bluuebunny/crossref_metadata_2025 huggingface dataset:
Different responses of localized and extended excitons to exciton–exciton scattering manifested in excitation density-dependent photoluminescence excitation spectra
Authors: Lan Sheng , Nishikawa Satoshi , Wada Osamu , Akahane Kouichi , Okada Yoshitaka , Kawabe Mitsuo
Published: July 2002

Excitation density-dependent photoluminescence excitation (PLE) measurements are performed for a disordered two-dimensional (2D) In0.40Ga0.60As/GaAs(311)B quantum dot superlattice (QDSL) containing localized and extended states separated by a mobility edge. For low excitation densities, the PLE spectrum is independent of excitation density. In the extended state region, it takes the shape of the absorption spectrum of the 2D QDSL. With increasing excitation density, the PLE spectral shape is dramatically changed. For excitation densities at which the state filling can be excluded, it is found that the different responses of localized and extended excitons to exciton–exciton scattering are responsible for the observed change in the PLE spectrum. A qualitative explanation for the evolution of the PLE spectrum is presented based on the excitation density-dependent capture, relaxation and recombination times obtained previously in time-resolved photoluminescence (TRPL) experiments. The mobility edge extracted from the modification of the PLE spectrum is in good agreement with that determined by TRPL measurements. To show the effect of coupling strength, a comparison of the excitation density-dependent PLE spectra is made between the 2D QDSL and a weekly coupled quantum dot array. It further confirms the existence of highly extended states in the 2D QDSL.
Angular dispersion of surface acoustic waves on (001), (110), and (111) GaAs
Authors: Kuok M. H. , Ng S. C. , Zhang V. L.
Published: July 2002

The generalized surface, pseudosurface, and high-frequency pseudosurface acoustic waves propagating on (001), (110), and (111) surfaces of GaAs have been examined using Brillouin spectroscopy. The phase velocities of the surface acoustic waves were measured as a function of their propagating directions over the entire angular periods on the respective basal planes. Unlike the single dispersion curve arising from the branching from the generalized surface mode to the pseudosurface mode, commonly observed for anisotropic cubic crystals, two separate and distinct dispersion branches were recorded for these surface modes on the (001) surface. The complete dispersion branch of high-frequency pseudosurface acoustic waves on the (110) surface has been determined.
A radiation transport coupled particle-in-cell simulation. I. Description of the model
Authors: Lee Hae June , Verboncoeur J. P.
Published: July 2002

A one-dimensional radiation transport model is coupled with a particle-in-cell simulation in order to incorporate the resonance trapping effect of photons and the kinetic effect of plasma in fluorescent-lamp-like discharges. Electrons and ions are treated with the conventional particle-in-cell method, and neutral species such as ground, radiative, and metastable state atoms are treated with a fluid model combined with the Holstein–Biberman equation. Also included are the atomic collisions among neutral species and the effect of nonuniform ground state density on photon transport. The general scheme of the model and the numerical methods for Doppler and pressure line broadenings are described in a one-dimensional planar model for the nonuniform ground-state density.
D=4
chiral string compactifications from intersecting branes
Authors: Aldazabal G. , Franco S. , Ibáñez L. E. , Rabadán R. , Uranga A. M.
Published: July 2002

Intersecting Dp-branes often give rise to chiral fermions living on their intersections. We study the construction of four-dimensional chiral gauge theories by considering configurations of type II D(3+n)-branes wrapped on nontrivial n-cycles on T2n×(R2(3−n)/ZN), for n=1, 2, 3. The gauge theories on the four noncompact dimensions of the brane world-volume are generically chiral and nonsupersymmetric. We analyze consistency conditions (RR tadpole cancellation) for these models, and their relation to four-dimensional anomaly cancellation. Cancellation of U(1) gauge anomalies involves a Green–Schwarz mechanism mediated by RR partners of untwisted and/or twisted moduli. This class of models is of potential phenomenological interest, and we construct explicit examples of SU(3)×SU(2)×U(1) three-generation models. The models are nonsupersymmetric, but the string scale may be lowered close to the weak scale so that the standard hierarchy problem is avoided. We also comment on the presence of scalar tachyons and possible ways to avoid the associated instabilities. We discuss the existence of (meta)stable configurations of D-branes on 3-cycles in (T2)3, free of tachyons for certain ranges of the six-torus moduli.
Electrophoretic deposition of pure and MgO-modified Ba0.6Sr0.4TiO3 thick films for tunable microwave devices
Authors: Ngo E. , Joshi P. C. , Cole M. W. , Hubbard C. W.
Published: July 2002

We report on the structural, dielectric, and optical properties of barium strontium titanate–magnesium oxide composite (BST-MgO) thick films fabricated by the electrophoretic deposition technique. The structure and morphology of the films were analyzed by x-ray diffraction and scanning electron microscopy, respectively. The films were optically characterized via FT-Raman spectroscopy. The dielectric measurements were conducted on metal–insulator–metal capacitors using platinum as the top and bottom electrodes. The typical small signal dielectric constants of Ba0.60Sr0.40TiO3 and Ba0.60Sr0.40TiO3–20 wt % MgO thick films, measured at an applied frequency of 1 MHz, were 603 and 327, respectively. The corresponding values of dissipation factor were 0.029 and 0.002, respectively. A high dielectric tunability of 17.3% was obtained for Ba0.60Sr0.40TiO3 thick films at an applied electric field of 20 kV/cm. The dielectric properties of the present thick films were comparable to those of the bulk ceramics; suggesting their suitability for high-frequency wide-band voltage-tunable device applications.
The classification of plane symmetric spacetimes by isometries
Authors: Feroze Tooba , Qadir Asghar , Ziad M.
Published: July 2002

