bluuebunny/crossref_metadata_2025 huggingface dataset:
Long ropes of boron nitride nanotubes grown by a continuous laser heating
Authors: Laude Thomas , Matsui Yoshio , Marraud Alain , Jouffrey Bernard
Published: July 2002

A continuous CO2 laser beam was focused on a surface of hexagonal boron nitride under low nitrogen pressure, inducing a stiff radial temperature gradient, rising locally over the dissociation temperature. Boron nitride nanotubes and faceted boron nitride onions were found on the heated surface, segregated in specific zones. Nanotubes were grown in macroscopic quantity, stoichiometric, well crystallized, mostly with few layers, and self-assembled in ropes as long as 40 μm. The cavity of boron nitride onions was often occupied by a boron nanocrystal. The recombination of boron and nitrogen both in plasma and on surface is suggested for the nucleation process.
Er–carrier interaction and its effects on the Er3+ luminescence of erbium-doped Si/SiO2 superlattices
Authors: Shin Jung H. , Jhe Ji-Hong , Seo Se-Young , Ha Yong Ho , Moon Dae Won
Published: July 2002

The Er–carrier interaction and its effects on the Er3+ photoluminescence properties of erbium-doped Si/SiO2 superlattices are investigated. The interaction between the erbium atoms and the electronic carriers was controlled by doping erbium into the SiO2 layers only and by depositing buffer layers of pure SiO2 between the erbium-doped SiO2 layers and the Si layers. We demonstrate that by controlling the erbium-carrier interaction, a three orders of the magnitude enhancement of the Er3+ luminescence intensity and a nearly complete suppression of the temperature-induced quenching of Er3+ luminescence can be achieved while still allowing the Er3+ ions to be excited by the carriers. We identify the asymmetry between the dominant carrier-mediated excitation and the de-excitation paths of Er3+ ions as the possible cause for the observed effects.
Current injection emission from a transparent p–n junction composed of p-SrCu2O2/n-ZnO
Authors: Ohta Hiromichi , Kawamura Ken-ichi , Orita Masahiro , Hirano Masahiro , Sarukura Nobuhiko , Hosono Hideo
Published: July 2002

An ultraviolet light-emitting diode (LED) operating at room temperature was realized using a p–n heterojunction composed of transparent conductive oxides, p-SrCu2O2 and n-ZnO. Multilayered films prepared by a pulsed-laser deposition technique were processed by conventional photolithography with the aid of reactive ion etching to fabricate the LED device. A rather sharp emission band centered at 382 nm was generated when a forward bias voltage exceeding the turn-on voltage of 3 V was applied to the junction. The emission may be attributed to a transition associated with the electron–hole plasma of ZnO.
Geometric modular action, wedge duality, and Lorentz covariance are equivalent for generalized free fields
Authors: Gaier Johanna , Yngvason Jakob
Published: July 2002

The Tomita–Takesaki modular groups and conjugations for the observable algebras of spacelike wedges and the vacuum state are computed for translationally covariant, but possibly not Lorentz covariant, generalized free quantum fields in arbitrary space–time dimension d. It is shown that for d⩾4 the condition of geometric modular action (CGMA) of Buchholz, Dreyer, Florig, and Summers [Rev. Math. Phys. (2000)], Lorentz covariance and wedge duality are all equivalent in these models. The same holds for d=3 if there is a mass gap. For massless fields in d=3, and for d=2 and arbitrary mass, CGMA does not imply Lorentz covariance of the field itself, but only of the maximal local net generated by the field.
Applications of a variational coupled-electron pair approach to the calculation of intermolecular interaction in the framework of the VB theory: Study of the van der Waals complex He–CH4
Authors: Specchio Roberto , Famulari Antonino , Martinazzo Rocco , Raimondi Mario
Published: July 2002

A general nonorthogonal coupled electron pair approach for the evaluation of electron correlation contribution is presented in details. The self-consistent field for molecular interactions wave function is used as reference state for a multistructure valence bond (VB) calculation. The central idea of the method is the optimization of the virtual space of the VB wave function by means of a procedure very close to the independent electron pair approach (IEPA) scheme. All the orbitals employed are expanded in the basis set of their fragment so as to exclude the basis set superposition error (BSSE) in a priori fashion. As an example, the application to the study of the van der Waals complex He–CH4 is reported. The equilibrium geometry of the system occurs at a He–C distance of 3.6 Å , with the He atom pointing to the center of one of the faces of the CH4 molecule, with a well depth of 19 cm−1. The potential energy surface of the He–CH4 complex is used to determine the parameters of a potential model which is employed in close-coupling calculations of integral state-to-state cross sections for rotationally inelastic scattering of methane molecules with helium atoms. The predicted values are compared with the available experimental data.
Nonlinear Schrödinger-type equations from multiscale reduction of PDEs. I. Systematic derivation
Authors: Calogero F. , Degasperis A. , Xiaoda Ji
Published: July 2002

In this article we begin a systematic investigation via multiscale expansions of nonlinear evolution PDEs (partial differential equations). In this first article we restrict consideration to a single, autonomous, but otherwise generic, PDE in 1+1 variables (space+time), of first order in time, whose linear part is dispersive, and to solutions dominated by a single plane wave satisfying the linear part of the PDE. The expansion parameter is an, assumedly small, coefficient multiplying this plane wave. The main (indeed, asymptotically exact) effect of the (weak) nonlinearity is then to cause a modulation of the amplitude of the plane wave and of its harmonics, which is generally described, in (appropriately defined) coarse-grained time and space variables, by evolution equations of nonlinear Schrödinger type. A systematic analysis of such equations is presented, corresponding to various assumptions on the “resonances” occurring for the first few harmonics.
Invariant subspaces of the periodic Navier–Stokes and magnetohydrodynamics equations: Symmetries and inverse cascades
Authors: Núñez Manuel
Published: July 2002

