using an x-ray microbeam with a diameter of 5–10 μm originated from synchrotron radiation. Quasiparallel intense synchrotron radiation enables one to obtain the full two-dimensional images of junctions with dimensions of 200×200 μm2 in about 1 h. The LTSSM results indicate that the standard quasiparticle diffusion and edge loss model for the spatial distribution of the junction response to x rays is evidently inadequate for intermediate or large junctions (with respect to a Josephson penetration depth). On this basis, it is argued that the models proposed for the signal creation and loss mechanism should be reconsidered.
Photoluminescence and structural characteristics of CdS nanoclusters synthesized by hydrothermal microemulsion
Authors: Liu Bing , Xu G. Q. , Gan L. M. , Chew C. H. , Li W. S. , Shen Z. X.
Published: July 2002

Spherical and uniform CdS nanoclusters were synthesized by hydrothermal microemulsion. The reaction of Cd2+ ions with S2− ions generated from the decomposition of thioacetamide proceeded in water microdroplets. The mean diameter of the CdS nanoclusters can be varied from 20 to 80 nm by increasing the reaction temperature from 30 to 120 °C. XRD results indicate that the resulting CdS nanoclusters have a reduced and distorted hexagonal lattice compared to bulk materials. Two intense luminescence bands, i.e., green and red, were observed to coexist in the CdS nanoclusters. Their peak positions and relative intensities were found to be sensitive to the size and structure of nanoclusters. These emissions are attributed to surface defects (green emission) and the Cd–Cl composite vacancies (red emission).
Ultrafast soft x-ray two-dimensional plasma imaging system based on gas electron multiplier detector with pixel readout
Authors: Pacella D. , Pizzicaroli G. , Gabellieri L. , Leigheb M. , Bellazzini R. , Brez A. , Gariano G. , Latronico L. , Lumb N. , Spandre G. , Massai M. M. , Reale S.
Published: July 2002

In the present article a new diagnostic device in the soft x-ray range, for magnetic fusion plasmas, is proposed based on a gas electron multiplier detector with 2.5×2.5 cm active area, equipped with a true two-dimensional readout system. The readout printed circuit board, designed for these experiments, has 128 pads. Each pad is 2 mm square and covers a roughly circular area. The operational conditions of the detector are settled to work in the x-ray range 3–15 keV at very high counting rates, with a linear response up to 2 MHz/pixel. This limitation is due to the electronic dead time. Images of a wrench and two pinholes were done at rates of 2.5 MHz/pixel with a powerful x-ray laboratory source showing an excellent imaging capability. Finally preliminary measurements of x-ray emission from a magnetic fusion plasma were performed on the Frascati tokamak upgrade experiment. The system was able to image the plasma with a wide dynamic range (more than a factor of 100), with a sampling frequency of 20 kHz and with counting rates up to 4 MHz/pixel, following the changes of the x-ray plasma emissivity due to additional radio frequency heating. The spatial resolution and imaging properties of this detector have been studied in this work for conditions of high counting rates and high gain, with the detector fully illuminated by very intense x-ray sources (laboratory tube and tokamak plasma).
Hydrogen storage in aligned carbon nanotubes
Authors: Chen Yan , Shaw David T. , Bai X. D. , Wang E. G. , Lund C. , Lu W. M. , Chung D. D. L.
Published: July 2002

Aligned carbon nanotubes (CNTs) with diameters of 50–100 nm, synthesized by plasma-assisted hot filament chemical vapor deposition, were employed for hydrogen adsorption experiments in their as-prepared and pretreated states. Quadruple mass spectroscopy and thermogravimetric analysis show a hydrogen storage capacity of 5–7 wt% was achieved reproducibly at room temperature under modest pressure (10 atm) for the as-prepared samples. Pretreatments, which include heating the samples to 300 °C and removing of the catalyst tips, can increase the hydrogen storage capacity up to 13 wt% and decrease the pressure required for storage. The weight gains were measured after the samples moved out of the hydrogen environment. The release of the adsorbed hydrogen can be achieved by heating the samples up to 300 °C.
Application of periodically modulated driving force to the transition kinetics in vinylidene fluoride/trifluoroethylene copolymers
Authors: Toda Akihiko , Takahashi Yoshiyuki , Arita Takeshi , Hikosaka Masamichi , Furukawa Takeo
Published: July 2002