A complete classification of plane symmetric Lorentzian manifolds according to their additional isometries and metrics (or classes of metrics) is obtained by solving the Killing equations. We obtain all metrics (or classes of metrics), that admit the group of motions Gr (where r=3,4,5,6,7 and 10) containing SO(2)⨷R2, the minimal symmetry inherited by the plane symmetric manifolds.
Chemical-pressure tailoring of low-field, room-temperature magnetoresistance in (Ca, Sr, Ba)Fe0.5Mo0.5O3
Authors: Kim Bog-Gi , Hor Yew-San , Cheong S-W.
Published: July 2002

Low-field intergrain magnetoresistance (IMR) in the polycrystalline materials of double perovskite SrFe0.5Mo0.5O3 is found to be highly tunable by doping either Ca or Ba into the Sr site. The dopant exerts a chemical pressure, changing the Curie temperature and the magnetic softness. The room-temperature IMR at optimal doping (Sr0.2Ba0.8Fe0.5Mo0.5O3) is approximately 3.5% in 100 Oe, and increases further in high fields. The unprecedented magnitude of the IMR in this highly spin polarized system provides future grounds for employing magnetic materials for innovative magnetic technology.
Lekner summations
Authors: Mazars Martial
Published: July 2002

The method of evaluating the electrostatic energies in partially periodic systems, proposed by J. Lekner [Physica A 157, 826 (1989)] and extensively developed by Grønbech-Jensen and co-workers [N. Grønbech-Jensen, Int. J. Mod. Phys. C 8, 1287 (1997) and N. Grønbech-Jensen, G. Hummer, and K. M. Beardmore, Mol. Phys. 92, 941 (1997)] is examined in the context of numerical simulations. The main practical problem in its applications is described analytically and illustrated on very simple numerical computations on a system of two charged hard spheres. The main results are a study of the convergence of these summations for the simple system of two charged hard spheres and to warn against possible systematic bias in numerical simulations (Monte Carlo and molecular dynamics) that may be induced by straightforward applications of this method.
Low energy electron scattering in CF2Cl2 and CF3Cl
Authors: Field D. , Jones N. C. , Lunt S. L. , Ziesel J.-P. , Gulley R. J.
Published: July 2002

Experimental data are presented for the scattering of electrons by CF2Cl2 and CF3Cl for both integral and backward scattering, from 2 eV (for CF2Cl2) and 0.65 eV (for CF3Cl) down to energies of 10 meV to 20 meV, with an energy resolution ranging from 0.75 meV to 1.5 meV (full width at half maximum) in the electron beam. Both molecules have dipole moments of ∼0.5 D and are expected to show very similar rotationally inelastic scattering cross sections. Cross sections for CF2Cl2 are, however, much larger at low energy than for CF3Cl, attributed to short-lived attachment of electrons to CF2Cl2. CF3Cl displays powerful suppression of rotational excitation in the forward direction, most strongly around 70 meV impact energy. This is ascribed to interference between a direct channel of rotational excitation and indirect excitation via a short-lived negative ion state.
Fine structure of granular banding in two-phase rimming flow
Authors: Thomas P. J. , Riddell G. D. , Kooner S. , King G. P.
Published: July 2002

Solid–liquid two-phase flow inside a partially filled horizontally rotating cylinder is investigated. We document the observation of a new, secondary banding pattern developing in the flow when the solid phase comes out of suspension to accumulate as regularly spaced, circumferential bands on the inner cylinder wall. This secondary pattern, the fine structure, is superposed on the primary pattern which we previously described [O. A. M. Boote and P. J. Thomas, Phys. Fluids 11, 2020 (1999)]. The fine structure is characterized by each of the primary bands adopting a compound structure consisting of three individual, narrower secondary bands. New results on the influence of the physical properties of the solid phase on transitions between characteristic flow states are briefly discussed. It is reported that state-transition boundaries in the phase plane and the wavelength of the primary instability are insensitive to particle size and shape while there exist influences due to the particle density.
Partitioning behavior and the effect of Co on the Curie temperature of nanocomposite PrFeCoB hard magnetic alloys
Authors: Zhang Y. , Warren P. J. , Cerezo A. , Harland C. L. , Davies H. A.
Published: July 2002

The microstructure and phase chemistry of melt-spun nanocomposite Pr9.7Fe76.6Co7.8B5.9 and Pr9.2Fe69.4Co15.4B6.0 ribbons have been studied using three-dimensional atom probe (3DAP) and transmission electron microscopy. The microstructure of these alloys consists of two phases, bcc α-Fe–Co and tetragonal Pr2(FeCo)14B. Practically all of the B and Pr atoms are rejected from the α-Fe–Co phase and are concentrated into the 2/14/1 hard magnetic phase. However, no significant difference of Co concentration between the two phases is observed. From the measured Co concentration in the 2/14/1 phase, it is explained why the effect of Co content on Curie temperature (Tc) is greater in the nanocomposite alloys than in single phase alloys. Predictions of Tc for the nanocomposite alloys based on the 3DAP composition data show excellent agreement with experimental measurements.
Electrical current distribution across a metal–insulator–metal structure during bistable switching
Authors: Rossel C. , Meijer G. I. , Brémaud D. , Widmer D.
Published: July 2002

