bluuebunny/crossref_metadata_2025 huggingface dataset:
Suppression/reversal of natural convection by exploiting the temperature/composition dependence of magnetic susceptibility
Authors: Seybert C. D. , Evans J. W. , Leslie F. , Jones W. K.
Published: July 2002

Natural convection, driven by temperature or concentration gradients or both, is an inherent phenomenon during solidification of materials on Earth. This convection has practical consequences (e.g., effecting macrosegregation) but also renders difficult the scientific examination of diffusive/conductive phenomena during solidification. It is possible to halt, or even reverse, natural convection by exploiting the variation (with temperature, for example) of the susceptibility of a material. If the material is placed in a vertical magnetic field gradient, a buoyancy force of magnetic origin arises and, at a critical field gradient, can balance the normal buoyancy forces to halt convection. At higher field gradients the convection can be reversed. The effect has been demonstrated in experiments at Marshall Space Flight Center where flow was measured by particle image velocimetry in MnCl2 solution in a superconducting magnet. In auxiliary experiments the field in the magnet and the properties of the solution were measured. Computations of the natural convection, its halting and reversal, using the commercial software FLUENT® were in good agreement with the measurements.
Preparation of transparent Nb/two-dimensional electron gas contacts by using electron cyclotron resonance plasma cleaning
Authors: Schäpers Th. , Müller R. P. , Crecelius G. , Hardtdegen H. , Lüth H.
Published: July 2002

The effect of electron cyclotron resonance plasma cleaning on the contact resistance between a superconducting Nb layer and a two-dimensional electron gas in a strained InxGa1−xAs/InP heterostructure is investigated. Cleaning by a He/H plasma results in a rough semiconductor surface and a high interface resistance. In contrast, by using a pure He plasma a smooth semiconductor surface with a considerably lower interface resistance is obtained.
Magneto-optical spectra of excitons and Landau subbands in strained CdTe epitaxial films under high magnetic fields
Authors: Watanabe K. , Miura N.
Published: July 2002

High quality CdTe thin epitaxial films were grown on (100)-GaAs substrates by hot wall epitaxy. Doublet structures consisting of a heavy hole exciton and a light hole exciton were observed in reflection and absorption spectra. This splitting is caused by biaxial thermal strain. Magnetoreflection and magnetoabsorption spectra were measured in magnetic fields up to 40 T. It was found from the diamagnetic shift of the excitons that the hole effective masses are drastically changed from unstrained crystals. Interband transitions between Landau subbands were clearly observed in the magnetoabsorption spectra at strong magnetic fields.
Ion emission from ferroelectrics
Authors: Sroubek Zdenek
Published: July 2002

Ion emission from lead–zirconium–titanate ferroelectrics under the application of voltage pulses has been observed and analyzed by a time-of-flight mass spectrometer. Mass spectra and relative timing of negative and positive ions and electron emission processes have been determined. The ion spectra are compared with time-of-flight secondary ion mass spectroscopy data obtained in situ. Based on the experimental results, possible microscopic mechanisms of ion formation are discussed.
Ultraviolet reflection–absorption spectroscopy of benzene on metal surfaces
Authors: Peng X. L. , Peck J. R. , Anderson L. W. , Goebel C. J. , Lawler J. E.
Published: July 2002

This article reports on ultraviolet reflection–absorption spectroscopy of C6H6 molecules physisorbed on Ag, Cu, and Au surfaces. The electronic 1A1g→1B2u transition of C6H6 at wavelengths near 250 nm is observed. We model the system and obtain center frequencies, linewidths, and estimates of the oscillator strengths for the various vibronic absorptions.
Geometry of multisymplectic Hamiltonian first-order field theories
Authors: Echeverria-Enrı́quez Arturo , Muñoz-Lecanda Miguel C. , Román-Roy Narciso
Published: July 2002

In the jet bundle description of field theories (multisymplectic models, in particular), there are several choices for the multimomentum bundle where the covariant Hamiltonian formalism takes place. As a consequence, several proposals for this formalism can be stated, and, on each one of them, the differentiable structures needed for setting the formalism are obtained in different ways. In this work we make an accurate study of some of these Hamiltonian formalisms, showing their equivalence. In particular, the geometrical structures (canonical or not) needed for the Hamiltonian formalism, are introduced and compared, and the derivation of Hamiltonian field equations from the corresponding variational principle is shown in detail. Furthermore, the Hamiltonian formalism of systems described by Lagrangians is performed, both for the hyper-regular and almost-regular cases. Finally, the role of connections in the construction of Hamiltonian field theories is clarified.
A semiquantitative analytic model for the magnetic response of a Terfenol-D impregnated composite specimen under tension
Authors: Feickert C. A. , Berman J. , Kamphaus J. , Trovillion J. , Quattrone R.
Published: July 2002

Magnetostrictive composites have found increasing use as a tagging agent for structural integrity monitoring. A simple magnetic model based on Heisenberg–Weiss theory is put forth that provides guidance as to the physical parameters that govern the magnetic response of a Terfenol-D impregnated composite specimen in tension. Further, the model provides an effective analytic parameterization of the magnetic response, resulting from unidirectional tension, and affords estimates of these parameters.
The ejection distribution of solvated electrons generated by the one-photon photodetachment of aqueous I− and two-photon ionization of the solvent
Authors: Kloepfer J. A. , Vilchiz V. H. , Lenchenkov V. A. , Germaine A. C. , Bradforth S. E.
Published: July 2002

