bluuebunny/crossref_metadata_2025 huggingface dataset:
Displacement of surface arsenic atoms by insertion of oxygen atoms into As–Ga backbonds
Authors: Sexton J. Z. , Yi S. I. , Hale M. , Kruse P. , Demkov A. A. , Kummel A. C.
Published: October 2003

Stable and metastable oxide structures resulting from the reaction of GaAs(001)-(2×4) with O atoms are investigated using scanning tunneling microscopy (STM). The relative stabilities of these oxide structures are examined using density functional theory calculations (DFT). STM images show that when GaAs(001)-(2×4) is exposed to O atoms, the O atom will either displace an arsenic atom from its original dimer position by taking its place or chemisorb subsurface and create a metastable site. As the O atom coverage increases, O atoms increasingly occupy the position of two displaced arsenic atoms across two neighboring dimers, while the number of subsurface sites remains constant. These experiments show that oxygen preferentially removes an arsenic pair (As2), with one atom originating from each of two neighboring As–As dimers instead of from the same As–As dimer. DFT calculations demonstrate the relative stability of the final chemisorption state by calculating the most stable oxygen configurations as functions of the oxygen and arsenic chemical potentials.
Adaptive design of nanoscale dielectric structures for photonics
Authors: Chen Yu , Yu Rong , Li Weifei , Nohadani Omid , Haas Stephan , Levi A. F. J.
Published: October 2003

Using adaptive algorithms, the design of nanoscale dielectric structures for photonic applications is explored. Widths of dielectric layers in a linear array are adjusted to match target responses of optical transmission as a function of energy. Two complementary approaches are discussed. The first approach uses adaptive local random updates and progressively adjusts individual dielectric layer widths. The second approach is based on global updating functions in which large subgroups of layers are adjusted simultaneously. Both schemes are applied to obtain specific target responses of the transmission function within selected energy windows, such as discontinuous cutoff or power-law decay filters close to a photonic band edge. These adaptive algorithms are found to be effective tools in the custom design of nanoscale photonic dielectric structures.
Band structures of GaAs, InAs, and Ge: A 24-k.p model
Authors: Ben Radhia S. , Boujdaria K. , Ridene S. , Bouchriha H. , Fishman G.
Published: October 2003

We present a generalized theoretical description of the 24×24 k.p approach for determining the band structure of the direct-band-gap semiconductors (GaAs, InAs) as well as the indirect-band-gap semiconductor (Ge), including far-level contribution (essentially the d levels). We extend the sp3s* basis functions by the inclusion of sV* orbitals. We find that the sp3“d”(s*)2 k.p model is fairly sufficient to describe the electronic structure of these systems over a wide energy range, obviating the use of any d orbitals. Finally, the comparison with available experimental and theoretical results shows that the present model reproduces known results for bulk GaAs, InAs, and Ge, that is, their band structure, including s and p valence bands and the lowest two conduction bands.
Near-infrared nonlinear properties of a glass–ceramic containing sodium niobate nanocrystals
Authors: Bosco C. A. C. , Falcão-Filho E. L. , Maciel G. S. , Acioli L. H. , de Araújo Cid B. , Lipovskii A. A. , Tagantsev D. K.
Published: October 2003

We have investigated the third-order nonlinear optical response of a niobic–silicate glass–ceramic containing sodium niobate (NaNbO3) nanocrystals using picosecond Z-scan and femtosecond optical Kerr gate techniques at 1064 and 800 nm, respectively. The results show a large dependence of the nonlinear refractive index (n2) on the volume fraction occupied by NaNbO3 crystallites. Nonlinear absorption is negligible for all samples and the response time of the nonlinearity was shorter than 100 fs demonstrating that this material is a promising candidate for ultrafast optical switching.
Structural motifs and stability of small argon–nitrogen clusters
Authors: Hewage Jinasena W. , Amar François G.
Published: October 2003

The molecular dynamics (MD) simulation method is used to study Arm(N2)n clusters. Using realistic pair potentials for the argon–argon, nitrogen–nitrogen, and argon–nitrogen interactions, the structures and thermodynamics of these clusters are investigated. The initial focus of the study is the series of thirteen particle clusters of Arm(N2)13−m (0⩽m⩽13). These icosahedral argon–nitrogen clusters display systematic changes in energetics when argon is substituted by nitrogen in the central position. The relative stability of argon-centered clusters over nitrogen-centered clusters is further investigated by defining and calculating a “species-centric” order parameter which can be monitored during a MD simulation. These results are interpreted in terms of frustration effects due to anisotropy in the N2–N2 and N2–Ar potentials. The consequences of these observations for cluster stability and for dynamical behavior, such as melting and evaporation, are investigated. The dynamical studies of larger clusters reveal that the mixed clusters evolve towards a structure with an argon core coated by a nitrogen shell.
Zero electron kinetic energy photoelectron spectroscopy and density functional theory calculations of gallium–methylamine complexes
Authors: Li Shenggang , Fuller Jason F. , Sohnlein Bradford R. , Yang Dong-Sheng
Published: October 2003