Combining scanning electron microscopy and electron-beam-induced current imaging with transport measurements, it is shown that the current flowing across a two-terminal oxide-based capacitor-like structure is preferentially confined in areas localized at defects. As the thin-film device switches between two different resistance states, the distribution and intensity of the current paths, appearing as bright spots, change. This implies that switching and memory effects are mainly determined by the conducting properties along such paths. A model based on the storage and release of charge carriers within the insulator seems adequate to explain the observed memory effect.
Two-body coupled cluster expansions
Authors: Van Voorhis Troy , Head-Gordon Martin
Published: July 2002

We show that the exact ground state wave function for an arbitrary two-body Hamiltonian can be exactly represented by a single reference coupled cluster wave function employing a general two-particle cluster operator. This can be used to construct a set of approximate methods that converge to the exact result and are in some sense complementary to the standard approach of including successively double, triple, quadruple, …, excitations. We present exploratory variational results for the neon atom and the dissociation of N2 to demonstrate the strengths and weaknesses of these generalized coupled cluster approximations.
Photo-stimulated desorption of rare gas atoms adsorbed on Si(100) surfaces modified with oxygen and deuterium
Authors: Watanabe Kazuya , Matsumoto Yoshiyasu
Published: July 2002

Photo-stimulated desorption of rare gas atoms (Xe and Kr) adsorbed on Si(100) surfaces has been investigated in the photon energy range of 1.16 eV to 6.43 eV. Rare gas atoms are photodesorbed from a clean surface and their velocity distribution is well represented by a Maxwellian with the average kinetic energy, 〈EK〉=0.07 eV. When the surface is modified by oxygen or deuterium, the desorption yield increases and the velocity distribution changes dramatically. Upon oxidation with one monolayer oxygen the velocity distribution shows two nonthermal components with 〈EK〉=0.85 and 0.25 eV, while only one component with 〈EK〉=0.17 eV for the mono-deuterated surface. The velocity component with 〈EK〉=0.85 eV has a threshold at hν∼3.5 eV. The origin of the component is understood by assuming the excitation from the ground state of a rare gas adsorbate to an excited state where electron transfer occurs from the adsorbate to the substrate. On the other hand, the other nonthermal desorption components do not show such remarkable wavelength dependence; they appear in the entire range of photon energies used in this study and do not show any significant changes in their velocity distributions. The origins of these components are discussed in terms of substrate-mediated excitation.
Stochastic model of CO oxidation on platinum surfaces and deterministic limit
Authors: Reichert Christian , Starke Jens , Eiswirth Markus
Published: July 2002

We present a general model for the oxidation of CO on low-index platinum single crystal surfaces. In order to take fluctuations into account, it is first formulated in terms of stochastic birth–death processes. A corresponding deterministic limit for large particle numbers can be derived rigorously. The dynamical behavior of the reaction kinetics is investigated by means of a numerical bifurcation analysis of the deterministic limit and stochastic simulations. The computed bifurcation diagrams for Pt(110) and Pt(100) are in qualitative and reasonable quantitative agreement with experimental results. For Pt(110), several types of noise-induced oscillations are obtained.
Photoelectron spectroscopy of GaX2−, Ga2X−, Ga2X2−, and Ga2X3−(X=P,As)
Authors: Taylor Travis R. , Gómez Harry , Asmis Knut R. , Neumark Daniel M.
Published: July 2002

Anion photoelectron spectra taken at various photodetachment wavelengths have been obtained for GaX2−, Ga2X−, Ga2X2−, and Ga2X3− (X=P,As). The incorporation of a liquid nitrogen cooled channel in the ion source resulted in substantial vibrational cooling of the cluster anions, resulting in resolved vibrational progressions in the photoelectron spectra of all species except Ga2X2−. Electron affinities, electronic term values, and vibrational frequencies are reported and compared to electronic structure calculations. In addition, similarities and differences between the phosphorus and arsenic-containing isovalent species are discussed.
Prototypes of bifunctional photochromic and electro-optical systems
Authors: Chidichimo G. , Formoso P. , Manfredi S. , Favaro G. , Mazzucato U. , Romani A
Published: July 2002

Two types of bifunctional films having both electro-optical and photochromic properties have been prepared and investigated. These films are able to modulate the light transmission by means of suitable electric fields and to change their color under light irradiation. One of the films is fluid-like, being an emulsion of a liquid crystal and a photochrome in an organic monomer. It is defined as a photochromic emulsion dispersed liquid crystal (PEDLC). The other film is solid, having the structure of a polymer dispersed liquid crystal and containing a small percentage of photochrome; it is defined as a photochromic polymer dispersed liquid crystal (PPDLC). The solid films were obtained by photopolymerizing the fluid one. The electro-optical and photochromic properties of these films were investigated. To choose suitable photochromic components, the color-forming and color-bleaching kinetics of some compounds of the classes of spiro-oxazines and spiropyrans were preliminarily investigated in the components of the matrix (liquid crystals and monomers). They generally exhibited a better colorability in the matrix, as compared with organic solvents, due to decreasing of the bleaching rate. Their presence did not alter the electro-optical properties of the films with respect to those of PDLC and EDLC systems. Some significant variations in the color-forming and color-bleaching processes, observed for some of the photochromes dispersed in PDLC films, are discussed. Two of them were found to have the prerequisites of colorability, reversibility, and durability to be used in the bifunctional devices. The results are promising in view of the future involvement of these films in technology.
Optimization of Xe adsorption kinetics in single walled carbon nanotubes
Authors: Kuznetsova A. , Yates J. T. , Simonyan V. V. , Johnson J. K. , Huffman C. B. , Smalley R. E.
Published: July 2002