The ultrafast dynamics following one-photon UV photodetachment of I− ions in aqueous solution are compared with those following two-photon ionization of the solvent. Ultrafast pump–probe experiments employing 50 fs ultraviolet pulses reveal similar and very rapid time scales for electron ejection. However, the electron ejection process from water pumped into the conduction band and from iodide ions detached at threshold are readily distinguishable. The observed picosecond timescale geminate recombination and electron escape dynamics are reconstructed using two different models, a diffusion-limited return of the electron from ∼15 Å to its parent and a competing kinetics model governed by the reverse electron transfer rate. We conclude that the “ejected” electron in the halide detachment is merely separated from the halogen atom within the same solvent shell. The assignment of detachment into a contact pair is based on the recombination profile rather than by the postulate of any new spectral absorption due to an electron in a contact pair. The contact pair is surprisingly long-lived and the nonadiabatic recombination is rather slow considering the proximity of the partners. Experiments in mixed solvents confirm our assignment of the two distinct ejection mechanisms. The detachment mechanism is therefore fundamentally different in the resonant (one photon) charge-transfer-to-solvent (CTTS) process from the multiphoton detachment of aqueous iodide ions, which bears more similarity to the direct solvent ionization.
Performance of a CsI photocathode in a hard x-ray streak camera
Authors: Hara Toru , Tanaka Yoshihito , Kitamura Hideo , Ishikawa Tetsuya
Published: July 2002

The performance of a CsI photocathode for hard x rays (8–46 keV) was compared to a Au photocathode using synchrotron radiation. We found that the sensitivity of CsI (100 nm thick) is 20–100 times higher than that of Au (30 nm thick), depending on the photon energy. The time response for 40 ps pulses is also improved slightly using CsI due to its narrower secondary electron energy spread. The study was performed in a streak camera environment; thus the results obtained include the instrumental effects of the camera and directly show the real performance of the cathodes for streak cameras. Although the question of material stiffness still remains for CsI, no degradation of cathode performance was observed during the present study.
Two-dimensional phase-locked antiguided vertical-cavity surface-emitting laser arrays
Authors: Zhou D. , Mawst L. J.
Published: July 2002

4×4 antiguided phase-locked vertical-cavity surface-emitting laser (VCSEL) arrays have been fabricated by a selective etching process and metalorganic chemical vapor deposition regrowth. Stable, diffraction-limited output is observed corresponding to either in-phase or out-of-phase mode operation, depending on the interelement spacing width. Calculations indicate resonant leaky-wave coupling occurs for interelement spacings corresponding to an integral number of half-waves of the radiation leakage from each VCSEL region, and the use of interelement loss is effective in suppressing nonresonant modes.
Sum-frequency generation (SFG) vibrational spectroscopy of side alkyl chain structures of polyimide surfaces
Authors: Oh-e Masahito , Lvovsky Alexander I. , Wei Xing , Shen Y. R.
Published: July 2002

Sum-frequency generation (SFG) vibrational spectroscopy was used to study how side alkyl chains of a polyimide are oriented at the air–polymer interface and how they are affected by mechanical rubbing and adsorption of a liquid crystal monolayer. The spectra of polyimides with various alkyl chain lengths all indicate that the chains protrude out of the polyimide surface with a very broad distribution. Longer alkyl chains appear to contain more gauche defects. While the polyimide backbones are aligned by rubbing, the side chain orientation is hardly affected. Adsorption of a liquid crystal monolayer on the surface significantly reduces the gauche defects in the alkyl chains.
Naphthalene dimer: Electronic states, excimers, and triplet decay
Authors: East Allan L. L. , Lim Edward C.
Published: July 2002

Computations have been performed for the singlet and triplet electronic states of varying orientations of naphthalene dimer. The dependence of exciton splitting upon orientation and intermonomer distance was explored. Splittings of triplet states are seen to be nontrivial at typical bonding distances, commensurate with the splittings of weakly allowed singlet states. Charge-transfer interaction with the excimer states is seen to be most significant in face-to-face orientations which can allow closer approach of the two monomers. Predictions of the prominent features of the singlet–singlet and triplet–triplet absorption spectra agree well with experimental findings. A spin-orbit channel-counting scheme is introduced to account for observed radiative and nonradiative decay of the T1 triplet state of the monomer, and then applied to the dimer. The mechanism has been found for the observed more rapid phosphorescence of the T1 state of the dimer when placed in orientations lacking inversion symmetry.
Full color light emission from amorphous SiCx:H with organic–inorganic structures
Authors: Ma Tianfu , Xu Jun , Du Jiafang , Li Wei , Huang Xinfan , Chen Kunji
Published: July 2002

Hydrogenated amorphous silicon carbide films with an organic–inorganic compound structure were fabricated by using organic carbon source xylene (C8H10), in a traditional radio frequency plasma-enhanced chemical vapor deposition system. Strong full-color light emission ranging from 630 to 420 nm was observed at room temperature. The results of photoluminescence and photoluminescence excitation experiments showed the emission was originated from the radiative recombination at band tails of the inorganic SiC matrix. Moreover, luminescence from the transition between discrete energy levels, which were associated with organic π-conjugated carbon systems, was also observed at resonant excitation conditions.
The properties and magnetically softened grain boundaries of Nd10Fe84−xB6Inx (x=0,1,2) exchange coupling magnets
Authors: Ji Q. G. , Gu B. X. , Du Y. W.
Published: July 2002

