bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrical characterization of two deep electron traps introduced in epitaxially grown n-GaN during He-ion irradiation
Authors: Auret F. D. , Goodman S. A. , Koschnick F. K. , Spaeth J-M. , Beaumont B. , Gibart P.
Published: July 2002

Epitaxial n-GaN was irradiated with 5.4-MeV He ions. Capacitance–voltage (C–V) measurements showed that 5.4-MeV He ions remove free carriers at a rate of 6200±300 cm−1 in the first micron below the surface. Deep level transient spectroscopy (DLTS) revealed that, in addition to the radiation-induced defects previously detected by DLTS at 0.18–0.20 eV below the conduction band, He-ion irradiation introduced two additional prominent defects, ER4 (EC-0.78 eV) and ER5 (EC-0.95 eV) at rates of 1510±300 and 3030±500 cm−1, respectively. Capture cross-section measurements revealed that electron capture kinetics of ER5 is similar to that of a line defect.
Solvent-induced phase transition in thermally evaporated pentacene films
Authors: Gundlach D. J. , Jackson T. N. , Schlom D. G. , Nelson S. F.
Published: July 2002

We report a solvent-induced phase transition in pentacene thin films, from a “thin film” phase to a bulk-like phase. X-ray diffraction indicates that as-deposited thermally evaporated pentacene films consist mainly of (001)-oriented pentacene with an elongated (001) plane spacing of 15.5±0.1 Å, and a minor amount with a (001) plane spacing of 14.5±0.1 Å. When such films are exposed to solvents such as acetone, isopropanol, or ethanol, the plane spacing of the entire layer shifts abruptly from the elongated (001) plane spacing to the bulk value and this shift is accompanied by a macroscopic change in film morphology. While molecular ordering is maintained as indicated by x-ray diffraction, thin film transistor performance is severely degraded, most likely as a result of the morphological changes in the film.
Reexamination of N composition dependence of coherently grown GaNAs band gap energy with high-resolution x-ray diffraction mapping measurements
Authors: Uesugi Katsuhiro , Morooka Nobuki , Suemune Ikuo
Published: July 2002

GaNAs films grown on GaAs(001) substrates by metalorganic molecular beam epitaxy were studied by high-resolution x-ray diffraction (XRD) mapping measurements. The lattice constants of epitaxial films are usually estimated from symmetric and asymmetric XRD 2θ−θ measurements. In this study, it is pointed out that the consideration of the tilt angle between the GaAs(115) and GaNAs(115) planes caused by elastic deformation of the films is crucial to determine the lattice constants of the GaNAs films coherently grown on GaAs substrates. Mapping measurements of (115) XRD (2θ−θ)−Δω were performed for this purpose. The band gap energy of the films was determined by Fourier transform absorption spectroscopy measurements. The band gap energy bowing measured up to the N composition of 4.5% will be discussed by comparing with other measurements and theoretical calculations.
Highly c-axis oriented Pb(Zr, Ti)O3 thin films grown on Ir electrode barrier and their electrical properties
Authors: Lee K. B. , Tirumala S. , Desu S. B.
Published: July 2002

We have investigated the structural and electrical properties of sol-gel derived Pb(Zr, Ti)O3 (PZT) thin films deposited on Ir electrode barrier (Ir/poly-Si/SiO2/Si). Owing to the interface-controlled growth, highly c-axis oriented perovskite PZT thin films were obtained for the postdeposition annealing temperature of 580 °C. Additionally, we found that the ferroelectric properties of IrO2/PZT/Ir/poly-Si capacitors were remarkably changed by the partial pressure of oxygen during the deposition of IrO2 top electrodes, which could be due to the enhanced reaction of IrO2 with PZT by the oxygen ion bombardments. Remanent polarization and coercive field of IrO2/PZT/Ir/poly-Si capacitor with the top electrodes deposited at PO2=1 mTorr was 20 μC/cm2 and 30 kV/cm, respectively, and showed negligible polarization fatigue up to 1011 switching repetitions. The leakage current density at a field of 80 kV was 5×10−8 A/cm2.
Very high-efficiency green organic light-emitting devices based on electrophosphorescence
Authors: Baldo M. A. , Lamansky S. , Burrows P. E. , Thompson M. E. , Forrest S. R.
Published: July 2002

We describe the performance of an organic light-emitting device employing the green electrophosphorescent material, fac tris(2-phenylpyridine) iridium [Ir(ppy)3] doped into a 4,4′-N,N′-dicarbazole-biphenyl host. These devices exhibit peak external quantum and power efficiencies of 8.0% (28 cd/A) and 31 lm/W, respectively. At 100 cd/m2, the external quantum and power efficiencies are 7.5% (26 cd/A) and 19 lm/W at an operating voltage of 4.3 V. This performance can be explained by efficient transfer of both singlet and triplet excited states in the host to Ir(ppy)3, leading to a high internal efficiency. In addition, the short phosphorescent decay time of Ir(ppy)3 (<1 μs) reduces saturation of the phosphor at high drive currents, yielding a peak luminance of 100 000 cd/m2.
Microstructure-electromigration correlation in a thin stripe of eutectic SnPb solder stressed between Cu electrodes
Authors: Liu C. Y. , Chen Chih , Liao C. N. , Tu K. N.
Published: July 2002

