bluuebunny/crossref_metadata_2025 huggingface dataset:
Oxygen interaction with disordered and nanostructured Ag(001) surfaces
Authors: Vattuone L. , Burghaus U. , Savio L. , Rocca M. , Costantini G. , Buatier de Mongeot F. , Boragno C. , Rusponi S. , Valbusa U.
Published: July 2002

We investigated O2 adsorption on Ag(001) in the presence of defects induced by Ne+ sputtering at different crystal temperatures, corresponding to different surface morphologies recently identified by scanning tunneling microscopy. The gas-phase molecules were dosed with a supersonic molecular beam. The total sticking coefficient and the total uptake were measured with the retarded reflector method, while the adsorption products were characterized by high resolution electron energy loss spectroscopy. We find that, for the sputtered surfaces, both sticking probability and total O2 uptake decrease. Molecular adsorption takes place also for heavily damaged surfaces but, contrary to the flat surface case, dissociation occurs already at a crystal temperature, T, of 105 K. The internal vibrational frequency of the O2 admolecules indicates that two out of the three O2− moieties present on the flat Ag(001) surface are destabilized by the presence of defects. The dissociation probability depends on surface morphology and drops for sputtering temperatures larger than 350 K, i.e., when surface mobility prevails healing the defects. The latter, previously identified with kink sites, are saturated at large O2 doses. The vibrational frequency of the oxygen adatoms, produced by low temperature dissociation, indicates the formation of at least two different adatom moieties, which we tentatively assign to oxygen atoms at kinks and vacancies.
Theoretical predictions of unstable two-phase regions in wurtzite group-III-nitride-based ternary and quaternary material systems using modified valence force field model
Authors: Takayama Toru , Yuri Masaaki , Itoh Kunio , Harris James S.
Published: July 2002

Group-III (B, Al, Ga, and In)-nitride quaternary alloys and group-III (Al, Ga, and In)-nitride-based mixed anion (As, P, and Sb) quaternary alloys are useful for blue and green light emitting devices and high-temperature, high-power, and high-frequency electronic devices. It is known that these alloys are very difficult to grow in certain compositional regions. The thermodynamical stability of these alloys is studied with respect to an unstable two-phase region in the phase field. The unstable two-phase region is predicted based on a strictly regular solution model. The interaction parameter used in our model is obtained analytically using the valence force field (VFF) model modified for wurtzite structures. The calculated interaction parameters, which are required to predict the unstable two-phase region, agree well with experimental results for related alloy systems. The modified VFF model can also be used to predict the microscopic crystal structure, such as first neighbor anion–cation bond lengths. In order to verify our calculations, we compare the calculated and experimental results for the first neighbor anion–cation bond lengths in the InGaN system. The calculated results agree well with the experimental results. From our calculation results, the unstable two-phase regions for four A1−x−yBxCyD type group-III-nitride quaternary alloys and nine A1−xBxC1−yDy type group-III-nitride mixed anion quaternary alloys are calculated. The predicted unstable two-phase regions agree well with experimentally observed regions of phase separation in ternary alloys, which suggests our model calculations can provide useful guidance in ternary and quaternary systems where there is no experimental data.
In-plane tensile-strained interfacial structure in a GaN nucleation layer on sapphire(0001)
Authors: Kim C. C. , Je J. H. , Yi M. S. , Noh D. Y. , Ruterana P.
Published: July 2002

Interfacial microstructure in GaN nucleation layers was investigated using synchrotron x-ray scattering and transmission electron microscopy. We find that tensile-strained, aligned, interfacial domains coexist with misaligned domains in an as-grown nucleation layer of mostly cubic stacking. The tensile strain originates in a 6/7 matched interfacial structure, wherein 6-Ga atomic distances in GaN match to 7-Al atomic distances in sapphire. The tensile state of the aligned, interfacial domains is preserved during annealing to 1100 °C, while the stacking sequence changes from cubic to hexagonal order. The correlation length of the stacking order is rather short, ∼9 Å in the hexagonal phase, compared to that of the cubic phase in the as-grown nucleation layer, ∼25 Å, due to stacking faults generated during the kinetically limited transformation.
Particle distribution study for low-volume fraction magnetostrictive composites
Authors: Duenas T. A. , Carman G. P.
Published: July 2002

We present experimental results for [1–3] magnetostrictive composites containing a 20% volume-fraction of Terfenol-D coarse particles (≫1 μm) suspended in a nonmetallic binder. Results from magnetostrictive strain and magnetoelastic measurements are provided for particle distributions that differ in particle size and range of size distribution. Particle size and packing density are shown to effect strain response and are attributed to demagnetization and loading (prestress) effects. Both a decreasing and increasing ΔE effect are measured for low and high fields, respectively, and found to be independent of particle distribution. The response of an optimized bimodal distribution increases packing density and produces 15% larger magnetostrictive strains than the other widely dispersed (polydispersed) (<106, <212, and <300 μm) and narrowly dispersed (monodispersed) (<45, 98, and 275 μm) distributions at comparable magnetic field levels.
Continuous-wave two-photon excitation of individual CdS nanocrystallites
Authors: van Oijen A. M. , Verberk R. , Durand Y. , Schmidt J. , van Lingen J. N. J. , Bol A. A. , Meijerink A.
Published: July 2002