The magnetic behavior of Nd10Fe84−xB6Inx (x=0,1,2) nanocomposites was investigated. It was found that a certain amount of indium (x=1) added to Nd10Fe84B6 magnets enhances the hard magnetic properties. The optimum coercivity and remanence ratio was increased from 5.0 kOe and 0.71 to 5.8 kOe and 0.83, respectively. The squareness of the hysteresis loop is improved greatly. The maximum magnetic energy product (BH)max increases remarkably from 12 to 18 MGOe. The origin for those enhancements is mainly attributed to the magnetically softened grain boundaries and enhanced crystallographical coherency by indium addition. The magnetic annealing is found to be helpful to enhance the coherency and coupling between hard and soft phases.
Carbon nitride thin film synthesized on iron buffer layers
Authors: Lu Y. F. , He Z. F. , Mai Z. H. , Ren Z. M.
Published: July 2002

Carbon nitride thin films were deposited on iron buffer layers by pulsed laser deposition assisted with ion implantation. Two types of samples (A) and (B) were prepared with and without iron layers. Several techniques were used to study the properties of the samples. Scanning tunneling microscopy (STM) was used to observe the surface structures of the samples. The difference in their surface morphologies was studied. The STM measurements also provided the relation between tunneling current and bias voltage to study the local density of states of the sample surface by calculating (dI/dV)/(I/V). Three band edges were observed from the calculated curve. Measurements by Raman and Fourier transform infrared (FTIR) spectra were carried out to study the electronic properties of the samples. The Raman spectra showed the presence of triply bonded carbon nitride bonds (C≡N) in sample (A), while only single bonds were observed in sample (B) by FTIR spectra. The mechanical properties were studied by nanoindentation. Both hardness and bulk modulus of the thin films were measured.
Determination of arsenic diffusion parameters by sulfur indiffusion in gallium arsenide
Authors: Scholz R. F. , Werner P. , Gösele U. , Engler N. , Leipner H. S.
Published: July 2002

Indiffusion profiles of sulfur in gallium arsenide were determined by secondary ion mass spectroscopy. In order to evaluate the shape of the profiles, a set of coupled reaction–diffusion equations was solved numerically. From the simulated nonequilibrium indiffusion profiles of sulfur, which diffuses into gallium arsenide via the kick-out mechanism, both the diffusion coefficient and the equilibrium concentration of arsenic self-interstitials were simultaneously determined. Transmission electron microscopy revealed that, due to an arsenic supersaturation, extrinsic dislocation loops have formed. The Fermi-level effect is more pronounced at lower diffusion temperatures and provides an additional driving force for the loop formation, agreeing well with the occurrence of larger faulted loops at a diffusion temperature of 950 °C rather than at 1100 °C. The complex behavior of the sulfur indiffusion can be quantitatively described by taking into account extended defects.
Spatial coherence of azimuthally symmetric Gaussian electromagnetic beams
Authors: Seshadri S. R.
Published: July 2002

The spatial coherence properties of azimuthally symmetric electromagnetic beams launched by planar Gaussian sources are investigated. The intensity distribution and the coherence properties of the source do not vary in the azimuthal direction. A procedure is postulated for obtaining the root mean squared coherence length as a function of position in a plane transverse to the propagation direction. The variation of the root mean squared coherence length with the position in a transverse plane as well as along the propagation direction is treated. The dependence of these variations on the source coherence parameter relative to the beam waist at the source plane is discussed. On propagation, the root mean squared coherence length at any point in a transverse plane other than on the beam axis is found to vary in the azimuthal direction although cylindrical symmetry is preserved with respect to the variation of the location of the point in the azimuthal direction.
Effect of temperature on carbon nitride films synthesized by ion-beam-assisted pulsed laser deposition
Authors: Chen Z. Y. , Zhao J. P. , Yano T. , Ooie T. , Yoneda M. , Sakakibara J.
Published: July 2002

Carbon nitride thin films were deposited by pulsed laser deposition with nitrogen ion beam assistance at a substrate temperature varying from room temperature to 800 °C. The effect of the substrate temperature on the nitrogen content, surface morphology, structure, and electrical property of the carbon nitride films was investigated. The deposited films were characterized by atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy, x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and four-probe resistance. The nitrogen content of the deposited films reached its maximum value of 25% at a substrate temperature of 400 °C. AFM images revealed that an island structure occurred and developed on the surface of the films deposited at the high substrate temperature. FTIR and XPS spectra showed the existence of sp3C–N and sp2C=N bonds in the deposited films. The deposited carbon nitride films had an amorphous structure with two carbon nitride phases inclusions, which had a stoichiometry near C3N4 and a variable stoichiometry from C5N to C2N, respectively. With the increase in substrate temperature, the relative content of the sp3C–N bonds, i.e., the C3N4 phase, increased and the crystallization degree of the deposited films enhanced, which were confirmed by the Raman analysis. Very few C≡N bonds in the films were found as compared to other carbon–nitrogen bonds. Electrical resistivity exhibited the highest value for the film deposited at 400 °C. Investigation results indicated that the high substrate temperature could promote the formation of C3N4 phase.
Formation and control of two-dimensional deoxyribonucleic acid network
Authors: Kanno Takashi , Tanaka Hiroyuki , Miyoshi Norio , Kawai Tomoji
Published: July 2002