Room-temperature electromigration occurs in a thin stripe of eutectic SnPb solder stressed by a current density of 105 amp/cm2. Hillocks and voids grow at the anode and the cathode, respectively. While the dominant diffusion species is Sn in this two-phase alloy, the growth of the hillocks, surprisingly, originates from the Pb grains.
Influence of residual stress on thermal expansion behavior
Authors: Wang X.-L. , Hoffmann C. M. , Hsueh C. H. , Sarma G. , Hubbard C. R. , Keiser J. R.
Published: July 2002

We demonstrate that the thermal expansion behavior of a material can be substantially modified by the presence of residual stresses. In the case of a composite tube made of two layers of dissimilar steels, in situ neutron diffraction measurements revealed a significant difference in the coefficients of thermal expansion along the radial and tangential directions. It is shown that the observed difference in thermal expansion is due to the change of residual stresses with temperature.
First principles study of Pb vacancies in PbTiO3
Authors: Pöykkö S. , Chadi D. J.
Published: July 2002

Electronic and ionic structures of lead vacancies in PbTiO3 were studied using an ab initio approach. Even though the lead vacancy is found to be an acceptor with stable charge states ranging from 2- to 4- it does not form a tightly bound defect pair with a double donor oxygen vacancy. The formation of distant and nearby lead-vacancy–oxygen-vacancy pairs is shown to be exothermic under certain growth conditions.
Sm(Co, Fe, Cu, Zr)
z
magnets fabricated by simple processing
Authors: Tang W. , Zhang Y. , Hadjipanayis G. C.
Published: July 2002

Optimization studies in Sm(Co, Cu, Fe, Zr)z magnets for high-temperature applications led to some compositions which develop their high coercivity with simple processing. Homogenized magnets with higher Cu and Zr content acquire a coercivity of above 20 kOe after a short aging (3 h) at 850 °C without the traditional slow cooling to 400 °C which is required for the commercial magnets. Microstructure studies showed that the homogenized magnets consist of a mixture of Sm(Co, Cu)5 precipitates in a disordered 2:17 matrix as compared to a uniform and featureless microstructure of the traditional homogenized magnets. Because of this, the time required for the full development of uniform cellular and lamellar structures with the right microchemistry is much shorter in the magnets.
Optical gap solitons: Past, present, and future; theory and experiments
Authors: Aceves Alejandro B.
Published: July 2002

Optical gap solitons refer to nonlinear waves propagating in optical fibers whose linear refractive index has a periodic variation. Stationary gap solitons came to light first in 1987 [Chen and Mills, Phys. Rev. Lett. 58, 160 (1987)]; two years later, they re-emerge in Christodoulides and Joseph [Phys. Rev. Lett. 62, 1746 (1989)] and are first extended to a more general traveling wave form in Aceves and Wabnitz [Phys. Lett. A 141, 37 (1989)]. But it was not until seven years later, that the first experimental demonstration [Eggleton et al., Phys. Rev. Lett. 76, 1627 (1996); J. Opt. Soc. Am. B 14, 2980 (1997)] was reported. Since then, there has been an increase in the study of the dynamics and applications of such solitons. This paper is a brief survey of some of the ongoing and future research on optical gap solitons.
Thickness dependence of magnetoelastic properties and of crystallization behavior in Fe–Tb–Dy/Fe and Fe–Tb–Dy/nanocrystalline Fe–Si–B–Nb–Cu multilayers
Authors: Farber P. , Kronmüller H.
Published: July 2002

Fe–Tb–Dy/Finemet™ and Fe–Tb–Dy/Fe multilayers have been prepared by ion beam sputtering on room temperature sapphire substrates (Finemet is a nanocrystalline Fe–Si–B–Nb–Cu magnetic alloy). After production, the Fe–Tb–Dy and the Finemet layers are amorphous, whereas the Fe layers are polycrystalline. Subsequent annealing leads to relaxation of the amorphous Fe–Tb–Dy layers in both cases and to crystallization of the Finemet layers for the Fe–Tb–Dy/Finemet multilayers. In this regime, the properties of the multilayers can be well described by a mixture rule of the two perfectly coupled components. In an optimum thickness range for both layers, they show very soft magnetic properties and high magnetoelasticity. Furthermore, it has been observed that crystallization of the Fe–Tb–Dy layers within the multilayer is hindered by interface stabilization as recently predicted in U. Herr, H. Geisler, H. Ippach, and K. Samwer, Phys. Rev. B 59, 13719 (1999).
Electron Interactions With SF6
Authors: Christophorou L. G. , Olthoff J. K.
Published: July 2002