By use of low-temperature confocal microscopy, continuous-wave two-photon fluorescence images are obtained of individual CdS nanocrystallites embedded in a polymer matrix. The quadratic dependence of the emission rate on the applied laser power proves that the observed fluorescence originates from the simultaneous absorption of two photons. From the experimental data the two-photon absorption cross-section σ(2) could be determined, resulting in a value smaller than that known from literature. The work presented is a first step towards high-resolution fluorescence-excitation spectroscopy on the electronic states in the band edge, inaccessible by conventional one-photon spectroscopy.
Epitaxial growth of ferromagnetic Ni2MnIn on (001) InAs
Authors: Xie J. Q. , Dong J. W. , Lu J. , Palmstrøm C. J. , McKernan S.
Published: July 2002

The ferromagnetic Heusler alloy Ni2MnIn has been grown on InAs (001) by molecular-beam epitaxy. In situ reflection high-energy electron diffraction, ex situ x-ray diffraction, Rutherford backscattering spectrometry, and transmission electron microscopy indicate the high-quality epitaxial growth of Ni2MnIn with the B2 crystal structure on InAs (001). Superconducting quantum interference device magnetometry shows that the Ni2MnIn film is ferromagnetic with a Curie temperature ∼170 K.
Organic μ cavities based on thermally evaporated TeOx-LiF distributed Bragg reflectors
Authors: Anni M. , Gigli G. , Cingolani R. , Patanè S. , Arena A. , Allegrini M.
Published: July 2002

We report on the realization of high-quality organic microcavities consisting of distributed Bragg reflectors based on lithium fluoride (LiF) and tellurium dioxide (TeOx) deposited by thermal evaporation. The materials are transparent in the range of 350 nm–5 μm and have an evaporation temperature of about 1000 K. The large difference in the refractive index (about 0.9 in the visible and near-infrared range) allows one to obtain reflectivity higher than 99% over a spectral region about 200 nm wide with a small number of periods. The mirror deposition technique is suitable for the fabrication of organic quantum microcavities in a single deposition process. Three fully evaporated organic λ cavities with Phyrrometene 580 as the active material are described. The cavities show a Q value of up to 300, good uniformity, and reproducibility.
The falling-needle viscometer
Authors: Davis Anthony M. J. , Brenner Howard
Published: July 2002

The determination of fluid viscosities from measured falling-needle velocities depends upon accurate prediction of the drag forces acting on a thin cylinder translating axisymmetrically and axially down a tube. Without a complete hydrodynamic theory of the device, the manufacturer’s calibrations must rely on certain empirical estimates to take account of the shape and finite length of the needle. Here, the advantages of slender-body theory are exploited to calculate drag coefficients in the presence of the tube wall, for shapes varying from spheroidal toward cylindrical. Our results suggest an alternative means of calibration.
Opacity of KCl single crystal shocked above the transition pressure
Authors: Zaretsky Eugene
Published: July 2002

Strong absorption of light by shock-transformed KCl (potassium chloride) single crystal has been used for measurement of electrical conductivity of its high-pressure B2 phase. The obtained conductivity values allow the determination of the width W of the gap between the valence and the conduction bands and its dependence W=W0(1−bε) on the compressive strain ε. The values of the gap width W0=0.49±0.06 eV and of the parameter b=1.4±0.4 lead to the conclusion that KCl in its high-pressure modification is a semiconductor.
Contribution of injection in current noise due to generation and recombination of carriers in p–n junctions
Authors: Tejada J. A. Jiménez , Godoy A. , Palma A. , Cartujo P.
Published: July 2002

A theory for the current noise associated with carrier generation and recombination in the space-charge region of p–n junctions is presented. We propose a noise model based on the response of the electric field to fluctuations of the trapped charge in this region, and make use of a collective transport-noise theory. The effects of the fluctuations of the space-charge region borders due to the fluctuations of the trapped charge are now taken into account. This new contribution is negligible when generation–recombination current governs the diode current. However, it is significant when diffusion current dominates, allowing the analytical study of generation–recombination noise to be extended to wider ranges of bias and temperature. Experimental results at low and high temperatures are explained with our theory. Empirical formulas of current noise density are also explained according to this complete theory of current noise calculation.
Isotropic-nematic phase separation in asymmetrical rod-plate mixtures
Authors: Wensink H. H. , Vroege G. J. , Lekkerkerker H. N. W.
Published: July 2002

Recent experiments on mixtures of rodlike and platelike colloidal particles have uncovered the phase behavior of strongly asymmetrical rod-plate mixtures. In these mixtures, in which the excluded volume of the platelets is much larger than that of the rods, an extended isotropic (I)–plate-rich nematic (N−)–rod-rich nematic (N+) triphasic equilibrium was found. In this paper, we present a theoretical underpinning for the observed phase behavior starting from the Onsager theory in which higher virial terms are incorporated by rescaling the second virial term using an extension of the Carnahan–Starling excess free energy for hard spheres (Parsons’ method). We find good qualitative agreement between our results and the low concentration part of the experimental phase diagram.
Yellow-to-violet upconversion in neodymium oxide nanocrystal/titania/ormosil composite sol–gel thin films derived at low temperature
Authors: Que Wenxiu , Kam C. H. , Zhou Y. , Lam Y. L. , Chan Y. C.
Published: July 2002