Gallium–dimethylamine and –trimethylamine were produced in pulsed laser vaporization molecular beams and studied by threshold photoionization and pulsed-field ionization zero electron kinetic energy (ZEKE) photoelectron spectroscopies and density functional theory calculations. Analyses of the ZEKE spectra yield molecular adiabatic ionization potentials and metal–ligand and ligand-based vibrational frequencies. Comparisons of the experimental and theoretical results establish the ground electronic states of the neutral and ionic complexes. The ionization potentials, Ga+/Ga–N stretching, and Ga+/Ga–N–C bending frequencies are 38 790, 206/177, and 132/128 cm−1 for the dimethylamine complex and 38 081 cm−1, 188/133, and 111/92 cm−1 for the trimethylamine species. The electronic ground states are A′1/2A′ for Ga+/Ga–NH(CH3)2 and A11/2A″ for Ga+/Ga–N(CH3)3. The calculated metal–ligand binding energies of Ga+/Ga–NH(CH2)2 and –N(CH3)3 are 36.2/9.7 and 37.1/8.5 kcal mol−1, respectively.
Potential energy surfaces and bound states for the open-shell van der Waals cluster Br–HF
Authors: Meuwly Markus , Hutson Jeremy M.
Published: October 2003

Semiempirical potential energy surfaces for the lowest three electronic states of the open-shell complex Br–HF are constructed, based on existing empirical potentials for Kr–HF and Kr–Ne and coupled-cluster electronic structure calculations for Br–Ne. Coupled cluster calculations are also described for He–F, Ne–F and Ar–F. Electrostatic interactions that arise from the quadrupole of the Br atom and the permanent multipoles of HF are also included in the Br–HF surfaces. The well depth of the lowest adiabatic surface is found to be 670 cm−1 at a linear equilibrium geometry. The results of helicity decoupled and full close-coupling calculations of the bound states of the complex are also described. The ground state, with total angular momentum projection quantum number |P|=3/2, is found 435 cm−1 below dissociation to Br (2P3/2)+HF (j=0). The lowest-frequency intermolecular bending and stretching vibrations are predicted around 145 and 211 cm−1, respectively. Parity splittings are found to be extremely small for bound states with projection quantum number |P|=3/2. The relevance of the results to recently recorded spectra of Br–HF is discussed.
First principles study of oxygen vacancy defects in tantalum pentoxide
Authors: Ramprasad R.
Published: October 2003

First principles total energy calculations were performed to characterize oxygen vacancy defects in tantalum pentoxide (Ta2O5). A simplified version of the crystalline orthorhombic phase of Ta2O5 was used in this study. Results indicate that O vacancies in Ta2O5 can be broadly classified based on their location in the lattice. One type of vacancy that occupies the “in-plane” sites displays deep or midgap occupied states and shallow unoccupied states, while a second type occupying “cap” sites results in shallow occupied states. For a wide range of Fermi levels or chemical potentials, the neutral and +2 charged states of the in-plane type vacancy and the +2 charge state of the cap type vacancy are found to be most stable.
Density functional characterization of N2 dissociation on the step of ruthenium clusters
Authors: Cao Zexing , Wan Huilin , Zhang Qianer
Published: October 2003

Mechanisms of N2 dissociative adsorption on small ruthenium clusters are studied by density functional calculations. The calculations indicate that the step of a ruthenium cluster has high activity for N2 activation, where an ensemble of five Ru atoms on the stepped surface of clusters is responsible for the active site. Such high activity arises from a strong charge-transfer interaction due to local phase adaptation between the π* orbital of N2 and the filled cluster valence orbital over the step region. Results from cluster models with different size show that the activation mechanism and the barrier are sensitive to the structural environment of the step. N2 dissociation over the step of the 11-atom cluster is a two-step process, where the rate-determining step has a barrier of 22 kcal mol−1. N2 dissociative adsorption on the stepped surface of 15-atom and 21-atom clusters is a one-step process, and the barrier is ∼7–10 kcal mol−1. Theoretical calculations on the 11-atom Os and Fe cluster models reveal a general activity of the stepped sites for N2 activation.
Surface wave propagation in steady ideal Hall-magnetohydrodynamic magnetic slabs
Authors: Miteva Rossitsa , Zhelyazkov Ivan , Erdélyi Robert
Published: October 2003

This paper studies the dispersion characteristics of sausage and kink surface waves traveling along a plasma layer within the framework of Hall magnetohydrodynamics in steady state. While in a static plasma slab these waves are Alfvén ones (their phase velocities are close to the Alfvén speed in the layer); in a slab with steady flows they may become super Alfvénic waves. Moreover, there exist two types of waves: forward and backward ones bearing in mind that the flow velocity defines the positive (forward) direction. As a typical representative of a magnetic slab in steady state here is considered a solar wind flux rope with a finite β plasma flow (typically β∼1).The forward sausage surface mode exhibits an increased dispersion at small wave numbers while the forward kink waves become practically non-dispersive. Both backward propagating sausage and kink surface modes show an increased dispersion for large wave numbers.
Controlling edge plasma rotation through poloidally localized refueling
Authors: Helander P. , Fülöp T. , Catto Peter J.
Published: October 2003

The transport of angular momentum due to neutral atoms in the tokamak edge is calculated and shown to be sensitive to the poloidal location of the neutrals. In the absence of external momentum sources, the edge plasma is predicted to rotate spontaneously in the opposite direction to the plasma current, at a speed proportional to the radial ion temperature gradient. If the plasma is collisional, this countercurrent rotation is largest if the neutrals are concentrated on the inboard side of the torus, while the opposite holds in a collisionless plasma. The presence of heavy impurity ions also promotes countercurrent rotation. The rotation caused by an external momentum source, such as neutral-beam injection, is found to be larger when the neutrals are on the inboard rather than the outboard side.
Temperature dependence of the optical properties in hexagonal AlN
Authors: Jiang L. F. , Shen W. Z. , Ogawa H. , Guo Q. X.
Published: October 2003