Closed end (10, 10) single walled carbon nanotubes (SWNTs) have been opened by oxidation at their ends and at wall defect sites, using ozone. Oxidation with ozone, followed by heating to 973 K to liberate CO and CO2, causes etching of the nanotube surface at carbon atom vacancy defect sites. The rate of adsorption of Xe has been carefully measured as a function of the degree of nanotube etching by ozone. It is found that a level of etching corresponding to wall openings of about 5–7 Å radius is optimal for maximizing the rate of Xe adsorption. Beyond this level of etching, the rate of Xe adsorption decreases as the surface area of the SWNTs decreases due to further carbon atom removal. Both experiment and modeling show that the presence of polar oxidized groups, such as –COOH or –COR groups, with dipole moments in the range 1.5–3.0 D at the perimeter of the defect sites, causes a retardation of the rate of Xe adsorption due to dipole-induced dipole interactions. This effect is larger for smaller radius defect sites and decreases as the defect sites increase in size beyond about 7 Å radius. At large defect radii, the energetic profile of the adsorption pathway controls the physisorption rate. Modeling shows that after Xe adsorption has been completed inside the nanotubes, then Xe clusters begin to form on the outer surface of the nanotubes at the defect sites where polar groups are present. The Xe clustering effect also occurs to a smaller degree when the defect sites are not decorated by polar groups. The experiments and modeling demonstrate how one may optimize the rate of adsorption of a gas into nanotubes by the adjustment of the size and polar character of the vacancy-site entry ports in the walls of the nanotubes.
Miniature high-resolution thermometer for low-temperature applications
Authors: Welander Paul B. , Hahn Inseob
Published: July 2002

We report on a new miniature, high-resolution, susceptibility thermometer that employs LaxGd1−xCl3 as the paramagnetic material for temperature measurement below 4 K. The thermometer utilizes small permanent disk magnets to provide the required magnetic field, and has a total mass of only 2 g. The thermometer shows a resolution of 0.2 nK in a 1 Hz bandwidth and an absolute drift rate less than 50 fK/s at 2.2 K. In this article, we describe the thermometer design, assembly procedure, and experimental test results, including its thermal time constant and susceptibility to external magnetic fields.
Analog fiber optic transmission link
Authors: McKinney Collin J. , Karwowski Hugon J.
Published: July 2002

An analog fiber optic transmission link (FOTL) has been designed for relaying signals from equipment held at high voltage potentials to equipment at ground potential. The need for a fiber link arises from the requirements of low energy, light ion experiments conducted at the Triangle Universities Nuclear Laboratory. In these experiments, ions are accelerated and transported to a high voltage scattering chamber and the FOTL is used to transmit signals from the scattering chamber to remote data acquisition systems. The performance of the link is mainly driven by the stringent requirements needed for successful transmission of analog pulses from radiation detectors located in the scattering chamber. The FOTL consists of a transmitter, receiver, and baseline restorer that are capable of transmitting radiation detector pulses without pulse height distortion, thus allowing high resolution energy measurements. The FOTL is also suitable for use with a wide variety of other analog signal types.
Fusion fuel ion temperature diagnostic for directly driven implosions
Authors: Chen J. B. , Zheng Z. J. , Peng H. S. , Zhang B. H. , Ding Y. K. , Chen M. , Chen H. S. , Wen T. S.
Published: July 2002

An ultrafast quenched plastic scintillation detector was used to measure the fusion fuel ion temperature at low implosion (DT) neutron yield (5×108–3×109) in the initial experiment performed at the Shenguang II laser facility. The typical temperatures of exploding pusher targets for direct drive were around 4 keV and the uncertainties were ±15–23%. The detection efficiency of the detector to DT neutrons was calibrated at a K-400 accelerator. The time response function of the detection system was calibrated by implosion neutrons from a DT-filled capsule, which can be regarded as a δ function pulsed neutron source due to its much narrower pulse width than that of the measured neutron time-of-flight spectrum.
Direct comparisons of rates for low temperature diffusion of hydrogen and deuterium on Cu(001) from quantum mechanical calculations and scanning tunneling microscopy experiments
Authors: Kua Jeremy , Lauhon Lincoln J. , Ho Wilson , Goddard William A.
Published: July 2002