Neodymium (III) oxide nanocrystals synthesized by the inverse microemulsion technique were dispersed in sol–gel titania/organically modified silane (ormosil) composite thin films for photonic applications. X-ray diffraction analysis and transmission electron microscopy observation show that the neodymium oxide nanoparticles have a nanocrystalline structure and that the size of the nanoparticles is in the range of 5–60 nm. An intense room-temperature yellow-to-violet upconversion emission at 402 nm (4D3/2→4I13/2) was measured from the composite thin film heated at 100 °C upon excitation with a yellow light at 587 nm. In addition to this violet emission, ultraviolet emission at 372 nm and blue emission at 468 nm were also observed. The lifetime of the violet emission was measured and the mechanism of the upconversion emission is proposed, and they indicate excited-state absorption originating from the long-lived F3/24 excited state should be responsible for the yellow-to-violet upconversion.
Resistance decrease in spin tunnel junctions by control of natural oxidation conditions
Authors: Zhang Z. G. , Freitas P. P. , Ramos A. R. , Barradas N. P. , Soares J. C.
Published: July 2002

Spin-dependent tunnel junctions with AlOx barriers were fabricated by in situ natural oxidation of a 7 Å thick Al film. Oxygen pressure was varied from 0.5 to 100 Torr, and oxidation time ranged from 5 min to 2 h. Junction resistances as low as 10 to 12 Ω μm2 were obtained with corresponding tunnelling magnetoresistance values (TMR) ranging from 14% to 17%, for the junctions oxidized at the lower pressure (0.5 Torr). Rutherford backscattering analysis (RBS) indicates an O/Al ratio of 1.29±0.34 denoting incomplete oxidation of the Al. Junctions oxidized at higher pressures (⩾10 Torr) can reach 25% to 30% TMR, with resistances ranging from 30 to 70 Ω μm2. RBS shows near-stoichiometric Al2O3 oxide composition (O/Al=1.51±0.43) in these barriers.
Amorphous/microcrystalline silicon superlattices—the chance to control isotropy and other transport properties
Authors: Kočka J. , Stuchlı́k J. , Stuchlı́ková Ha , Švrček V. , Fojtı́k P. , Mates T. , Luterová K. , Fejfar A.
Published: July 2002

Preparation of amorphous silicon/microcrystalline silicon superlattices allowed us a systematic study of transition from isotropic amorphous silicon to microcrystalline silicon with anisotropic (columnar) microstructure. The fact that just a few nm of amorphous interlayers are sufficient to interrupt columnar growth of crystallites is reflected in a clearly demonstrated isotropy of transport properties of all superlattice samples. Values of dark conductivity and diffusion length as well as grain size vary with changing crystallinity and so we can tailor the properties of the resulting material by adjusting thicknesses of amorphous and microcrystalline layers repeated to achieve a total desired thickness. Properly selected design of superlattice can lead to transport properties more suitable for solar cells than with pure microcrystalline silicon.
Magnetotransport properties of a single grain boundary in a bulk La–Ca–Mn–O material
Authors: Vertruyen B. , Cloots R. , Rulmont A. , Dhalenne G. , Ausloos M. , Vanderbemden Ph.
Published: July 2002

Besides the “intrinsic” colossal magnetoresistance effect observed in single crystals, the polycrystalline manganate compounds also exhibit an “extrinsic” magnetoresistance related to the presence of grain boundaries. We report electrical transport and magnetic measurements carried out on a bigrain sample extracted from a floating zone method-grown rod of calcium doped lanthanum manganate. Electrical resistance was measured both within a grain and across the grain boundary, between 20 and 300 K and from 0 to 8 T. Magnetoresistance values up to 99% are reached within the grain. The temperature dependence of the resistance across the grain boundary displays a “foot-like” feature towards the bottom of the transition. Low field and high field magnetoresistance effects are examined. We compare our results for a “bulk” grain boundary to those obtained by other authors for bicrystal thin films and bulk polycrystalline materials.
Reference system master equation approaches to condensed phase charge transfer processes. I. General formulation
Authors: Golosov Andrei A. , Reichman David R.
Published: July 2002

Two complimentary master equation approaches for the study of condensed phase charge transfer processes are developed. The first approach is appropriate when the (diabatic) activation energy is larger than the electronic coupling strength, and thus localized tight binding states form a zeroth-order set of electronic states. The second approach, the “pure dephasing reference system” (PDRS) master equation method, is designed to study the case where charge is delocalized over the donor and acceptor sites, and a zeroth-order set of “quasiadiabatic” states are used as a reference system. It is argued that a large majority of the electron transfer parameter space may be accurately covered with these two methods. Particular emphasis is placed on the treatment of nonequilibrium initial preparation, short-time nonexponential behavior, and backflow. The methods outlined in this work are general enough to treat anharmonic environments. A semiclassical implementation of the master equation approaches appropriate for anharmonic environments is outlined. The relationship between the methods developed herein and previous work, including the “nonequilibrium golden rule”-type methods of Coalson and co-workers [J. Chem. Phys. 101, 436 (1994); 102, 5658 (1995)] is analyzed. The limitations of the methods are discussed.
Intersubband absorption dynamics in coupled quantum wells
Authors: Müller T. , Bratschitsch R. , Strasser G. , Unterrainer K.
Published: July 2002

