bluuebunny/crossref_metadata_2025 huggingface dataset:
Laser-induced fluorescence of OH radicals in a dielectric barrier discharge
Authors: Sankaranarayanan R. , Pashaie B. , Dhali S. K.
Published: July 2002

We discuss the results of laser-induced fluorescence measurements of OH radicals in a dielectric-barrier discharge. The discharge is in parallel plate geometry in atmospheric pressure air and argon. Although the air discharge consists of discrete microdischarges, two-dimensional images show the spatial uniformity of the OH radical. Results show that with increasing power, the OH production decreases due to gas heating and increased ozone production. The addition of O2 increases the OH production at low concentrations; however, at higher O2 concentration the OH concentration decreases due to increased electron attachment.
Dynamics of the orientation of active and passive scalars in two-dimensional turbulence
Authors: Lapeyre G. , Hua B. L. , Klein P.
Published: July 2002

The active nature of vorticity is investigated in order to understand its difference with a passive scalar. The direct cascade down to small scales is examined through both classical and new diagnostics (based on tracer gradient properties) in numerical simulations of freely decaying two-dimensional (2D) turbulence. During the transient evolution of turbulence, the passive scalar possesses a stronger cascade due to different alignment properties with the equilibrium orientations obtained in the adiabatic approximation by Lapeyre et al. [Phys. Fluids 11, 3729 (1999)] and Klein et al. [Physica D 146, 246 (2000)]. In strain-dominated regions, the passive scalar gradient aligns better with the equilibrium orientation than the vorticity gradient does, while the opposite is true in effective-rotation-dominated regions. A study of the kinematic alignment properties shows that this is due to structures with closed streamlines in the latter regions. However, in the final evolutionary stage of turbulence, both active and passive tracer gradients have identical orientations (i.e., there is a perfect alignment between the two gradients, all the more so when they are stronger). The effect of diffusion on the cascade is also studied.
Strain modification of epitaxial perovskite oxide thin films using structural transitions of ferroelectric BaTiO3 substrate
Authors: Lee M. K. , Nath T. K. , Eom C. B. , Smoak M. C. , Tsui F.
Published: July 2002

Effects of induced biaxial strain on the electrical transport and magnetic properties of epitaxial thin films of SrRuO3 and La0.67Sr0.33MnO3 by structural transitions of ferroelectric BaTiO3 substrates have been studied. Large jumps of electrical resistivity (∼5% in SrRuO3 and ∼12% in La0.67Sr0.33MnO3) and low field magnetization (∼70% in La0.67Sr0.33MnO3) have been observed in the films at the structural transition temperatures of BaTiO3 substrate. The hysteretic jumps are reproducible through many thermal cycles, and they can be attributed to strain effects induced by the substrate. The use of phase transitions of ferroelectric substrates to manipulate lattice strain of epitaxial thin film heterostructures can be a useful way to modify the properties of perovskite oxides.
Variable operation of Hall thruster with multiple segmented electrodes
Authors: Fisch N. J. , Raitses Y. , Dorf L. A. , Litvak A. A.
Published: July 2002

Variable plasma jet velocity with low beam divergence over a range of mass flow rates can be achieved through segmented electrode operation of the Hall plasma accelerator. With the use of just a cathode side electrode at the cathode potential, the beam divergence can be decreased substantially, at some cost in efficiency. However, the additional use of an anode side electrode retains the same reduced plume divergence, but at efficiencies comparable to the nonsegmented operation. The high efficiency persists also when the anode side electrode is biased at an intermediate potential, thus producing two-stage Hall accelerator operation.
Surface potential measurements of electron-irradiated insulators using backscattered and secondary electron spectra from an electrostatic toroidal spectrometer adapted for scanning electron microscope applications
Authors: Jbara O. , Belhaj M. , Odof S. , Msellak K. , Rau E. I. , Andrianov M. V.
Published: July 2002

A technique for the accurate determination of the surface potential US and its evolution during irradiation, is proposed. The technique is based on detecting both backscattered (BSE) and secondary electrons (SE) in a scanning electron microscope (SEM). The (BSE+SE) spectra are measured using a compact, highly sensitive electrostatic toroidal spectrometer (ETS), specially adapted for SEM applications. The use of an ETS analyzer set in a SEM for deducing the surface potential from (SE+BSE) spectra of electron irradiated insulators is introduced here. The surface potential is determined, either from the measured maximum energy of the secondary electron peak, or from its beginning. Various dielectric materials such as MgO, Al2O3, Y2O3, mica (potassium silicate aluminum), and Teflon were studied by this technique. Experimental investigations of the beam energy and current effects on the surface potential of bare insulators are reported. The change, due to this surface potential, in some physical quantities such as the amount of BSEs and the most probable energy of their spectral distribution is also studied. The results concerning coated and grounded insulators highlight the influence of the internal electric field on the BSEs energy distribution. An account of the various benefits of using the toroidal spectrometer in the surface potential measurement is also given.
Evidence for negatively charged vacancy defects in 6H-SiC after low-energy proton implantation
Authors: Britton D. T. , Barthe M.-F. , Corbel C. , Hempel A. , Henry L. , Desgardin P. , Bauer-Kugelmann W. , Kögel G. , Sperr P. , Triftshäuser W.
Published: July 2002