Recent experiments by Lauhon and Ho using scanning tunneling microscopy (STM) observed the direct hopping of H and D on Cu(001) as a function of temperature. They found nearly temperature independent tunneling for H below 60 K, but could not detect the tunneling threshold for D (it is at least 1000 times lower than for H). The availability of such direct and accurate measurements provides the opportunity for validating the level of theory required to predict the diffusion of adsorbates on surfaces. Thus, we carried out density functional theory (DFT) using the generalized gradient approximation (GGA-II) on periodic slabs. The calculated tunneling rate of 4.74×10−4 s−1 for H is in close agreement with the experimental value of 4.4×10−4 s−1. We predict 4.66×10−9 s−1 for the tunneling rate of D (one hop every 83 months!). Between 60 and 80 K, the calculated thermally activated diffusion rate of H is 1012.88 exp(−0.181 eV/kT) s−1 in close agreement with the STM value: 1012.9±0.3 exp(−0.197 eV/kT). For deuterium, between 50 and 80 K, the calculated rate is 1012.70 exp(−0.175 eV/kT) s−1 in close agreement with the STM value: 1012.7±0.2 exp(−0.194 eV/kT) s−1. These results validate that such first principles theory can be used to predict the diffusion (including tunneling) for adsorbates on surfaces, providing important data needed to unravel surface processes in catalysis and crystal growth.
Four-dimensional lattice gauge theory with ribbon categories and the Crane–Yetter state sum
Authors: Pfeiffer Hendryk
Published: July 2002

Lattice Gauge Theory in four-dimensional Euclidean space–time is generalized to ribbon categories which replace the category of representations of the gauge group. This provides a framework in which the gauge group becomes a quantum group while space–time is still given by the “classical” lattice. At the technical level, this construction generalizes the spin foam model dual to lattice gauge theory and defines the partition function for a given triangulation of a closed and oriented piecewise-linear four-manifold. This definition encompasses both the standard formulation of d=4 pure Yang–Mills theory on a lattice and the Crane–Yetter invariant of four-manifolds. The construction also implies that certain classes of spin foam models formulated using ribbon categories are well-defined even if they do not correspond to a topological quantum field theory.
Silicon deposition from disilane on Si(100)-2×1: Microscopic model including adsorption
Authors: Ferguson B. A. , Reeves C. T. , Safarik D. J. , Mullins C. B.
Published: July 2002

We present a model for the calculation of homoepitaxial film growth rates during silicon deposition on Si(100)-2×1 from disilane. Central to this model is the use of thermalized gaseous disilane adsorption probabilities that have been determined as a function of gas and surface temperature by convoluting supersonic molecular beam adsorption probability data with a Maxwell–Boltzmann distribution of incident kinetic energies and angles. These calculations show that the primary adsorption pathway over the entire range of conditions investigated is the so-called trapping-mediated mechanism, in which dissociative chemisorption occurs via a physisorbed intermediate. A second adsorption mechanism, direct chemisorption, is activated by translational energy and does in fact contribute somewhat to adsorption, but only at high gas and surface temperatures. Hydrogen coverages and silicon film growth rates are calculated from a simple surface decomposition kinetic model together with a phenomenological thermal desorption model and compare favorably to experimental measurements. Under conditions of high flux or low surface temperature, the growth rate is limited by hydrogen desorption and therefore increases with increasing surface temperature. In the flux-limited or adsorption-limited growth regime, the growth rate is predicted to decrease with increasing surface temperature due to a drop in the adsorption probability, resulting in a maximum in the growth rate for a given set of deposition conditions.
Nonadiabatic donor–acceptor electron transfer mediated by a molecular bridge: A unified theoretical description of the superexchange and hopping mechanism
Authors: Petrov E. G. , Shevchenko Ye. V. , Teslenko V. I. , May V.
Published: July 2002

Nonadiabatic bridge-assisted electron transfer (ET) is described by a set of kinetic equations which simultaneously account for the sequential (hopping) as well as the superexchange mechanism. The analysis is based on the introduction of a certain reduced density operator describing a particular set of electron-vibrational levels of the molecular units (sites) involved in the transfer act. For the limiting case of intrasite relaxations proceeding fast compared to intersite transitions a set of rate equations is obtained. This set describes the time evolution of the electronic site populations and is valid for bridges with an arbitrary number of units. If the rate constants for the transition from the bridge to the donor as well as to the acceptor exceed those for the reverse transitions the ET reduces to a single-exponential process with an effective forward and backward transfer rate. These effective rates contain a contribution from the sequential and a contribution from the superexchange mechanisms. A detailed analysis of both mechanisms is given showing their temperature dependence, their dependence on the number of bridge units, and the influence of the energy gap and the driving force. It is demonstrated that for integral bridge populations less than 10−3 the complicated bridge-mediated ET reduces to a donor–acceptor ET with an effective overall transfer rate. This transfer rate contains contributions from the sequential as well as the superexchange mechanisms, and thus can be used for a quantitative analysis of the efficiency of different electron pathways. For room-temperature conditions and even at a very small bridge population of 10−4–10−10 the superexchange mechanism is superimposed by the sequential one if the number of bridge units exceeds 4 or 5.
Superconducting properties of nanocrystalline MgB2 thin films made by an in situ annealing process
Authors: Zeng X. H. , Sukiasyan A. , Xi X. X. , Hu Y. F. , Wertz E. , Li Qi , Tian W. , Sun H. P. , Pan X. Q. , Lettieri J. , Schlom D. G. , Brubaker C. O. , Liu Zi-Kui , Li Qiang
Published: July 2002

We have studied the structural and superconducting properties of MgB2 thin films made by pulsed-laser deposition followed by in situ annealing. The cross-sectional transmission electron microscopy reveals a nanocrystalline mixture of textured MgO and MgB2 with very small grain sizes. A zero-resistance transition temperature (Tc0) of 34 K and a zero-field critical current density (Jc) of 1.3×106 A/cm2 were obtained. The irreversibility field was ∼8 T at low temperatures, although severe pinning instability was observed. The result is a step towards making the in situ deposition process a viable technique for MgB2 Josephson junction technologies.
Intramolecular isotope effects in the reactions of CF32+ and CO22+ with HD
Authors: Tafadar Nurun , Kearney Dominic , Price Stephen D.
Published: July 2002