The method of “modulated driving force” has been applied to the kinetics of ferroelectric–paraelectric transition in copolymers of vinylidene fluoride (VDF) with trifluoroethylene (TrFE). The method examines the response of transition kinetics to a periodically modulated driving force, e.g., supercooling or superheating. The response to the modulation in temperature appears in the apparent heat capacity obtained by a temperature-modulated differential scanning calorimeter. By examining the frequency dispersion and its dependence on underlying linear heating (or cooling) rate, the mean time required for the completion of transition in each crystallite and the dependence of transition rate on superheating (or supercooling) are obtainable. In VDF/TrFE copolymers, it is known that the transition behavior undergoes a drastic change from reversible transition with low VDF content to nucleation-controlled transition with higher content. Several types of compositions (VDF/TrFE=47/53, 52/48, 59/41, 65/35, 69/31 and 73/27 by mol %) have been examined experimentally with this method in terms of the crossover of transition behaviors.
Radiative recombination from InP quantum dots on (100) GaP
Authors: Hatami F. , Masselink W. T. , Schrottke L.
Published: July 2002

We describe the growth and optical emission from strained InP quantum dots grown on GaP using gas-source molecular beam epitaxy. Self-organized island formation takes place for InP coverage greater than 1.8 monolayers on the (100) GaP surface. Intense photoluminescence from the dots is peaked at about 2.0 eV, blueshifted by 0.6 eV from the band gap of bulk InP due to strain, quantum size effects, and possibly Ga interdiffusion.
Domain wall propagation in a Fe-rich glass-coated amorphous microwire
Authors: Zhukov A.
Published: July 2002

The temperature and field dependencies of the velocity of domain wall propagation have been measured in the range of temperatures 5–300 K in amorphous Fe65B15Si15C5 glass-coated microwires. The domain wall velocity within the experimental range of magnetic fields (65–100 A/m) and temperatures (5–300 K) was found between 35 and 65 m/s. Consequently, the domain wall mobility was estimated from these dependencies. The critical field for the domain wall propagation is below the switching field reflecting the existence of an energy barrier for the domain wall depinning.
Spectroscopic properties of Pr3+-doped Ca4GdO(BO3)3 (GdCOB)
Authors: Brenier A. , Kityk I. V.
Published: July 2002

The main spectroscopic properties (absorption, emission, decay times) of Pr3+-doped Ca4GdO(BO3)3 (GdCOB) have been determined for the three X, Y, Z polarizations. The Judd–Ofelt analysis, extended to anisotropic crystals, normalized and modified to take into account the influence of the 4f5d lowest energy level, has been performed and gives reasonable agreement with experimental data. The band energy approach is used for estimations of probabilities of relative cross relaxations and multiphonon relaxation.
Quartz tuning forks as sensors for attractive-mode force microscopy under ambient conditions
Authors: King G. M. , Lamb J. S. , Nunes G.
Published: July 2002

We present investigations of the frequency versus distance behavior of a quartz tuning-fork-based atomic force microscope. We show that if the amplitude of the motion A of the tip is large, then the apparent shape of the tip–surface interaction curve depends on A. For smaller amplitudes of oscillation (A≲3 nm), we find that the shape of the interaction curve becomes independent of A. In this low amplitude limit, a simple relation between the observed frequency shift and the underlying interaction allows quantitative determination of tip–sample forces. Tuning fork sensors open a window for dynamic-mode force microscopy in a regime where conventional microfabricated sensors are overwhelmed by long range capillary forces.
Detection of single x-ray photons by an annular superconducting tunnel junction
Authors: Frunzio L. , Li L. , Prober D. E. , Vernik I. V. , Lisitskii M. P. , Nappi C. , Cristiano R.
Published: July 2002

We present an experiment that detects photons by use of an annular Nb-based superconducting tunnel junction (STJ). In one magnetic field configuration, we stably trapped a single magnetic fluxon in the STJ barrier during a transition to the superconducting state. This is an innovative configuration which avoids the use of an externally applied field during detector operation. It offers potential benefits for STJs used in imaging arrays. In this configuration, and also in the conventional one with an externally applied parallel magnetic field, we observed current pulses produced by single 6 keV x-ray photons. The pulses were the same for the two configurations.
Quantifying electrical spin injection: Component-resolved electroluminescence from spin-polarized light-emitting diodes
Authors: Jonker B. T. , Hanbicki A. T. , Park Y. D. , Itskos G. , Furis M. , Kioseoglou G. , Petrou A. , Wei X.
Published: July 2002