It is shown that when the initial condition and the forcing term of the periodic Navier–Stokes or magnetohydrodynamics equations have Fourier coefficients which vanish outside a certain semigroup of frequencies, the same happens to the solutions for all time. Subgroups of frequencies correspond to solutions possessing certain symmetries. By taking as a semigroup the frequencies whose Fourier components are non-negative integers, we get a class of solutions for which the higher modes do not influence the evolution of the lower ones; therefore, the phenomenon of inverse cascading cannot occur for them.
A spinor reformulation of the null surface treatment of GR
Authors: Frittelli Simonetta , Mason Lionel , Newman Ezra T.
Published: July 2002

We begin with a four-dimensional manifold, M, that possesses a two parameter family of (local) foliations by three-surfaces, with the two parameters being the coordinates on the sphere of directions at each point of the manifold expressed via homogeneous coordinates πA and πA′. By then requiring that each foliation (of the two parameter set of foliations) be a one parameter family of null surfaces for some (as yet unkown) conformal Lorentzian metric—we derive an explicit expression, in terms of the foliation description, for this conformal metric. We then show (1) how a conformal factor can be chosen to convert the conformal metric into a metric and (2) how to impose on the foliation and conformal factor conditions so that the metric satisfies the vacuum Einstein equations. The material described here is very much connected to the null surface formulation (NSF) of GR developed earlier. The advantages of the present formulation are that one can much more easily see the logical structure of the NSF, one can calculate with much greater ease and finally it allows [because of the use of (spinor) index calculus] generalizations of the NSF so that the study of the evolutionary development of the null surface singularities (caustics, etc.) can be developed.
A high-bandwidth electrical waveform generator based on an aperture-coupled stripline
Authors: Skeldon M. D.
Published: July 2002

An electrical waveform generator based on an aperture-coupled stripline is described. The device is a four-port electrical directional coupler consisting of two striplines coupled through an aperture in their common ground plane. Replacing the aperture layer of the device can produce arbitrarily shaped electrical waveforms with 100 ps structure over a pulse envelope of several nanoseconds. A numerical solution of the telegraph equations using the method of characteristics is used to model and design devices. Excellent agreement is obtained between the model and device performance.
Application of Thomson scattering to identify ion acoustic waves stimulated by the Langmuir decay instability
Authors: Depierreux S. , Labaune C. , Fuchs J. , Baldis H. A.
Published: July 2002

In this article, we present how ion acoustic waves can be unambiguously identified and can be used to definitely demonstrate the existence of the Langmuir decay instability (LDI) of electron plasma waves associated with stimulated Raman scattering. This observation is very important in the context of inertial confinement fusion because the LDI is a mechanism that may influence the stimulated Raman scattering saturation and growth. The experimental identification of those waves relies on Thomson scattering of a short wavelength probe beam. The Thomson scattered light was analyzed by two sets of spectrometer and streak camera providing information on the density fluctuations associated with the electron plasma waves and ion acoustic waves with temporal and spectral resolutions. Selection of a narrow range of angles (4° of aperture) inside the probe and scattered light apertures provided necessary resolution in wave vector. The method used for the design of the Thomson scattering diagnostic is described as well as the technique used to clearly identify the observed ion acoustic waves as one of the Langmuir decay instability products.
Pulsed amplification of cw dye laser with undetectable amplified spontaneous emission
Authors: Ni Chi-Kung , Kung A. H.
Published: July 2002

We have constructed a cw dye master oscillator/dye pulse-amplifier system that generates 4 ns, 100 mJ pulses with a repetition rate of 30 Hz. The output pulse has a bandwidth of 275 MHz. Backward-stimulated Brillouin scattering is used to control the growth of amplified spontaneous emission (ASE). The content of ASE in the final output is under our detection limit (<10−4) for the entire tuning range.
Atomic ordering and magnetic properties in Nd57Fe20B8Co5Al10 solids
Authors: Xing L. Q. , Eckert J. , Löser W. , Roth S. , Schultz L.
Published: July 2002

Nd 57 Fe 20 B 8 Co 5 Al 10 bulk cylinders and ribbons were quenched from the melt at different cooling rates using copper mold casting and melt spinning, respectively. Both the melt-spun ribbons and the cast cylinders display x-ray diffraction patterns without obvious crystalline peaks. However, the ribbons show soft magnetic properties, the cylinders are hard magnetic, and the crystallized alloys are not ferromagnetic at all. On the other hand, the cylinders show different exothermic transformation behavior than the ribbons upon heating to elevated temperatures. While the ribbons are regarded as amorphous, the structure of the cylinders prepared at slower cooling rate is considered to consist of metastable ordered clusters. This structure forms in the undercooled melt most likely due to intensive generation of ordered nuclei and slow growth kinetics. The magnetic coherence length, which is equivalent to the dimension of the ordered clusters, may be larger than the exchange length. Therefore, the cylinders shows much larger coercivity than the ribbons.
Static zero field splitting effects on the electronic relaxation of paramagnetic metal ion complexes in solution
Authors: Rast S. , Fries P. H. , Belorizky E.
Published: July 2002

