bluuebunny/crossref_metadata_2025 huggingface dataset:
Hydrogen bonding and collective proton modes in clusters and periodic layers of squaric acid: A density functional study
Authors: Rovira Carme , Novoa Juan J. , Ballone Pietro
Published: July 2002

Hydrogen bonding in clusters and extended layers of squaric acid molecules has been investigated by density functional computations. Equilibrium geometries, harmonic vibrational frequencies, and energy barriers for proton transfer along hydrogen bonds have been determined using the Car–Parrinello method. The results provide crucial parameters for a first principles modeling of the potential energy surface, and highlight the role of collective modes in the low-energy proton dynamics. The importance of quantum effects in condensed squaric acid systems has been investigated, and shown to be negligible for the lowest-energy collective proton modes. This information provides a quantitative basis for improved atomistic models of the order–disorder and displacive transitions undergone by squaric acid crystals as a function of temperature and pressure.
Substrate effects on the fractal kinetics of a simple surface reaction
Authors: Moiny F. , Dumont Martine
Published: July 2002

The reversible monomer–monomer model occurring without diffusion on irregular substrates (probabilistic supports and Sierpinski carpets) is investigated by means of Monte Carlo simulations. The strong influence of the catalyst surface structure on the kinetics of the reaction and its fractal like features such as noninteger reaction rate orders is investigated for steady-state regimes controlled by the reaction. We succeed in separating the dependence of the reaction rate orders on adsorbate cluster parameters from that on bare surface geometry specifics. The inactivity ratio ρ appears to be the relevant parameter to characterize the bare surface structure effects on the Langmuir–Hinshelwood reaction kinetics.
Microstructures of FePt–Al–O and FePt–Ag nanogranular thin films and their magnetic properties
Authors: Ping D. H. , Ohnuma M. , Hono K. , Watanabe M. , Iwasa T. , Masumoto T.
Published: July 2002

The microstructure and magnetic property relationship in L10 ordered FePt–Al–O and FePt–Ag granular thin films have been studied. As-sputtered FePt–Al–O films, composed of isolated fine spherical FePt particles of ∼2 nm diameter with a disordered face-centered-cubic (fcc) structure, exhibit superparamagnetism. Annealing above 650 °C induces a transformation from a disordered fcc structure to ordered L10 phase and the films become magnetically hard. The microstructures of these films change greatly depending on the film compositions and annealing conditions, which are correlated with the magnetic properties. It was found that FePt particles smaller than 5 nm do not order at 500 °C, while the continuous FePt film orders perfectly at the same temperature, suggesting that the ordering temperature, Tc, decreases significantly when the particle size becomes less than 5 nm. In the FePt–Ag granular thin film, when the Ag composition is around 50 at. %, high coercivity (∼10 kOe) and fine uniform microstructure are obtained by annealing the films at 550 °C for 1 h. The changes in the magnetic properties of these granular films are discussed based on microstructural observation results.
Relativistic bounce-averaged quasilinear diffusion equation for low-frequency electromagnetic fluctuations
Authors: Brizard Alain J. , Chan Anthony A.
Published: July 2002

A relativistic bounce-averaged quasilinear diffusion equation is derived to describe stochastic particle transport associated with low-frequency electromagnetic fluctuations in a nonuniform magnetized plasma. Expressions for the relativistic quasilinear diffusion coefficients are calculated explicitly for magnetically-trapped particle distributions in axisymmetric magnetic geometry in terms of drift-bounce resonant contributions associated with low-frequency fluctuations which conserve the first adiabatic invariant.
Weakly primitive vectors of Kac-modules of the Lie superalgebras sl(m/n)
Authors: Su Yucai
Published: July 2002

It was conjectured by Hughes et al. [J. Math. Phys. 33, 470–491 (1992)] that there exists a bijection between the composition factors of a Kac-module and the so-called permissible codes. In a previous paper it was proved that to any unlinked code, there corresponds a composition factor of the Kac-module. Here it is proved that to any linked code, there corresponds a composition factor of the Kac-module.
Vibronic interactions and superconductivity in acene anions and cations
Authors: Kato Takashi , Yamabe Tokio
Published: July 2002