Sulfur hexafluoride (SF6) is commonly used as a gaseous dielectric and as a plasma etching gas. In this work, the state of knowledge on electron-interaction cross sections and electron-swarm parameters in SF6 is comprehensively reviewed and critically assessed. Cross sections are presented and discussed for the following scattering processes: total electron scattering; differential elastic; elastic integral; elastic momentum; total vibrational; total and partial ionization; total dissociative and nondissociative electron attachment; and dissociation into neutrals. Coefficients for electron-impact ionization, effective ionization, electron attachment, electron drift, and electron diffusion are also reviewed and assessed. In addition, complementary information on the electronic and molecular structure of the SF6 molecule and on electron detachment and ion transport in parent SF6 gas is provided that allows a better understanding of the nature of the cross sections and swarm parameters. The assessed data are used to deduce cross sections and coefficients for which there exist no direct measurements at the present time. The present work on electron interactions with the SF6 molecule reveals a rather simple picture which can be summarized as follows: (1) Elastic electron scattering is the most significant electron scattering process over the electron energy range from ∼0.01 to ∼1000 eV. (2) Below 15 eV the most distinct inelastic energy-loss process is vibrational excitation—direct dipole excitation involving the ν3 mode and indirect vibrational excitation via negative ion states involving the ν1 mode. (3) Below ∼0.1 eV electron attachment forming SF6− is the most dominant interaction (along with elastic scattering). Above this energy, the cross sections for dissociative electron attachment forming fragment anions [principally SFx− (x=3, 4, and 5) and F−] are appreciable, with the room temperature total electron attachment cross section dominated by the formation of SF5− between ∼0.3 and 1.5 eV and by the formation of F− beyond ∼2.0 eV. (4) Above ∼16 eV dissociative ionization becomes significant, generating principally SFx+ (x=1, 3, 4, and 5) and F+ positive-ion fragments which, together with elastic electron scattering, makes up most of the total electron scattering cross section. (5) Electron-impact dissociation into neutral fragments SFx (x=1, 2, and 3) and F occurs above ∼15 eV, with cross section values potentially exceeding those for ionization for electron energies near 20 eV. (6) The total electron scattering cross section exhibits distinct structure due to negative-ion resonances near 0.0, 2.5, 7.0, and 11.9 eV. The most significant data needs are for direct measurements of vibrational excitation cross sections, for cross sections for electron-impact dissociation into neutral fragments, and for the momentum transfer cross section at low energies.
Revisiting freely decaying two-dimensional turbulence at millennial resolution
Authors: Bracco A. , McWilliams J. C. , Murante G. , Provenzale A. , Weiss J. B.
Published: July 2002

We study the evolution of vortex statistics in freely decaying two-dimensional turbulence at very large Reynolds number. The results obtained here confirm that the peak vorticity inside vortex cores is conserved and that the number of vortices as a function of time, N(t), decreases as a power law. In addition, the numerical findings are consistent with the predictions of the scaling theories proposed by Carnevale et al. [Phys. Rev. Lett. 66, 2735 (1991)] and Weiss and McWilliams [Phys. Fluids A 5, 608 (1993)]. We also obtain new evidence for a self-similar distribution of vortex radii and circulations, that suggests the possibility of a generic statistical behavior of the decaying phase of two-dimensional turbulence at high Reynolds number.
A detailed study on the symmetry breaking and its effect on the potential surface of NO3
Authors: Eisfeld Wolfgang , Morokuma Keiji
Published: July 2002

The tenacious symmetry breaking of the electronic wave function of the nitrate radical (NO3) and its effect on the ground-state potential energy surface is investigated in detail. The symmetry breaking of Hartree–Fock wave functions results from a dominance of the orbital localization effect over the resonance effect and leads to three different solutions, one symmetrical and two distorted ones, for the same electronic state. The respective equilibrium geometries of these solutions are points on different potential surfaces, making their comparison meaningless. The resonance effect is promoted by dynamic as well as static electron correlation. However, the dynamic correlation methods [e.g., many-body perturbation theory (MBPT) and coupled-cluster single double (CCSD)] cannot overcome the symmetry breaking of the reference function and the problem of multiple solutions persists. The symmetry breaking can be avoided by the complete active space self-consistent field (CASSCF) approach that yields unique, single-valued surfaces for all electronic states. However, a sufficiently large and appropriately selected active space has to be used to avoid unphysical distortion of the wave function. Still the orbital localization effect leads to equilibrium geometries of C2v symmetry which strongly depend on the state-averaging of the CASSCF wave function. Multireference single double configuration interaction (MR-SDCI) wave functions are also free of symmetry breaking, if the reference orbitals are and if the configuration space is invariant under the symmetry operations. MRCI geometry optimizations only result in D3h symmetric structures with bond lengths and harmonic frequencies in close agreement with experimental data.
Giant magnetostriction in an ordered Fe3Pt single crystal exhibiting a martensitic transformation
Authors: Kakeshita Tomoyuki , Takeuchi Tetsuya , Fukuda Takashi , Tsujiguchi Masato , Saburi Toshio , Oshima Ryuichiro , Muto Shunsuke
Published: July 2002

Magnetostriction measurements have been made in an ordered Fe3Pt single crystal with degree of order of about 0.8, which exhibits a cubic-tetragonal martensitic transformation at 97 K. The specimen was cooled down to 4.2 K without magnetic field, and then a magnetic field of 4 T is applied to the specimen along 〈001〉 at 4.2 K and removed. As a result, a reversible giant magnetostriction of about 0.5% is observed. This reversible magnetostriction will be caused by the rearrangement of crystallographic domains, being three times as large as that of Terfenol-D (Fe2DyxTb1−x: typical magnetostrictive materials).
How accurate are Stoney’s equation and recent modifications
Authors: Klein Claude A.
Published: July 2002

Stoney’s equation has been—and still is—extensively used to evaluate the macrostress acting in a coating deposited on a thick substrate. In principle, the formula applies only in the “thin-film approximation,” that is, for coatings much thinner than the substrate. The main purpose of this communication is to demonstrate that, based on a general theory of elastic interactions in multilayer laminates, the correct formula for the stress can be expressed in terms of Stoney’s equation and a correction factor equal to (1+γ δ3)/(1+δ), where γ designates the ratio of the biaxial moduli and δ is the ratio of the layer thicknesses. In this light, it is shown that (a) Stoney’s equation does not cause serious errors for thickness ratios δ⩽0.1; (b) Atkinson’s recently proposed modification, which does not require information on the coating’s modulus, yields much improved results for thickness ratios up to δ≃0.4; and (c) Brenner–Senderoff-type expressions can be very misleading and should be avoided.
Dielectric studies of phase transitions in (Pb1−xBax)ZrO3
Authors: Pokharel Bhadra P. , Pandey Dhananjai
Published: July 2002