We have used pulsed-slow-positron-beam-based positron lifetime spectroscopy to investigate the nature of acceptors and charge states of vacancy-type defects in low-energy proton-implanted 6H-SiC(H). We can infer from the temperature dependence of the lifetime spectra that neutral and negatively charged vacancy clusters exist in the track region. Depending on annealing, they give rise to positron lifetimes of 257±2, 281±4, and 345±2 ps, respectively. The 281 ps cluster likely has an ionization level near the middle of the band gap. By comparison with theory, the 257 and 280 ps are identified as (VC–VSi)2 and (VC–VSi)3 clusters, respectively. In addition, other acceptors of ionic type act as strong trapping centers at low temperature (T<150 K). Neutral monovacancy-like complexes are also detected with a lifetime of 160±2 after 900 °C annealing.
Uniform deposition of ultrathin polymer films on the surfaces of Al2O3 nanoparticles by a plasma treatment
Authors: Shi Donglu , Wang S. X. , van Ooij Wim J. , Wang L. M. , Zhao Jiangang , Yu Zhou
Published: July 2002

Surface modification of nanoparticles will present great challenges due to their extremely small dimensions, high surface areas, and high surface energies. In this research, we demonstrate the uniform deposition of ultrathin polymer films of 2 nm on the surfaces of alumina nanoparticles. The deposited film can also be tailored to multilayers. Time-of-flight secondary ion mass spectroscopy was used to confirm the pyrrole thin film on the nanoparticle surfaces. Using such a nanocoating, it is possible to alter the intrinsic properties of materials that cannot be achieved by conventional methods and materials.
Nonlinear dynamics of small-scale magnetic islands in high temperature plasmas
Authors: Sydora Richard D.
Published: July 2002

The growth and nonlinear evolution of small-scale magnetic islands having a characteristic width of the order of the skin depth and smaller than an ion gyroradius is studied using nonlinear gyrokinetic particle simulations. In the sheared slab geometry it is demonstrated that in the absence of plasma pressure gradients, in a collisionless plasma, skin-size magnetic islands are spontaneously generated and form a nonlinear Bernstein–Greene–Kruskal-type asymptotic state. In the presence of electron thermal gradients skin-size drift-magnetic islands also form. When shorter wavelength electron-temperature-gradient-driven modes are included in the simulation nonlinear coupling to the drift-magnetic island is observed, which has a significant impact on its temporal evolution.
Production of ordered silicon nanocrystals by low-energy ion sputtering
Authors: Gago Raúl , Vázquez Luis , Cuerno Rodolfo , Varela Marı́a , Ballesteros Carmen , Albella José Marı́a
Published: July 2002

We report on the production of ordered assemblies of silicon nanostructures by means of irradiation of a Si (100) substrate with 1.2 keV Ar+ ions at normal incidence. Atomic force and high-resolution transmission electron microscopies show that the silicon structures are crystalline, display homogeneous height, and spontaneously arrange into short-range hexagonal ordering. Under prolonged irradiation (up to 16 h) all dot characteristics remain largely unchanged and a small corrugation develops at long wavelengths. We interpret the formation of the dots as a result of an instability due to the sputtering yield dependence on the local surface curvature.
Lateral current spreading in GaN-based light-emitting diodes utilizing tunnel contact junctions
Authors: Jeon Seong-Ran , Song Young-Ho , Jang Ho-Jin , Yang Gye Mo , Hwang Soon Won , Son Sung Jin
Published: July 2002

InGaN/GaN multiple-quantum-well light-emitting-diode structures utilizing tunnel contact junctions grown by metalorganic chemical vapor deposition have been demonstrated. The p+/n+ GaN tunnel junctions are located in the upper cladding layers of conventional devices, allowing n-type GaN instead of p-type GaN as a top contact layer. Thus, metal ohmic contacts are done at the same time on the top and the lower contact layers. The reverse-biased tunnel contact junction provides lateral current spreading without semitransparent electrode and spatially uniform luminescence exhibiting an improved radiative efficiency. The tunnel contact junction is shown to be an effective method to make possible hole injection via a lateral electron current, with only a small penalty in voltage drop compared to conventional devices.
Gauge factor enhancement driven by heterogeneity in thick-film resistors
Authors: Grimaldi C. , Ryser P. , Strässler S.
Published: July 2002

We present a simple picture of the gauge factor (GF) enhancement in highly heterogeneous materials such as thick-film resistors. We show that when the conducting phase is stiffer than the insulating one, the local strains within the latter are enhanced with respect to the averaged macroscopic strain. Within a simple model of electron tunneling processes, we show that the enhanced local strain leads to values of GF higher than those expected for a homogeneous system. Moreover, we provide formulas relating the enhancement of GF to the elastic and microstructural characteristics of thick-film resistors.
Highly efficient electroluminescent materials based on fluorinated organometallic iridium compounds
Authors: Wang Y. , Herron N. , Grushin V. V. , LeCloux D. , Petrov V.
Published: July 2002

We report a class of highly efficient electroluminescent materials based on fluorinated iridium compounds. Using aluminum as the cathode, a device, using fac-tris[5-fluoro-2(5-trifluoromethyl-2-pyridinyl)phenyl-C,N]iridium (Ir-2h) as the luminescent layer, displayed intense electroluminescence at 525 nm with an efficiency of 20 cd/A and a maximum radiance of 4800 cd/m2. Differing from the previously reported Ir(ppy)3, Ir-2h can be used in the undiluted form without the use of a charge-transporting host. This indicates that Ir-2h by itself has good enough charge-transporting properties. Photoluminescence studies at room temperature and 77 K revealed that electroluminescence originates from the metal-to-ligand charge transfer state with a quantum yield of 0.56 for Ir-2h and 0.5 for Ir(ppy)3 in toluene at room temperature. In the thin-film form, photoluminescence quantum yield of Ir-2h is a factor of 10 greater than that of Ir(ppy)3 due to the larger self-quenching effect of Ir(ppy)3.
Quenching rate constants for reactions of Ar(4p′[1/2], 4p[1/2], 4p[3/2]2, and 4p[5/2]2) atoms with 22 reagent gases
Authors: Sadeghi N. , Setser D. W. , Francis A. , Czarnetzki U. , Döbele H. F.
Published: July 2002