We apply an interband pump/intersubband probe technique to monitor the temporal evolution of the electron population in the first and second subband of an undoped GaAs/AlGaAs asymmetric double quantum well after interband optical excitation. The spacing between the two subbands is smaller than the longitudinal optical phonon energy. The time dependence of the intersubband absorption can be explained by a simple rate equation model. We extract an intersubband lifetime of T21=100 ps and a recombination time of τ=410 ps at an excitation density of nS=2×1011 cm−2.
Band structure treatment of the influence of nonstoichiometric defects on optical properties in LiNbO3
Authors: Kityk I. V. , Makowska-Janusik M. , Fontana M. D. , Aillerie M. , Abdi F.
Published: July 2002

We present a band structure approach in connection with a molecular dynamics cluster optimization that accounts for various structural modifications related to the nonstoichiometry of LiNbO3 crystals. Variation of optical properties with the deviation from the stoichiometric composition can be understood within this approach. The particular role of the electron-phonon contributions to the electro-optic coefficient is shown. Model calculations yield a large dependence of the electro-optic coefficient r22 on the crystal composition, in agreement with the experimental data. The observed minimum of the r22 coefficient versus nonstochiometry is explained within a microscopic approach. The r33 tensor component shows substantially different behavior versus the stoichiometry.
Algebro-geometric solution of the 2+1 dimensional Burgers equation with a discrete variable
Authors: Cao Cewen , Geng Xianguo , Wang Hongye
Published: July 2002

The quasiperiodic solution of the 2+1 dimensional Burgers equation with a discrete variable is obtained through three steps: (a) decomposition into a symplectic map plus two finite-dimensional Hamiltonian systems; (b) straightening out of both the discrete and the continuous flows in the Jacobian variety; (c) inversion into the original variables. Inner relation with the modified Kadomtsev–Petviashvili equation is presented. The explicit theta function solutions for these two 2+1 integrable models are given.
All-solid-state triggerless repetitive pulsed power generator utilizing a semiconductor opening switch
Authors: Teramoto Yusuke , Deguchi Daisuke , Lisitsyn Igor V. , Namihira Takao , Katsuki Sunao , Akiyama Hidenori
Published: July 2002

The repetitive pulsed power generator was constructed with the semiconductor opening switch realizing the pulse compression by an inductive energy storage system. For the preliminary pulse compression, the magnetic pulse compression system was employed. The saturable inductors and transformers were used to transfer the electrical energy from the primary storage capacitor to the load compressing the pulse. For the final output of the generator, the semiconductor opening switch (SOS) was used. The SOS assembly used in the system consists of 80 diodes that were designed to conduct high reverse current and have short reverse recovery time. By tuning the current fed to the SOS, the amplitude and pulse width of the generated voltage at the 300 Ω resistive load were 150 kV and 60 ns, respectively. Although the present generator system operates repetitively, the system has no triggered gas-discharge or semiconductor closing switch. Also, all the devices used in the system are in solid sate. The system provides stable operation and long lifetime.
A spectral quadruple for de Sitter space
Authors: Kopf Tomáš , Paschke Mario
Published: July 2002

A set of data supposed to give possible axioms for spacetimes with a sufficient number of isometries in spectral geometry is given. These data are shown to be sufficient to obtain 1+1 dimensional de Sitter spacetime. The data rely at the moment somewhat on the guidance given by a required symmetry, in part to allow explicit calculations in a specific model. The framework applies also to the noncommutative case. Finite spectral triples are discussed as an example.
Shock–planar curtain interactions in two dimensions: Emergence of vortex double layers, vortex projectiles, and decaying stratified turbulence
Authors: Zabusky Norman J. , Zhang Shuang
Published: July 2002

Vortex double layers and vortex projectiles (VPs) are the essential coherent structures which emerge in the shock excited planar-parallel “curtain” (s/f/s) simulations of a two-dimensional shock tube with the piecewise parabolic method. These opposite signed layers, formed by shock induced baroclinic deposition of vorticity, “bind” and are strongly affected by secondary reflected shocks and vortex interactions. This versatile configuration, easily obtained in the laboratory, allows us to study the formation, evolution, and reacceleration of VPs and stratified turbulence and mixing.
Radio-frequency single-electron transistor: Toward the shot-noise limit
Authors: Aassime A. , Gunnarsson D. , Bladh K. , Delsing P. , Schoelkopf R.
Published: July 2002

We have fabricated an aluminum single-electron transistor and characterized it at frequencies up to 10 MHz by measuring the reflected signal from a resonant tank in which the transistor is embedded. We measured the charge sensitivity of this radio-frequency single-electron transistor to be 3.2×10−6 e/Hz, which corresponds to the uncoupled energy sensitivity of 4.8 ℏ. Our measurements indicate that with further improvements, the radio-frequency single-electron transistor could reach the shot-noise limit estimated to be about 1 ℏ.
Ultrathin epitaxial ferroelectric films grown on compressive substrates: Competition between the surface and strain effects
Authors: Zembilgotov A. G. , Pertsev N. A. , Kohlstedt H. , Waser R.
Published: July 2002

The mean-field Landau-type theory is used to analyze the polarization properties of epitaxial ferroelectric thin films grown on dissimilar cubic substrates, which induce biaxial compressive stress in the film plane. The intrinsic effect of the film surfaces on the spontaneous polarization is taken into account via the concept of the extrapolation length. The theory simultaneously allows for the influence of the misfit strain imposed on the film lattice by a thick substrate. Numerical calculations are performed for PbTiO3 and BaTiO3 films under an assumption of the polarization reduction in surface layers. The film mean polarization is calculated as a function of film thickness, temperature, and misfit strain. It is shown that the negative intrinsic size effect is reduced in epitaxial films due to the in-plane compression of the film lattice. At room temperature, strong reduction of the mean polarization may take place only in ultrathin films (thickness ∼1 nm). Theoretical predictions are compared with the available experimental data on polarization properties of BaTiO3 films grown on SrRuO3 coated SrTiO3.
High-resolution Fourier transform infrared and cw-diode laser cavity ringdown spectroscopy of the ν2+2ν3 band of methane near 7510 cm−1 in slit jet expansions and at room temperature
Authors: Hippler Michael , Quack Martin
Published: July 2002