The spin-polarized light-emitting diode (spin-LED) is a very effective tool for accurately quantifying electrical spin injection in a model independent manner. We resolve and identify various components which occur in the electroluminescence (EL) spectra of GaAs quantum-well-based spin-LEDs, and examine the circular polarization of each. While a number of components exhibit significant circular polarization, the values do not necessarily reflect the electrical spin injection efficiency. We show that a reliable measure of spin injection efficiency can be obtained only if one takes care to spectroscopically resolve and accurately identify the free exciton or free carrier components of the EL spectrum, and exclude other components.
Transport cross sections relevant to cool hydrogen plasmas bounded by graphite
Authors: Schultz D. R. , Krstić P. S.
Published: July 2002

Atomic collision quantities relevant to transport in hydrogen plasmas bounded by graphite walls are considered. Fully quantal, ab initio calculations of the differential and integral elastic scattering cross sections for H+, D+, and T+ colliding with C at center of mass energies between 0.1 and 200 eV are described. The computed elastic cross section and its transport moments, the momentum transfer, and viscosity cross sections, are compared with those from a simple analytical model (the Massey–Mohr approximation) and with a three-body classical scattering approach in order to extend the data to higher collision energies. For energies typical of the edge plasma, the elastic scattering cross section is found to be as much as 10 times larger than that estimated from the widely used analytical approximation. The highly accurate quantal results are also tabulated and made available to the plasma science community through the world wide web.
Containerless solidification of Si, Zr, Nb, and Mo by electrostatic levitation
Authors: Sung Yeon Soo , Takeya Hiroyuki , Togano Kazumasa
Published: July 2002

Containerless solidification of Si (Tm=1687 K), Zr (Tm=2128 K), Nb (Tm=2750 K), and Mo (Tm=2896 K) has been carried out by the electrostatic levitation (ESL) process in which a bulk sample was electrostatically charged and levitated, then melted by laser irradiation in high vacuum. Multiple cycles of melting–undercooling–recalescence–cooling of Si (23.1 mg), Zr (41.9 mg), Nb (40.4 mg), and Mo (45.7 mg) were reproducibly repeated. Among them, Mo is a refractory material with the highest melting temperature ever containerlessly melted and solidified by electrostatic levitation. The ESL-processed Mo sample showed a thermal dendritic structure throughout the entire surface uniformly without any apparent grain boundaries implying heterogeneous nucleation must have been suppressed due to the containerless condition in the ESL process. The back reflection Laue pattern also revealed a homogeneous quality of the ESL-processed Mo sample.
Transport equations for concentrated electrolyte solutions: Reference frame, mutual diffusion
Authors: Dufrêche J.-F. , Bernard O. , Turq P.
Published: July 2002

Transport coefficients of dissociated electrolytes in aqueous solution are studied in the framework of the primitive model of ionic solutions in which the solvent is a dielectric continuum. Simple explicit expressions are obtained by using a combination of Smoluchowski equation for the dynamics and the mean spherical approximation (MSA) for the equilibrium pair correlation function. The case of symmetrical and dissymmetrical electrolytes is examined. In the case of the mutual diffusion, a reference-frame correction, that can be obtained from density measurements has to be taken into account for molar concentrations. The combined Smoluchowski/MSA theory of the primitive model is found to be self-consistent since it is able to describe simultaneously the different transport and equilibrium properties up to high concentrations (1–2 mol l−1).
Sound wave propagation in transition-regime micro- and nanochannels
Authors: Hadjiconstantinou Nicolas G.
Published: July 2002

We present an extension of the existing continuum theory for sound wave propagation in dilute gases in “narrow” two-dimensional channels to arbitrary Knudsen numbers; the theory provides predictions for the wavelength and attenuation coefficient as a function of the oscillation frequency. A channel is considered narrow in the context of wave propagation when its height is much smaller than the characteristic diffusion length based on the wave frequency. This criterion is easily satisfied by small scale (transition-regime) channels for most frequencies of interest. Numerical simulations for a dilute monoatomic gas using the direct simulation Monte Carlo are used to verify the theoretical results. Good agreement is found between theory and simulation.
A hierarchical family of global analytic Born–Oppenheimer potential energy surfaces for the H+H2 reaction ranging in quality from double-zeta to the complete basis set limit
Authors: Mielke Steven L. , Garrett Bruce C. , Peterson Kirk A.
Published: July 2002