A general theory of the electronic relaxation of an S state complexed paramagnetic metal ion (Mn2+, Gd3+) in solution is developed. Contrarily to the usual assumption, the electronic relaxation is not only due to the effects of the transient zero field splitting, but is strongly influenced by the static crystal field effect which is modulated by the random Brownian rotation of the complex. The electron paramagnetic resonance measured linewidths of three Gd3+ (S=7/2) complexes at various temperatures and fields [D. H. Powell et al. J. Am. Chem. Soc. 118, 9333 (1996)] are well interpreted in the framework of this model and show the contributions of both mechanisms. The fitted crystal field parameters, the correlation times, and the activation energies are in good agreement with their expected values from the underlying processes. Moreover, our interpretation does not require the addition of any field independent contribution to the linewidth like the spin-rotation mechanism. The longitudinal relaxation function is well approximated using a single relaxation time, whereas the transverse relaxation function is a superposition of four decreasing exponentials.
Direct calculation of the one-electron density matrix for closed-shell systems
Authors: Matsuoka Osamu , Matsufuji Takaharu , Sano Tatsuji
Published: July 2002

It has been found that independent parameters in the variation of a one-electron density matrix (DM) for closed-shell systems are elements of its unitary transformed matrix and, in a special case, reduce to the rotation parameters that connect the occupied and virtual orbital spaces in the exponential transformed self-consistent field method. To obtain the unitary matrix of transformation, a simpler method of orthogonalizing the column vectors of the DM has been proposed instead of its diagonalization. An iterative method has been formulated to determine these independent parameters. Several test calculations using this method reproduced the results using the Hartree–Fock–Roothaan method.
Coupling of kinetic and transport phenomena in self-organization of C–B–N nanotube growth into sandwich structures
Authors: Louchev Oleg A. , Sato Yoichiro , Kanda Hisao , Bando Yoshio
Published: July 2002

A mechanism for self-organization of C–B–N sandwich-like nanotubes with well-separated phases of C and BN [K. Suenaga, C. Colliex, N. Demoncy, A. Loiseau, H. Pascard, F. Willaime, Science 278, 653 (1997)] in arc-discharge is proposed. This mechanism includes (i) the microkinetic self-organization of nanotube growth in step-flow mode combined with a C and BN phase segregation at growth steps and (ii) the self-organization of “macro”-oscillations of impinging fluxes of C and B on the surface of nanotubes, which are caused by the nonuniformity of C and B content in the gas combined with the onset of natural convection, providing repeated transport of emerging nanotubes through regions with abundance in C and B, and leading to nucleation and growth of C and BN layers.
Entropy, correlations, and ordering in two dimensions
Authors: Saija F. , Prestipino S. , Giaquinta P. V.
Published: July 2002

The ordering of simple fluids in two dimensions was investigated using the residual multiparticle entropy (RMPE) as a measure of the relevance of correlations involving more than two particles in the configurational entropy of the system. To this end, we performed Monte Carlo simulations of two prototype systems, i.e., Lennard-Jones particles and hard discs. Consistent with previous studies, we found that, on approaching the freezing transition, the RMPE of the fluid undergoes a change from negative to positive values. However, in two dimensions the vanishing of the RMPE appears to be more directly related to the formation of six-fold orientationally ordered patches, a process which foreshadows the freezing transition. The specificity of the structural condition attained by the fluid in a state corresponding to a vanishing RMPE was further corroborated by an analysis of the shape of the radial distribution function (RDF): in fact, it turns out that the spatial profiles of the RDF of the Lennard-Jones fluid along a zero-RMPE locus can be superimposed at medium and large distances notwithstanding the difference of density and/or temperature of the corresponding thermodynamic states. The same long-range profile of the RDF is shared also by hard discs in the cited condition. Such a “scaling” property also holds in three dimensions where it provides a suggestive nexus between the ordering criterion based on the vanishing of the RMPE and the Hansen–Verlet freezing rule.
Phase behavior of polymer containing colloidal dispersions: The integral equation theory
Authors: Shusharina Nadezhda P. , Khalatur Pavel G. , Khokhlov Alexei R.
Published: July 2002

We present an integral equation theory based on the polymer reference interaction site model (PRISM) formalism to describe a phase behavior of polymer containing colloidal dispersion. We focus on the thermodynamic stability of dispersed component, i.e., on the conditions of percolation and phase separation in this subsystem. Different phase diagrams for the colloid subsystem are constructed in the variables: volume fraction of the colloid component–temperature. We analyze the behavior of the spinodal line and the percolation threshold. It was shown that the spinodal lines do not exhibit a critical point. The additivity of a polymer induced and a direct attraction between the colloidal particles is examined.
Resonance-state selective photodissociation of OCS (2 1Σ+): Rotational and vibrational distributions of CO fragments
Authors: Itakura Ryuji , Hishikawa Akiyoshi , Yamanouchi Kaoru
Published: July 2002