The total quenching rate constants of argon atoms in the 4p′[1/2]0, 4p[1/2]0, 4p[3/2]2, and 4p[5/2]2 states (2p1, 2p5, 2p6, and 2p8, respectively, in the Paschen numbering system) by rare gases, H2, D2, N2, CO, NO, O2, F2, Cl2, CO2, NO2, CH4, C2H2, C2H4, C2H6, CF4, CHF3, and SF6 have been determined at room temperature. These four excited states of argon (energy 13.09–13.48 eV) were selectively prepared by two-photon excitation from the ground state using VUV (184–190 nm range) laser pulses. The total quenching rates were deduced from the pressure dependence of the decay times of the excited-state atoms, measured by observing their fluorescence emission intensities in the presence of added reagents. The quenching constants increase from values of ≅0.01×10−10 cm3 atom−1 s−1 for Ne, to ≅0.1×10−10 cm3 atom−1 s−1 for He and Ar, and to very large values, (5–15)×10−10 cm3 atom−1 s−1, for most polyatomic molecules, F2, Cl2, and O2. The quenching mechanisms of the Ar(4p,4p′) atoms are briefly discussed and compared to the reactions of the Ar(4s,4s′) metastable and resonance state atoms, 11.55–11.83 eV, which can serve as a reference.
Optimized active-matrix drives for liquid crystal displays
Authors: Nakamura H. , Crain J. , Sekiya K.
Published: July 2002

A combination of computer simulation and experimental methods is used to provide a reliable and systematic means of optimizing the response time of active-matrix liquid crystal displays. Optical transients are measured and simulated (within a continuum elastic framework including hydrodynamic backflow) and are thereby related to director reorientation processes under various driving conditions. We find that an unconventional “overdrive” method, in which the device is briefly biased to a voltage exceeding the target voltage, leads to dramatic improvements in device response time which approach a factor of 6 in some cases. The computational model is capable of accurately predicting the optimum overdrive voltage. The physical origins of these findings and the importance of hydrodynamic backflow are discussed.
Current and current density of a finite-width, space-charge-limited electron beam in two-dimensional, parallel-plate geometry
Authors: Watrous John J. , Luginsland John W. , Frese Michael H.
Published: July 2002

The current carried by a steady-state, finite-width beam produced by space-charge-limited emission in two-dimensional parallel plate geometry is known to diverge strongly from estimates based on the classic one-dimensional Child–Langmuir problem. The two-dimensional problem presents formidable analytic difficulties, but a numerical approach to this problem has been developed. The approach simultaneously determines the electrostatic potential and the unknown current density profile. Calculations show that the total current is a function of the dimensionless ratio of beam width (w) to anode–cathode gap (d), but that the current density profile varies with both w and d separately.
Weak magnetic moment on IrMn exchange bias pinning layers
Authors: Hase T. P. A. , Fulthorpe B. D. , Wilkins S. B. , Tanner B. K. , Marrows C. H. , Hickey B. J.
Published: July 2002

We present evidence from soft x-ray resonant magnetic scattering measurements at the Mn L3 edge for the existence of a small magnetic moment on the antiferromagnetic IrMn pinning layer in a NiFe/Cu/Co/IrMn spin valve structure. The variation of the signal in an applied magnetic field shows that the moment lies antiparallel to the Co moment. Changes in the Mn L3 edge signal as the Co moment is rotated into the hard direction are rapid and do not appear to be associated with thermal reordering of the antiferromagnetic domain structure.
Effects of substrate bias voltage on plasma parameters in temperature control using a grid system
Authors: Bai K. H. , Hong J. I. , You S. J. , Chang H. Y.
Published: July 2002

In this paper we investigate the effects of substrate bias voltage on plasma parameters in temperature control using a grid system in inductively coupled plasma. Electron temperature can be controlled from 2.5 eV to 0.5 eV at 1 mTorr Ar plasma using grid bias voltage, and the electron temperature is a strong function of substrate bias voltage. The main control parameter determining the electron temperature is the potential difference between grid-biased voltage and the plasma potential in the temperature controlled region (ΔφII,g). When substrate bias voltage is negative, plasma parameters do not vary with substrate bias voltage due to constant ΔφII,g
Two-photon photopolymerization and diagnosis of three-dimensional microstructures containing fluorescent dyes
Authors: Sun Hong-Bo , Tanaka Tomokazu , Takada Kenji , Kawata Satoshi
Published: July 2002

We report in this letter two-photon-absorption photopolymerization using fluorescent dye-doped resin, by which complex fluorescence-active three-dimensional (3D) microstructures were readily produced. Structures were imaged by means of two-photon confocal techniques. The realization of dye-doping photopolymerization would be an important step towards production of photonic-crystal-based high-efficiency polymer optoelectronic devices. Furthermore, we found that the doped dye acted as label molecules to stain solidified structures, which provided an effective two-photon fluorescence technique for internal microdiagnosis of polymerized 3D microstructures.
Diamagnetic double-loop method for a highly sensitive measurement of energy stored in a Stellarator plasma
Authors: Besshou S. , Aizawa K. , Tomiyama K. , Kondo K. , Mizuuchi T. , Nagasaki K. , Obiki T. , Okada H. , Sano F.
Published: July 2002