The ν2+2ν3 combination band of CH412 near 7510 cm−1 was studied with the recently introduced technique of cavity ring-down spectroscopy employing a cw-diode laser in a pulsed supersonic slit jet expansion and with Doppler-limited Fourier-transform infrared spectroscopy at room temperature. ν2+2ν3 is the strongest absorption band in the high-wave-number region of the N=2.5 icosad of methane. First assignments of the combination band are provided. The vibrational origin of ν2+2ν3 at 7510.3378±0.0010 cm−1, the integrated band strength G=(1.3±0.2)×10−4 pm2 and the vibrational transition moment |μν|=(1.0±0.1)×10−3 D have been determined. The values represent benchmarks to test effective vibrational Hamiltonians and ab initio calculations for methane. Although an isolated band analysis was possible at low J-values, the influence of strong perturbations becomes evident at higher rotational excitation. The F1-component of ν2+2ν3 interacting by a strong Coriolis resonance with the IR-active F2-component appears to be a dominant perturber.
Thermodynamics and phase separation of a de-ionized colloidal system in the symmetric Poisson–Boltzmann and mean spherical approximation theories
Authors: Bhuiyan L. B. , Outhwaite C. W.
Published: July 2002

A model system of charged hard spheres and point counterions is used to analyze a de-ionized colloidal dispersion. Thermodynamic properties are calculated using a symmetric Poisson–Boltzmann equation, a linear form, and the Mean Spherical Approximation theory. The theories are found to predict a gas–liquid type transition and the spinodals computed. The results are compared among themselves and with recent Monte Carlo simulations for the model.
Qualitative dynamics of generalized Langevin equations and the theory of chemical reaction rates
Authors: Martens Craig C.
Published: July 2002

In this paper, we present an analysis of condensed phase chemical reactions from the perspective of qualitative dynamical systems theory. Our approach is based on a phenomenological phase space representation of the generalized Langevin equation (GLE). In general, the GLE with memory requires an infinite-dimensional phase space for its description. The phenomenological phase space is constructed by augmenting the physical phase plane (q,p) with additional variables defined as the convolution of the system momentum with the memory kernel and its time derivatives. The qualitative dynamics in this representation are then characterized in terms of the eigenvalues and eigenvectors of the linear system near the barrier top. The phase space decomposes into a single unstable direction and a complementary stable subspace. The rate of exponential growth along the unstable eigenvector is directly related to the rate of chemical reaction, and our linear analysis reproduces the Grote–Hynes expression for the reaction rate [R. F. Grote and J. T. Hynes, J. Chem. Phys. 73, 2715 (1980)]. In the presence of noise, the stable subspace can be identified with the stochastic separatrix, a manifold of initial conditions with a reaction probability of 0.5. Other dynamical processes, such as solvent caging, can also be given a simple geometric interpretation in terms of the qualitative dynamical analysis.
The reaction of C2H with H2: Absolute rate coefficient measurements and ab initio study
Authors: Peeters Jozef , Ceursters Benny , Nguyen Hue Minh Thi , Nguyen Minh Tho
Published: July 2002

In this work, a pulsed laser photolysis/chemiluminescence (PLP/CL) technique was used to measure absolute rate coefficients for the reaction of C2H+H2→products over the temperature range 295–666 K. Ethynyl radicals were produced pulsewise by excimer laser photolysis of acetylene at 193 nm and real-time pseudo-first-order decays of C2H were monitored by the CH(A 2Δ→X 2Π) chemiluminescence resulting from their reaction with O2. Over the experimental temperature range, the results indicate that the rate coefficient exhibits a non-Arrhenius behavior in line with theoretical predictions, khydrogen(T)=3.92×10−19 T2.57±0.30 exp[−(130±140) K/T] cm3 molecule−1 s−1. Experiments were supplemented by ab initio molecular orbital calculations up to the coupled-cluster theory including all single and double excitations plus perturbative corrections for the triples, UCCSD(T), with the 6-311++G(d,p) basis set for geometry optimizations and the aug-cc-pVTZ for electronic energy single points, revealing that the direct hydrogen abstraction yielding HC≡CH+H is the only product channel of any importance. There is also no important crossing between the doublet and quartet energy surfaces. Finally, geometry optimizations at the UCCSD(T)/6-311++G(2df,2p) level have shown that the transition structure for H-abstraction is linear; harmonic vibration frequencies at this level, and single-point UCCSD(T)/aug-cc-pVTZ energies for these geometries result in an adiabatic barrier height for H-abstraction, including harmonic vibration zero point energies, of 12.8 kJ/mol, while the classical potential energy barrier is 9.2 kJ/mol.
Effects of void size and gas content on electrical breakdown in lightweight, mechanically compliant, void-filled dielectrics
Authors: Anderson R. A. , Lagasse R. R. , Russick E. M. , Schroeder J. L.
Published: July 2002