Phase transitions in semiwet derived (Pb1−xBax)ZrO3 ceramics for the composition range 0⩽x⩽0.30 have been investigated by dielectric measurements at various frequencies during heating and cooling cycles. The paraelectric (PE) to ferroelectric (FE) to antiferroelectric (AFE) sequence of phase transitions is observed for 0⩽x<0.20. On increasing the Ba2+ content from x=0 to x=0.05, the thermal hysteresis associated with the AFE–FE phase transition increases from 11 to 100 °C. This is attributed to the increase in the piezoelectric coupling between the strain and polarization. For x=0.20, the FE phase does not transform into the AFE phase on cooling. Pronounced deviations from the regular FE behavior are observed on increasing the Ba2+ content to x=0.25. For x=0.30, the temperatures corresponding to the peak values of the real and imaginary parts of the dielectric constant become frequency dependent indicating relaxor FE behavior. It is shown that the polar clusters present in the PE phase undergo Vogel–Fulcher type relaxational freezing in the relaxor FE phase. The results of temperature dependent polarization measurements confirm the findings of the dielectric studies. It is proposed that Ba2+ substitution modifies the AFE and FE interactions of the PbZrO3 matrix in such a manner that their strengths become comparable for x=0.30 leading to the glassy or relaxor behavior.
Hugoniot data for iron
Authors: Brown J. M. , Fritz J. N. , Hixson R. S.
Published: July 2002

A definitive set of the Los Alamos Hugoniot data for iron in a pressure regime extending to 442 GPa is given. Earlier standards data, obtained using conventional explosive systems, were thoroughly reprocessed. All original film records were reread. On the basis of more recent experiment and theory, some data were culled because the experimental designs were found to be insufficiently conservative. The analysis was also modified to take into account preheating of the explosively driven flyer plates. Minor clerical errors in transcription of measurements were corrected. An improved algorithm for the flash-gap time correction was incorporated. Higher-pressure data were obtained using a conventional 13-pin target assembly on a two-stage light gas gun. Several polynomial representations of the data are given. A linear fit to the data (Us=3.935+1.578 Up, where the shock velocity Us and the particle velocity Up are in km/s) has a root-mean-square misfit of 62 m/s. The quadratic fit (Us=3.691+1.788 Up−0.038 Up2) has a root-mean-square misfit of 39 m/s.
Electromigration in Sn–Pb solder strips as a function of alloy composition
Authors: Liu C. Y. , Chen Chih , Tu K. N.
Published: July 2002

Using thin film solder strips, we have investigated the electromigration of six different compositions of Sn–Pb solders at current density of 105 A/cm2 near ambient temperature. The six compositions are pure Sn, Sn80Pb20, Sn70Pb30, Sn62Pb38 (eutectic), Sn40Pb60, and Sn5Pb95. The eutectic alloy, with the lowest melting point and a high density of lamella interfaces, was found to have the fastest hillock growth. As composition moving toward the two terminal phases, the hillock growth rate decreases; but it increases again in pure Sn. The interface between Sn and Pb, being the fastest kinetic path of mass transport, also serves as the place to initiate hillock and void formation.
Nuclear spin–spin coupling constants from regular approximate relativistic density functional calculations. II. Spin–orbit coupling effects and anisotropies
Authors: Autschbach Jochen , Ziegler Tom
Published: July 2002

Based on our recently published two-component relativistic formulation of the nuclear spin–spin coupling hyperfine terms, we present a full implementation into the Amsterdam Density Functional program. The scalar relativistic code has been extended to include the relativistic analogue of the spin–dipole operator in the coupling calculations, which can now in addition be based on two-component spin–orbit coupled Kohn–Sham orbitals. One-bond coupling constants for some plumbanes are in good agreement with experiment, slightly improving the scalar relativistic values. Coupling constants and anisotropies for the XF (X=Cl, Br, I) and TlX (X=F, Cl, Br, I) series are compared to experimental data and for ClF additionally to recently published ab initio calculations. The spin–dipole term contributes largely to the coupling constants in XF. Spin–orbit effects are essential for the TlX couplings, where they can yield the most important contributions. In addition, data is reported for the benchmark systems ethane, ethene, and ethyne.
Magnetic properties and large magnetic-field-induced strains in off-stoichiometric Ni–Mn–Al Heusler alloys
Authors: Fujita A. , Fukamichi K. , Gejima F. , Kainuma R. , Ishida K.
Published: July 2002

Magnetic properties and magnetic-field-induced strains (MFIS) have been investigated for off-stoichiometric Ni–Mn–Al Heusler alloys with an ordered L21 structure. A clear martensitic transformation in Ni53Mn25Al22 alloy was revealed below the Curie temperature. In the polycrystalline specimen, an irreversible relative change due to the MFIS was confirmed between the martensite start and finish temperatures Ms and Mf, and a maximum relative length change ΔL/L|7T of about −100 ppm was observed at just above Mf. On the other hand, a large irreversible relative length change of about 1000 ppm has been demonstrated in the magnetic field of 7 T for a single crystal cut from the polycrystalline specimen. A delay of the response of strains against the magnetic field was also confirmed.
Label-free probing of the binding state of DNA by time-domain terahertz sensing
Authors: Brucherseifer M. , Nagel M. , Haring Bolivar P. , Kurz H. , Bosserhoff A. , Büttner R.
Published: July 2002