In this article we describe a diamagnetic double-loop method for measurement of the energy stored in a magnetized plasma. Two concentric diamagnetic loops enclose a plasma column. The advantage of this method is the capability to eliminate to a high extent the background magnetic field and the effect of eddy currents in the measurement of relatively small diamagnetic signals. This method was applied to initial plasmas produced in the helical axis stellarator Heliotron J. A stored energy of 0.7 kJ was measured with a resolution of ±0.02 kJ at rippling magnetic field of 1 T in a plasma heated by rf discharges up to 400 kW. This diamagnetic double-loop method improved the signal to noise ratio of the conventional single loop method sixfold. The magnetic diagnostics hardware, experimental results, and an error estimate are presented.
The dynamics of vibrational excitations on surfaces: CO on Ru(001)
Authors: Bonn Mischa , Hess Christian , Wolf Martin
Published: July 2002

We present an experimental and theoretical study of vibrational excitation of the C–O stretch vibration of carbon monoxide adsorbed on a ruthenium Ru(001) surface with ultrashort femtosecond infrared laser pulses. After broadband excitation leading to transfer of a significant fraction of the CO molecules to their first (∼15%) and second (∼5%) vibrationally excited states, we observe a competition between vibrational energy relaxation and energy delocalization through dipole–dipole coupling. We reproduce the observed excited state spectra by solving the three-level Bloch equations and accounting for intermolecular vibrational energy transfer on a picosecond time scale. The rate of vibrational energy transfer, and its coverage-dependence, can be described by a Förster energy transfer mechanism. We discuss possibilities to optimize the degree of localized vibrational excitation of a specific bond of molecules at surfaces through chirped pulse IR excitation.
Plastic Josephson junctions
Authors: Schön Jan Hendrik
Published: July 2002

The electrical properties of polythiophene thin films can be varied from insulating to superconducting using the field effect. A spatial modulation of the properties can be achieved by patterning of the gate electrode leading to the preparation of Josephson junctions based on solution cast polymer films. These junctions can be used to build a polymer superconducting quantum interference device or to study the superconducting properties of the material itself. The observations are in accordance with the idea of electron–phonon-mediated superconductivity in this organic polymer.

At the request of the authors and editor, this article is being retracted effective 24 February 2003.
Subnanosecond, time-resolved, broadband infrared spectroscopy using synchrotron radiation
Authors: Lobo R. P. S. M. , LaVeigne J. D. , Reitze D. H. , Tanner D. B. , Carr G. L.
Published: July 2002

A facility for subnanosecond time-resolved (pump-probe) infrared spectroscopy has been developed at the National Synchrotron Light Source of Brookhaven National Laboratory. A mode-locked Ti:sapphire laser produces 2 ps duration, tunable near-infrared pump pulses synchronized to probe pulses from a synchrotron storage ring. The facility is unique on account of the broadband infrared from the synchrotron, which allows the entire spectral range from 2 cm−1 (0.25 meV) to 20 000 cm−1 (2.5 eV) to be probed. A temporal resolution of 100 ps, limited by the infrared synchrotron-pulse duration (full width at half maximum), is achievable. A maximum time delay of 170 ns is available without gating the infrared detector. To illustrate the performance of the facility, a measurement of electron–hole recombination dynamics for an HgCdTe semiconductor film in the far- and mid-infrared range is presented.
Effective pinhole-collimated ultrasmall-angle x-ray scattering instrument for measuring anisotropic microstructures
Authors: Ilavsky J. , Allen A. J. , Long G. G. , Jemian P. R.
Published: July 2002

Small-angle scattering is widely used for measuring materials microstructure in the 1–100 nm size range. Ultrasmall-angle x-ray scattering (USAXS), typically achieved through crystal collimation, extends this size range to include features over 1 μm in size. This article reports on USAXS on the UNICAT beam line 33-ID at the Advanced Photon Source. The instrument makes use of a six-reflection crystal pair as a collimator and another six-reflection crystal pair as an analyzer. First principle absolute calibration and a broad scattering vector range make this a very effective instrument, limited only by the fact that the measurement of anisotropic microstructures is excluded due to slit smearing from the crystal collimation. This limitation has recently been removed by adding a horizontally reflecting crystal before and another after the sample. This creates a USAXS instrument with collimation in two orthogonal directions. We call this configuration effective pinhole USAXS. Now, anisotropic materials are probed using 9–17 keV photons in the same physically-relevant (from 50 nm to over 1 μm) microstructural size range as that available for materials which scatter isotropically.
X-ray photoelectron spectroscopy and high resolution electron microscopy studies of Aurivillius compounds: Bi4−xLaxTi3O12(x=0, 0.5, 0.75, 1.0, 1.5, and 2.0)
Authors: Chu Ming-Wen , Ganne Marcel , Caldes Maria Teresa , Brohan Luc
Published: July 2002