Dielectric potting materials (encapsulants) are used to prevent air breakdown in high-voltage electrical devices. We report breakdown strengths in void-filled encapsulants, stressed with unipolar voltage pulses of the order of 10 μs duration. High strengths, on the order of 100 kV mm−1, are measured under these test conditions. The materials studied include low-density open celled gel-derived foams with cell sizes of 4 μm or less, closed celled CO2-blown polystyrene and urethane foams, and epoxies containing 48 vol % of hollow glass microballoon (GMB) fillers. These last specimens varied the void gas (N2 or SO2) and also the void diameters (tens to hundreds of μm). Our measurements are thought to be directly sensitive to the rate of field-induced ionization events in the void gas; however, the breakdown strengths of the materials tested appeared to vary in direct proportion with the conventional Paschen-law gas-discharge inception threshold, the electric stress at which gas-ionization avalanches become possible. The GMB-epoxy specimens displayed this type of dependence of breakdown strength on the void-gas density and void size, but the measurements were an order of magnitude above the conventional predictions. Small-celled foams also showed increased breakdown strengths with decreased cell size, although their irregular void geometry prevented a direct comparison with the more uniformly structured microballoon-filled encapsulants. The experimental observations are consistent with a breakdown mechanism in which the discharge of a few voids can launch a full breakdown in the composite material.
Passive scalar mixing in a planar shear layer with laminar and turbulent inlet conditions
Authors: Pickett L. M. , Ghandhi J. B.
Published: July 2002

The effect of inlet conditions on the mixing of a passive scalar was investigated in a planar shear layer with inlet boundary layers that were laminar, tripped and naturally turbulent–transitional. Planar laser-induced fluorescence measurements of acetone were used to directly evaluate the shear layer structure, and were processed to determine probability density functions (PDFs) of the mixture fraction. The results agree well with previous studies in aqueous and gaseous systems for laminar inlet conditions. Large-scale structures of a nearly homogeneous composition were found, and the structures spanned the mixing layer width giving rise to a nonmarching style PDF. A high-speed boundary layer that differed from the laminar state (produced by tripping a laminar boundary layer, from the naturally turbulent–transitional state at high flow rates, or by tripping the turbulent–transitional condition) gave rise to a hybrid-style PDF that was markedly different from either the laminar nonmarching case, or the marching shape that has been found by other investigators. The hybrid PDF shape had a marching character on the high-speed side of the mixing layer, and a distributed nature not favoring any specific composition on the low-speed side of the mixing layer. Tripping the low-speed boundary layer produced no change in the hybrid PDF shape, confirming that the difference observed between the high- and low-speed sides was the result of the shear layer development with turbulent inlet conditions, not an inlet condition effect. In addition, with turbulent and transitional inlet conditions all turbulent passive scalar profiles were found to be self-similar and the velocity power spectra displayed a −5/3 slope indicating well-developed turbulent conditions prevailed at a relatively low (104) Reynolds number. Secondary structures were observed in images with turbulent–transitional inlet conditions, but not with tripped inlet conditions.
Epitaxial growth and valence control of strained perovskite SrFeO3 films
Authors: Yamada Hiroyuki , Kawasaki M. , Tokura Y.
Published: July 2002

We have fabricated single-crystalline thin films of perovskite-type iron oxide SrFeO3 which include high valent iron (Fe4+). The fabrication is accomplished on the lattice-mismatched substrates of SrTiO3 [(STO), 1.4% tensile strain] and (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 [(LSAT), 0.5% tensile strain] by pulsed-laser deposition of SrFeO2.5 with Fe3+ state and subsequent annealing in ozone. The SrFeO2.5 films are grown in a layer-by-layer mode, exhibiting atomically smooth surfaces. The ozone annealed films on LSAT show metallic conductivity as observed in bulk SrFeO3, whereas those on STO remain semiconducting due to large tensile strain. A pseudomorphically strained epitaxial structure can be maintained by the proper choice of substrates and growth conditions through the huge volume change accompanied by the valence change from Fe3+ to Fe4+.
Space-charge-limited current in cylindrical diodes with finite-length emitter
Authors: Kostov K. G. , Barroso J. J.
Published: July 2002

In the present paper we extend to two dimensions the classical Langmuir–Blodgett law, which determines the maximum current ILB that can be accelerated by a given voltage across two infinitely long coaxial cylinders. Generalization of the ILB law is established by performing two-dimensional 2D particle-in-cell numerical experiments on a variety of cylindrical diode configurations with a finite-length emitter. It is found that the limiting current in two dimensions I2D follows a monotonically decreasing function of the emitter length-to-outer electrode radius ratio L/R as expressed by the fitting function I2D/ILB=1+0.1536/(L/R)+0.0183/(L/R)2 within 2.5% accuracy for simulation data in which the ratio of the outer to inner radii exceeds 3.
Influence of segregation on the measurement of stress in thin films
Authors: Thomas O. , Müller P. , Gergaud P. , Labat S.
Published: July 2002