We present a promising approach for the label-free characterization of genetic material. Time-resolved terahertz (THz) transmission analysis of polynucleotides demonstrate a strong dependence of the complex refractive index on the binding state (hybridized/denatured) of deoxyribonucleic acid (DNA) molecules. By monitoring THz transients, one can thus infer the binding state of oligo- and polynucleotides, and hence identify polynucleotides by detecting the binding of unknown polynucleotide DNA sequences to known probe molecules. A broadband experimental demonstration in a free-space configuration, as well as a discussion of the potential application for next generation gene chips is presented.
CO oxidation on Pt(111)—Scanning tunneling microscopy experiments and Monte Carlo simulations
Authors: Völkening S. , Wintterlin J.
Published: July 2002

The catalytic reaction between adsorbed oxygen atoms and CO molecules on Pt(111) was investigated by scanning tunneling microscopy and modeled by Monte Carlo simulations. Experiments were performed by dosing preadsorbed Oad layers with CO between 237 and 274 K. Two stages were observed during dosing with CO, an initial reordering and compression of (2×2)Oad islands, and a subsequent shrinking of the islands by the reaction of Oad to give CO2. The reaction occurs exclusively at boundaries between (2×2)Oad and c(4×2)COad domains. The reaction order with respect to the oxygen coverage is 0.5; the reactivity of the boundary increases during the reaction. The Monte Carlo simulations included surface diffusion of Oad atoms, attractive interactions between Oad atoms, the Oad–COad reaction probability (with parameters from quantitative scanning tunneling microscopy measurements), adsorption/desorption of CO, and a high mobility of COad. The experimentally observed domain shapes, the reaction order of 0.5, and the increasing boundary reactivity could only be reproduced by additionally including an Oad coordination-dependent activation energy ΔEreact* of 25 meV per Oad neighbor that accounts for the attractive Oad–Oad interactions. The initial ordering stage could be modeled by incorporating an additional repulsive interaction between Oad and COad. The fact that no reaction occurs in the interior of the (2×2)Oad domains, although they are covered by a layer of interstitial COad molecules, is attributed to the crucial role of reactive Oad–COad configurations that only exist at the domain boundaries.
Fast microchannel plate detector with an impedance matched anode in suspended substrate technology
Authors: Schletti Reto , Wurz Peter , Scherer Stefan , Siegmund Oswald H.
Published: July 2002

Fast particle detectors with subnanosecond pulse widths are key elements in modern time-of-flight mass spectrometers. Typically, an impedance matched transmission line from the extended anode to the coaxial cable is necessary to obtain fast pulses. We present an approach using a planar geometry for the transmission line. Thereby, the impedance match is realized with a transition from a 50 Ω suspended substrate microstrip line to a coaxial 50 Ω line. A prototype with an active area of 1.8 cm diameter including a strategy to reduce peak ringing was built and tested. The mean pulse width measured with a 1 GHz analog oscilloscope was 520 ps (full width at half maximum) with a rise time of 380 ps. The robust, compact, and low-weight design of the detector is well suited for an application in space, where weight, space, and power consumption are very limited resources.
On the development of large-scale structures of a jet normal to a cross flow
Authors: Lim T. T. , New T. H. , Luo S. C.
Published: July 2002

It is well known that vortex rings are the dominant flow structures in the near field of a free jet, and this has led many researchers to believe that they also occur in a jet in cross flow (JICF). Previous studies have postulated that these vortex rings deform and fold as they convect downstream, which culminates in the formation of vortex loops at both the upstream and the lee-side of the jet column. In this paper, we take a fresh look at the vortical structures of JICF in water by releasing dye at strategic locations around the jet exit. The results show that there is no evidence of ring vortices in JICF, and the postulation that vortex loops are formed from the folding of the vortex rings does not reflect the actual flow behavior. The presence of a counter-rotating vortex pair (CVP) at the jet exit is found to inhibit the formation of the vortex rings. Instead, vortex loops are formed directly from the deformation of the cylindrical vortex sheet or jet column, without going through the vortex rings, in a process similar to the buoyant jet and wake structures studied by Perry and Lim [J. Fluid Mech. 88, 451 (1978)].
Green-induced infrared absorption in MgO doped LiNbO3
Authors: Furukawa Y. , Kitamura K. , Alexandrovski A. , Route R. K. , Fejer M. M. , Foulon G.
Published: July 2002

Green-induced infrared absorption (GRIIRA) was investigated by a photothermal technique for undoped and Mg-doped LiNbO3 crystals that have different Li/Nb ratios. Threshold effect on GRIIRA was found, threshold MgO concentrations being the same for GRIIRA and photorefraction. We suggest that GRIIRA is associated with the formation of the small polaron that is located on Nb antisite defect. The remarkable decrease of GRIIRA in Mg:LiNbO3 can then be attributed to the elimination of this intrinsic defect, Nb in Li, following the incorporation of Mg on Li sites. For nonlinear optical applications, LiNbO3 doped with MgO at concentrations over threshold has a combined advantage of having almost no GRIIRA and photorefraction.
All-orders spectral calculation of radio-frequency heating in two-dimensional toroidal plasmas
Authors: Jaeger E. F. , Berry L. A. , D’Azevedo E. , Batchelor D. B. , Carter M. D.
Published: July 2002