Recently, we have successfully fabricated large-scale deoxyribonucleic acid (DNA) networks on mica surfaces using a simple fabrication method. This report describes how we fabricated a variety of structures depending on the type of DNA, and controlled these structures using a post-treatment ethanol which we observed using atomic force microscopy. We found that the height of fiber in the DNA network depended on the type of DNA and its original length, and that the fiber height and mesh diameter could be shortened and widened, respectively, with ethanol treatment.
Fine wire thermometer for air temperature measurement
Authors: Harrison R. G. , Pedder M. A.
Published: July 2002

A resistive temperature sensor constructed from 25 μm platinum wire has been optimized for atmospheric measurements of air temperature, operated with a precision resistance-to-voltage converter. The sensor is able to operate in the atmosphere without a radiation shield, and calibration to better than 0.1 K accuracy can be obtained, temporally stable for the typical life of the sensor. The time response is also sufficiently rapid (lag time 40 ms) to allow it to be used for turbulent measurements in the atmospheric surface layer. The electronics system supplies sufficient signal processing for a ±5 V output to directly correspond to ±50 °C.
Quantum dot solar cells
Authors: Aroutiounian V. , Petrosyan S. , Khachatryan A. , Touryan K.
Published: July 2002

The quantum dot solar cell concept is proposed as a scheme for increased solar cell efficiency. A theoretical model is presented for a practical p–i–n quantum dot solar cell, based on the self-organized InAs/GaAs system. The advantages of using the quantum dot in the active region for photon absorption in the long-wavelength part of the spectrum, leading to cell efficiency, is discussed.
A parallel-detection frequency-domain near-infrared tomography system for hemoglobin imaging of the breast in vivo
Authors: McBride Troy O. , Pogue Brian W. , Jiang Shudong , Österberg Ulf L. , Paulsen Keith D.
Published: July 2002

A novel near-infrared frequency-domain system designed for tomographic breast imaging is described. The setup utilizes five optical wavelengths, from 660 to 826 nm, and parallel detection with 16 photomultiplier tubes. Direct fiberoptic coupling with the tissue is achieved with a high precision positioning device using 16 motorized actuators (0.5 μm precision) arranged radially in a circular geometry. Images of breast tissue optical absorption and reduced scattering coefficients are obtained using a Newton-type reconstruction algorithm to solve for the optimal fit between the measurement data and predicted data from a finite element solution to the frequency-domain diffusion equation. The design, calibration, and performance of the tomographic imaging system are detailed. Data acquisition from the system requires under 30 s for a single tomographic slice at one optical wavelength with a measurement repeatability for a single phantom on average of 0.5% in ac intensity and 0.4° in phase. Absorbing and scattering phantoms typical of in vivo breast optical properties were imaged over a range of object-to-background contrasts (50%–250%). The reconstructed absorption and scattering images are accurate to within 20% of the expected value.
Improved plasma performance on Large Helical Device
Authors: Komori A. , Ohyabu N. , Yamada H. , Kaneko O. , Kawahata K. , Ashikawa N. , deVaries P. , Emoto M. , Funaba H. , Goto M. , Ida K. , Idei H. , Ikeda K. , Inagaki S. , Inoue N. , Isobe M. , Kado S. , Khlopenkov K. , Kobuchi T. , Kubo S. , Kumazawa R. , Liang Y. , Masuzaki S. , Matsumoto Y. , Minami T. , Miyazawa J. , Morisaki T. , Morita S. , Murakami S. , Muto S. , Mutoh T. , Nagayama Y. , Nakamura Y. , Nakanishi H. , Narihara K. , Narushima Y. , Nishimura K. , Noda N. , Notake T. , Ohdachi S. , Oka Y. , Okamoto M. , Osakabe M. , Ozaki T. , Pavlichenko R. O. , Peterson B. J. , Sagara A. , Saito K. , Sakakibara S. , Sakamoto R. , Sasao H. , Sasao M. , Sato K. , Sato M. , Seki T. , Shimozuma T. , Shoji M. , Suzuki H. , Takechi M. , Takeiri Y. , Tamura N. , Tanaka K. , Toi K. , Tokuzawa T. , Torii Y. , Tsumori K. , Yamada I. , Yamaguchi S. , Yamamoto S. , Yokoyama M. , Yoshimura Y. , Watanabe K. Y. , Watanabe T. , Watari T. , Hamada Y. , Itoh K. , Matsuoka K. , Ohkubo K. , Satow T. , Sudo S. , Yamazaki K. , Motojima O. , Fujiwara M.
Published: July 2002

Since the start of the Large Helical Device (LHD) experiment, various attempts have been made to achieve improved plasma performance in LHD [A. Iiyoshi et al., Nucl. Fusion 39, 1245 (1999)]. Recently, an inward-shifted configuration with a magnetic axis position Rax of 3.6 m has been found to exhibit much better plasma performance than the standard configuration with Rax of 3.75 m. A factor of 1.6 enhancement of energy confinement time was achieved over the International Stellarator Scaling 95. This configuration has been predicted to have unfavorable magnetohydrodynamic (MHD) properties, based on linear theory, even though it has significantly better particle-orbit properties, and hence lower neoclassical transport loss. However, no serious confinement degradation due to the MHD activities was observed, resolving favorably the potential conflict between stability and confinement at least up to the realized volume-averaged beta 〈β〉 of 2.4%. An improved radial profile of electron temperature was also achieved in the configuration with magnetic islands, minimized by an external perturbation coil system for the Local Island Divertor (LID). The LID has been proposed for remarkable improvement of plasma confinement like the high (H) mode in tokamaks, and the LID function was suggested in limiter experiments.
Josephson-junction arrays as high-efficiency sources of coherent millimeter-wave radiation
Authors: Vasilić B. , Shitov S. V. , Lobb C. J. , Barbara P.
Published: July 2002