Intramolecular isotope effects in the bond forming reactions following collisions of both CO22+ and CF32+ with HD have been investigated experimentally. For the CO22++HD system the bond-forming pathway forming XCO+ (X=H, D) exhibits a strong intramolecular isotope effect favoring the formation of DCO+ at low collision energies. For the CF32++HD system the bond-forming pathway forming XCF2+ also exhibits a strong intramolecular isotope effect favoring the formation of DCF2+ at low collision energies. However, in the CF32++HD system a weak, and previously unobserved, channel, forming XF+ exhibits no intramolecular isotope effect over the collision energy regime (0.2–0.5 eV) investigated. The absence of an intramolecular isotope effect in the formation of XF+ casts doubt on the previous explanation of such isotope effects as resulting from orientation effects in the approach of the dication to the HD molecule. Using a recently proposed mechanism for the reaction of CO22+ with H2, an analysis of the statistical and zero-point factors affecting the competition between the bond-forming channels is presented. This analysis shows that such factors can readily explain the intramolecular isotope effects observed in these reactive systems.
Design of impurity band-based photonic crystal waveguides and delay lines for ultrashort optical pulses
Authors: Lan Sheng , Nishikawa Satoshi , Ishikawa Hiroshi , Wada Osamu
Published: July 2002

We investigate the transmission of ultrashort pulses through impurity band-based photonic crystal waveguides. It is found that in the general case the transmission behavior depends strongly on the pulse width with respect to the resonance linewidth in impurity bands. By controlling the configuration of the waveguides, quasiflat impurity bands can be obtained in which the dependence of transmission on pulse width is very weak. As long as the pulse width is much narrower than the bandwidth, pulses can transmit through the quasiflat impurity bands with negligible distortion and attenuation. The conditions necessary for achieving quasiflat impurity bands are derived by examining waveguides of different configurations and properties. The mechanism responsible for the formation of quasiflat impurity bands is revealed from the discussion of the symmetry of single defect and their coupling.
Evidence of blocking effect on carrier trapping process by necking region in very narrow AlGaAs/GaAs V-grooved quantum wire structure
Authors: Liu X. Q. , Sasaki A. , Ohno N. , Li Z. F. , Lu W. , Shen S. C. , Fu Y. , Willander M. , Tan H. H. , Jagadish C.
Published: July 2002

Transient band-gap renormalization (BGR) effects are investigated in AlGaAs/GaAs V-grooved quantum structures. The temperature-dependent transient BGR effects in the sidewall quantum well (SQWL) provide direct evidence of the existence of the blocking effect by the necking region barrier on the carrier trapping process. These effects provide a useful method to show the existence of the necking region, particularly for very thin SQWL structures. The temperature-dependent lifetimes of the SQWL and quantum wire (QWR) provide further proof of the carrier trapping process from the SQWL to the QWR.
Covariant geometric quantization of nonrelativistic time-dependent mechanics
Authors: Giachetta G. , Mangiarotti L. , Sardanashvily G.
Published: July 2002

We provide geometric quantization of the vertical cotangent bundle V*Q→Q→R, equipped with the canonical Poisson structure and treated as a momentum phase space of nonrelativistic time-dependent mechanics. We show that this quantization is equivalent to fiberwise quantization of symplectic fibers of V*Q→R and that the quantum algebra of time-dependent mechanics is an instantwise algebra. Quantization of the classical evolution equation defines a connection on this instantwise algebra and describes quantum evolution in time-dependent mechanics as a parallel transport.
Effects of In and Ga interdiffusion on the optical gain of InGaN/GaN quantum well
Authors: Chen Chii-Chang , Hsueh Tao-Hung , Ting Yi-Sheng , Chi Gou-Chung , Chang Chin-An
Published: July 2002

In this study, we analyze the effects of thermal annealing by calculating the optical gain in the InGaN/GaN quantum well. The interdiffusion of Ga and In atoms across the interface of the well and the barrier resulting from thermal treatments is described by Fick’s law. The strong piezoelectric effect due to lattice mismatch in the InGaN/GaN quantum well is also considered in the calculation. The results confirm that the thermal annealing can induce an increase of the optical gain. However, an excessive annealing might result in decreasing the optical gain in the InGaN/GaN quantum well. The maximum optical gain can be obtained at a diffusion length of 4Å of In and Ga atoms. There is a good agreement between the experimental data in literature and the optimized diffusion length studied in this work.
Instability of hydrogenated amorphous carbon films towards defect creation at high disorder
Authors: Lejeune M. , Bouzerar R. , Benlahsen M. , Durand-Drouhin O. , Zeinert A.
Published: July 2002

We report a complete characterization of hydrogenated amorphous carbon (a-C:H) thin films through Raman spectroscopy, photothermal deflection spectroscopy, and mechanical studies. The interpretation of the optical absorption data within the Dasgupta’s model offers an estimate of the disorder intensity through a characteristic energy scale of the model. The vibrational data (Raman spectra) and the stress data together with the optical absorption data give strong evidence for a sharp transition between a low disorder state dominated by aromatic rings and a strong disorder state likely in keeping with the creation of some topological defects, such as odd rings.
Effect of temperature, pressure and volume on long time relaxation dynamics in fragile glass-forming liquid
Authors: Paluch Marian
Published: July 2002