The rotational and vibrational state distributions of the CO fragments produced through the photodissociation of OCS in the vacuum ultraviolet (VUV) region (150–155 nm), OCS (2 1Σ+)→CO (X 1Σ+)+S(1S), are derived for the three lowest quasi-bound vibrational resonances (v*=0−2) in the 2 1Σ+ state. The rotational state distributions of the CO fragments in the vCO=0 and 1 vibrational states are determined, respectively, by the analysis of the rotational structures in the laser-induced fluorescence (LIF) spectra of the A1Π–X 1Σ+(0,0) and (1,1) transitions of CO. The rotational temperatures of CO in the vCO=0 state are low (∼100 K) for all the three resonances, while those in the vCO=1 state are substantially higher, i.e., 2210, 940, and 810 K for v*=0, 1, and 2, respectively. The vibrational state distributions of CO are derived from the Doppler spectroscopy of the counterpart S(1S) fragments. From the analysis of the observed Doppler profiles, it is found for all the three lowest vibrational resonances of OCS that the vibrational distributions are represented well by the Boltzmann-type distribution with a vibrational temperature of around 7000 K. On the basis of these new findings, the energy partitioning in the photodissociation process through these three vibrational resonances in the 2 1Σ+ state is discussed.
Distinguishing between interface dipoles and band bending at metal/tris-(8-hydroxyquinoline) aluminum interfaces
Authors: Hill I. G. , Mäkinen A. J. , Kafafi Z. H.
Published: July 2002

Using a combination of photoelectron spectroscopies and ellipsometry to study the thin-film growth of Alq3 (tris-(8-hydroxyquinoline) aluminum) on Ag, we have been able to distinguish between molecular orbital energy shifts due to “band bending” and interface dipole formation. We have observed large binding energy shifts (≈0.5 eV) of the organic molecular levels at low coverages. We conclude that these shifts are consistent with the formation of a polarized molecular layer, or interface dipole, and are inconsistent with an electrostatic band-bending model.
Spreading of liquids on gel surfaces
Authors: Szabó D. , Akiyoshi S. , Matsunaga T. , Gong J. P. , Osada Y. , Zrinyi M.
Published: July 2002

In this work we present a simple model for the kinetics of spreading of film-forming liquids on polymer gels. The model is compared with experiments and fair agreement is obtained. The spreading process can be theorized by considering the gel as a thick liquid layer containing a fibrous material (i.e., the polymer network). The spreading theory of Joos and Pintens is extended in such a way that the penetration of the flow into the bulk of the gel—which plays the major role in the kinetics of the spreading—is described by the Debye–Brinkman equation. It is also shown that spreading experiments can provide information on the surface structure of the underlying gel.
Structure and stability of Na6Pb clusters
Authors: Han Young-Kyu , Hirao Kimihiko
Published: July 2002

The geometric and electronic structures of the bimetallic cluster, Na6Pb, have been calculated using the density functional and coupled-cluster approaches, with a relativistic effective core potential and one-electron spin–orbit operators. Six possible isomeric Na6Pb structures (Oh, D3h, D3d, C5v, C3v, and C2v) have been examined, and the highly symmetrical Oh structure was found to be the most stable form. The stabilities of the clusters correlate well with the Pb–Na bond distances and with the charge transfer from the Na6 subsystem to the more electronegative Pb atom. The binding energies were found to vary according to the structure, with a spread of about 0.1 eV and 0.3 eV for the density functional and coupled-cluster calculations, respectively. These are of the same order, and somewhat smaller, as are found for the Na6Mg clusters (about 0.5 eV), demonstrating that the stabilization of the Na6Pb cluster is not particularly dependent on the geometric structures. At the same time, we found that the Na6Pb cluster is more stable as the symmetry is higher.
Linear predictive filtering in a numerically simulated turbulent flow
Authors: Amonlirdviman Keith , Breuer Kenneth S.
Published: July 2002

A numerical investigation is made into the use of linear predictive (Wiener) filters to predict flow quantities in the near-wall region of a turbulent boundary layer for use in active control algorithms. Optimal filters for the prediction of Reynolds stress and fluctuating streamwise velocity components using wall-shear stress information are developed and their dependence on the number and location of shear sensors is explored. It is found that a densely populated, wall-based, sensor system can predict the near-wall Reynolds stress with good accuracy, and that 76% of the optimal performance can be achieved with as few as four sensors whose locations coincide with the strongest weights of the Wiener filter derived using a very dense network of input sensors.
Microwave spectra, density functional theory calculations and molecular structure of acetylenemethyldioxorhenium
Authors: Kukolich S. G. , Drouin B. J. , Indris O. , Dannemiller J. J. , Zoller J. P. , Herrmann W. A.
Published: July 2002

The molecular structure for acetylenemethyldioxorhenium, (C2H2)(CH3)ReO2, was obtained by measuring and analyzing the rotational spectra for 14 isotopomers. This appears to be the first gas-phase measurement for a rhenium metallacyclopropene, and the first structural data on this compound. This complex is closely related to reaction intermediates in methyltrioxorhenium and osmium tetroxide catalyzed oxidation reactions, which are important in industrial chemical production and syntheses of chiral products. The microwave spectra were measured in the 4–11 GHz range using a Flygare–Balle-type pulsed-beam microwave spectrometer. The structural parameters obtained using the 42 measured rotational constants are in very good agreement with results from the new density functional theory calculations. The structure of the acetylene ligand is modified through interaction with the metal atom, and exhibits partial sp2 hybridization in the complex. The C–C bond length is increased by 0.09 Å to 1.29 Å. The H–C–C interbond angles are reduced from 180° to 146°, and 147°. The measured rhenium, methyl–carbon bond length is 2.116(2) Å, and rhenium–acetylene carbon bond lengths are 2.043(2) Å and 2.067(2) Å. Large quadrupole splitting patterns arising from the two rhenium isotopes [χcc(185 Re)=812.62 MHz, and χcc(187 Re)=769.023 MHz], with large off-diagonal components [|χab(185 Re)|=725.40 MHz, and (|χab(187 Re)|=686.384 MHz] caused some initial difficulty in assigning transitions. When the assignments and analysis were completed, the systematic changes in the measured quadrupole coupling tensor components due to rotations of the principal inertial axes on isotopic substitution were obtained, and helped to confirm assignments for the various isotopomers.
Explicit core-hole localization and relaxation effects in the calculation of inner-shell spectrum of C2H4
Authors: Rocha Alexandre B. , Bielschowsky Carlos E.
Published: July 2002