Recent experiments have indicated that Josephson-junction arrays can radiate coherently in a laser-like fashion, as predicted by theoretical work in the 1970s. We present results from measurements of high-efficiency Josephson-junction arrays coupled to resonant cavities. In one of our samples with four columns and 36 rows, the dc to ac (180 GHz) conversion efficiency reaches an asymptotic value of about 32%. Using a simple circuit model we show that we have achieved optimal dc to ac conversion in this sample.
Self-modulation of an intense laser beam in near-critical homogeneous plasma
Authors: Lin Hai , Chen Li-Ming , Zhang Jie , Zhang Ping , Fan Wen-Bing
Published: July 2002

An investigation of longitudinal intensity modulation of a relativistic laser beam in near-critical homogeneous plasma (ne>14ncr) is performed in this paper. On the basis of a nonlinear Schrödinger equation of electromagnetic vector potential, a model equation of the intensity profile is derived via the variational approach from Action and Lagrangian. Longitudinally uniform and longitudinally modulated pulse structures are viewed as different solutions of this model equation. For these two different types of pulse structure, their relative stability under different structure parameters is numerically studied. Numerical results indicate that this relative stability depends closely on the transverse structures of the laser pulse.
Pair potential from structure data in dusty plasmas
Authors: de Angelis U.
Published: July 2002

It is suggested that experimental data on the dust structure factor in dusty plasmas can be used to find the interaction potential between dust particles using the theory relating structure and forces in charged fluids.
A study of built-in filter for some eddy viscosity models in large-eddy simulation
Authors: Magnient Jean-Christophe , Sagaut Pierre , Deville Michel
Published: July 2002

Traditional large-eddy simulation (LES) provides good results on standard benchmark turbulent flows, but engineering applications involving more complex geometries are restricted. This is mainly due to the physical hypotheses of homogeneity, isotropy, or local equilibrium of the subgrid-scale (SGS) models, which are questionable in more realistic configurations. It appears essential to qualify the built-in filters of LES models before performing coarse mesh/high Reynolds number computations. A study of the Smagorinsky and the Schumann models with a mesh-independent filtering length larger than the mesh size is presented in this paper. The forms of the associated built-in filters are identified, and a control of the filter length through the SGS model is performed. Finally an asymptotic behavior of the filter is found to be independent of the spatial scheme for large filter length. This work is partly an assessment of previous theoretical studies on LES filters and kinetic energy spectra.
Mechanical and friction properties of thermoplastic polyurethanes determined by scanning force microscopy
Authors: Mailhot B. , Komvopoulos K. , Ward B. , Tian Y. , Somorjai G. A.
Published: July 2002

The mechanical and friction properties of additive-free polyurethane (PUR) consisting of polyether soft segments of polytetramethyleneoxide and hard segments (HS) comprising 4,4'-diphenylmethane diisocyanate and 1,4-butanediol as a chain extender were studied by scanning force microscopy. The hardness and elastic modulus of PUR containing 42, 57, 69, and 100 wt % HS were determined from nanoindentation experiments performed with a diamond tip of nominal radius of curvature equal to ∼20 μm and maximum indentation load in the range of 10–500 μN. The mechanical properties of PUR were found to strongly depend on the HS content and contact load. The elastic modulus of PUR with 42 wt % HS was found to decrease with increasing load due to the high softness of this material that influenced the measurements. The friction characteristics of the samples with 42, 57, and 69 wt % HS were studied in order to evaluate the dependence of the coefficient of friction on load and sliding speed. The PUR with 57 wt % HS exhibited the lowest coefficient of friction. Depending on the PUR composition, remarkably different friction behaviors were obtained. The observed mechanical behavior and friction characteristics are interpreted in terms of the chemical composition and microstructure of the samples.
Modeling anisotropic chemical wave patterns in the NO+H2 reaction on a Rh(110) surface
Authors: Makeev A. , Hinz M. , Imbihl R.
Published: July 2002

A realistic mathematical model describing the formation of anisotropic chemical wave patterns in the NO+H2 reaction on a Rh(110) surface is presented. For the point model describing the local reaction kinetics a bifurcation analysis has been conducted. In order to take into account the state-dependent anisotropy of surface diffusion site-blocking effects through coadsorbates for the diffusing species were introduced. The spatially distributed model reproduces well the experimentally determined excitability and bistability range and the existence ranges for the different types of chemical wave patterns: elliptical and rectangular target patterns, travelling wave fragments, and varying front geometries in the range of double metastability were modeled. In addition, the dependencies of the pulse/front velocity on the hydrogen partial pressure and temperature were simulated.
Multiple electron stripping of 3.4 MeV/amu Kr7+ and Xe11+ in nitrogen
Authors: Mueller D. , Grisham L. , Kaganovich I. , Watson R. L. , Horvat V. , Zaharakis K. E. , Armel M. S.
Published: July 2002