A hierarchical family of analytical Born–Oppenheimer potential energy surfaces has been developed for the H+H2 system. Ab initio calculations of near full configuration interaction (FCI) quality (converged to within ≈1 μEh) were performed for a set of 4067 configurations with the aug-cc-pVDZ, aug-cc-pVTZ, and aug-cc-pVQZ basis sets. The complete basis set (CBS) limit energies were obtained using a highly accurate many-body basis set extrapolation scheme. Surfaces were fitted for the estimated CBS limit, as well as for the aug-cc-pVDZ, aug-cc-pVTZ, and aug-cc-pVQZ basis sets using a robust new functional form. The mean unsigned fitting error for the CBS surface is a mere 0.0023 kcal/mol, and deviations for data not included in the fitting process are of similarly small magnitudes. Highly accurate calculations of the saddle point and van der Waals minimum configurations were performed using basis sets as large as aug-mcc-pV7Z, and these data show excellent agreement with the results of the extrapolated potential surface. The remaining errors from fitting, correlation treatment, and basis set incompleteness for the new CBS-limit surface are lower by over an order of magnitude compared to any prior analytic surface, and are all now significantly smaller than non-Born–Oppenheimer effects. We expect that this new family of potential surfaces will prove useful in studies elucidating the sensitivity of dynamical quantities to the quality of the potential surface.
Ferroelectric properties of laser-ablated Sr1−xAxBi2Ta2O9 thin films (where A=Ba, Ca)
Authors: Das Rasmi R. , Bhattacharya P. , Pérez W. , Katiyar Ram S. , Desu Seshu B.
Published: July 2002

Bismuth-layered ferroelectric thin films of Sr1−xAxBi2Ta2O9, with composition x=0 and 0.2, were fabricated by using the pulsed-laser deposition technique. Structural characterization of the films by x-ray diffraction and atomic force microscopy, revealed that the films are polycrystalline in nature with average grain size of 180 nm. The films displayed spherical grains with a surface roughness of 12 nm. The ferroelectric measurements of Sr0.8Ba0.2Bi2Ta2O9, SrBi2Ta2O9, and Sr0.8Ca0.2Bi2Ta2O9 showed saturated hysteretic behavior with remanent polarization (2Pr) of 23.5, 17.9, 14 μC/cm2 and coercive field (Ec) of 31.06, 74.2, 86.3 kV/cm for a maximum applied electric field of 360 kV/cm. Films exhibited minimal (⩽17%) degradation of polarization for up to 1010 switching cycles. It was observed that the coercive field decreased with increase in the ionic size of partially substituted cations. The leakage current density of films were found to be of the order of ∼10−8 A/cm2 for up to a breakdown field of about 75 kV/cm.
Self-assembled monolayer formation on magnetic hard disk surface and friction measurements
Authors: Choi Junho , Kawaguchi Masahiro , Kato Takahisa
Published: July 2002

Self-assembled monolayers (SAMs) of 1H,1H,2H,2H-perfluorodecyltriethoxysilane were deposited on the magnetic hard disk surface by immersion. SAM formation was confirmed by contact angle measurement, ellipsometry, and x-ray photoelectron spectroscopy. Friction of unlubricated, SAM-coated, and perfluoropolyether (PFPE) Zdol-coated disk surfaces was measured with lateral force microscopy, spin-stand tester, and scratch tester. The SAM-coated disk surfaces had lower friction as compared to the unlubricated disk surfaces. This result is in agreement with the hydrophobic nature of the SAM-coated surface. On the other hand, friction of the SAM-coated surface was larger than the 2-nm-thick PFPE-coated surface, even though the hydrophobicity of the SAM-coated surface was higher than that of the PFPE-coated surface. This would be attributed to the mobility of the multilayered PFPE lubricant, whereas the friction of SAM-coated surface was comparable or even better when compared to that of 1.1-nm-thick PFPE-coated surface, which had a small mobile portion.
Pressure-induced magnetic transitions in LaMn2Si2
Authors: Hofmann M. , Campbell S. J. , Knorr K. , Hull S. , Ksenofontov V.
Published: July 2002

The effects of applied hydrostatic pressure on the structural and magnetic behavior of ferromagnetic LaMn2Si2 of tetragonal I4/mmm structure have been investigated by x-ray diffraction (0–4.1 GPa) and neutron diffraction (0–6.1 GPa) measurements. We have observed a pressure-induced ferromagnetic to antiferromagnetic transition at ∼6.1 GPa in LaMn2Si2 by neutron diffraction.
Electrical alternans and spiral wave breakup in cardiac tissue
Authors: Karma Alain
Published: July 2002