We report on the reflectance results of high-quality single crystal AlN thin films at temperature T from 23 to 300 K. With the aid of a detailed procedure developed for analyzing reflection spectra, together with a band tail theory based on calculation of the density of occupied states and carrier–phonon interaction, we have obtained the temperature-dependent optical properties of AlN thin films, such as band gap Eg(T)=6.280–1.700×10−3T2/(1480+T) eV, absorption coefficient in both the Urbach region α0=1.02×104 cm−1 and intrinsic absorption region αd=2.10×105–2.36×102T cm−1, Urbach band tail parameter EU(T)=63.5+4.71×10−3T3/2+1.61 coth(717/T) meV and refractive index. The wavelength- and temperature-dependent refractive index dispersion below the band gap can be described by the Sellmeier equation. Furthermore, we unify the various experimental refractive index data reported in the literature with our refractive index results. These optical properties provide an experimental basis for further theoretical investigation and device design of AlN.
Cluster coarsening and luminescence emission intensity of Ge nanoclusters in SiO2 layers
Authors: Lopes J. M. J. , Zawislak F. C. , Behar M. , Fichtner P. F. P. , Rebohle L. , Skorupa W.
Published: October 2003

SiO 2 layers 180 nm thick are implanted with 120 keV Ge+ ions at a fluence of 1.2×1016 cm−2. The distribution and coarsening evolution of Ge nanoclusters are characterized by Rutherford backscattering spectrometry and transmission electron microscopy and the results are correlated with photoluminescence measurements as a function of the annealing temperatures in the 400 °C⩽T⩽900 °C range. At 400 °C we observe a monomodal array of clusters characterized by a mean diameter 〈φ〉=2.2 nm which increases to 〈φ〉=5.6 nm at 900 °C. This coarsening evolution occurs concomitantly with a small change of the total cluster–matrix interface area and an increase of the Ge content trapped in observable nanoclusters. However, at 900 °C a significant fraction of up to about 20% of the Ge content still remains distributed in the matrix around the nanoparticles. The results are discussed in terms of possible atomic mechanisms involved in the coarsening behavior that lead to the formation of the oxygen deficiency luminescence centers.
A note on Anderson localization for the random hopping model
Authors: Klopp Frédéric , Nakamura Shu
Published: October 2003

This short note is devoted to the proof of Lifshitz tails and a Wegner estimate, and thus, band edge localization, for the random hopping model.
Weak Hopf algebras corresponding to Uq[sln]
Authors: Aizawa N. , Isaac P. S.
Published: October 2003

We investigate the weak Hopf algebras of Li based on Uq[sln] and Sweedler’s finite dimensional example. We give weak Hopf algebra isomorphisms between the weak generalizations of Uq[sln] which are “upgraded” automorphisms of Uq[sln] and hence give a classification of these structures as weak Hopf algebras. We also show how to decompose these examples into a direct sum which leads to unexpected isomorphisms between their algebraic structure.
Parametric phenomena of the particle dynamics in a periodic gravitational wave field
Authors: Balakin Alexander B. , Kurbanova Veronika R. , Zimdahl Winfried
Published: October 2003

We establish exactly solvable models for the motion of neutral particles, electrically charged point and spin particles [U(1) symmetry], isospin particles [SU(2) symmetry], and particles with color charges [SU(3) symmetry] in a gravitational wave background. Special attention is devoted to parametric effects induced by the gravitational field. In particular, we discuss parametric instabilities of the particle motion and parametric oscillations of the vectors of spin, isospin, and color charge.
Reflectivity in surface transverse wave gratings
Authors: Yanchev Ventsislav M. , Strashilov Vesseline L.
Published: October 2003

An effective method for improving the grating reflectivity in surface transverse wave quartz resonators is described. Deposition of a thin polymer layer on the electrode grating surface leads to improvement in the wave trapping by reducing the mode conversion into radiative bulk waves. The effect is additionally enhanced by increased acoustic impedance mismatch at the polymer/metal interfaces. Combined with an optimum layout design, the method can yield a considerably higher resonator Q factor compared to conventional surface acoustic waves.
Photoelectron emission microscopy observation of inversion domain boundaries of GaN-based lateral polarity heterostructures
Authors: Yang W.-C. , Rodriguez B. J. , Park M. , Nemanich R. J. , Ambacher O. , Cimalla V.
Published: October 2003

An intentionally grown GaN film with laterally patterned Ga- and N-face polarities is studied using in situ UV-photoelectron emission microscopy (PEEM). Before chemical vapor cleaning of the surface, the emission contrast between the Ga- and N-face polarities regions was not significant. However, after cleaning the emission contrast between the different polarity regions was enhanced such that the N-face regions exhibited increased emission over the Ga-face regions. The results indicate that the emission threshold of the N-face region is lower than that of the Ga face. Moreover, bright emission was detected from regions around the inversion domain boundaries of the lateral polarity heterostructure. The PEEM polarity contrast and intense emission from the inversion domain boundary regions are discussed in terms of the built-in lateral field and the surface band bending induced by the polarization bound surface charges.
X-ray photoelectron spectroscopic study on sapphire nitridation for GaN growth by hydride vapor phase epitaxy: Nitridation mechanism
Authors: Dwikusuma F. , Kuech T. F.
Published: October 2003