In order to determine the fatigue-free origin of the ferroelectric Bi3.25La0.75Ti3O12, a series of x-ray photoelectron spectroscopy and high resolution electron microscopy studies were performed on the polycrystalline Bi4−xLaxTi3O12 (BLTx, x=0, 0.5, 0.75, 1.0, 1.5, and 2.0) powders. From the XPS study, the surfaces of all La-containing compounds are found to consist of one outermost Bi-rich region and followed by a La-rich region, instead of the chemical stoichiometry in the bulk. An HREM study on Bi3.25La0.75Ti3O12 further confirms that this surface configuration arises from some intergrowth defects with a thickness of 5 nm appearing on the crystal edge, which is also observed in compounds with x=1.0 and 2.0, but not in the poor fatigue-resistant Bi4Ti3O12 (x=0). This La-induced defect locally changes the chemical composition of the crystal surface, which probably possesses a different physical characteristic as compared to the bulk. Consequently, it could can be the physical nature of the interface, when in contact with the metal electrode, Pt, in ferroelectric nonvolatile memory. Since the fatigue phenomenon on a ferroelectric capacitor arises predominately from the pinning of domain walls on the metal-ferroelectric interface, the surface configurations of La-containing Bi4Ti3O12 compounds should be considered as one of the fatigue-free factors as well as the self-regulation of the Bi2O2 layer and the chemical stability of the perovskite slabs. In addition, the band gap of Bi3.25La0.75Ti3O12 is also estimated by UV absorption spectrum to be 3.9±0.1 eV.
Systematic measurement of AlxGa1−xN refractive indices
Authors: Özgür Ümit , Webb-Wood Grady , Everitt Henry O. , Yun Feng , Morkoç Hadis
Published: July 2002

Dispersion of the ordinary and extraordinary indices of refraction have been measured systematically for wurtzitic AlxGa1−xN epitaxial layers with 0.0⩽x⩽1.0 throughout the visible wavelength region. The dispersion, measured by a prism coupling waveguide technique, is found to be well described by a Sellmeier relation. Discrepancies among previous measurements of refractive index dispersion, as a consequence of different growth conditions and corresponding band gap bowing parameter, are reconciled when the Sellmeier relation is parameterized not by x but by band gap energy.
Perturbative corrections to the equation-of-motion spin–flip self-consistent field model: Application to bond-breaking and equilibrium properties of diradicals
Authors: Krylov Anna I. , Sherrill C. David
Published: July 2002

We present perturbative corrections to a recently introduced spin–flip self-consistent field (SF-SCF) wave function. The spin–flip model describes both closed and open shell singlet states within a single reference formalism as spin–flipping, e.g., α→β, excitations from a triplet (Ms=1) reference state for which both dynamical and nondynamical correlation effects are much smaller than for the corresponding singlet state. The simplest spin–flip model employs a SCF wave function for the reference state, and the resulting equations for target states are therefore identical to configuration interaction singles (in spin–orbital form). While being a qualitatively correct zero-order wave function, SF-SCF should be augmented by dynamical correlation corrections to achieve a quantitative accuracy. The results demonstrate that the second-order approximation to the more theoretically complete spin–flip coupled-cluster model (truncated at double substitutions) represents a systematic improvement over the SF-SCF model.
Nanoscale organic transistors based on self-assembled monolayers
Authors: Schön J. H. , Bao Z.
Published: July 2002

A device structure is reported for the fabrication of nanoscale organic transistors. In this structure, a self-assembled monolayer is used to define the channel length, as well as acting as the semiconducting material. High current modulation and high current output are demonstrated with 4,4′-dithiolbiphenylene. Various dielectric materials, such as SiO2,Al2O3, and a self-assembled silane monolayer, have been shown to result in high-performance transistors. Finally, nanopatterning can also be achieved by using an insulating alkanethiol to define the channel length of a conventional organic field-effect transistor.

At the request of the authors, this article is being retracted effective 24 February 2003.
Development of transferable interaction models for water. IV. A flexible, all-atom polarizable potential (TTM2-F) based on geometry dependent charges derived from an ab initio monomer dipole moment surface
Authors: Burnham C. J. , Xantheas S. S.
Published: July 2002

In this work we examine the consequences of incorporating the ab initio derived monomer potential-energy surface and nonlinear dipole surface of Partridge and Schwenke [J. Chem. Phys. 106, 4618 (1997)] into the previously developed TTM2-R model of Burnham et al. [J. Chem. Phys. 116, 1500 (2002)] in order to develop a new, all-atom polarizable, flexible model for water (TTM2-F). We found that the use of the nonlinear dipole surface is essential in modeling the change in the internal geometry of interacting water molecules and, in particular, the increase in the internal H–O–H bend angle with cluster size. This is the first demonstration of a flexible model which shows an increase in the bending angle in clusters. An explanation for this behavior is presented using the concept of geometric polarizabilities. The model furthermore reproduces the n=2–6 cluster binding energies to within an RMS deviation of 0.05 kcal/mol per hydrogen bond with respect to the MP2 complete basis set estimates. Preliminary results for the liquid with the new model are also presented.
Electron energy deposition in an electron-beam pumped KrF amplifier: Impact of beam power and energy
Authors: Petrov G. M. , Giuliani J. L. , Dasgupta A.
Published: July 2002