This paper reports the results of a theoretical investigation of spiral wave breakup in model equations of action potential propagation in cardiac tissue. A general formulation of these equations is described in which arbitrary experimentally determined restitution and dispersion curves can in principle be fitted. Spiral wave behavior is studied in two-dimension as a function of a parameter Re which controls the steepness of the restitution curve at short diastolic intervals. Spiral breakup is found to occur when the minimum period Tmin, below which a periodically stimulated tissue exhibits alternans in action potential duration, exceeds by a finite amount the spiral rotation period TS. At this point, oscillations in action potential duration are of sufficiently large amplitude to cause a spontaneous conduction block to form along the wavefront. The latter occurs closer to the initiation point of reentry (spiral tip) with increasing steepness and, hence, in smaller tissue sizes. Spiral breakup leads to a spatially disorganized wave activity which is always transient, except for tissues larger than some minimum size and within a very narrow range of Re which increases with dispersion.
Lattice models of advection-diffusion
Authors: Pierrehumbert Raymond T.
Published: July 2002

We present a synthesis of theoretical results concerning the probability distribution of the concentration of a passive tracer subject to both diffusion and to advection by a spatially smooth time-dependent flow. The freely decaying case is contrasted with the equilibrium case. A computationally efficient model of advection-diffusion on a lattice is introduced, and used to test and probe the limits of the theoretical ideas. It is shown that the probability distribution for the freely decaying case has fat tails, which have slower than exponential decay. The additively forced case has a Gaussian core and exponential tails, in full conformance with prior theoretical expectations. An analysis of the magnitude and implications of temporal fluctuations of the conditional diffusion and dissipation is presented, showing the importance of these fluctuations in governing the shape of the tails. Some results concerning the probability distribution of dissipation, and concerning the spatial scaling properties of concentration fluctuation, are also presented. Though the lattice model is applied only to smooth flow in the present work, it is readily applicable to problems involving rough flow, and to chemically reacting tracers.
Reaction kinetic surfaces and isosurfaces of the catalytic hydrogenolysis of ethane and its self-poisoning over Ni and Pd catalysts
Authors: Kristyan Sandor , Szamosi Janos
Published: July 2002

The significance of three-dimensional visualization is exhibited in the analysis of chemical reaction kinetics. The particular example used is from heterogeneous catalysis. It is also shown that this approach can be useful in exploring the mechanistic aspects of kinetics.
A study on thin film Fe2O3 photoanodes by measuring laser-induced photopotential
Authors: Itoh Kiminori , Nakao Masashi , Honda Kenichi
Published: July 2002

Laser-induced fast transient photopotentials were measured at Fe2O3 film electrodes. A photopotential rise within 300 ns was interpreted in terms of charge separation processes in the film, considering the contributions from both the electron and the hole. The time course of the photopotential up to 10 μs was attributed to the electron–hole recombination process and the interfacial charge transfer.
Formation of iron silicide thin films
Authors: Zhu Qi-Gao , Iwasaki Hiroshi , Williams Ellen D. , Park Robert L.
Published: July 2002

The reaction of thin films of Fe with a silicon substrate to form iron silicides has been studied using Auger electron spectroscopy (AES) and electron energy loss spectroscopy. The formation of FeSi and FeSi2 is confirmed by comparison of plasmon losses with those reported for the bulk silicides. Changes in the Fe-AES spectrum upon silicide formation result primarily from changes in the plasmon losses. Changes in the AES intensity are correlated with the formation of the two silicide phases. FeSi forms during heating around 400 °C and is stable at temperatures up to 530 °C. Further heating results in the formation of FeSi2 at around 550 °C. This phase is stable up to 670 °C.
Classical spin clusters: Integrability and dynamical properties
Authors: Srivastava N. , Kaufman C. , Müller G. , Magyari E. , Weber R. , Thomas H.
Published: July 2002

A pair of exchange-coupled classical spins with biaxial exchange and single-site anisotropy represents a Hamiltonian system with two degrees of freedom for which the integrability question is nontrivial. We have found that such a system is completely integrable if the model parameters satisfy a certain condition. For the integrable cases, the second integral of the motion (in addition to the Hamiltonian), which guarantees integrability, is determined explicitly. It can be reconstructed numerically by means of time averages of dynamical variables over all trajectories. In the nonintegrable cases, the existence of the time averages is still guaranteed, but they no longer define an analytic invariant, and their determination is subject to long-time anomalies. Our numerical calculation of time averages for two lines of initial conditions reveals a number of interesting features of such nonanalytic invariants.
Effects of oxygen partial pressure during deposition on the properties of ion-beam-sputtered indium-tin oxide thin films
Authors: Bregman J. , Shapira Yoram , Aharoni H.
Published: July 2002