Theoretical results for K-shell spectrum (C 1s) of ethylene are presented. Core-hole localization and relaxation effects are explicitly considered by means of a three structures generalized multistructural (GMS) wave function. Values for the generalized (GOS) and optical (OOS) oscillator strengths for the low-lying C 1s states are reported.
Prediction of a weakly bound excited state in the He24–7Li molecule
Authors: Delfino A. , Frederico T. , Tomio Lauro
Published: July 2002

A scale-independent approach, valid for weakly bound three-body systems, is used to analyze the existence of excited Thomas–Efimov states in molecular systems with three atoms: a helium dimer together with isotopes of lithium (Li6 and Li7) and sodium (Na23). With the present study and the available data, we can clearly predict that the He24−Li7 system supports an excited state with binding energy close to 2.31 mK.
Theoretical investigation of the initial reaction of the NO decomposition on the Si (100) (2×1) reconstructed surface
Authors: Korkin Anatoli A. , Demkov Alexander A. , Tanpipat Noppawan , Andzelm Jan
Published: July 2002

We have investigated theoretically the initial reaction of nitric oxide (NO) with the Si(001)(2×1) surface, followed by N and O insertion into the silicon film during the initial growth of the oxynitride film. We use quantum chemical [ab initio and density functional theory (DFT) cluster approach] and solid state physics (DFT with periodic boundary conditions) computational methods. Our study suggests a low barrier reaction path for NO decomposition on the Si(100)(2×1) reconstructed silicon surface.
Symmetry group methods for heat kernels
Authors: Craddock M. J. , Dooley A. H.
Published: July 2002

We apply methods of symmetry groups to heat equations on ↛ with drift terms and to heat equations for Lie groups. In particular, we are able to characterize those functions f for which equations of the form (∂2u/∂x2)+f(x)ux=ut have a point symmetry which takes a constant solution to the fundamental solution. Further, we apply symmetry methods to the heat equation of the ax+b group.
A set of standard enthalpies of formation for benchmarking, calibration, and parametrization of electronic structure methods
Authors: Cioslowski Jerzy , Schimeczek Michael , Liu Guang , Stoyanov Vesselin
Published: July 2002

A comprehensive set of 600 experimental standard enthalpies of formation (ΔHf0) is presented. With its diverse species, many possessing less usual geometries and bonding situations, this compilation is capable of uncovering deficiencies in approaches of quantum chemistry that are not detectable with smaller sets of ΔHf0 values. Its usefulness in benchmarking, calibration, and parametrization of new electronic structure methods is illustrated with the development of the B3LYP/6-311++G** bond density functional scheme. This scheme, which is sufficiently inexpensive in terms of computer time and memory to allow predictions even for molecules as large as the C60 fullerene, requires only single point calculations at optimized geometries. It yields values of ΔHf0 with the average absolute error of 3.3 kcal/mol, rivaling more expensive methods in accuracy (especially for larger systems). A list of species that are poorly handled by a typical hybrid density functional used in conjunction with a moderate-size basis set is given. This list is intended for rigorous testing of new density functionals.
Ordering dependence of carrier lifetimes and ordered states of Ga0.52In0.48P/GaAs with degree of order ⩽0.55
Authors: Sasaki A. , Tsuchida K. , Narukawa Y. , Kawakami Y. , Fujita Sg. , Hsu Y. , Stringfellow G. B.
Published: July 2002

Photoluminescence (PL) properties and carrier lifetimes of spontaneously ordered Ga0.52In0.48P/GaAs are measured at low temperatures. Samples with values of degree of order S from 0.22 to 0.55 are used. The full width at half maximum of the PL increases from 9.36 to 30.6 meV with increasing S. The PL peak of the indirect transition is blueshifted with increasing excitation power, whereas the PL peak of the direct transition is not. The energy separation between the PL peaks for the direct and the indirect transitions becomes greater with increasing S. The carrier lifetime increases dramatically with increasing S, from 0.68 ns at S=0.22 to 13 800 ns at S=0.55 at the peak energy of the PL spectrum. The ordering dependence of these characteristics is interpreted using a simple physical model of the ordered state. Variations in the ordered state with increasing ordering are discussed.
Fluctuations in the number of irreversibly adsorbed particles
Authors: Adamczyk Zbigniew , Szyk-Warszyńska Lilianna , Siwek B. , Weroński P.
Published: July 2002