Spectral calculations of radio-frequency (rf) heating in tokamak plasmas are extended to two dimensions (2-D) by taking advantage of new computational tools for distributed memory, parallel computers. The integral form of the wave equation is solved in 2-D without any assumption regarding the smallness of the ion Larmor radius (ρ) relative to the perpendicular wavelength (λ⊥). Results are therefore applicable to all orders in k⊥ρ, where k⊥=2π/λ⊥. Previous calculations of rf wave propagation and heating in 2-D magnetized plasmas have relied on finite Larmor radius expansions (k⊥ρ≪1) and are thus limited to relatively long wavelengths. In this paper, no such assumption is made, and we consider short wavelength processes such as the excitation and absorption of ion Bernstein waves in 2-D with k⊥ρ>1. Results show that this phenomenon is far more complex than simple one-dimensional plasma models would suggest. Other applications include fully self-consistent 2-D solutions for high-harmonic fast-wave heating in spherical tokamaks. These calculations require the storage and inversion of a very large, dense matrix, but numerical convergence can be improved by writing the plasma current in the laboratory frame of reference. To accurately represent the wave spectrum in this frame, the local plasma conductivity is corrected to first order in ρ/L, where L is the equilibrium scale length. This correction is necessary to ensure accuracy in calculating the wave spectrum and hence the fraction of power absorbed by ions and electrons.
Continuous wave operation of a 9.3 μm quantum cascade laser on a Peltier cooler
Authors: Hofstetter Daniel , Beck Mattias , Aellen Thierry , Faist Jérôme , Oesterle Ursula , Ilegems Marc , Gini Emilio , Melchior Hans
Published: July 2002

High average power quantum cascade lasers at 9.3 μm using InP top cladding layers and both junction up and junction down mounting are presented. A 3 mm long, junction up mounted device emitted 54 mW average power at 30 °C and 11.5% duty cycle with a threshold current density of 3.72 kA/cm2. A similar, but only 1.5 mm long device with high reflection coating on both facets was mounted junction down and tested at even higher duty cycles. At −27 °C, we achieved continuous wave operation with a threshold current density of 3.3 kA/cm2.
Electromechanical coupling coefficient of 〈001〉-oriented Pb(Mg1/3Nb2/3)O3–PbTiO3 cystals: Stress and temperature independence
Authors: Viehland D. , Powers J.
Published: July 2002

The electromechanical coupling coefficient (k33) of 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 〈001〉-oriented crystals have been investigated under unaxial stress (σ) and temperature (T). The results demonstrate that 〈001〉-oriented crystals have a k33 of ∼0.92, which is nearly independent of σ between 0 and 6×107 N/m2 (∼4% decrease) and relatively independent of temperature between 5 and 40 °C (∼4% increase).
Water at supercritical conditions: A first principles study
Authors: Boero Mauro , Terakura Kiyoyuki , Ikeshoji Tamio , Liew Chee Chin , Parrinello Michele
Published: July 2002

We analyze, via first principles molecular dynamics, the structural and electronic properties of water close to and above the critical point. Contrary to the ordinary liquid state, at supercritical conditions the hydrogen bond network is destabilized to various extents and the continuous breaking and reformation of hydrogen bonded structures allow large density and dipole fluctuations that, in turn, can significantly affect the dielectric properties of the solvent. Close to the critical point, where the density is very low, small clusters, mainly dimers and trimers, are the dominant features, but many molecules exhibit no H-bond. On the other hand, at higher densities, more extended structures appear, but still a continuous network cannot form. In both cases, H-bond configurations that are anomalous with respect to the normal liquid phase appear. These features strongly affect the solvent properties of supercritical water with respect to those of ambient water. They most likely vary continuously as a function of temperature, pressure and density and, hence, can be tuned to optimize the desired chemical process.
Potential energy curves and transport properties for the interaction of He with other ground-state atoms
Authors: Partridge Harry , Stallcop James R. , Levin Eugene
Published: July 2002

The interactions of a He atom with a heavier atom are examined for 26 different elements, which are consecutive members selected from three rows (Li–Ne, Na–Ar, and K,Ca, Ga–Kr) and column 12 (Zn, Cd) of the Periodic Table. Interaction energies are determined using high-quality ab initio calculations for the states of the molecule that would be formed from each pair of atoms in their ground states. Potential energies are tabulated for a broad range of interatomic separation distances. The results show, for example, that the energy of an alkali interaction at small separations is nearly the same as that of a rare-gas interaction with the same electron configuration for the closed shells. Furthermore, the repulsive-range parameter for this region is very short compared to its length for the repulsion dominated by the alkali-valence electron at large separations (beyond about 3–4 a0). The potential energies in the region of the van der Waals minimum agree well with the most accurate results available. The ab initio energies are applied to calculate scattering cross sections and obtain the collision integrals that are needed to determine transport properties to second order. The theoretical values of Li–He total scattering cross sections and the rare-gas atom–He transport properties agree well (to within about 1%) with the corresponding measured data. Effective potential energies are constructed from the ab initio energies; the results have been shown to reproduce known transport data and can be readily applied to predict unknown transport properties for like-atom interactions.
Tellurium antisites in CdZnTe
Authors: Chu Muren , Terterian Sevag , Ting David , Wang C. C. , Gurgenian H. K. , Mesropian Shoghig
Published: July 2002