Vibronic interaction and its role in the occurrence of superconductivity in the monoanions and cations of benzene and acenes are studied. The vibrational frequencies and the vibronic coupling constants for benzene and acenes are computed and analyzed and electron—phonon coupling constants in the monoanions and cations are evaluated. The C–C stretching E2g mode of 1656 cm−1 affords large electron—phonon coupling constants in the monoanion and cation of benzene. The C–C stretching Ag modes of 1400–1600 cm−1 and the lowest frequency Ag mode play an important role in the electron–phonon coupling in the monoanions of acenes, while the only C–C stretching Ag modes of 1400–1600 cm−1 afford large electron–phonon coupling constants in the monocations of acenes. We estimate possible superconducting transition temperatures Tcs for the monoanions and cations of benzene and acenes. The predicted Tcs for the monocations of benzene and acenes are much lower than those for the monoanions. Possible high-temperature superconductivity in the monoanion and cation of benzene is suggested.
Asymptotics for the condensate multivortex solutions in the self-dual Chern–Simons CP(1) model
Authors: Nam Hee-Seok
Published: July 2002

In this paper we study the asymptotics for the condensate multivortex solutions in the self-dual Chern–Simons CP(1) model. When the breaking parameter s belongs to (−12,12), we show that for any sequence of multivortex solutions which lies between suitable super- and subsolutions with respect to the Chern–Simons coupling constant κ, we can find a subsequence which converges to a constant depending only on s as κ goes to zero. Also we investigate the locally uniform convergence speed.
Democratic supersymmetry
Authors: Devchand Chandrashekar , Nuyts Jean
Published: July 2002

We present generalizations of N-extended supersymmetry algebras in four dimensions, using Lorentz covariance and invariance under permutation of the N supercharges as selection criteria.
Residual donors and compensation in metalorganic chemical vapor deposition as-grown n-GaN
Authors: Xu Xiaoliang , Liu Hongtu , Shi Chaoshu , Zhao Youwen , Fung S. , Beling C. D.
Published: July 2002

In our recent report, [Xu et al., Appl. Phys. Lett. 76, 152 (2000)], profile distributions of five elements in the GaN/sapphire system have been obtained using secondary ion-mass spectroscopy. The results suggested that a thin degenerate n+ layer at the interface is the main source of the n-type conductivity for the whole film. The further studies in this article show that this n+ conductivity is not only from the contribution of nitride-site oxygen (ON), but also from the gallium-site silicon (SiGa) donors, with activation energies 2 meV (for ON) and 42 meV (for SiGa), respectively. On the other hand, Al incorporated on the Ga sublattice reduces the concentration of compensating Ga-vacancy acceptors. The two-donor two-layer conduction, including Hall carrier concentration and mobility, has been modeled by separating the GaN film into a thin interface layer and a main bulk layer of the GaN film. The bulk layer conductivity is to be found mainly from a near-surface thin layer and is temperature dependent. SiGa and ON should also be shallow donors and VGa–O or VGa–Al should be compensation sites in the bulk layer. The best fits for the Hall mobility and the Hall concentration in the bulk layer were obtained by taking the acceptor concentration NA=1.8×1017 cm−3, the second donor concentration ND2=1.0×1018 cm−3, and the compensation ratio C=NA/ND1=0.6, which is consistent with Rode’s theory. Saturation of carriers and the low value of carrier mobility at low temperature can also be well explained.
Magnetic properties of Fe–Rh alloys
Authors: Paduani C.
Published: July 2002

We performed first-principles calculations on molecular clusters with the discrete variational method, in the spin-polarized case, to study the magnetic properties of Fe–Rh alloys. In ordered FeRh, our results showed a nearly constant local moment for both iron and rhodium atoms, in the ferromagnetic phase, for a wide range of lattice spacings. By shrinking the lattice, one observes the collapse of the magnetic moment for Fe and Rh atoms, in both ferromagnetic and antiferromagnetic phases. Our results showed a strong dependence of the hyperfine magnetic field at the iron sites on the atomic configurations in their neighborhood. A loss of magnetic coupling is expected by adding Rh atoms in the immediate vicinity of the iron sites.
Influence of the distribution function shape and the band structure on impact ionization modeling
Authors: Grasser T. , Kosina H. , Selberherr S.
Published: July 2002

Accurate modeling of impact ionization is a critical issue for deep submicron devices. All established analytical models for the distribution function based on the local electric field or on the local average carrier energy give rather poor results. Therefore, theoretically sound microscopic scattering rates cannot be properly transformed into accurate macroscopic models. We show that by accounting for the average square energy an accurate analytical description of the distribution function can be given. Together with a proper band structure model the analytical distribution function can then be used to evaluate microscopic models in a macroscopic device simulator. The present model is accurate for bulk and sub 100 nm feature size devices and involves only local quantities, which makes it a good choice for inclusion in conventional device simulators.
2–5 pyrochlore relaxor ferroelectric Cd2Nb2O7 and its Fe2+/Fe3+ modifications
Authors: Kolpakova N. N. , Syrnikov P. P. , Lebedev A. O. , Czarnecki P. , Nawrocik W. , Perrot C. , Szczepanska L.
Published: July 2002