The nitridation of c-plane sapphire within the hydride vapor phase epitaxy system was systematically studied as a function of time and ammonia partial pressure using ex situ x-ray photoelectron spectroscopy, reflection high-energy electron diffraction, and atomic force microscopy. During the nitridation process, nitrogen was incorporated into the sapphire surface. There were two different nitrogen chemical bonding states, which can be attributed to N–Al bonds and nitrogen in oxygen-rich environment (‘N–O’). As the nitridation continued, the N 1s intensity increased while the O 1s intensity decreased indicating the growth of a nitrogen-rich layer. The sapphire nitridation process can be modeled as a diffusion couple of AlN and Al2O3, where N3− and O2− interdiffuse in the rigid Al3+ framework. Nitrogen diffuses into sapphire and substitutes for oxygen to bond with aluminum. The bond substitution is accompanied by structural changes where the AlN in-plane direction is rotated 30° with respect to the sapphire direction. The replaced oxygen diffuses out to the surface, combines with hydrogen and desorbs as H2O. The overall nitridation rate is determined by the slower of the two moving anions. From the x-ray photoelectron spectroscopy data, the chemical diffusion coefficient of nitrogen (D̃N) and oxygen (D̃O), were estimated. D̃N was found to be higher than D̃O, which suggested that the overall nitridation rate was controlled by the diffusion of oxygen to the surface. After nitridation, no protrusions were observed on the surface and no significant changes in the surface roughness were measured when compared to the as-received sapphire.
Linear perturbation growth at the trailing edge of a rarefaction wave
Authors: Wouchuk J. G. , Carretero R.
Published: October 2003

An analytic model for the perturbation growth inside a rarefaction wave is presented. The objective of the work is to calculate the growth of the perturbations at the trailing edge of a simple expanding wave in planar geometry. Previous numerical and analytical works have shown that the ripples at the rarefaction tail exhibit linear growth asymptotically in time [Yang et al., Phys. Fluids 6, 1856 (1994), A. Velikovich and L. Phillips, ibid. 8, 1107 (1996)]. However, closed expressions for the asymptotic value of the perturbed velocity of the trailing edge have not been reported before, except for very weak rarefactions. Explicit analytic solutions for the perturbations growing at the rarefaction trailing edge as a function of time and also for the asymptotic perturbed velocity are given, for fluids with γ<3. The limits of weak and strong rarefactions are considered and the corresponding scaling laws are given. A semi-qualitative discussion of the late time linear growth at the trailing edge ripple is presented and it is seen that the lateral mass flow induced by the sound wave fluctuations is solely responsible for that behavior. Only the rarefactions generated after the interaction of a shock wave with a contact discontinuity are considered.
Start currents in an overmoded gyrotron
Authors: Yeddulla M. , Nusinovich G. S. , Antonsen T. M.
Published: October 2003

High-power long-pulse millimeter-wave gyrotrons operate in high-order modes. The spectral density of these modes is very high. Therefore, self-excitation conditions can be fulfilled for several modes simultaneously. Correspondingly, in order to determine which mode will be excited first in such a device, the starting currents of competing modes should be calculated much more accurately than in gyrotrons with a rarer spectrum. In the present paper, an existing linear theory is generalized to take into account effects of magnetic field tapering, cavity profile, finite beam thickness, velocity spread and axially dependent beam coupling to the fields of competing modes. Starting currents are calculated for the operating and the most dangerous competing mode in a 140 GHz gyrotron, which is under development at Communication and Power Industries. Calculations show that the radial position of the electron beam plays a critical role in deciding which mode dominates the mode competition.
Ferroelectric phase transition in Pb0.92Gd0.08(Zr0.53Ti0.47)0.98O3 nanoceramic synthesized by high-energy ball milling
Authors: Parashar S. K. S. , Choudhary R. N. P. , Murty B. S.
Published: October 2003

The nanocrystalline Gd-doped lead titanium zirconate (PGZT) with the composition Pb0.92Gd0.08(Zr0.53Ti0.47)0.98O3 has been synthesized by a combination of high-energy ball milling [mechanical alloying (MA)] and sintering. PGZT prepared by MA followed by high-temperature sintering has a higher dielectric constant compared to that of the conventional technique. PGZT shows a nonrelaxor ferroelectric behavior but has diffuse ferroelectric–paraelectric phase transition. The activation energy of the material was found sensitive to its crystallite size.
Microstructural evolution and nanocrystal formation in Pb+-implanted ZrSiO4 single crystals
Authors: Lian Jie , Rı́os Susana , Boatner Lynn A. , Wang L. M. , Ewing Rodney C.
Published: October 2003

Single crystals of ZrSiO4 (zircon) with a (110) orientation were implanted with 300 keV Pb+ at room temperature to fluences ranging from 1014 to 1017 ions/cm2. The damage accumulation and microstructural evolution were analyzed by cross-sectional transmission electron microscopy (TEM) and glancing-angle x-ray diffraction (XRD). The experimental damage profiles as observed by TEM and XRD methods were compared to Monte Carlo simulations using the SRIM-2000 code. At the lowest ion fluence (1014 ions/cm2), a buried amorphous layer formed in the zircon matrix. The surface layer is highly damaged and consists of zircon nanocrystals. The critical amorphization dose for zircon implanted with 300 keV Pb+ was in the range of 0.25–0.43 displacements per atom. With increasing ion fluence, the thickness of the amorphous layer increased. When the Pb concentration in the substrate exceeded ∼3.5 at. % (i.e., at 1017 ions/cm2), Pb nanoparticles precipitated at room temperature and formed a layer ∼90 nm thick embedded within the amorphous zircon matrix. Effects of the displacement energies employed in the SRIM-2000 simulation on the damage profiles and the critical amorphization dose were also analyzed.
Optical transmission and Faraday rotation spectra of a bismuth iron garnet film
Authors: Kahl S. , Popov V. , Grishin A. M.
Published: October 2003