The electron deposition in an Ar–Kr–F2 mixture, based on a solution of the electron Boltzmann equation, is presented. The model is relevant to an electron-beam generated KrF* laser amplifier at atmospheric pressure. Sets of cross sections for Ar, Kr, and F2 have been compiled. Calculations have been performed to determine the electron energy distribution function, energy per electron–ion pair and the ionization and excitation rates. It is found that the inclusion of inner shell ionization and the subsequent Auger emission are essential for matching known results on both the energy per electron–ion pair Wei and the stopping power in pure Ar or Kr target gases. For the chosen Ar–Kr–F2 mixture, Wei is calculated to be 24.6 eV. The excitation-to-ionization ratio is calculated to be 0.38 for Ar and 0.54 for Kr at low input power density Pbeam (1 kW/cm3). Both ratios increase with Pbeam, particularly for Kr which attains 0.8 at 1 MW/cm3. The dependency on Pbeam and the excitation efficiency for Kr is significantly higher than previously assumed in KrF* kinetic models. Results are also compared with the continuous slowing down approximation to demonstrate that this approach is limited to the regime of low power deposition.
Highly correlated calculations with a polynomial cost algorithm: A study of the density matrix renormalization group
Authors: Chan Garnet Kin-Lic , Head-Gordon Martin
Published: July 2002

We study the recently developed Density Matrix Renormalization Group (DMRG) algorithm in the context of quantum chemistry. In contrast to traditional approaches, this algorithm is believed to yield arbitrarily high accuracy in the energy with only polynomial computational effort. We describe in some detail how this is achieved. We begin by introducing the principles of the renormalization procedure, and how one formulates an algorithm for use in quantum chemistry. The renormalization group algorithm is then interpreted in terms of familiar quantum chemical concepts, and its numerical behavior, including its convergence and computational cost, are studied using both model and real systems. The asymptotic convergence of the algorithm is derived. Finally, we examine the performance of the DMRG on widely studied chemical problems, such as the water molecule, the twisting barrier of ethene, and the dissociation of nitrogen. In all cases, the results compare favorably with the best existing quantum chemical methods, and particularly so when the nondynamical correlation is strong. Some perspectives for future development are given.
Efficient electrical spin injection from a magnetic metal/tunnel barrier contact into a semiconductor
Authors: Hanbicki A. T. , Jonker B. T. , Itskos G. , Kioseoglou G. , Petrou A.
Published: July 2002

We report electrical spin injection from a ferromagnetic metal contact into a semiconductor light emitting diode structure with an injection efficiency of 30% which persists to room temperature. The Schottky barrier formed at the Fe/AlGaAs interface provides a natural tunnel barrier for injection of spin polarized electrons under reverse bias. These carriers radiatively recombine, emitting circularly polarized light, and the quantum selection rules relating the optical and carrier spin polarizations provide a quantitative, model-independent measure of injection efficiency. This demonstrates that spin injecting contacts can be formed using a widely employed contact methodology, providing a ready pathway for the integration of spin transport into semiconductor processing technology.
Magnetic phase diagram of HoMnO3
Authors: Fiebig M. , Degenhardt C. , Pisarev R. V.
Published: July 2002

The phase diagram of hexagonal HoMnO3 in the magnetic-field/temperature plane is investigated by second harmonic generation (SHG). Four different magnetic phases of the frustrated Mn sublattice are identified on the basis of the selection rules for SHG. In-plane or interplane reordering of Mn spins at the phase boundaries occurs almost independent of each other which leads to a tetracritical point in the phase diagram. Interactions of the Mn and Ho sublattices are discussed as possible origin of the phase transitions.
Comparison of ensemble and single molecule approaches to probing polymer relaxation dynamics near Tg
Authors: Deschenes Laura A. , Vanden Bout David A.
Published: July 2002

Single molecule spectroscopy and polarization holeburning (PHB) are used to measure the dynamics of rotation for rhodamine 6G in poly(methylacrylate). It is found that the ensemble average correlation time obtained from the single molecule experiments is the same as that obtained from bulk fluorescence anisotropy decay. Both reflect the same underlying dynamics as the mechanically measured α relaxation. Fitting α relaxation and PHB data to the Vogel–Tammann–Fulcher (VTF) equation, it is found that the two measurements follow the same temperature dependence, but with absolute times that differ by nearly four orders of magnitude. This difference is discussed in two contexts. In the first the relative ratio of the correlation and α-relaxation times depend on probe size, in which case the large difference is not unexpected given the size of the probe. In the second, the difference is attributed to specific probe/polymer interactions. In this case, a modified VTF equation that includes an activation energy term can be used to explain this phenomenon; when all parameters but the activation energy are fixed to the α-relaxation values, it is found that an activation energy of 21 KJ/mol could cause the observed slowing of the rhodamine 6G rotational correlation times.

At the request of the authors and editor, this article is being retracted 27 April 2006.
Propagation and post-acceleration of a pseudospark-sourced electron beam
Authors: Yin H. , Cross A. W. , Phelps A. D. R. , Zhu D. , He W. , Ronald K.
Published: July 2002

Propagation and post-acceleration of a pseudospark-sourced electron beam from a three-gap pseudospark discharge chamber were studied in recent experiments. The pseudospark produced an electron beam of two phases, an initial 22 kV, 50 A hollow cathode phase beam of brightness 109−10 Am−2 rad−2 followed by a 200 V, 200 A conductive phase (CP) beam of brightness 1011−12 Am−2 rad−2. The aim of these experiments was to post accelerate the lower-voltage, higher-current CP beam using an acceleration unit driven by a 40 kV, 125 ns voltage pulse produced by a cable Blumlein. The experiments were realized by attaching an acceleration unit to the downstream side of the anode of the discharge chamber. Both the pseudospark discharge and the cable Blumlein were triggered to ensure time correlation between initiation of the pseudospark discharge and post-acceleration of the beam.
Two-dimensional model of a stationary plasma thruster
Authors: Hagelaar G. J. M. , Bareilles J. , Garrigues L. , Boeuf J. -P.
Published: July 2002