The electrical properties of CdTe and Cd1−xZnxTe crystals grown under excess tellurium by a modified Bridgman technique are critically dependent on the zinc content. Below an x value of 0.07, the as-grown CdZnTe crystals are n type while, above this value, CdZnTe crystals are p type. The origin of the shallow donor level at 0.01 eV below the conduction band is most likely singly ionized Te antisites (Te at Cd sites). The origin of the deep donor level at 0.75 eV below the conduction band is therefore doubly ionized tellurium antisites. Based on this model, the conduction type of CdZnTe crystals is determined by the results of compensation between the shallow donors of Te antisites and the shallow acceptors of Cd vacancies. High resistivity Cd0.9Zn0.1Te crystals are produced by compensating the p-type crystals with indium impurity at a low doping level of 1–5×1015 cm−3. At room temperature, CdZnTe radiation detectors can resolve the six low energy peaks in the Am241 spectrum, a performance comparable to that of the best CdZnTe detectors reported.
Geometrical derivatives and magnetic properties in atomic-orbital density-based Hartree–Fock theory
Authors: Larsen Helena , Helgaker Trygve , Olsen Jeppe , Jørgensen Poul
Published: July 2002

A reformulation of Hartree–Fock theory for time-independent molecular properties with perturbation-dependent basis sets and which refers strictly to the atomic-orbital basis is presented. The formalism is based on a recently proposed exponential parametrization of the one-electron atomic-orbital density matrix. In the presented formulation, only multiplications and additions of sparse matrices are needed. Linear scaling with system size is therefore obtainable, making this formulation ideally suited to large molecular systems. The paper contains general formulas for molecular energy derivatives up to fourth order, with special attention given to molecular gradients, molecular Hessians, magnetizabilities, and nuclear magnetic shieldings.
Adsorption of NH3 and NO2 molecules on carbon nanotubes
Authors: Chang Hyunju , Lee Jae Do , Lee Seung Mi , Lee Young Hee
Published: July 2002

Adsorption of NH3 and NO2 molecules on semiconducting single-walled carbon nanotubes is investigated using density functional theory. Both NH3 and NO2 molecules are found to bind to carbon nanotubes via physisorption. Electron charge transfer is found to be a major mechanism determining the conductivity change in carbon nanotubes upon exposure to NH3 and NO2 molecules. The calculated density of states is also considered to elucidate the differences in the NO2 and NH3 gas detection mechanism of carbon nanotubes.
Visual characteristics and initial growth rates of round cryogenic jets at subcritical and supercritical pressures
Authors: Chehroudi B. , Talley D. , Coy E.
Published: July 2002

Cryogenic liquids initially at a subcritical temperature were injected through a round tube into an environment at a supercritical temperature and at various pressures ranging from subcritical to supercritical values. Pure N2 and O2 were injected into environments composed of N2, He, Ar, and various mixtures of CO+N2. The results were photographically observed and documented near the exit region using a CCD camera illuminated by a short duration backlit strobe light. At low subcritical chamber pressures, the jets showed surface irregularities that amplified downstream, exhibiting intact, shiny, but wavy (sinuous) surface features that eventually broke up into irregularly shaped small entities. A further increase of chamber pressure at constant jet initial and ambient temperatures caused the formation of many small droplets to be ejected from the surface of the jet similar to what is observed in the second wind-induced jet breakup regime. As the chamber pressure was further increased, the transition to a full atomization regime was inhibited near but slightly below the critical pressure. The jet structure at this point changed and began to resemble a turbulent gas jet with no detectable droplets. The reason was attributed to the reduction of the surface tension and enthalpy of vaporization as the critical pressure of the injectant is approached. The initial divergence angle of the jet was measured at the jet exit and compared with the divergence angle of a large number of other mixing layer flows, including atomized liquid sprays, turbulent incompressible gaseous jets, supersonic jets, and incompressible but variable density jets. The divergence angle for all these cases was plotted over four orders of magnitude in the gas-to-liquid density ratio, the first time such a plot has been reported over this large a range of density ratios. At and above the critical pressure of the injectant, the jet growth rate measurements agreed quantitatively with the theory for incompressible but variable density gaseous mixing layers. This is the first time a quantitative parameter has been used to demonstrate that the similarity between the two flows extends beyond a mere qualitative physical appearance. Finally, as the pressure is reduced to progressively more subcritical values, the spreading rate approaches that measured by others for liquid sprays.
Electron transport properties and collision cross sections in C2F4
Authors: Yoshida K. , Goto S. , Tagashira H. , Winstead C. , McKoy B. V. , Morgan W. L.
Published: July 2002

We have measured the electron drift velocity, longitudinal diffusion coefficient, and ionization coefficient in tetrafluoroethene (C2F4). Using these data and the results of ab initio calculations of the elastic, momentum-transfer, and neutral-excitation cross sections, along with measurements of the partial ionization cross sections, we have performed a swarm analysis in order to construct a self-consistent set of electron impact cross sections for C2F4. The swarm analysis consists of solutions to Boltzmann’s equation for electrons in C2F4 for values of E/N⩽500 Td and direct Monte Carlo simulation of electron transport in C2F4 for 500 Td⩽E/N⩽2000 Td. We present an analysis and discussion of the sensitivity of cross sections derived from swarm data to uncertainties in the electron transport measurements. We also discuss the failure of the two-term spherical harmonic solution to Boltzmann’s equation for E/N>500 Td, which necessitated the use of Monte Carlo simulations for high values of E/N.
k-constraint for the modified Kadomtsev–Petviashvili system
Authors: Chen Deng-yuan
Published: July 2002

By imposing constraint (Lk)−=q∂−1r∂ on the pseudo-differential operator Lk, the k constrained modified Kadomtsev–Petviashvili (KP) hierarchy and their corresponding Lax pair are obtained from the linear problem and its adjoint of the modified KP system. Especially, the modified KdV system, the GNS system with derivative coupling, the Burgers system, and a new 3×3 integrable system are presented as examples.
Alfvén wave cascades in a tokamak
Authors: Sharapov S. E. , Alper B. , Berk H. L. , Borba D. N. , Breizman B. N. , Challis C. D. , Fasoli A. , Hawkes N. C. , Hender T. C. , Mailloux J. , Pinches S. D. , Testa D. ,   EFDA–JET work programme
Published: July 2002