The weak-field dielectric dispersion (100 Hz–1.8 GHz) studies both of pure and Fe2+/Fe3 modified Cd2Nb2O7 ceramics over the temperature range of 90–380 K are presented and discussed from the viewpoint of relaxor and glassy properties of the system. It is revealed that Cd2Nb2O7 pyrochlore is intolerant of the addition of 25 mol % Fe2+ or Fe3+ for Cd2+. From the x-ray diffraction analysis, pure Cd2Nb2O7 forms a single-phase pyrochlore, while the compositions Cd1.5Fe0.52+Nb2O7 and Cd1.5Fe0.53+Nb2O7 give CdNb2O6 columbite doped with Fe2+ or Fe3+ on the Cd sites (<8 and <2 mol %, respectively), except for minor amount of parasitic hematite. The novel CdNb2O6 type compounds are not ferroelectrics, unlike Cd2Nb2O7. In the latter, at TC=196 K the dielectric relaxation due to the motion of ferroelectric domain walls driven by an external ac electric field is observed. A polydispersive dielectric response of Cd2Nb2O7 around 188 K has characteristics of the relaxor ferroelectrics with glassy behavior (like PMN). Near the characteristic freezing temperature of the zero-field-cooled state (Tf=183 K) the dielectric absorption spectra and the relaxation-time distribution strongly broaden and tend to flatten out, while below Tf the imaginary part of the dielectric permittivity becomes nearly frequency independent. The dielectric response of Cd2Nb2O7 dominating far below TC (around 150 K) and that of Fe2+/Fe3+ doped CdNb2O6 between 90 and 380 K are typical of glass-forming systems at temperature far above Tglass. The relaxational process is characterized by (i) a significant frequency dependence of the peak permittivity position, (ii) non-Arrhenius behavior, and (iii) increasing asymmetry of the dielectric absorption spectrum at the low-frequency side with decreasing temperature, without broadening the relaxation-time distribution and freezing the peak-absorption frequency. It is proposed that although the nature of structural disorder in Cd2Nb2O7 pyrochlore and Fe2+/Fe3+ doped CdNb2O6 columbite is different, in both systems the off-center displacements of the A-site ions act as a random field and are responsible for the relaxor and dipolar glass-like behavior upon cooling. The Debye-like HF dielectric relaxation (1 MHz–1.8 GHz) observed both in Cd2Nb2O7 and its isostructural analog Cd2Ta2O7 at RT and higher (a centrosymmetric phase) is attributed to fluctuations in polarization of the dynamically reoriented O(7th)–Cd–O(7th) dipoles due to dynamical off-center location of Cd ions.
On the proposed second law paradox in a nonzero floating potential
Authors: Cruden Brett A.
Published: July 2002

A second law paradox was previously proposed for a plasma contained within an infinite blackbody. The proposed second law paradox was dependent on the plasma having a nonzero floating potential [D. P. Sheehan and J. D. Means, Phys. Plasmas 5, 2469 (1998)]. This work demonstrates that a nonzero floating potential is indicative of some energy contained within the plasma that can be withdrawn from the plasma without violation of the second law. Furthermore, it is shown from the probe theory that the plasma in this hypothetical configuration must have a floating potential of zero at steady state.
Photoemission study of the solid-state interdiffusion in hybrid Fe/ZnSe/GaAs(001) heterostructures
Authors: Mosca D. H. , Abbate M. , Schreiner W. H. , Etgens V. H. , Eddrief M.
Published: July 2002

We report the chemical stability of the hybrid Fe/ZnSe/GaAs heterostructure prepared by molecular-beam epitaxy. Analyses of photoemission spectra show a remarkable chemical stability of the Fe/ZnSe(001) interface up to 380 °C, where the effective disruption of the heterostructure occurs, together with the appearance of Ga atoms near the film surface. This suggests that the ZnSe/GaAs interface is surprisingly less stable than the Fe/ZnSe one.
Large magnetoresistance in postannealed Bi thin films
Authors: Cho Sunglae , Kim Yunki , Freeman A. J. , Wong G. K. L. , Ketterson J. B. , Olafsen L. J. , Vurgaftman I. , Meyer J. R. , Hoffman C. A.
Published: July 2002