The force that a deposited film A exerts on its substrate B is usually written as the sum of volumetric and surface (and interface) contributions. In the simplest case (pseudomorphous film, absence of segregation), volumetric and surface contributions can be easily separated through simple force measurements. In this article, we show that, in the presence of segregation, only simultaneous in situ stress, strain, and composition measurements allow the proper analysis of stress establishment in thin films. For this purpose, we discuss how segregation influences both volumetric and surface stresses. More precisely, we show that (1) the separation between a surface and a volumetric stress can be meaningless when segregation occurs; (2) one should carefully distinguish between the true physical thickness and the deposited thickness; (3) surface strain should be accounted for in the analysis of the experimental diffraction data; and (4) when the elastic modulus misfit between film and substrate cannot be neglected the total stress depends explicitly on the shape of the concentration profile.
Coercive field of 0.955Pb(Zn1/3Nb2/3)O3–0.045PbTiO3 single crystal and its frequency dependence
Authors: Yin Jianhua , Cao Wenwu
Published: July 2002

Domain switching behavior of 0.955Pb(Zn1/3Nb2/3)O3–0.045PbTiO3 ferroelectric single crystal has been investigated using the ultrasonic technique in addition to the polarization hysteresis measurements. The ultrasonic technique makes it possible to extend the coercive field measurement to frequencies as low as 10−5 Hz, revealing the existence of a saturated coercive field Esc, which representing the intrinsic switching barrier.
Nanocrystalline diamond formation during argon ion irradiation of graphite
Authors: Wang Zhenxia , Yu Guoqing , Yu Liping , Zhu Fuying , Zhu Dezhang , Xu Hongjie , Ruan Meilling
Published: July 2002

In the present work, diamond nanoparticles with sizes of ∼2–70 nm have been produced by Ar+ ion irradiation at the fluence of ∼1022 cm−2. With the help of transmission electron microscope, electron diffraction, and Raman spectroscopy, these nanoparticles’ shape and size were determined and their crystal structure was also solved. From these results, our discussion, based on the viewpoint of beam-solid interaction, may be critical for understanding the growth process of nanocrystalline diamonds.
Monte Carlo simulation of electron dynamics in superlattice quantum cascade lasers
Authors: Compagnone F. , Di Carlo A. , Lugli P.
Published: July 2002

A theoretical investigation of electron dynamics in superlattice InGaAs/AlInAs quantum cascade lasers (QCLs) is presented, based on a Monte Carlo simulation that includes both electron–electron and electron–phonon scattering. Nonequilibrium phonons and phonon quantization effects have been explicitly considered. Calculated luminescence and gain spectra are presented. Our analysis provides a clear physical insight into the QCL operational mode.
Sulfur doping of diamond films: Spectroscopic, electronic, and gas-phase studies
Authors: Petherbridge James R. , May Paul W. , Fuge Gareth M. , Robertson Giles F. , Rosser Keith N. , Ashfold Michael N. R.
Published: July 2002

Chemical vapor deposition (CVD) has been used to grow sulfur doped diamond films on undoped Si and single crystal HPHT diamond as substrates, using a 1% CH4/H2 gas mixture with various levels of H2S addition (100–5000 ppm), using both microwave (MW) plasma enhanced CVD and hot filament (HF) CVD. The two deposition techniques yield very different results. HFCVD produces diamond films containing only trace amounts of S (as analyzed by x-ray photoelectron spectroscopy), the film crystallinity is virtually unaffected by gas phase H2S concentration, and the films remain highly resistive. In contrast, MWCVD produces diamond films with S incorporated at levels of up to 0.2%, and the amount of S incorporation is directly proportional to the H2S concentration in the gas phase. Secondary electron microscopy observations show that the crystal quality of these films reduces with increasing S incorporation. Four point probe measurements gave the room temperature resistivities of these S-doped and MW grown films as ∼200 Ω cm, which makes them ∼3 times more conductive than undoped diamond grown under similar conditions. Molecular beam mass spectrometry has been used to measure simultaneously the concentrations of the dominant gas phase species present during growth, for H2S doping levels (1000–10 000 ppm in the gas phase) in 1% CH4/H2 mixtures, and for 1% CS2/H2 gas mixtures, for both MW and HF activation. CS2 and CS have both been detected in significant concentrations in all of the MW plasmas that yield S-doped diamond films, whereas CS was not detected in the gas phase during HF growth. This suggests that CS may be an important intermediary facilitating S incorporation into diamond. Furthermore, deposition of yellow S was observed on the cold chamber walls when using H2S concentrations >5000 ppm in the MW system, but very little S deposition was observed for the HF system under similar conditions. All of these results are rationalized by a model of the important gas phase chemical reactions, which recognizes the very different gas temperature profiles within the two different types of deposition reactor.
Irreducible Brillouin conditions and contracted Schrödinger equations for n-electron systems. II. Spin-free formulation
Authors: Kutzelnigg Werner , Mukherjee Debashis
Published: July 2002

Recently [W. Kutzelnigg and D. Mukherjee, Chem. Phys. Lett. 317, 567 (2000); D. Mukherjee and W. Kutzelnigg, J. Chem. Phys 114, 2047 (2001)] the irreducible k-particle Brillouin conditions IBCk and the irreducible k-particle contracted Schrödinger equations ICSEk were derived. These permit the definition of a hierarchy of k-particle approximations for the direct calculation of the cumulants λk of the k-particle density matrices. Now, the spin-free form of these conditions, appropriate for a spin-free Hamiltonian, is given. This is particularly useful for open-shell states. The definition of the cumulants of the reduced densities has to be generalized for these anyway, making use of irreducible tensor operators with respect to SU2. There are two alternative definitions of spin-free cumulants, of which the one in terms of spin-free reduced density matrices appears to be preferable. Alternatively to the straight spin-free formulation, we also present a theory in terms of spin-free operators adapted to the symmetric group. Partial trace relations that relate the elements of the cumulants of different particle rank are derived. There are partial trace relations for “exchange elements,” which are determined by the total spin quantum number S. From these relations the individual exchange elements of Λ2 can be obtained in special cases. This allows a simple formulation of the stationarity conditions for open-shell states of any spin multiplicity.
Energy conservation in a nonlinear gyrokinetic particle-in-cell code for ion-temperature-gradient-driven modes in θ-pinch geometry
Authors: Hatzky Roman , Tran Trach Minh , Könies Axel , Kleiber Ralf , Allfrey Simon J.
Published: July 2002