We prepared an epitaxial 530-nm-thick bismuth iron garnet (BIG) film on a Gd3Ga5O12(GGG) (111) substrate by pulsed laser deposition and measured spectra of optical transmission and magneto-optical Faraday rotation θF(λ) in visible light. Both spectra are shaped by the dispersion relations of the dielectric tensor and the effects of multiple-beam interference in the thin film. From fitting of the transmission spectrum, dispersion relations of the real and imaginary parts of the refractive index were found for the wavelength range from 515 to 1000 nm, n(λ)=2.36+(413 nm/λ)2 and k(λ)=(λ/4π nm)×exp[(1660 nm/λ)2−15.2]. With these data as input information, a single diamagnetic line centered at λ=486 nm was fitted to the experimentally recorded magneto-optical spectrum. The effects of thin film interference and surface roughness were included in order to closely reproduce the measured spectrum. The roughness determined from spectral analysis was compared to the rms roughness measured by atomic force microscopy. The values agree within a deviation of less than 20% and are approximately 3% of film thickness. The magneto-optical figure of merit 2|θF|/α (α is the absorption coefficient) of our film increases strongly with wavelength and exceeds 100° at 740 nm. The optical efficiency for magneto-optical visualization |exp(−2αd)sin(4θFd)| (d is film thickness) possesses a maximum value of 29% at 600 nm. Both quantities were calculated using the obtained dispersion relations for absorption and Faraday rotation.
Simulation of semiconductor intersubband Raman laser
Authors: Wu B. H. , Cao J. C. , Xia G. Q.
Published: October 2003

A Raman laser [Liu et al., Appl. Phys. Lett. 78, 3580 (2001)] that operated on quantum well intersubband transitions was reported recently. Properties of this optically pumped double quantum well structure are investigated numerically in this article. The carrier lifetime of each subband in the structure is calculated and the pump-induced carrier distributions for each subband are obtained. Using the density matrix method we calculate the stimulated Raman gain and find that it does not always increase linearly with the pumping intensity as the former analysis suggested, but, rather, only when the pumping intensity is low. When the pumping intensity is high, the Raman gain saturates. The calculated results are qualitatively in agreement with the experimental ones of Liu et al.
Quantum property of solid hydrogen as revealed by high-resolution laser spectroscopy
Authors: Katsuki H. , Fushitani M. , Momose T.
Published: October 2003

Pure vibrational overtone transitions of solid parahydrogen are studied using high-resolution laser spectroscopy. Extremely narrow spectral linewidth (∼20 MHz) allows us to observe rich spectral structure that originates in subtle intermolecular interactions in the crystal. It is found that anisotropy of the distribution of zero-point lattice vibration of hydrogen molecules perturbs the energy levels of the vibrationally excited states significantly. A large amplitude of zero-point lattice vibration, an intrinsic propoerty of quantum solids, is directly observed from the present high-resolution spectroscopy. The first observation of a pure vibrational overtone transition of solid orthodeuterium is also discussed.
Temperature dependence of hot luminescence in solid xenon: theory and experiment
Authors: Selg M. , Kink R.
Published: October 2003

Vibrational relaxation of self-trapped excitons in solid xenon has been investigated both experimentally and theoretically. Hot luminescence spectra of the crystal have been measured in a wide temperature range (from liquid helium temperature to 100 K), using x-ray and two-photon excimer laser (ArF and KrF) excitation. The theoretical analysis is based on a recent nonperturbative relaxation theory and on a rigorous quantum-mechanical Franck–Condon approach.
The vibrational relaxation of CO2 isolated in solid argon
Authors: Cenian A. , Śliwiński G. , Grigorian G.
Published: October 2003

The vibrational relaxation of CO2 molecules embedded in an Ar matrix is described using a model based on multiphonon transitions. Rates for the VT and VV processes are determined from a fitting of simulated and experimental data. The calculations confirmed that radiative processes influence significantly the vibrational energy relaxation of CO2 embedded in solid Ar; e.g., the rate determined for energy transfer between the ν3 and ν2 modes is significantly lower than that predicted under the assumption of nonradiative relaxation.
Matrix-isolation FTIR study of azidoacetone and azidoacetonitrile
Authors: Frankowski M. , Fox B. S. , Smith-Gicklhorn A. M. , Beyer M. K. , Bondybey V. E. , Algarra M. , Costa M. L. , Rodrigues P. , Barros M. T. , Cordeiro M. N. D. S.
Published: October 2003

Azidoacetonitrile (N3CH2CN) and azidoacetone (N3CH2COCH3) are studied by matrix-isolation FTIR spectroscopy in solid neon, argon, and nitrogen. The IR spectra calculated using the density-fuctional theoretical method are discussed in comparison with the experimental data. Significant broadening of the recorded azide bands indicate an awkward fit of these compounds into the solid environment. The strongest absorption is observed for both compounds in the regions of asymmetric and symmetric stretches of the N3 azide group. Strong band splittings in the N3 asymmetric stretch region can be most likely explained by very strong Fermi resonances with the CN stretch and combinations and overtones of the numerous lower-frequency vibrational modes.
Effect of grain-size distribution on the energy loss of a pair of charged projectiles in a dust-contaminated plasma
Authors: Mirza Arshad M. , Sarwar M. Adnan , Qaisar M. S.
Published: October 2003

Analytical and numerical results are presented for the slowing down of a pair of heavy test charge projectiles through a multicomponent, dust-contaminated plasma. The correlation and interference effects of two collinear and noncollinear projectiles on electrostatic potential and energy loss are studied for a Maxwellian distribution and a special class of physically reasonable size distributions. The energy loss behavior versus projectile velocity of noncollinear projectiles is also examined for various orientations. It is found that the energy loss for Maxwellian distribution (for large value of spectral index κ) is larger compared to that for generalized Lorentzian distribution. It is also observed that for smaller values of κ, the test charge projectile gains energy instead of losing. These results would be useful for the understanding of the energy loss mechanism, which might be responsible for the coagulation of dust particles in molecular clouds, in the ion-beam driven inertial confinement fusion scheme and in dust plasma crystal formation, etc.
Large anomalous enhancement of perpendicular exchange bias by introduction of a nonmagnetic spacer between the ferromagnetic and antiferromagnetic layers
Authors: Garcia F. , Sort J. , Rodmacq B. , Auffret S. , Dieny B.
Published: October 2003