Experiments designed for generating internal transport barriers in the plasmas of the Joint European Torus [JET, P. H. Rebut et al., Proceedings of the 10th International Conference, Plasma Physics and Controlled Nuclear Fusion, London (International Atomic Energy Agency, Vienna, 1985), Vol. I, p. 11] reveal cascades of Alfvén perturbations with predominantly upward frequency sweeping. These experiments are characterized by a hollow plasma current profile, created by lower hybrid heating and current drive before the main heating power phase. The cascades are driven by ions accelerated with ion cyclotron resonance heating (ICRH). Each cascade consists of many modes with different toroidal mode numbers and different frequencies. The toroidal mode numbers vary from n=1 to n=6. The frequency starts from 20 to 90 kHz and increases up to the frequency range of toroidal Alfvén eigenmodes. In the framework of ideal magnetohydrodynamics (MHD) model, a close correlation is found between the time evolution of the Alfvén cascades and the evolution of the Alfvén continuum frequency at the point of zero magnetic shear. This correlation facilitates the study of the time evolution of both the Alfvén continuum and the safety factor, q(r), at the point of zero magnetic shear and makes it possible to use Alfvén spectroscopy for studying q(r). Modeling shows that the Alfvén cascade occurs when the Alfvén continuum frequency has a maximum at the zero shear point. Interpretation of the Alfvén cascades is given in terms of a novel-type of energetic particle mode localized at the point where q(r) has a minimum. This interpretation explains the key experimental observations: simultaneous generation of many modes, preferred direction of frequency sweeping, and the absence of strong continuum damping.
Elasticity and magnetoelasticity of Fe–Ga solid solutions
Authors: Wuttig Manfred , Dai Liyang , Cullen James
Published: July 2002

Continuous-wave measurements of the shear elastic constants of bcc Fe–Ga solid solutions show that the constant C44 is independent of the gallium composition while the constant 12(C11–C12) decreases linearly and extrapolates to zero at 26 at. % Ga. This result indicates that the Fe–Ga solid solution is inhomogeneous and contains clusters consisting of embryos of a martensitic phase. The increasing softness of 12(C11–C12) then reflects an increase of the embryo density. A model assuming that the clusters are centered around next-nearest Ga pairs reproduces the known composition dependence of the magnetostrictive constant λ100 in the composition range 0<XGa<0.23 with only one adjustable parameter. Its value suggests a cluster size of 12 atoms.
Numerical studies of edge localized instabilities in tokamaks
Authors: Wilson H. R. , Snyder P. B. , Huysmans G. T. A. , Miller R. L.
Published: July 2002

A new computational tool, edge localized instabilities in tokamaks equilibria (ELITE), has been developed to help our understanding of short wavelength instabilities close to the edge of tokamak plasmas. Such instabilities may be responsible for the edge localized modes observed in high confinement H-mode regimes, which are a serious concern for next step tokamaks because of the high transient power loads which they can impose on divertor target plates. ELITE uses physical insight gained from analytic studies of peeling and ballooning modes to provide an efficient way of calculating the edge ideal magnetohydrodynamic stability properties of tokamaks. This paper describes the theoretical formalism which forms the basis for the code.
A fundamental-measure theory for inhomogeneous associating fluids
Authors: Yu Yang-Xin , Wu Jianzhong
Published: July 2002

The fundamental-measure theory (FMT) of Rosenfeld for hard spheres is extended to inhomogeneous associating fluids on the basis of Wertheim’s first-order thermodynamic perturbation theory (TPT1). The excess intrinsic Helmholtz energy, which includes contributions from hard-sphere repulsion and from intermolecular bonding, is represented as a functional of three weighted densities that are related to the geometry of spherical particles. In the absence of association, this theory is the same as the original FMT, and at bulk conditions it reduces to TPT1. In comparison with Monte Carlo simulation results, the extended fundamental-measure theory provides good descriptions of the density profiles and adsorption isotherms of associating hard spheres near a hard wall. Calculated results indicate that the critical temperatures for the vapor–liquid equilibria of associating fluids in hard slit pores are suppressed compared with that for the bulk fluid and the confinement has more significant impact on the liquid side than the vapor side of the coexistence curve. Unlike nonpolar fluids at similar conditions, saturated associating liquids in hard slit pores do not exhibit strong layering near the solid surface.
On the relation of protein dynamics and exciton relaxation in pigment–protein complexes: An estimation of the spectral density and a theory for the calculation of optical spectra
Authors: Renger Thomas , Marcus R. A.
Published: July 2002

A theory for calculating time– and frequency–domain optical spectra of pigment–protein complexes is presented using a density matrix approach. Non-Markovian effects in the exciton–vibrational coupling are included. A correlation function is deduced from the simulation of 1.6 K fluorescence line narrowing spectra of a monomer pigment–protein complex (B777), and then used to calculate fluorescence line narrowing spectra of a dimer complex (B820). A vibrational sideband of an excitonic transition is obtained, a distinct non-Markovian feature, and agrees well with experiment on B820 complexes. The theory and the above correlation function are used elsewhere to make predictions and compare with data on time–domain pump–probe spectra and frequency–domain linear absorption, circular dichroism and fluorescence spectra of Photosystem II reaction centers.