Use of heavy ions beams with ∼10 MeV/amu mass ∼200, and average charge state of 1+ has been proposed as a driver for heavy ion fusion. Stripping of the ion beam by background gas can lead to an increase in the space charge density of the beam, which may make focusing the intense ion beam onto small targets more complex. Knowledge of the electron loss cross sections is essential to understand and address the problem. Currently, there are no 10 MeV/amu mass=200, charge state=1 beams available, and the theories that calculate electron loss cross sections can be experimentally tested only by using available beams of somewhat lower energy and higher initial charge state. The charge state distribution of ions produced in single collisions of 3.4 MeV/amu Kr7+ and 3.4 MeV/amu Xe11+ in N2 have been measured at the Texas A&M Cyclotron Institute using a windowless gas cell. The charge states of the outgoing ions are determined by magnetic analysis using a position-sensitive microchannel-plate detector. The cross sections for single and multiple electron loss are determined, and the results indicate that substantial multiple-electron loss occurs. The relative cross section for loss of i+1 electrons is 0.3–0.7 times that for i electron loss. The average number of electrons removed per one collision (sum of the electron-weighted cross sections normalized to the total cross section) is 1.86 for Kr and 1.97 for Xe.
A new crystalline phase of the electroluminescent material tris(8-hydroxyquinoline) aluminum exhibiting blueshifted fluorescence
Authors: Braun M. , Gmeiner J. , Tzolov M. , Coelle M. , Meyer F. D. , Milius W. , Hillebrecht H. , Wendland O. , von Schütz J. U. , Brütting W.
Published: July 2002

We report on two different crystalline phases of tris(8-hydroxyquinoline) aluminum (Alq3) which were obtained by thermal sublimation in a horizontal glass tube. These phases are investigated by x-ray powder diffraction, Raman and infrared spectroscopy, and low temperature photoluminescence measurements. Apart from the already known α phase we could identify a new crystalline phase of Alq3 (δ-Alq3) showing blueshifted fluorescence. As compared to the α phase this new phase is characterized by a larger unit cell volume, a reduced number of Raman lines in the energy range between 70 and 700 cm−1, a blueshift of the photoluminescence maximum by about 0.2 eV, and a decreased intersystem crossing to the triplet state. These differences are interpreted in terms of the isomery of the Alq3 molecule. It is supposed that the new phase contains the facial isomer, whereas in the other phases only the meridianal isomer was reported. Low temperature photoluminescence spectra show a well-resolved vibronic progression with about the same spacing of 550 cm−1 for both crystalline phases of Alq3. Site-selective photoluminescence measurements reveal the existence of an additional redshifted featureless emission, which is ascribed to energy relaxation into low-lying states.
Investigating a stretched vortex with ultrafast two-dimensional ultrasonic speckle velocimetry
Authors: Manneville S. , Sandrin L. , Fink M.
Published: July 2002

Ultrasonic speckle velocimetry (USV) was recently introduced as a new ultrafast tool for measuring fluid velocities in two dimensions by analyzing the speckle signal backscattered from particles moving with the flow [Sandrin et al., Appl. Phys. Lett. 78, 1155 (2001)]. An experimental study of a stationary stretched vortex is presented, which demonstrates the efficiency of ultrafast 2D USV in terms of quantitative assessment of vortical flows. The full velocity field is recovered and compared to the Burgers model for two different fluids: water and milk. The dependence of the vortex characteristics on the main control parameter is investigated.
Chain-link-fence structures produced in a plane jet
Authors: Sakakibara Jun , Anzai Tomokuni
Published: July 2002

Vortical structures of the chain-link-fence type have been experimentally produced in a plane jet. The jet was excited by temporal periodic disturbances with spanwise phase variations added in the initial shear layer. Chain-link-fence-like structures, which significantly differed from ordinary vortices such as “roller and rib” structures, are observed experimentally by stereoscopic particle image velocimetry when the temporal phase difference of disturbances with the spanwise direction is 180°. Some of the original vortices remain downstream as stretched lambda-type vortices, although the destruction of the orderly vortices into complex turbulence was significant.
Dynamic yield and tensile strength of aluminum single crystals at temperatures up to the melting point
Authors: Kanel G. I. , Razorenov S. V. , Baumung K. , Singer J.
Published: July 2002

This article presents experimental results of the dynamic yield strength and dynamic tensile strength (“spall strength”) of aluminum single crystals at shock-wave loading as a function of temperature. The load duration was ∼40 and ∼200 ns. The temperature varied from 20 to 650 °C which is only by 10 °C below the melting temperature. A linear growth of the dynamic yield strength by more than a factor of 4 was observed within this temperature range. This is attributed to the phonon drag effect on the dislocation motion. High dynamic tensile strength was maintained over the whole temperature range, including the conditions at which melting should start in a material under tension. This could be an indication of the existence of superheated states in solid crystals.
Ab initio molecular orbital prediction of the Renner–Teller effect for the first excited state ∏2 of MgNC: Reinterpretation of the laser-induced fluorescence spectrum based on the predicted spectroscopic constants
Authors: Odaka Tina Erica , Taketsugu Tetsuya , Hirano Tsuneo , Nagashima Umpei
Published: July 2002