In (Pt/Co)n/FeMn multilayers, the magnitude of exchange bias, HE, can be considerably enhanced by placing an ultrathin nonmagnetic Pt spacer between the multilayer (ML) and the antiferromagnetic (AFM) layer. The bias is maximum for a spacer layer thickness, t, of a few angstroms and it decreases progressively as t is further increased. This bias enhancement is accompanied by an increase of coercivity, HC. This behavior is due to the role of the Pt spacer in enhancing the perpendicular effective anisotropy of the last Co layer in the ML, which has the effect of increasing the net ferromagnetic (FM)/AFM spin projection, thus leading to the HE and HC enhancements. The decrease of HE and HC for thicker spacer layers is due to the limited range of the FM–AFM proximity effect.
Absence of saturation for finite injected currents in axially symmetric cavity diode
Authors: Biswas Debabrata , Kumar Raghwendra , Puri R. R.
Published: October 2003

The Child–Langmuir law is investigated numerically using a fully electromagnetic particle-in-cell code for a closed axially symmetric diode. It is found that the average current transmitted to the anode (JTR) increases with the injected current (JIN) even after the formation of virtual cathode in both the nonrelativistic and relativistic cases. The increase is found to be a power law, JTR∼JIN1−β. In other words, the time averaged fraction f of electrons reaching the anode varies with the input current as, f∼JIN−β, where β<1. In contrast, for an infinite parallel plate diode, f∼JIN−1. The possibility of asymptotic saturation is also discussed.
Large room-temperature spin-dependent tunneling magnetoresistance in polycrystalline Fe3O4 films
Authors: Liu Hui , Jiang E. Y. , Bai H. L. , Zheng R. K. , Wei H. L. , Zhang X. X.
Published: October 2003

Polycrystalline Fe3O4 films have been prepared by reactive sputtering at room temperature. Transmission electron microscopy images show that the films consist of quite uniform Fe3O4 grains well separated by grain boundaries. It was found that the tunneling of spin-polarized electrons across the antiferromagnetic coupled grain boundaries dominates the transport properties of the films. Magnetoresistance (MR) {=[ρ(H)−ρ(0)]/ρ(0)} shows linear and quadratic magnetic-field dependence in the low-field range when the field is applied parallel and perpendicular to film plane, which is similar to the behaviors observed in the epitaxial Fe3O4 films consisting of a large fraction of antiferromagnetic antiphase domain boundaries. At 300 K, the size of the MR reaches −7.4% under a 50-kOe magnetic field, which is a very large MR for polycrystalline Fe3O4 films.
Tailoring structure and electrical properties of carbon nanotubes using kilo-electron-volt ions
Authors: Wei B. Q. , D’Arcy-Gall J. , Ajayan P. M. , Ramanath G.
Published: October 2003

We report the effects of 30 and 50 keV Ga+ ion irradiation on the structure and electrical properties of arc-evaporated multiwalled carbon nanotubes (MWNTs). For 50 keV ions with doses of ∼1013 ions/cm2 the outer shells of the MWNTs remain intact, while the inner layers reorganize into highly ordered pillbox-like ∼5-nm-diam nanocompartments of varying lengths between 2 and 20 nm. Increasing the dose to ∼1014 ions cm−2 results in the gradual disordering of the graphitic shells and destroys the nanocapsules, while at doses of 1015 ions cm−2 the graphitic shells collapse into the hollow, resulting in the formation a homogenous amorphous rod. Irradiating nanotubes with 30 keV ions yields similar results, but at higher doses. Irradiated nanotubes exhibit a decrease in electron activation energy from 194 to 112 meV, while the semiconducting behavior is essentially preserved for ion doses up to 5×1015 ions cm−2. Ion irradiation could be a useful tool to locally modify nanotube structure and tailor properties for device applications.
Donor–donor binding in semiconductors: Engineering shallow donor levels for ZnTe
Authors: Janotti A. , Wei Su-Huai , Zhang S. B.
Published: October 2003

In the past, codoping by mixing donors with acceptors has been proposed to lower the dopant ionization energy. However, the level repulsion between donor and acceptor states is weak due to symmetry considerations. Here, we propose an innovative approach to lower the donor ionization energy by combining donor with donor. Using first-principles band structure method, we demonstrated this concept with n-type doping in ZnTe. For example, we find that the BrTe–SnZn pair has a binding energy of 0.9 eV and a shallow ε(+/0) donor level at 70 meV below the conduction-band minimum, compared to 240 meV for the isolated BrTe impurity.
Structure and growth of monoclinic Mo2S3 nanorods
Authors: Che R. C. , Bai N. , Peng L.-M.
Published: October 2003

We report the synthesis of sulfide nanostructures with trivalent molybdenum, i.e., crystalline Mo2S3 nanorods via a solid-gas reaction between porous Al2O3 impregnated with MoO3 and H2S gas. We show that the introduction of additional 8% H2 gas results in MoS2 nanotubes of the same size. A growth model is proposed for the formation of Mo2S3 nanorods, and the effect of H2 is discussed and demonstrated.
Femtosecond laser quenching of the ε phase of iron
Authors: Sano Tomokazu , Mori Hiroaki , Ohmura Etsuji , Miyamoto Isamu
Published: October 2003