Fluctuations in the number of colloid particles adsorbed irreversibly under pure diffusion transport conditions were determined as a function of surface density and ionic strength of the suspension. The experiments were carried out for monodisperse polystyrene latex particles of micrometer size range adsorbing irreversibly at mica surface. The surface density of adsorbed particles at various areas was determined using the direct microscope observation method. A new experimental cell was used enabling in situ observations of particles adsorption under conditions of negligible gravity effects. It was found that the particle density fluctuations for high ionic strength were in a good agreement with the theoretical results derived from the random sequential adsorption (RSA) model. Also, the theoretical results stemming from the equilibrium scaled particle theory reflected the experimental data satisfactorily. For lower ionic strength a deviation from the hard sphere behavior was experimentally demonstrated. This effect due to the repulsive electrostatic interactions was interpreted in terms of the effective hard particle concept. The universal dependence of variance on particle density obtained in this way was found in a good agreement with the RSA model for all ionic strength. These results proved that fluctuations in particle density of monolayer formed under diffusional conditions differ fundamentally from these obtained under ballistic transport conditions.
Hyperfunction quantum field theory: Analytic structure, modular aspects, and local observable algebras
Authors: Nagamachi S. , Brüning E.
Published: July 2002

This paper addresses the following problem of relativistic quantum field theory: Given a relativistic quantum field, construct a net of local observable algebras over space–time with “natural” properties. A few years ago we started a project which suggests to look at this problem in the framework of relativistic quantum field theory in terms of Fourier hyperfunctions. Accordingly we present the relevant analyticity results, some modular aspects of our theory for the Bisognano–Wichman argument, and a concrete suggestion for the definition of the local observable algebras. Finally, we construct a class of models of hyperfunction quantum fields for which the algebras of bounded operators assigned to nonempty regions in space–time are not trivial. It is remarkable that our models are not tempered quantum fields and do not admit “test functions” of compact support.
Influence of particle adsorption probability on the stoichiometry of thin films grown by pulsed laser deposition
Authors: Itina T. E.
Published: July 2002

An expansion of a bicomponent laser plume into a dilute ambient gas is simulated using a combined direct simulation-random trajectory Monte Carlo method. The stoichiometry of thin films deposited from laser-desorbed material on a flat substrate is examined. In the case of energy-dependent particle adsorption probability, the dependencies of deposition rate on the background pressure are shown to be nonmonotonic with maximums at low gas pressure. In addition, an increase in the ratio of light to heavy species was obtained at low pressure. We demonstrate that these results can be attributed to the interplay between the effects of collisions with the background gas on the fluxes of particles arriving at the substrate and on the adsorption probability of the species. The calculation results are consistent with recent experiments. The study is of interest for the optimization of the experimental conditions during pulsed laser deposition of multicomponent materials.
Strain state in GaN epilayers from optical experiments
Authors: Deleporte E. , Guénaud C. , Voos M. , Beaumont B. , Gibart P.
Published: July 2002

The intrinsic optical properties of GaN thin epilayers grown on a sapphire substrate and by Mg-enhanced lateral overgrowth (ELO) are studied as a function of temperature (from 20 up to 250 K) by means of photoluminescence excitation spectroscopy. The comparison between the experimental and calculated results allows us to determine the strain state of the GaN layers. At low temperature (T=20 K) the strain is stronger in the ELO epilayers than in the GaN layers grown on sapphire, but, contrary to the GaN layers grown on the sapphire substrate, the strain state remains constant in the overgrown GaN layers when temperature varies.
Exactly calculable field components of electric dipoles in planar boundary
Authors: Margetis Dionisios , Wu Tai Tsun
Published: July 2002

The Sommerfeld integrals for the electromagnetic fields in the planar boundary between air and a homogeneous, isotropic medium, due to a horizontal and a vertical electric dipole each lying along the interface, are examined in detail. In the case of the horizontal dipole, the tangential electric field is given in terms of series that involve confluent hypergeometric functions, namely, the Fresnel and exponential integrals. A similar exposition is presented for the magnetic and vertical electric fields of the vertical dipole. When the index of refraction of the adjacent space is of a sufficiently large magnitude, the derived series converge rapidly and uniformly with the distance from the source. Specifically, their rates of convergence are shown to be independent of distance. It is pointed out that the corresponding formulas of King et al. are valid down to any distance close to the source, where they smoothly connect to known “quasi-static” approximations.
Transient electron transport in indium-doped semiinsulating GaAs
Authors: Kažukauskas V. , Storasta J. , Vaitkus J.-V.
Published: July 2002

We investigated the effect of In-doping up to a concentration of 2×1020 cm−3 on transient transport phenomena in semiinsulating liquid-encapsulated-Czochralski grown GaAs. The changes in time after a strong laser excitation of the photoconductivity and nonequilibrium Hall mobility were analyzed. We did not find extra energy levels caused by indium. Nevertheless, In-doping caused significant changes in the behavior of the nonequilibrium mobility in the temperature range of 300–420 K, which were not observed in other crystals, undoped or doped by other dopants. They could not be explained merely by the reduction of dislocation density caused by In. Besides, the In-doping was demonstrated to cause the rearrangement of defect inhomogeneities. It is considered that lattice defects become distributed more homogeneously and appear more probably as short-range inhomogeneities instead of accumulations around dislocations. This diminishes the role of percolation phenomena and intensifies the effect of smaller defects on carrier transport.
Interface morphology of a Cr(001)/Fe(001) superlattice determined by scanning tunneling microscopy and x-ray diffraction: A comparison
Authors: Schmidt C. M. , Bürgler D. E. , Schaller D. M. , Meisinger F. , Güntherodt H.-J. , Temst K.
Published: July 2002