Spectroscopic constants of the first excited state Ã 2∏ of MgNC have been predicted from three-dimensional potential energy surfaces determined by ab initio molecular orbital calculations at the multi-reference singles and doubles configuration interaction level. The equilibrium structure of Ã 2∏ MgNC is linear with re(Mg-N) of 1.8843 Å and re(N-C) of 1.1812 Å. The predicted spectroscopic constants are as follows: the rotational constant B0=6127 MHz, ω1=2101 cm−1, ω2=163 cm−1, ω3=586 cm−1, and the spin-orbit coupling constant ASO=34.9 cm−1. The Renner parameter ε was calculated to be 0.259 from the lower 12A″ and the upper 22A′ potential energy surfaces. Based on the thus predicted spectroscopic constants and ε, the observed laser-induced fluorescence spectra have been reinterpreted purely from the theoretical point of view within errors of a few wavenumbers. Present assignments indicate that only lines for κ states have been observed.
Comparison of quantum scattering on an ab initio potential surface with experimental total differential scattering measurements for Li++N2
Authors: Siska P. E.
Published: July 2002

A recently calculated high-level ab initio potential energy surface for Li++N2 [J. Chem. Phys. 109, 6615 (1998)] is used in quantum close-coupling rigid-rotor calculations of the total differential scattering at a collision energy E=2.47 eV. The results are compared with experimental rainbow scattering measurements of Böttner, Ross, and Toennies [J. Chem. Phys. 65, 733 (1976)] and good agreement is found for the position, intensity, and damping of the rainbow. A new hybrid sudden approximation (HSA) also agrees well with both close coupling and experiment.
Investigation of the amorphous-to-crystalline transition in Mo/Si multilayers
Authors: Bajt Saša , Stearns Daniel G. , Kearney Patrick A.
Published: July 2002

The microstructure of Mo/Si multilayers grown by magnetron and ion beam sputter deposition has been characterized over a range of Mo layer thicknesses. We observe an abrupt amorphous-to-crystalline transition in the Mo layers at a thickness of ∼2 nm. The transition exhibits several interesting features including a large decrease in the thickness of the Si-on-Mo interlayer and a significant increase in the roughness of the multilayer. We present an explanation for the transition behavior in terms of a critical thickness for the nucleation of Mo crystallites.
NO
2
sensitive Au gate metal–oxide–semiconductor capacitors
Authors: Filippini D. , Aragón R. , Weimar U.
Published: July 2002

Au gate metal-oxide-semiconductor capacitors are sensitive to NO2 in air up to 200 ppm, depending on operating temperature (100 °C to 200 °C), gate thickness (50 to 900 nm), and morphology. In the absence of catalytic properties or lattice diffusivity, a model invoking molecular surface adsorption and grain boundary diffusion is proposed, which quantitatively describes the transient and steady state response of the devices. Sensitivity is given by the arrival of the diffusing species to the gate–dielectric interface, where capacitive coupling of the adsorbed molecules induces work function changes, which shift the flat band voltage positively, opposite that observed for H2 with Pd gates, consistently with an oxidizing, rather than reducing, character.
On the orientational effects in monolayers of diatomic molecules
Authors: Patrykiejew A. , Sałamacha A. , Sokołowski S. , Zientarski T. , Binder K.
Published: July 2002

The Monte Carlo simulation method is used to study orientational ordering in monolayer films of diatomic molecules on the (100) plane of face centered cubic crystal. Systems of admolecules with different elongation are studied and their orientational and positional ordering discussed. It is shown that in the case of small elongations the adsorbed monolayer orders into a simple (1×1) structure. When the elongation of adsorbed molecules increases, the film orders into more complex structures. In such cases, the adsorbate lattice decomposes into four interpenetrating sublattices.
Interfaces of Fe/Si multilayered films with a strong antiferromagnetic coupling analyzed by optical and magneto-optical spectroscopies
Authors: Kudryavtsev Y. V. , Nemoshkalenko V. V. , Lee Y. P. , Kim K. W. , Rhee J. Y. , Dubowik J.
Published: July 2002

Fe/Si multilayered films (MLF) exhibiting a strong antiferromagnetic (AF) coupling were investigated by optical and magneto-optical (MO) spectroscopies. The results were compared with the computer-simulated spectra based on various structural models of MLF. It was shown that neither semiconducting FeSi2 nor ε-FeSi can be considered as the spacer layers in the Fe/Si MLF for the strong AF coupling. The optical properties of the spacer extracted from the effective optical response of the MLF strongly support its metallic nature. A reasonable agreement between experimental and simulated equatorial-Kerr-effect spectra was obtained with the fitted optical parameters of the spacer with the FeSi stoichiometry. Comparison of the extracted optical properties of the spacer with the calculated ones by using the first principles showed that a B2-phase metallic FeSi compound is spontaneously formed at the interfaces during deposition. For the Fe/Si system with ultrathin Fe and Si sublayers (thinner than 1 nm), our optical data indicate that the structure of the whole MLF is close to the amorphous and semiconducting ε-FeSi.
Microwave spectrum of the CH3CCS radical in the E3/22 electronic ground state
Authors: Habara Hideta , Yamamoto Satoshi
Published: July 2002

The rotational spectrum of a symmetrical-top radical CH3CCS in the E3/22 electronic ground state has been detected by microwave spectroscopy. The CH3CCS radical is produced by discharging a gaseous mixture of CH3CCH and CS2, and 50 spectral lines with the K range from −4 to 6 are observed in the 132–261 GHz region with a source modulation millimeter-wave spectrometer. In addition, 19 low-J lines with K=0 and 1 are measured in the 11–21 GHz region with a Fourier transform microwave spectrometer. Ten molecular constants including hyperfine interaction constants are determined accurately by a least-squares method. The obtained hyperfine constants are found to be about a half of those of a related radical, CH3S, indicating that the structure of CH3CCS would be expressed in a classical resonant form; H3C–C≡C–Ṡ↔H3C–Ċ=C=S.
Fabrication of a nanometric Zn dot by nonresonant near-field optical chemical-vapor deposition
Authors: Kawazoe Tadashi , Yamamoto Yoh , Ohtsu Motoichi
Published: July 2002