Relaxation dynamics of low molecular glass-forming liquid triphenylomethane triglycidyl ether was studied under isothermal and isobaric conditions using dielectric spectroscopy. In addition we also carried out pressure-volume-temperature, PVT, measurements. The data enable us to examine the relaxation rate as functions of temperature, pressure and volume. Among other observed properties, it was found that temperature dependence of the α-relaxation times in vicinity of glass transition exhibit non-Arrhenius behavior even at constant volume conditions. Our findings confirm the thesis postulated by M. L. Ferrer and co-workers in J. Chem. Phys. 109, 8010 (1998) that temperature is a dominant control variable, which is responsible for the spectacular non-Arrhenius increase of relaxation rate near Tg. The pressure dependence of the glass transition temperature has also been determined both from dielectric and volumetric data. Satisfactory agreement between results obtained from the two methods has been found.
Electrical conductivity of highly concentrated electrolytes near the critical consolute point: A study of tetra-n-butylammonium picrate in alcohols of moderate dielectric constant
Authors: Oleinikova A. , Bonetti M.
Published: July 2002

The electrical conductivity of highly concentrated solutions of tetra-n-butylammonium picrate (TBAP) in 1-dodecanol (dielectric constant ε=4.6) and 1,4-butanediol (ε=25.9), and in mixtures of both alcohols, is measured in an extended temperature range ≈10−5<τ<≈10−1, where τ=(T−Tc)/Tc is the reduced temperature with Tc, the critical temperature. The electrical conductivity Λ(T) obeys the Vogel–Fulcher–Tammann (VFT) law for the temperatures far from the critical one. In the temperature range τ<10−2 a systematic deviation of the electrical conductivity from the regular VFT behavior is observed. This deviation is attributed to a critical anomaly. At the critical point the amplitude of the critical anomaly is finite with a value which varies between ≈0.4 and ≈2.7% of Λ(Tc), depending on the solvent. The (1−α) critical exponent describes well the conductivity anomaly, α being the exponent of the specific heat anomaly at constant pressure. The value of the Walden product (Λeqvη), with Λeqv, the equivalent conductivity and η, the shear viscosity, allows the degree of dissociation αdiss of TBAP to be determined at the critical point. αdiss becomes larger for increasing values of ε: for TBAP in 1-dodecanol αdiss≈0.25 and in 1,4-butanediol αdiss≈0.73. When the degree of dissociation of the salt is accounted for the Debye screening length is found almost independent on ε.
Subpicosecond carrier dynamics in low-temperature grown GaAs as measured by time-resolved terahertz spectroscopy
Authors: Beard Matthew C. , Turner Gordon M. , Schmuttenmaer Charles A.
Published: July 2002

The transient photoconductivity in a 1 μm layer of low temperature grown GaAs (LT-GaAs) on a GaAs substrate was measured using time-resolved terahertz spectroscopy. When photoexcitation occurs at 400 nm we find a time-dependent mobility that increases from 400±100 to 1100±100 cm2 V−1 s−1 with a time constant of 2 ps. Photoexcitation at 800 nm produces a time-independent mobility of 3000±500 cm2 V−1 s−1. We determine the carrier lifetime in LT-GaAs to be 1.1 ± 0.1 ps.
The noncommutative harmonic oscillator in more than one dimension
Authors: Hatzinikitas Agapitos , Smyrnakis Ioannis
Published: July 2002

The noncommutative harmonic oscillator, with noncommutativity not only in position space but also in phase space, in arbitrary dimension is examined. It is shown that the ⋆-genvalue problem, which replaces the Schrödinger problem in this case, can be decomposed into separate harmonic oscillator equations for each dimension. The two-dimensional noncommutative harmonic oscillator (four noncommutative phase-space dimensions) is investigated in greater detail. The requirement of the existence of rotationally symmetric solutions leads to a two parameter harmonic oscillator which is completely solved in this case. The angular momentum operator is derived and its ⋆-genvalue problem is shown to be equivalent to the usual eigenvalue problem of the ⋆-genfunction related wave function. The ⋆-genvalues of the angular momentum are found to depend on the energy difference of the oscillations in the two dimensions. Furthermore two examples of a symmetric noncommutative harmonic oscillators are analyzed. The first is the noncommutative two-dimensional Landau problem with harmonic oscillator potential, which shows degeneracy in the energy levels for certain critical values of the noncommutativity parameters, and the second is the three-dimensional harmonic oscillator with noncommuting coordinates and momenta.
Proposed experiment to assess operation of quantum cellular automaton cells
Authors: Macucci M. , Gattobigio M. , Iannaccone G.
Published: July 2002

We propose an experiment for the detection of quantum cellular automaton (QCA) operation in a cell made up of four silicon quantum dots. We show that correlated switching between the two pairs of dots forming the cell can be clearly detected from the locking of the Coulomb blockade current peaks through each pair. We have performed numerical simulations on the basis of capacitance values obtained from experimental results on cells fabricated with silicon-on-insulator technology. This approach is shown to allow detection of QCA switching without the need for additional charge detectors, although fabrication parameters are rather critical.
Energy bottlenecks and the rotational dynamics of a water-like fluid
Authors: Evans Glenn T.
Published: July 2002