A global nonlinear simulation code for the time evolution of ion-temperature-gradient-driven modes in θ-pinch geometry as a first approximation to the stellarator Wendelstein 7-X (W7-X) [Grieger et al., Proceedings of the 13th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Washington, DC, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 3, p. 525] has been developed. A δf particle-in-cell (PIC) method is used to solve the coupled system of gyrokinetic equations for the ions, in the electrostatic approximation, and the quasineutrality equation, assuming adiabatically responding electrons. The focus has been on adherence to conservation laws, i.e., particle number and energy conservation. Besides other improvements it has been shown that a well-chosen initial distribution of the markers in reduced phase space makes optimal use of the δf PIC method to reduce the statistical noise for a given number of markers. In a model including all (1351) physically relevant modes, it has been possible to achieve energy conservation beyond the saturation of the instability.
Effective binding force calculation in a dimeric protein by molecular dynamics simulation
Authors: Sergi Alessandro , Ciccotti Giovanni , Falconi Mattia , Desideri Alessandro , Ferrario Mauro
Published: July 2002

A good example of macromolecular recognition is found in the interaction of the two monomers of the dimeric superoxide dismutase protein found in Photobacterium leiognathi. We have produced, by molecular dynamics simulation techniques, a specific path for the rupture of the dimer and calculated the effective force involved in the process by extending a well established free energy calculation scheme, the molecular dynamics blue moon approach to rare events. Within this picture we have generalized the approach to a vectorial reaction coordinate and performed a number of different simulations in function of the monomer-momomer separation, at fixed relative orientation. We find a deep minimum and we compute the height of the free energy barrier to break the dimer. As for the system characterization we have found that, when the separation distance increases, the protein structure is stable and the monomer-monomer interface is uniformly hydrated. Moreover, identifying the crucial contacts for the stabilization of the dimer, we have found the sequence of the different microscopic events in the monomer-monomer recognition and we have developed a view of the process which requires a merging of standard explanations, in agreement with the recent picture of recognition as a dynamical process mixing the various mechanisms previously considered [Kimura et al., Biophys. J. 80 635 (2001)].
Improvement of on/off ratio in single-shot multichannel demultiplexing by using an optical Kerr gate of a squarylium dye J aggregate film
Authors: Sato Yasuhiro , Furuki Makoto , Tian Minquan , Iwasa Izumi , Pu Lyong Sun , Tatsuura Satoshi
Published: July 2002

We demonstrated ultrafast single-shot multichannel demultiplexing by using a squarylium dye J aggregate film as an optical Kerr medium. High efficiency and fast recovery of the optical Kerr responses were achieved when a signal-pulse wavelength was close to the absorption peak of the J aggregate film with off-resonant excitation. The on/off ratio in demultiplexing of 1 Tb/s signals was improved to be approximately 5. By introducing time delay to both horizontal and vertical directions, we succeeded in directly observing the conversion of 1 Tb/s serial signals into two-dimensionally arranged parallel signals.
Effect of combined triangularity and ellipticity on the stability limit of the ideal internal kink mode in a tokamak
Authors: Eriksson H. G. , Wahlberg C.
Published: July 2002

The effect of combined triangular and elliptical shaping of the plasma cross section on the stability limit of the ideal, internal kink mode in a tokamak is analyzed by means of a computer algebra expansion of the magnetohydrodynamic equations. The work extends the result of a previous investigation of the effect of ellipticity alone on this mode in toroidal plasmas [Wahlberg, Phys. Plasmas 5, 1387 (1998)]. It is shown that, in contrast to the strongly destabilizing effect of (positive) ellipticity alone, the effect of combined positive ellipticity and positive triangularity is stabilizing. The effect is of the same importance as the effect of ellipticity alone if the triangularity of the q=1 surface is of the same order of magnitude as the inverse aspect ratio. When the safety factor at the magnetic axis is close to (but below) unity, the geometrical factor governing the stability of the ideal, internal kink mode is found to be the same as the corresponding factor in the Mercier criterion for shaped tokamaks.
Structural study of refractory-metal-free C40 TiSi2 and its transformation to C54 TiSi2
Authors: Yu T. , Tan S. C. , Shen Z. X. , Chen L. W. , Lin J. Y. , See A. K.
Published: July 2002

The structure of laser-induced refractory-metal-free C40 TiSi2 has been studied by glancing-angle x-ray diffraction (GAXRD) in detail. The result shows that laser-induced C40 TiSi2 has a hexagonal structure with the P6222 space group and lattice parameters a=0.467 nm and c=0.662 nm. The ordering effect and the stress effect on the TiSi2 film are also discussed based on the GAXRD and micro-Raman results. The C40 phase completely transforms to the technologically important C54 phase at a relatively low temperature of 700 °C.