We have observed a large increase in the magnetoresistance (MR) of molecular beam epitaxy grown Bi thin films, which were subjected to a postannealing procedure 3 °C below the Bi melting point. We have achieved an increase in the MR by a factor of 2560 at helium temperatures compared with of 343 for an as-grown film. The enhancement of the MR in the annealed films is due to higher electron and hole mobilities (μe≈1×106 cm2/V s at 5 K) relative to those of the as-grown films (μe≈9×104 cm2/V s at 5 K). The enhancement of the mobility in the annealed films is also supported by the observation of Shubnikov–de Haas oscillations.
A note on the generalized fractal dimensions of a probability measure
Authors: Guérin Charles-Antoine
Published: July 2002

We prove the following result on the generalized fractal dimensions Dq± of a probability measure μ on Rn. Let g be a complex-valued measurable function on Rn satisfying the following conditions: (1) g is rapidly decreasing at infinity, (2) g is continuous and nonvanishing at (at least) one point, (3) ∫g≠0. Define the partition function Λa(μ,q)=an(q−1)‖ga * μ‖ lim qq, where ga(x)=a−ng(a−1x) and  *  is the convolution in Rn. Then for all q>1 we have Dq±=1/(q−1)limr→0 infsup[log Λaμ(r,q)/log r].
Coherent state path integral for the Bloch particle
Authors: Shibata Junya , Niizeki Komajiro
Published: July 2002

We construct a coherent state path integral formalism for the one-dimensional Bloch particle within the single band model. The transition amplitude between two coherent states is a sum of transition amplitudes with different winding numbers on the two-dimensional phase space which has the same topology as that of the cylinder. Appearance of the winding number is due to the periodicity of the quasimomentum of the Bloch particle. Our formalism is successfully applied to a semiclassical motion of the Bloch particle under a uniform electric field. The wave packet exhibits not only the Bloch oscillation but also a similar breathing to the one for the squeezed state of a harmonic oscillator.
Temperature-dependent contactless electroreflectance and photoluminescence study of GaAlAs/InGaAs/GaAs pseudomorphic high electron mobility transistor structures
Authors: Lin D. Y. , Huang Y. S. , Shou T. S. , Tiong K. K. , Pollak Fred H.
Published: July 2002

Temperature-dependent contactless electroreflectance (CER) and photoluminescence (PL) measurements in the range of 30 K<T<300 K on two GaAlAs/InGaAs/GaAs pseudomorphic high electron mobility transistor structures with different well widths fabricated by molecular-beam epitaxy on (100) GaAs substrates are presented. For the CER measurement, the 11H transition is completely screened out by the two-dimensional electron gas and the prominent feature related to the Fermi energy edge singularity transition showed a Stokes shift to higher energy with respect to the 21H transition of the PL measurements at low temperature. From the Stokes shifts, the Fermi energy level of the system is evaluated, and hence, the density of the two-dimensional electron gas. The temperature-dependent PL measurements revealed two features, identified to be the 11H and 21H transitions. The relative intensities of the 11H and 21H transitions were analyzed taking into account the effect of the subband filling and the wave-function overlap factors. A good agreement is found between experimental data and theoretical calculation results.
Strain-induced insulator–metal transition and room-temperature colossal magnetoresistance in low-doped La1−xBaxMnO3 thin films
Authors: Zhang Jun , Tanaka Hidekazu , Kawai Tomoji
Published: July 2002

Magnetotransport properties in low-doped La1−xBaxMnO3 thin films were investigated. The bulk La1−xBaxMnO3 (x=0.05 and 0.1) compounds are insulating, but an insulator–metal transition was observed in the strained thin films deposited on SrTiO3(100) substrates. At x=0.05, magnetization measurements revealed a transition from spin canting in bulk to ferromagnetic order in thin film. Moreover, a large magnetoresistance effect was obtained in the thin film with x=0.1 at room temperature and in a low magnetic field. A strain-induced modification of eg orbital stability was proposed to explain these properties.
Scattering and extinction of ion beams in a dusty plasma device
Authors: Nakamura Y.
Published: July 2002

Collisions of ions with charged dust grains are important for the propagation of low frequency waves such as dust acoustic waves and dust ion-acoustic waves. The collision cross-sectional area of charged dust grains depends on the velocity of an ion beam. The collision cross-sectional area of charged dust grains with beam ions is measured. It is compared with the geometrical cross-sectional area of the grain. The experiment is performed in a dusty double-plasma device with glass beads of 8.9 μm in average diameter. The ion beam current and energy are measured with a directional retarding potential analyzer. It is observed that, when dust density inside the system is increased, the beam current ratio is reduced. From the reduction of the ion beam current, the effective cross-sectional area of the dust particle is estimated as a function of the beam energy.
On the wavelet formalism for multifractal analysis
Authors: Murguı́a J. S. , Urı́as Jesús
Published: July 2002