The quenching of the ε phase of iron, which has not been observed under a conventional shock compression, was attained using a femtosecond laser. The crystalline structure in a recovered iron sample was determined using an electron backscatter diffraction pattern system. The femtosecond laser driven shock may have the potential to quench high-pressure phases of other materials.
Scanbacks and direct rebates: manufacturer's tools against forward buying
Authors: Arcelus F. J. , Srinivasan G.
Published: October 2003

This paper evaluates the role of trade incentives specifically designed to fight forward‐buying practices on the part of the retailers, by examining the use of scanbacks and direct rebates as manufacturers' tools for the prevention of these practices. Scanner data allows the manufacturer to keep track of the retailer's pricing policies at the point of sale and hence tie its discount policy to the magnitude of the retailer's pass‐through to the final customers. Trade incentives of this type are called scanbacks, whereby the determination of the retailer's compensation is based on actual performance normally measured by scanner data. Another incentive is the direct rebate, whereby the manufacturer passes on directly to the final customer some discount, normally in the form of a coupon, upon proof of purchase. Rebates are one of the oldest trade incentives and certainly predate the advent of electronic commerce. Their relevance is enhanced by the fact that they can be easily adapted to the modern B2B marketplace. The economic effects of these incentives are evaluated in terms of their effect on the three basic links of the supply chain in question, namely (i) the manufacturer that offers the incentive; (ii) the retailer that develops the optimal pricing and ordering policy for each manufacturer's incentive; and (iii) the final customer who is the ultimate purchaser of the merchandise.
The Operating Performance of Firms that Switch Their Stock Listings
Authors: Papaioannou George J. , Travlos Nickolaos G. , Viswanathan K. G.
Published: October 2003

In this article we examine the operating performance of stocks that switch from NASDAQ to the American Stock Exchange (AMEX) or the New Stock Exchange (NYSE) and from AMEX to the NYSE. Specifically, we investigate whether post‐listing operating performance is consistent with the reported negative long‐term drift of post‐listing stock returns and whether there is evidence of self‐selection of the listing time. We find evidence of negative post‐listing changes in operating return on assets and sales, which, on a match‐adjusted basis, are significant for the relatively small NASDAQ stocks switching to AMEX. We also find evidence that firms self‐select the time of listing changes.
A New Class of Bacterial RNA Polymerase Inhibitor Affects Nucleotide Addition
Authors: Artsimovitch Irina , Chu Clement , Lynch A. Simon , Landick Robert
Published: October 2003

RNA polymerase (RNAP) is the central enzyme of gene expression. Despite availability of crystal structures, details of its nucleotide addition cycle remain obscure. We describe bacterial RNAP inhibitors (the CBR703 series) whose properties illuminate this mechanism. These compounds inhibit known catalytic activities of RNAP (nucleotide addition, pyrophosphorolysis, and Gre-stimulated transcript cleavage) but not translocation of RNA or DNA when translocation is uncoupled from catalysis. CBR703-resistance substitutions occur on an outside surface of RNAP opposite its internal active site. We propose that CBR703 compounds inhibit nucleotide addition allosterically by hindering movements of active site structures that are linked to the CBR703 binding site through a bridge helix.
Structure of Rab GDP-Dissociation Inhibitor in Complex with Prenylated YPT1 GTPase
Authors: Rak Alexey , Pylypenko Olena , Durek Thomas , Watzke Anja , Kushnir Susanna , Brunsveld Lucas , Waldmann Herbert , Goody Roger S. , Alexandrov Kirill
Published: October 2003

Rab/Ypt guanosine triphosphatases (GTPases) represent a family of key membrane traffic regulators in eukaryotic cells whose function is governed by the guanosine diphosphate (GDP) dissociation inhibitor (RabGDI). Using a combination of chemical synthesis and protein engineering, we generated and crystallized the monoprenylated Ypt1:RabGDI complex. The structure of the complex was solved to 1.5 angstrom resolution and provides a structural basis for the ability of RabGDI to inhibit the release of nucleotide by Rab proteins. Isoprenoid binding requires a conformational change that opens a cavity in the hydrophobic core of its domain II. Analysis of the structure provides a molecular basis for understanding a RabGDI mutant that causes mental retardation in humans.
Anti-Clumping Factor A Immunoglobulin Reduces the Duration of Methicillin-ResistantStaphylococcus aureusBacteremia in an Experimental Model of Infective Endocarditis
Authors: Vernachio John , Bayer Arnold S. , Le Thuan , Chai Yin-Li , Prater Bradley , Schneider Amy , Ames Brenda , Syribeys Peter , Robbins Jeffrey , Patti Joseph M.
Published: October 2003

SA-IGIV is a human polyclonal immunoglobulin containing elevated levels of antibodies specific for the fibrinogen-binding MSCRAMM protein clumping factor A (ClfA). In vitro, SA-IGIV specifically recognized ClfA that was expressed on the surface ofStaphylococcus aureusand inhibited bacterial adherence to immobilized human fibrinogen by >95%. Moreover, SA-IGIV efficiently opsonized ClfA-coated fluorescent beads and facilitated phagocytosis by human polymorphonuclear leukocytes. To determine its potential therapeutic efficacy, SA-IGIV was evaluated in combination with vancomycin in a rabbit model of catheter-induced aortic valve infective endocarditis (IE) caused by methicillin-resistantS. aureus(MRSA). The combination therapy was more effective than vancomycin alone in sterilizing all valvular vegetations when used therapeutically during early (12-h) IE. The combination therapy resulted in clearance of bacteremia that was significantly faster than that of vancomycin alone in animals with well-established (24-h) IE. Therefore, in both early and well-established MRSA IE, the addition of SA-IGIV to a standard antibiotic regimen (vancomycin) increased bacterial clearance from the bloodstream and/or vegetations.
Influence of VanD Type Resistance on Activities of Glycopeptides In Vitro and in Experimental Endocarditis Due to
Enterococcus faecium
Authors: Lefort Agnès , Garry Louis , Depardieu Florence , Courvalin Patrice , Fantin Bruno
Published: October 2003