Stationary plasma thrusters (SPTs) are advanced propulsion devices that use a gas discharge to ionize and accelerate the propellant. We present in detail a two-dimensional model of an SPT discharge. The model combines a particle simulation of neutral atoms and ions with a fluid description of electrons, where the electric field is obtained from imposing quasineutrality. The electron mobility and energy loss are treated in an empirical way and characterized by ad hoc parameters. Typical simulation results are shown.
Influence of the sample length and profile of the magnetoimpedance effect in FeCrSiBCuNb ultrasoft magnetic wires
Authors: Vázquez M. , Li Y.-F. , Chen D.-X.
Published: July 2002

The magnetoimpedance (MI) effect has been investigateded in Fe63.5Cr10Si13.5B9Cu1Nb3 alloy wire exhibiting an ultrasoft magnetic character after suitable thermal treatment at 525 °C. It has been measured as a function of the sample length and of the distance to the ends of the wire (MI profile). The observed dependence on both parameters is correlated with the local particularities of the magnetization process: complex domain structures emerging near the ends to decrease the magnetostatic energy modify the local magnetic susceptibility and consequently the MI response. The present study points out the importance of geometrical dimensions of samples and measuring conditions when reporting numerical values in MI studies. This wire, showing large magnetoimpedance together with optimized soft magnetic behavior and improved corrosion properties seems to be indicated as a sensing element in GMI based sensors.
Delayed phase explosion during high-power nanosecond laser ablation of silicon
Authors: Lu Quanming , Mao Samuel S. , Mao Xianglei , Russo Richard E.
Published: July 2002

An important parameter for high-irradiance laser ablation is the ablation crater depth, resulting from the interaction of individual laser pulses on a targeted surface. The crater depth for laser ablation of single-crystal silicon shows a dramatic increase at a laser intensity threshold of approximately 2×1010 W/cm2, above which, large (micron-sized) particulates were observed to eject from the target. We present an analysis of this threshold phenomenon and demonstrate that thermal diffusion and subsequent explosive boiling after the completion of the laser pulse is a possible mechanism for the observed dramatic increase of the ablation depth. Calculations based on this delayed phase explosion model provide a satisfactory estimate of the measurements. In addition, we find that the shielding of an expanding mass plasma during laser irradiation has a profound effect on this threshold phenomenon.
Differential method for the real time measurement of the diamagnetic β and internal inductance in Tore Supra
Authors: Joffrin E. , Defrasne P.
Published: July 2002

A differential method for real time measurements of the diamagnetic energy and poloidal β has been successfully installed on the Tore Supra tokamak. This diagnostic makes use of concentric diamagnetic loops compensated by precise measurements of toroidal current variations. This diamagnetic measurement is now routinely used in the feedback system to control the internal inductance of the Tore Supra plasma using non-inductive current drive generated by lower hybrid waves.
On the role of the purely transverse Weibel instability in fast ignitor scenarios
Authors: Silva Luı́s O. , Fonseca Ricardo A. , Tonge John W. , Mori Warren B. , Dawson John M.
Published: July 2002

The growth rate for the purely transverse Weibel instability is determined from relativistic kinetic theory using a waterbag distribution function in the momenta perpendicular to the main propagation direction of the beam. A parametric study is presented for conditions relevant to the fast ignitor. It is shown that for expected parameters the purely transverse Weibel instability will be significantly suppressed or even eliminated due to the transverse energy spread or emittance.
SiC and its bicrystalline nanowires with uniform BN coatings
Authors: Tang C. C. , Bando Y. , Sato T. , Kurashima K. , Ding X. X. , Gan Z. W. , Qi S. R.
Published: July 2002

Uniformly BN-coated SiC nanowires have been synthesized within the framework of vapor–liquid–solid growth mechanism. Nanoscale Ni–C alloy covering graphite sheet was used as catalysts and the mixture of boron and silica was heated to simultaneously generate B2O2 and SiO gas-phase precursors. 2 to 4 nm BN layers were found to coat the overall surface of the inside of the SiC nanowires with the diameters ranging from several nm to 80 nm. The bicrystalline structure of SiC nanowires with BN coating were also observed and had a [311] growth axis at least. The structures and possible coating mechanism have been studied by high-resolution transmission electron microscopy.
Radiative properties of puffed-gas mixtures: The case of optically thick plasmas composed of two elements with similar atomic numbers
Authors: Apruzese J. P. , Davis J.
Published: July 2002

Using detailed atomic and radiative transfer calculations, we consider the K-shell radiative properties of optically thick plasmas composed of two different elements with nearly the same atomic number. A sodium-neon mixture is chosen for specificity, and the plasma conditions considered are similar to those achieved in puffed-gas or exploding-wire Z pinches. The emissivity of an individual K-shell line is parameterized in terms of an ‘‘ultimate’’ photon escape probability which is a function of both the single-flight photon escape probability and the collisional quenching probability. Holding the total ion density (sodium plus neon) constant, we have evaluated the total K-shell emissivity as a function of sodium-to-neon ratio. In contrast to the optically thin case the emitted K-shell power peaks at values of this ratio intermediate between pure neon and pure sodium. The opacity effects responsible for this phenomenon are analyzed using the escape parameterization.
FMR linebroadening in metals due to two-magnon scattering
Authors: Heinrich B. , Cochran J. F. , Hasegawa R.
Published: July 2002