It is proved that the multifractal characterizations of diametrically regular measures that are provided by the wavelet and by the Hentschel–Procaccia formalisms are identical.
Acoustic dispersion in glow discharge plasma: A phenomenological analysis
Authors: Soukhomlinov V. S. , Kolosov V. Y. , Sheverev V. A. , Ötügen M. V.
Published: July 2002

A phenomenological analysis is carried out to investigate the propagation of sound wave in the positive column of a glow discharge plasma. Specifically, the effect of local energy transfer from the discharge into the gas is studied. The analysis shows that the density fluctuations cause variations in the energy transfer rate. This effect is found to be a function of the plasma parameters and acoustic wave frequency thus leading to acoustic dispersion. The conclusion confirms earlier mathematical treatment of the problem [Kolosov et al., AIAA Paper No. 99-4882, 1999] and provides a physical explanation of the phenomenon.
Problems in recent analysis of injected carrier dynamics in semiconductor quantum dots
Authors: Lang Roy
Published: July 2002

This brief letter discusses conceptual problems that appeared in recent analysis of injected carrier dynamics in semiconductor quantum dots. It is pointed out that the frequently adopted approximation of neglecting the upward relaxation transitions severely restricts the validity of rate equations analysis. The proposed alternative approach with master equations for the transitions between microstates has, in principle, validity equivalent with conventional rate equations, but the presented form of equations neglects the upward transitions and has inaccurate expressions for the downward flow contributions.
Projection-reduced close coupling calculations of cross sections for vibrational relaxation in collisions of diatomic molecules with atoms
Authors: Krems Roman V. , Nordholm Sture
Published: July 2002

This work investigates the possibility of performing converged close coupling calculations of cross sections for vibrational relaxation in atom–diatom collisions with basis sets reduced by neglecting states with high values of angular momentum projection. The calculations for the Ar+HF system show that if the molecule is initially in the state with vibrational and rotational quantum numbers v=1, ji, it may be possible to neglect states with projections of angular momentum greater than ji for all rotational levels in the basis set.
Acceleration of heavy ion beams by means of direct injection into RFQ Linac
Authors: Takeuchi T. , Katayama T. , Okamura M. , Yano K. , Sakumi A. , Hattori T. , Jameson R. A.
Published: July 2002

A heavy ion source based on a new principle of direct plasma injection system was developed at Tokyo Institute of Technology. This heavy ion source combines directly a laser ion source and a radio frequency quadrupole (RFQ) LINAC. We carried out an acceleration test of carbon ions on this source and proved this principle. An observed total carbon beam current of 8 mA at 214 keV/u was observed. Beam analysis using a bending magnet downstream of the RFQ LINAC showed that accelerated beam includes 4.0 and 0.8 mA C4+ and C3+, respectively.
Reduction restrictions of Darboux and Laplace transformations for the Goursat equation
Authors: Leble S. B. , Yurov A. V.
Published: July 2002

We study Darboux and Laplace transformations of the solutions and potentials of the Goursat equation which is equivalent to one of the Lax pair equations for the 2D-MKdV hierarchy. The reduction restrictions for these transformations are considered. The derived reduction equations are generalizations of the Liouville and sinh-Gordon equation. The integrability of these equations by the ST method is proved. The binary Darboux transformation for the Goursat equation is suggested. We find exact rational nonsingular solutions of the 2D-MKdV equations via the Moutard transformation for the Goursat equation.
Regularizing divergences in the von Neumann entropy
Authors: Retamal Juan C. , Vergara Lautaro
Published: July 2002

We study the decoherence process of a harmonic oscillator in a dissipative environment by considering the von Neumann entropy. Derivatives of the von Neumann entropy around the initial time exhibit divergences when the system is initially in a pure state. A regularization procedure based on the zeta function technique is considered in order to extract information about decoherence.
Dispersion management for solitons in a Korteweg–de Vries system
Authors: Clarke Simon , Malomed Boris A. , Grimshaw Roger
Published: July 2002