We demonstrate a technique for the deposition of nanometric Zn dots by photodissociation of gas-phase diethylzinc using an optical near field under nonresonant conditions. The observed deposited Zn dot was less than 50 nm in size. The photodissociation mechanisms are based on the unique properties of optical near fields, i.e., enhanced two-photon absorption, induced near-field transition, and a direct excitation of the vibration-dissociation mode of diethylzinc.
Global bifurcations in a laser with injected signal: Beyond Adler’s approximation
Authors: Zimmermann Martı́n G. , Natiello Mario A. , Solari Hernán G.
Published: July 2002

We discuss the dynamics in the laser with an injected signal from a perturbative point of view showing how different aspects of the dynamics get their definitive character at different orders in the perturbation scheme. At the lowest order Adler’s equation [Proc. IRE 34, 351 (1946)] is recovered. More features emerge at first order including some bifurcations sets and the global reinjection conjectured in Physica D 109, 293 (1997). The type of codimension-2 bifurcations present can only be resolved at second order. We show that of the two averaging approximations proposed [Opt. Commun. 111, 173 (1994); Quantum Semiclassic. Opt. 9, 797 (1997); Quantum Semiclassic. Opt. 8, 805 (1996)] differing in the second order terms, only one is accurate to the order required, hence, solving the apparent contradiction among these results. We also show in numerical studies how a homoclinic orbit of the S̆il’nikov type, bifurcates into a homoclinic tangency of a periodic orbit of vanishing amplitude. The local vector field at the transition point contains a Hopf-saddle-node singularity, which becomes degenerate and changes type. The overall global bifurcation is of codimension-3. The parameter governing this transition is θ, the cavity detuning (with respect to the atomic frequency) of the laser.
Ring closure in dioxin formation process: An ab initio molecular dynamics study
Authors: Farajian Amir A. , Mikami Masuhiro , Ordejón Pablo , Tanabe Kazutoshi
Published: July 2002

The four possible mechanisms of ring closure in dioxin formation from chlorophenols are studied using ab initio molecular dynamics, within generalized gradient approximation. Free energy barriers, derived as the potential of mean constraint force, directly lead to a static approximation for the transition rates. We show, however, that the static approximation overestimates the actual rates by 33%–345%, depending on the intermediate/temperature. Comparing our results with the available experimental data, we conclude that, contrary to what is widely assumed, the pathways starting from two chlorophenols are not among the most dominant pathways in the formation of highly toxic dioxins, at least in the gas phase. This signifies the role of catalysts and/or other precursors.
Evidence for high intergranular current flow in a single-phase polycrystalline MgB2 superconductor
Authors: Kawano K. , Abell J. S. , Kambara M. , Babu N. Hari , Cardwell D. A.
Published: July 2002

The distribution of a magnetic field in single-phase polycrystalline bulk MgB2 has been measured using a magneto-optical (MO) technique for an external magnetic field applied perpendicular to the sample surface. The MO studies indicate that an intergranular current network is readily established in this material and the current is not limited by weak-linked grain boundaries. The grain boundaries are observed to resist preferential magnetic-field penetration and the persistent current has an intergranular character at temperatures up to 30 K. The results provide clear evidence that the intragranular current flow is isotropic. A critical current density of ≈104 Acm−2 was estimated at 30 K in a field of 150 mT from the magnetic-field gradient using the standard Bean model. These results provide further evidence of the considerable potential for MgB2 for engineering applications.
InGaN multiple-quantum-well green light-emitting diodes on Si grown by metalorganic chemical vapor deposition
Authors: Egawa T. , Zhang B. , Nishikawa N. , Ishikawa H. , Jimbo T. , Umeno M.
Published: July 2002

We report the characteristics of InGaN multiple-quantum-well (MQW) green light-emitting diodes (LEDs) on Si (111) substrates. The MQW LEDs were grown on Si by metalorganic chemical vapor deposition using Al0.27Ga0.73N/AlN intermediate layers. The LED on Si showed an operating voltage of 7 V, a series resistance of 100 Ω, an optical output power of 20 μW, and a peak emission wavelength of 505 nm with a full width at half maximum of 33 nm at 20 mA drive current. The optical output power was half as compared to that of green LED on sapphire. The LED also exhibited a stable operation over 500 h under automatic current control (20 mA) condition at 27 °C.
Reduction of microtrenching and island formation in oxide plasma etching by employing electron beam charge neutralization
Authors: Watanabe M. , Shaw D. M. , Collins G. J.
Published: July 2002

During plasma etching of oxide thin-film patterns, nonuniform charge buildup within etching features results in formation of microtrenches. Near the etch endpoint, the underlying film layer adjacent to the feature edges is exposed first, leaving an oxide island in the feature center and potentially causing underlayer damage before the endpoint is reached. Herein, a directional electron flux is added to the plasma ion flux incident on the etching substrate with the goal of minimizing microtrenching and oxide island formation. Scanning electron microscopic images of patterns etched with added electron irradiation show a reduction in microtrenching and oxide island formation as compared to patterns etched under identical conditions without electron irradiation. A computer simulation shows that the added electron irradiation reduces microtrenching by allowing more uniform ion flux to reach the feature bottom.