Water is modeled as a spherical top with a hard, convex tetrahedral surface. In its collisions, water transfers linear and angular momentum according to the usual two-body kinematics; however, rotational energy transfer is diminished from the hard-body value since portions of the surface of water are blocked due to pre-existing hydrogen bonds. Owing to the bottleneck in transfer of rotational energy, convective effects normally important for water at subcritical densities are conjectured to be significant at densities where water forms hydrogen bond networks. The present theory is compared to experimental and computer simulation studies of orientational and angular momentum relaxation times, and the agreement or lack thereof is analyzed.
Geometric amplitude, adiabatic invariants, quantization, and strong stability of Hamiltonian systems
Authors: Bliokh K. Yu.
Published: July 2002

Considered is a linear set of ordinary differential equations with a matrix depending on a set of adiabatically varying parameters. Asymptotic solutions have been constructed. As has been shown, an important characteristic determining the qualitative portrait of the system is the real part of Berry’s complex geometric phase, which we call the geometric amplitude. For systems with purely imaginary eigenvalues the equivalence has been proven in the adiabatic approximation of system sets without geometric amplitude, Hamiltonian systems, quantizable systems, and strongly stable systems. Classification of the systems without geometric amplitude is given with respect to the kind of matrix of the initial system.
Deactivation effects of the lowest excited states of Er3+ and Ho3+ introduced by Nd3+ ions in LiYF4 crystals
Authors: Jagosich F. H. , Gomes L. , Tarelho L. V. G. , Courrol L. C. , Ranieri I. M.
Published: July 2002

The deactivation effects of the lowest excited states of Er3+ and Ho3+ introduced in Er-or Ho-LiYF4 (YLF) crystals, codoped with Nd ions, were observed by the activator’s fluorescence decays. In the case of Er:Nd:YLF, the I13/24→4I15/2 and I11/24→4I13/2 transitions at 1.5 and 2.7 μm, respectively, were analyzed. The I75→5I8 and I65→5I7 transitions at 2.1 and 2.9 μm, respectively, were investigated for the Ho:Nd:YLF system. Laser excitations generated by a tunable optical parametric oscillator were used in this investigation. The use of a resonant laser excitation to induce the fluorescence allowed accurate measurements of the donor fluorescence decay by a time-resolved infrared spectroscopic system with a time resolution of 0.5 μs. As a result, a general criterion for the migration mechanism, involved in the donor to acceptor energy transfer, was proposed and depends on a parameter R. This parameter was defined as the ratio between the transfer rate obtained from the best fit of the fluorescence decay and the theoretical transfer rate predicted by the diffusion model. It was observed that the donor to acceptor transfer is always dominated by a diffusion migration (R∼1) if the donor is in the second excited state, despite a great variation of the CDD/CDA ratio (from ∼1 to 371). Nevertheless, a discrete energy migration was found to dominate in the [Ho, Er]→Nd energy transfer when the first excited state of the activator is involved. In this case, the experimental value of the transfer rate is smaller than expected according to the hopping model. Introducing a finite trapping efficiency of an exciton migration in the hopping model, all the observed experimental results were explained. The presence of Nd ions, in addition to decreasing the lifetime of the first excited state of Er3+ and Ho3+ in YLF, also depopulates the second excited state (partially), depending on the Nd concentration used.
Spin-dependent transport in the double-perovskite Sr2CrWO6
Authors: Philipp J. B. , Reisinger D. , Schonecke M. , Marx A. , Erb A. , Alff L. , Gross R. , Klein J.
Published: July 2002

We report on the preparation and characterization of the double-perovskite compound Sr2CrWO6 with a Curie temperature of 390 K. We have fabricated both Sr2CrWO6 bulk sintered polycrystalline bars and high-quality epitaxial thin films on SrTiO3 substrates by pulsed-laser deposition. The samples were characterized by thermogravimetric analysis, x-ray diffraction, electrical transport, and magnetization measurements. Polycrystalline samples containing a large number of grain boundaries show a large low-field magnetoresistance of up to 100% at 5 K. At room temperature, this effect is reduced to a few percent. Our results show that Sr2CrWO6 is an interesting candidate for room-temperature magnetoelectronic materials.
A rigidity theorem for nonvacuum initial data
Authors: Etesi Gábor
Published: July 2002

In this note we prove a theorem on nonvacuum initial data for general relativity. The result presents a “rigidity phenomenon” for the extrinsic curvature, caused by the nonpositive scalar curvature. More precisely, we claim that in the case of an asymptotically flat nonvacuum initial data if the spatial metric has everywhere nonpositive scalar curvature, then the extrinsic curvature cannot be compactly supported.
Modeling the acceleration field and objective lens for an aberration corrected photoemission electron microscope
Authors: Feng J. , Padmore H. , Wei D. H. , Anders S. , Wu Y. , Scholl A. , Robin D.
Published: July 2002

The modeling of the optical properties of the acceleration field and objective lens of a photoemission electron microscope (PEEM) is presented. Theory to calculate the aberrations of the extraction field was derived, and extended to include relativistic effects. An analysis of the microscope’s electron optical performance and aberrations has been performed using an analytical model as well as a ray tracing method. Ray tracing has the flexibility needed for the assessment of aberrations where the geometry is too complex for analytical methods. This work shows that in the case of a simple PEEM front end of the acceleration gap and objective lens, the all orders ray tracing and full analytical treatments agree to very high precision. This allows us now to use the ray tracing method in situations where analytical methods are difficult, such as an aberration compensating electron mirror.