A Cr(001)/Fe(001) superlattice with ten bilayers grown by molecular beam epitaxy on a Ag(001) substrate is studied by in situ scanning tunneling microscopy (STM) and ex situ x-ray diffraction (XRD). Layer-resolved roughness parameters determined from STM images taken in various stages of the superlattice fabrication are compared with average values reported in the literature or obtained from the fits of our XRD data. Good agreement is found for the rms roughnesses describing vertical roughness and for the lateral correlation lengths characterizing correlated as well as uncorrelated interface roughness if peculiarities of STM and XRD are taken into account. We discuss in detail (i) the possible differences between the STM topography of a free surface and the morphology of a subsequently formed interface, (ii) contributions due to chemical intermixing at the interfaces, (iii) the comparison of XRD parameters averaged over all interfaces versus layer-resolved STM parameters, and (iv) the question of the coherent field of view for the determination of rms values.
Effects of Si-doping in the barriers on the recombination dynamics in In0.15Ga0.85N/In0.015Ga0.985N quantum wells
Authors: Ryu Mee-Yi , Yu Phil Won , Shin Eun-joo , Lee Joo In , Yu Sung Kyu , Oh Eunsoon , Nam Ok Hyun , Sone Chul Soo , Park Yong Jo
Published: July 2002

A systematic study of time-resolved photoluminescence spectra from In0.15Ga0.85N/In0.015Ga0.985N quantum wells (QWs) with different Si-doping concentration in the barriers has been carried out. The 10 K recombination lifetimes in the QWs depend strongly on the Si-doping in the barriers, decreasing from ∼80 to ∼20 ns as the Si-doping level increases from 2×1018 to 1×1019 cm−3. The separation between the spontaneous and stimulated emission (SE) peaks and a shift of the emission peak to longer wavelengths with delay time after a pulsed excitation decrease with increasing Si doping. The increased recombination rate, the decrease of peak shift with delay time, and the reduced separation between the spontaneous and SE peaks with increasing Si doping are attributed to the screening of piezoelectric field by carriers originated from Si-doped barriers.
Piezoelectric-field effect on electronic and optical properties of [111] InxGa1−xAs/GaAs superlattices
Authors: Kim B. W.
Published: July 2002

We theoretically analyze effects of the piezoelectric field and its screening on the electronic and optical properties including optical matrix element and spontaneous emission for InxGa1−xAs/GaAs superlattices (SLs). For the investigation we take [111] In0.15Ga0.85As/GaAs SL with three types of structures according to well/barrier widths of 40 Å/40 Å, 100 Å/100 Å, and 160 Å/160 Å. The carrier densities are assumed to be equivalent to 5×1016, 1×1017, 5×1017, 1×1018, and 5×1018 cm−3 over 40 Å width in the wells. In the numerical computations, we use the computation model presented recently by [B. W. Kim, J. H. Yoo, and S. H. Kim, ETRI Journal 21, 65 (1999)]. The model self-consistently solves 8×8 (conduction, heavy, light, and spin split-off valence bands) effective-mass Schrödinger equation and the Hartree and exchange-correlation potential equations through the variational procedure. The results show that piezoelectric field causes significant changes in the electronic and optical properties; however, the screening of piezoelectric field by the carriers in the wells seems not very effective for device application.
Visible and band edge electroluminescence from indium tin oxide/SiO2/Si metal–oxide–semiconductor structures
Authors: Chen Miin-Jang , Lin Ching-Fuh , Liu W. T. , Chang S. T. , Liu C. W.
Published: July 2002

Characteristics of electroluminescence from indium tin oxide (ITO)/SiO2/Si metal–oxide–semiconductor (MOS) structures fabricated on both p-type and n-type Si wafers were investigated. The ITO/SiO2/Si MOS on p-type Si could have both the visible and band edge electroluminescence, while the ITO/SiO2/Si MOS on n-type Si has only band edge emission. The reason for the difference is attributed to the impact ionization that only occurs for ITO/SiO2/Si(p) MOS. The study indicates that the band edge emission and visible luminescence are competing processes. The electroluminescence from ITO/SiO2/Si(n) is also discovered to be less than that from the Al/SiO2/Si(n). The reason is possibly due to the damage of the oxide bonding and the SiO2/Si interface during the ITO sputtering.
Growth of epitaxial {111} Ni0.82Fe0.18O and the exchange anisotropy of Ni0.82Fe0.18O/Ni80Fe20
Authors: Lai Chih-Huang , Lien Wei-Chih , White Robert L.
Published: July 2002