The existence of “dispersion-managed solitons,” i.e., stable pulsating solitary-wave solutions to the nonlinear Schrödinger equation with periodically modulated and sign-variable dispersion is now well known in nonlinear optics. Our purpose here is to investigate whether similar structures exist for other well-known nonlinear wave models. Hence, here we consider as a basic model the variable-coefficient Korteweg–de Vries equation; this has the form of a Korteweg–de Vries equation with a periodically varying third-order dispersion coefficient, that can take both positive and negative values. More generally, this model may be extended to include fifth-order dispersion. Such models may describe, for instance, periodically modulated waveguides for long gravity-capillary waves. We develop an analytical approximation for solitary waves in the weakly nonlinear case, from which it is possible to obtain a reduction to a relatively simple integral equation, which is readily solved numerically. Then, we describe some systematic direct simulations of the full equation, which use the soliton shape produced by the integral equation as an initial condition. These simulations reveal regions of stable and unstable pulsating solitary waves in the corresponding parametric space. Finally, we consider the effects of fifth-order dispersion.
Exciton confinement in organic dendrimer quantum wells for opto-electronic applications
Authors: Lupton J. M. , Samuel I. D. W. , Burn P. L. , Mukamel S.
Published: July 2002

Organic dendrimers are a fascinating new class of materials for opto-electronic applications. We present coupled electronic oscillator calculations on novel nanoscale conjugated dendrimers for use in organic light-emitting diodes. Strong confinement of excitations at the center of the dendrimers is observed, which accounts for the dependence of intermolecular interactions and charge transport on the degree of branching of the dendrimer. The calculated absorption spectra are in excellent agreement with the measured data and show that benzene rings are shared between excitations on the linear segments of the hyperbranched molecules. The coupled electronic oscillator approach is ideally suited to treat large dendritic molecules.
On the presence of odd transverse convective rolls in narrow-gap horizontal annuli
Authors: Dyko Mark P. , Vafai Kambiz
Published: July 2002

A numerical study of three-dimensional supercritical states of natural convection in a narrow-gap horizontal cylindrical annulus is presented. For the first time, an odd number of counter-rotating transverse rolls is shown to occur in a narrow-gap annulus. Flow and temperature fields associated with the odd roll state are compared to those of transverse roll states exhibiting even numbers of rolls, and factors influencing the occurrence and number of odd rolls are investigated.
Application of white x-ray microbeams for the analysis of dislocation structures
Authors: Barabash R. I. , Ice G. E. , Larson B. C. , Yang Wenge
Published: July 2002

The measurement of dislocation structures on mesoscopic length scales is a particularly important application of white-beam Laue microdiffraction. Near a Bragg reflection the intensity distribution in reciprocal space is sensitive to the organization of the dislocations, which occurs at several structural levels. Unpaired geometrically necessary dislocations (GND) and geometrically necessary boundaries (GNB) result in elongated streaks in the Laue image. The direction of the streaks depends on the average orientation of the dislocation arrays and the diffraction vectors. Laue images collected using synchrotron x-ray microbeams are sensitive to the detailed hierarchical distribution of dislocations and can be used to study the orientation and density of individual GNDs and GNBs.
Experimental analysis of an integrated optics vibrometer
Authors: Paone Nicola , Scalise Lorenzo
Published: July 2002

A multipoint vibration sensor realized in integrated optics is presented. The sensor is composed of four optical heads that have been conceived for the simultaneous measurement of vibrations (in terms of displacement) in several points of a vibrating mechanical system with the aim of an industrial diagnostic. The system presented here is realized on small silicon chips (28 mm×28 mm) using integrated optic (IO) technology, each chip realizing a dual Michelson interferometer. The conditioning and acquisition system for the signal, based on an A/D conversion board is reported together with a brief description of the optical circuit. Experimental tests carried out on the sensor have provided maximum velocity and maximum operating frequency of the sensing system. Frequency response function tests carried out with a commercial single-point laser Doppler displacement sensor are reported in the present article together with the laboratory setup and test results. The network of vibration sensors in integrated optics, the object of the study, have proven to be capable of measuring the vibration displacement of varnished metal surfaces optically untreated with good accuracy (1% standard uncertainty on the instantaneous amplitude) in a frequency range 0–200 Hz with surface velocities up to 100 mm/s. Limitation of the data processing unit have been discussed for vibration frequencies higher then 200 Hz.
Phase diagram and anomalous properties of La0.7M0.3MnO3 lanthanum manganites near the structural phase transition
Authors: Ulyanov A. N. , Yu S. C. , Starostyuk N. Yu. , Pismenova N. E. , Moon Y. M. , Lee K. W.
Published: July 2002