Ion-beam-sputtered indium-tin oxide (ITO) films were studied as a function of the oxygen pressure PO2, during deposition. Analysis of electrical transport measurements, Auger electron spectroscopy (AES), and x-ray photoelectron spectroscopy (XPS) show a self-consistent correlation of all the results. With increasing PO2 a monotonous decrease is observed in the carrier density, which is found to be directly proportional to the oxygen vacancy concentration. This is based on a direct evaluation of the concentration of In unoxidized species in the film using AES and XPS. The moderate decrease of the electron mobility as PO2 is increased is also attributed to the change in the film oxidation state. The systematic relations between all the parameters investigated in this study yield a better understanding of the deposition process and point in the direction of achieving the best ITO films.
SrB4O7:Sm2+ pressure optical sensor: Investigations in the megabar range
Authors: Leger J. M. , Chateau C. , Lacam A.
Published: July 2002

The previously determined optical properties of SrB4O7 ;Sm2+ make this compound a possible better pressure calibrant than ruby, particularly when used at elevated temperatures. Moreover, it can be used with the same experimental devices. In fact, SrB4O7 ;Sm2+ displays an intense emission line which is a nondegenerated singlet, 5D0−7F0, at 685.41 nm in the same spectral range as ruby, 694.22 nm. In the present study, its shift with pressure has been calibrated with the shift of the ruby R1 line up to 60 GPa and has also been recorded to over 100 GPa. No phase transition of SrB4O7 ;Sm2+ was found at least up to 108 GPa. The pressure profiles in the diamond anvil cell have been established at 60 and 105 GPa. In addition, the bulk modulus of SrB4O7 ;Sm2+, B0=170 GPa, has been determined from x-ray diffraction experiments up to 27 GPa. An analytical expression for the shift of the wavelength with pressure is derived and applications to ruby and strontium borate are given.
Oxidation induced decomposition of YBa2Cu3O7−x
Authors: Williams R. K. , Alexander K. B. , Brynestad J. , Henson T. J. , Kroeger D. M. , Lindemer T. B. , Marsh G. C. , Scarbrough J. O. , Specht E. D.
Published: July 2002

At low oxygen potentials, YBa2Cu3O7−x decomposes by chemical reactions involving reduction. The results of this study show that the compound also decomposes at higher oxygen potentials. The initial decomposition products were found to be Y2BaCuO5 and a Ba-Cu oxide phase. The Ba-Cu phase was found to be a peroxide-type and the decomposition can be described by a chemical reaction involving oxidation: 4(YBa2Cu3O7−x)+(1/2 − 3/2δ +2x)O2⇄2Y2BaCuO5+3Ba2Cu3O6−δ+CuO. At 800 °C the equilibrium pressure for this reaction is slightly <1 bar, and the enthalpy change is ∼80-kJ/mol Y123. This result is not consistent with the conclusions drawn from some other studies of the stability of the superconducting Y-Ba-Cu-O compounds, and the differences are discussed.
Effect of the hard magnetic inclusion on the macroscopic anisotropy of nanocrystalline magnetic materials
Authors: Navarro I. , Pulido E. , Crespo P. , Hernando A.
Published: July 2002

It is shown that the presence of highly anisotropic magnetic precipitates in a soft multiphased matrix can produce a remarkable hardening, even when the volume fraction of the precipitates is small. The exchange coupling between the matrix and the precipitates is the relevant parameter. In particular, the simplified analysis we develop in this paper accounts for the magnetic hardening observed in very soft Fe-rich nanocrystals after annealing at higher temperatures.
Quality factor in a-Si:H nip and pin diodes
Authors: van Berkel C. , Powell M. J. , Franklin A. R. , French I. D.
Published: July 2002

We analyze the forward characteristics of a-Si:H nip and pin diodes. At low bias, a well-defined exponential region exists, described by a noninteger quality factor n between 1.2 and 1.7. With increasing temperature, the quality factor decreases. This behavior can be understood with a model based on electron and hole recombination in the i layer, which relates the temperature dependence of the quality factor to the distribution of localized states in the amorphous silicon. The predictions of the model are supported by numerical calculations in which the diode device equations are solved for a given distribution of localized states. The different ideality factors are due to different energy dependencies of the density of deep states in the i layer.