Epitaxial {111} Ni0.82Fe0.18O films (NiFeO) were prepared by using solid-source metal-organic chemical vapor deposition on {0001} α-Al2O3. The exchange anisotropy between NiFeO and Ni80Fe20 (NiFe) was investigated. Structural and chemical analyses showed that NiFeO films were compositionally homogeneous. The 60-degree rotational twins formed in the NiFeO films. The epitaxial relationship between Ni0.82Fe0.18O and Al2O3 is the following: [111] NiFeO‖[0001]α-Al2O3, [11̄0]NiFeO‖[11̄00]α-Al2O3, and [112̄]NiFeO‖[112̄0]α-Al2O3. The orientation relationship between NiFeO and NiFe is cube-to-cube. Misfit dislocations pile up at the interfaces significantly reduce the stress, and result in semi-coherent interfaces. In-plane tension and out-of-plane compression were developed in NiFeO films due to the thermal stress, which may force the magnetic moment of NiFeO to lie in the {111} planes parallel to the film surface, and may lead to a strong interfacial coupling between NiFeO and NiFe. The relative small exchange field possibly results from (1) a low anisotropy energy compared to the interfacial coupling energy in the NiFeO {111} plane, (2) an orientation distribution of the easy axes in the NiFeO {111} plane, and (3) the large domain size of NiFeO. The Malozemoff model was used to estimate the exchange field in this system. Based on the assumption that the domain size of the NiFeO is approximately equal to the twin size, the calculated exchange field agrees well with the experimental results.
High-resolution electron microscopy investigations on stacking faults in SrBi2Ta2O9 ferroelectric thin films
Authors: Zhu X. H. , Li A. D. , Wu D. , Zhu T. , Liu Z. G. , Ming N. B.
Published: July 2002

Structural planar defects in the SrBi2Ta2O9 (SBT) films with 10 mol % excess Bi grown in Pt/TiO2/SiO2/Si substrates by metalorganic deposition have been observed by high-resolution electron microscopy. It was found that these stacking defects were planar defects with extra Bi–O planes normal to the c axis. These structural defects are expected to effectively improve the ferroelectric response and fatigue-resistance characteristics of SBT films because of the extra Bi–O planes having higher strucutral flexibility and alleviating the mechanical stresses and strains as well as injected-charge problems.
Excimer laser destruction of micrometer size particles in a gas volume
Authors: Hopp B. , Smausz T. , Rácz A. , Bor Zs.
Published: July 2002

ArF excimer laser ablation of micrometer size smoke grains was investigated. It was demonstrated that this procedure is suitable for destruction of these particulates in consequence of very fast heating, oxidation, burning, and evaporation. The time-resolved investigations revealed that the destruction process takes place between 0 and 7 ns after the maximum of the excimer pulse. The efficiency of breaking into small pieces was found to be dependent on the excimer laser fluence. Atomic force microscopic investigations of the sedimented smoke particles proved that the size of the grains significantly decreased by the excimer laser radiation.
Fabrication and lateral electronic transport measurements of gold nanowires
Authors: Ramsperger U. , Uchihashi T. , Nejoh H.
Published: July 2002

A technique for fabrication of gold nanowires on a Si(111) surface in ultrahigh vacuum and their electronic transport properties are presented. Gold wires with widths as small as 4 nm are produced by using a gold-coated piezoresistive cantilever in atomic force microscope contact mode. This technique allows patterns to be written at will. In situ electronic transport measurements of a gold wire as long as 7 μm and 4 nm wide show unambiguous metallic behavior. This fabrication method could become pivotal within the next generation of nanoscale microprocessors.
Second order nonlinear optical properties of In-doped lithium niobate
Authors: Xue D. , Betzler K. , Hesse H.
Published: July 2002

Second order nonlinear optical properties of In-doped lithium niobate crystals have been quantitatively studied from the chemical bond viewpoint. The results show that the second order nonlinear optical response of In-doped lithium niobate crystals at 1079 nm decreases remarkably with increasing In concentration in the crystal. This approximately linear composition-property correlation in In-doped lithium niobate crystals is quantitatively expressed in the current work. Furthermore, the different influences of Mg, Zn, and In dopants, respectively, on the nonlinear susceptibility of lithium niobate single crystals are also compared in the present work.
Quantum dynamics using pseudo-particle trajectories: A new approach based on the multiconfiguration time-dependent Hartree method
Authors: Worth G. A.
Published: July 2002

Quantum molecular-dynamics simulations are an important way of gaining information on the molecular level about chemical systems. In this paper, a new method for the approximate solution of the time-dependent Schrödinger equation is presented. This is a reformulation of the multiconfiguration time-dependent Hartree (MCTDH) wave packet propagation method, which is transformed so that the evolution of the wave function can be represented by pseudo-particle trajectories. In this way, the poor scaling of computational resources with system size attending all exact solutions of the time-dependent Schrödinger equation are circumvented. The equations of motion for the trajectories and the wave function expansion coefficients (importance of each trajectory for the representation) are derived using a variational principle. Other than the MCTDH ansatz, no major approximations have been introduced, and the method converges on the numerically exact solution. Importantly, the trajectories are not classical trajectories, and are coupled by nonlocal effects. A strategy for the practical solution of the equations of motion is then detailed.
An ab initio study of some noble gas monohalides
Authors: Hoffman Gerald J. , Colletto Mitchell
Published: July 2002

Results from high-level ab initio calculations on NeF, ArF, KrF, XeF, and XeCl are reported and compared to experiment and to previous calculations. CCSD(T) results for NeF and ArF ground state potentials show agreement with experimental potentials to within the probable uncertainty of the measurement. In addition to CCSD(T), multireference averaged coupled-pair functional calculations are performed on KrF, XeF, and XeCl as well as calculation of spin–orbit coupling of the noble gas atom. Very good agreement with experiment is observed for XeF using this technique, while poor agreement is observed for KrF; this casts some doubt on the experimental potential for KrF. Results for XeCl show semiquantitative agreement with experiment. Finally, the potentials for the charge-transfer states of XeF, XeCl, and KrF and their spectroscopic constants are presented. Improved agreement over previous calculations is observed with some experimental measurements of these constants, for XeCl in particular.