The La0.7Ca0.3−xBaxMnO3 (x=0; 0.03; 0.06…0.3) and La0.7Ca0.3−xSrxMnO3 (x=0.105, 0.12, 0.135, 0.15, 0.165, and 0.18) lanthanum manganites were synthesized and studied. The Curie temperature TC of Ba range showed a sharp change near the concentration structural orthorhombic–rhombohedral phase transition. The TC values for that range are under the “classical” ferromagnetic–paramagnetic line. The TC temperatures of Sr range of samples are independent on x value at the vicinity of structural transition in the rhombohedral phase. Temperature dependence of magnetization of the La0.7Ca0.165Sr0.135MnO3 composition looks like a two-step curve which is caused by the closeness of ferromagnetic ordering and structural phase transition temperatures.
Structure of recombination-induced stacking faults in high-voltage SiC p–n junctions
Authors: Liu J. Q. , Skowronski M. , Hallin C. , Söderholm R. , Lendenmann H.
Published: July 2002

The structure of stacking faults formed in forward-biased 4H- and 6H-SiC p–n− diodes was determined using conventional and high-resolution transmission electron microscopy. Typical fault densities were between 103 and 104 cm−1. All observed faults were isolated single-layer Shockley faults bound by partial dislocations with Burgers vector of a/3〈1–100〉-type.
Remarks on the distributional Schwarzschild geometry
Authors: Heinzle J. Mark , Steinbauer Roland
Published: July 2002

This work is devoted to a mathematical analysis of the distributional Schwarzschild geometry. The Schwarzschild solution is extended to include the singularity; the energy momentum tensor becomes a δ-distribution supported at r=0. Using generalized distributional geometry in the sense of Colombeau’s (special) construction the nonlinearities are treated in a mathematically rigorous way. Moreover, generalized function techniques are used as a tool to give a unified discussion of various approaches taken in the literature so far; in particular we comment on geometrical issues.
Demonstration of high gain amplification of femtosecond ultraviolet laser pulses
Authors: Osvay K. , Kurdi G. , Klebniczki J. , Csatári M. , Ross I. N.
Published: July 2002

Femtosecond pulses at 400 nm were amplified using a noncollinear optical parametric amplifier pumped by picosecond pulses at 267 nm. A flat spectral gain exceeding 3500 was achieved in single pass within the available 17 nm bandwidth of the signal pulse. The effect of pump depletion, group delay difference, and the geometry of the interacting pulses on the spectral gain are also investigated.
Dopant-dependent reflectivity and refractive index of microcrystalline molybdenum–bronze thin films
Authors: Hussain Zahid
Published: July 2002

Reflectivity spectra of HxMoO3 and LixMoO3 thin films were measured over the photon energy range from 0.4 to 4.2 eV. It was found that microcrystalline molybdenum bronzes have reflectances between 6% and 30% over the concentration x, range 0⩽x⩽0.64. Values for the real part of the refractive index n were also determined from the refined reflectivity data using different numerical techniques depending upon the dispersive and nondispersive regions in the data. The values of high-frequency dielectric constant εhf of ZxMoO3 (Z=H+,Li+) bronzes were determined from the refractive index data to estimate the effective electronic masses involved in the optical and/or polaronic transitions. We interpret the optical data using the modified Drude–Zener model together with a single-oscillator model to differentiate between bound and free electronic states. Using a single-oscillator model, the oscillator energy Ea and the dispersion energy Ed were found to increase and decrease, respectively, with increasing x values, opposite to what occurs in crystalline molybdenum bronzes. These findings support the fact that Bloch electrons are almost absent in the investigated microcrystalline bronzes. The small reflectivity crests and the observed shift of the minimum reflectivity (or minimum refractive index) toward higher frequency with increasing x value in ZxMoO3 appear to be consistent with polaronic hopping, which arises from the excitation of electrons from localized in-gap states of Mo5+ ions into higher impurity states of Mo6+ (or Mo4+) ions within the optical band gap.
Directed dynamic self-assembly of objects rotating on two parallel fluid interfaces
Authors: Grzybowski Bartosz A. , Whitesides George M.
Published: July 2002