The consequences of VanD type glycopeptide resistance on the activity of vancomycin and teicoplanin were evaluated in vitro and in a rabbit model of aortic endocarditis with VanD type clinical isolate
Enterococcus faecium
BM4339 (MICs: vancomycin, 64 μg/ml; teicoplanin, 4 μg/ml) and its susceptible derivative BM4459 (MICs: vancomycin, 1 μg/ml; teicoplanin, 1 μg/ml). The two antibiotics were inactive against BM4339 in vivo, in terms both of reduction of bacterial counts and of prevention of emergence of glycopeptide-resistant subpopulations, despite using teicoplanin at concentrations greater than the MIC for VanD strains. This could be due to the high inoculum effect also observed in vitro with BM4339 and two other VanD type isolates against both antibiotics. These results suggest that detection of VanD type resistance is of major importance because it abolishes in vivo glycopeptide activity and allows the emergence of mutants highly resistant to glycopeptides.
Extended-Spectrum β-Lactamases in
Klebsiella pneumoniae
Bloodstream Isolates from Seven Countries: Dominance and Widespread Prevalence of SHV- and CTX-M-Type β-Lactamases
Authors: Paterson David L. , Hujer Kristine M. , Hujer Andrea M. , Yeiser Bethany , Bonomo Michael D. , Rice Louis B. , Bonomo Robert A. ,   the International Klebsiella Study Group
Published: October 2003

A huge variety of extended-spectrum β-lactamases (ESBLs) have been detected during the last 20 years. The majority of these have been of the TEM or SHV lineage. We have assessed ESBLs occurring among a collection of 455 bloodstream isolates of
Klebsiella pneumoniae
, collected from 12 hospitals in seven countries. Multiple β-lactamases were produced by isolates with phenotypic evidence of ESBL production (mean of 2.7 β-lactamases per isolate; range, 1 to 5). SHV-type ESBLs were the most common ESBL, occurring in 67.1% (49 of 73) of isolates with phenotypic evidence of ESBL production. In contrast, TEM-type ESBLs (TEM-10 type, -12 type, -26 type, and -63 type) were found in just 16.4% (12 of 73) of isolates. The finding of TEM-10 type and TEM-12 type represents the first detection of a TEM-type ESBL in South America. PER (for
Pseudomonas
extended resistance)-type β-lactamases were detected in five of the nine isolates from Turkey and were found with SHV-2-type and SHV-5-type ESBLs in two of the isolates. CTX-M-type ESBLs (
bla
CTX-M-2
type and
bla
CTX-M-3
type) were found in 23.3% (17 of 73) of isolates and were found in all study countries except for the United States. We also detected CTX-M-type ESBLs in four countries where they have previously not been described—Australia, Belgium, Turkey, and South Africa. The widespread emergence and proliferation of CTX-M-type ESBLs is particularly noteworthy and may have important implications for clinical microbiology laboratories and for physicians treating patients with serious
K. pneumoniae
infections.
In Vitro and Ex Vivo Activities of Minocycline and EDTA against Microorganisms Embedded in Biofilm on Catheter Surfaces
Authors: Raad Issam , Chatzinikolaou Ioannis , Chaiban Gassan , Hanna Hend , Hachem Ray , Dvorak Tanya , Cook Guy , Costerton William
Published: October 2003

Minocycline-EDTA (M-EDTA) flush solution has been shown to prevent catheter-related infection and colonization in a rabbit model and in hemodialysis patients. We undertook this study in order to determine the activities of M-EDTA against organisms embedded in fresh biofilm (in vitro) and mature biofilm (ex vivo). For the experiment with the in vitro model, a modified Robbin’s device (MRD) was used whereby 25 catheter segments were flushed for 18 h with 10
6
CFU of biofilm-producing
Staphylococcus epidermidis
,
Staphyloccocus aureus
, and
Candida albicans
per ml. Subsequently, each of the catheter segments was incubated in one of the following solutions: (i) streptokinase, (ii) heparin, (iii) broth alone, (iv) vancomycin, (v) vancomycin-heparin, (vi) EDTA, (vii) minocycline (high-dose alternating with low-dose), or (viii) M-EDTA (low-dose minocycline alternating with high-dose minocycline were used to study the additive and synergistic activities of M-EDTA). All segments were cultured quantitatively by scrape sonication. For the experiment with the ex vivo model, 54 catheter tip segments removed from patients and colonized with bacterial organisms by roll plate were longitudinally cut into two equal segments and exposed to either saline, heparin, EDTA, or M-EDTA (with high-dose minocycline). Subsequently, all segments were examined by confocal laser electron microscopy. In the in vitro MRD model, M-EDTA (with a low concentration of minocycline) was significantly more effective than any other agent in reducing colonization of
S. epidermidis
,
S. aureus
, and
C. albicans
(
P
< 0.01). M-EDTA (with a high concentration of minocycline) eradicated all staphylococcal and
C. albicans
organisms embedded in the biofilm. In the ex vivo model, M-EDTA (with a high concentration of minocycline) reduced bacterial colonization more frequently than EDTA or heparin (
P
< 0.01). We concluded that M-EDTA is highly active in eradicating microorganisms embedded in fresh and mature biofilm adhering to catheter surfaces.