The theory of two-magnon scattering is extended to disordered ferromagnetic metals. In order to treat properly the role of two-magnon scattering in metals we have derived the formula for the rf susceptibility in the parallel (in plane) configuration by using the retarded Green’s function formalism. The roles of the elliptical polarization, the finite values of resonance k vectors, nonresonant spin-wave interactions, eddy current effects as well as the contribution of spin-orbit interaction to the intrinsic damping are included. We have evaluated the susceptibility in the presence of magnetostatic inhomogeneities, statistical variations of local properties and fluctuating exchange interactions.
Surface layer spinodal decomposition in In1−xGaxAsyP1−y and In1−xGaxAs grown by hydride transport vapor-phase epitaxy
Authors: Chu S. N. G. , Nakahara S. , Strege K. E. , Johnston W. D.
Published: July 2002

Composition modulation due to spinodal decomposition in In1−xGaxAsyP1−y quaternary and In1−xGaxAs ternary alloys is observed by transmission electron microscopy in epitaxial layers grown by hydride transport vapor phase epitaxy at 700 °C on (001) InP substrate. A quasi-periodic fine contrast oriented along [100] and [010] directions is found in all samples over a composition range of 0.20≤x≤0.53 and 0.37≤y≤1. Contrary to the result reported on materials grown by liquid-phase epitaxy, complete mixing at composition outside the miscibility gap predicted by thermodynamics for bulk crystals is not observed. The evidence indicates a rapid spinodal decomposition by surface diffusion during vapor phase deposition. Furthermore, transmission electron microscopy diffraction contrast experiments on the (110) cross-sectional view of planar and nonplanar epitaxial layers reveal a columnar structure oriented along the growth direction which is consistent with the surface layer spinodal formation. Additional coarse contrast is observed in InGaAs epilayers. TEM stereo investigation confirms a film thickness variation associated with the coarse contrast.
Simulations of temperature measurements of shock-heated solids
Authors: DaSilva L. , Ng A. , Parfeniuk D.
Published: July 2002

The rarefaction of a shock-heated solid surface into vacuum and the luminous radiation emitted by the surface were studied using a one-dimensional hydrodynamic code. The temporal evolution of the spectral and brightness temperatures as observed by a remote detector was determined. Moreover, the time-integrated spectral temperature was computed to simulate the actual experimental measurements. For shock speeds greater than 106 cm/s and for measurement times exceeding a few picoseconds, the code predicted a significiantly lower ‘‘observed’’ temperature, in good agreement with experiments.
Optical energy gaps of β-In2S3 thin films grown by spray pyrolysis
Authors: Kim Wha-Tek , Kim Chang-Dae
Published: July 2002

β-In2S3 thin films for which x varied from 2.0 to 3.9 with the x composition of the spray solution (In2Sx) were grown on thoroughly cleaned glass slides by the spray pyrolysis method with the substrate temperature maintained at 350 °C and a rate of spray of 6 ml/min. The energy gap of the grown β-In2S3 thin film was 2.20 eV, indicating an indirect transition, whereas that of In2S3.9 increased to 2.43 eV.
Temperature dependence of electrical properties of n- and p-type 3C-SiC
Authors: Yamanaka M. , Daimon H. , Sakuma E. , Misawa S. , Yoshida S.
Published: July 2002

Electrical properties of unintentionally doped n-type and Al-doped p-type 3C-SiC layers, epitaxially grown on Si by chemical vapor deposition, have been investigated at temperatures between 10 and 1000 K. Activation energies of Al acceptors and residual donors obtained from the temperature dependence of carrier density are 160 and 18 meV, respectively. 40%–60% of Al acceptors in the p-type epilayers are compensated, and hole mobility is limited by acoustic phonon scattering above 300 K and by ionized impurity scattering below 250 K.
Anisotropic magnetostriction in Tb0.27Dy0.73Fe1.95
Authors: Teter J. P. , Clark A. E. , McMasters O. D.
Published: July 2002

The magnetostriction of a single crystal of Tb0.27Dy0.73Fe1.95 grown by the Bridgman technique was measured over the temperature range of magnetic anisotropy compensation (300–330 K). Using a growth rate of 3 cm/h, twinned dendrites were suppressed. We looked for, but found no evidence of, modification of the cubic structure by the resulting cellular [112] growth. Magnetostriction measurements along all four 〈111〉 axis are nearly identical. However, the magnetostriction is highly anisotropic, with λ100/λ111=0.06±0.02 at 300 K. The effects of this large magnetostriction anistropy are discussed. Of particular importance is the magnetostriction along nonprincipal axes. Here we find that the magnetization direction for maximum strain is not collinear with the strain measurement direction. Measurements along the growth [112̄] direction revealed the maximum magnetostriction with the magnetization 14.75° from [112̄]. Expressions which relate the direction of magnetization for maximum strain to the measurement direction are derived for magnetization rotation in the (110) and (100) planes. The existence of noncollinear maximum magnetostriction axes leads to a region of 180° domain wall motion near H=0 in transducer rods of Tb0.27Dy0.73Fe1.95 under compressive prestress. This portion of the magnetization curve is not accompanied by magnetostriction. Magnetostriction curves in nonsaturating magnetic fields also provide a method to determine the magnetic anistropy. Anisotropy constants calculated in this way are compared to those determined earlier by torque and magnetization measurements.