This paper describes dynamic self-assembly of millimeter-sized objects rotating at two parallel fluid interfaces and interacting with one another hydrodynamically, both in the plane of the interface and between the interfaces. The nature of hydrodynamic interactions between the objects rotating on different interfaces and, consequently, the morphologies of the ordered structures that self-assemble, depend on the sizes and the three-dimensional shapes of the rotating particles. Large particles rotating on one interface (“templates”) can direct the self-assembly of smaller particles (“substrates”) on the other interface. Two examples of directed self-assembly are discussed: (i) selective dimerization of disk spinning on the lower interface under the influence of cylinders rotating on the upper interface, and (ii) assembly of “substrate” rectangles into a pattern of squares that is identical with the pattern formed by the “template” squares. General, qualitative characteristics of the vortex–vortex interactions between objects of complex shapes are discussed.
Temperature-dependent carrier dynamics in self-assembled InGaAs quantum dots
Authors: Urayama J. , Norris T. B. , Jiang H. , Singh J. , Bhattacharya P.
Published: July 2002

We measured the transient temperature-dependent carrier population in the confined states of self-assembled In0.4Ga0.6As quantum dots as well as those of the surrounding wetting layer and barrier region using differential transmission spectroscopy. Results show directly that thermal reemission and nonradiative recombination contribute significantly to the dynamics above 100 K. We offer results of an ensemble Monte Carlo simulation to explain the contribution of these thermally activated processes.
Neural networks for real time determination of radiated power in JET
Authors: Barana O. , Murari A. , Franz P. , Ingesson L. C. , Manduchi G.
Published: July 2002

This article describes the use of neural networks (NNs) for the on-line computation of the radiated power in JET. The NNs have been trained using a database of about 120 discharges, for which the emitted power had been calculated via tomographic inversion of JET bolometric signals. In addition to the bolometric data, elongation and triangularity have been used as input to the NN, since these provide useful complementary information. Dedicated NNs have been designed for the determination of the total radiated power, the power from the bulk, and from the divertor region. All the NNs have been tested with a set of about 30 discharges with positive results. Moreover, the NNs can operate at full sampling speed and are therefore suited to follow edge localized modes and other rapid phenomena. The sensitivity of the NNs to failures in the input signals has also been tested, proving their robustness. Their possible use in feedback applications is finally briefly discussed.
Nonlinear evolution of the lower hybrid drift instability: Current sheet thinning and kinking
Authors: Lapenta Giovanni , Brackbill J. U.
Published: July 2002

Through numerical plasma simulations using the implicit code CELESTE3D [G. Lapenta and J. U. Brackbill, Nonlinear Processes Geophys. 7, 151 (2000)], the development of kink modes in a Harris current sheet is investigated, and their possible nonlinear interaction with the lower hybrid drift instability (LHDI) is considered. Consistent with earlier work, the rapid development of a short wavelength LHDI is observed, followed by the slow development of long wavelength current sheet kinking. The growth of kink modes is in agreement with the linear theory for the drift kink instability only at very small mass ratios (mi/me=16). At more realistic mass ratios, the growth rate exceeds that predicted by linear theory. A thorough investigation of the dependence of current sheet kinking on ion/electron mass and temperature ratios, and current sheet thickness reveals that the growth of kink modes is unaffected by current sheet thinning, but is strongly dependent on the ion/electron temperature ratio. The saturation amplitude of the LHDI increases with decreasing electron temperature, as do the nonlinear modifications of the initial equilibrium. In particular, the ion diamagnetic drift velocity of the ions decreases sufficiently on the flanks of the current sheet to support a Kelvin–Helmholtz instability, especially with cold electrons, whose properties are completely consistent with the kink modes observed in the simulations.
Anisotropic Coulomb effects on bremsstrahlung emission from bi-Maxwellian plasmas
Authors: Jung Young-Dae , Kim Chang-Geun
Published: July 2002

Anisotropy of the electron-ion bremsstrahlung spectrum from bi-Maxwellian plasmas is investigated including the Coulomb correction effects. The Coulomb correction effects on the bremsstrahlung emission rate for T∥>T⊥ are found to be more significant than those for T∥<T⊥ where T∥ and T⊥ are temperatures for the parallel and perpendicular motion to the magnetic field. An increase of the parallel or perpendicular temperature causes a decrease of the Coulomb effects on the bremsstrahlung emission power.
Origin of the vertical-anticorrelation arrays of InAs/InAlAs nanowires with a fixed layer-ordering orientation
Authors: Sun Zhongzhe , Yoon Soon Fatt , Wu Ju , Wang Zhanguo
Published: July 2002

InAs/In 0.52 Al 0.48 As nanowire multilayer arrays were grown on (001) InP substrate by molecular-beam epitaxy. The structural property of the arrays was investigated by transmission electron microscopy. The results clearly showed the formation of InAs nanowires, evolution of InAs/InAlAs interface, and composition and thickness modulations in the InAlAs spacer layer. A fixed spatial ordering of InAs/InAlAs nanowires was revealed for all the samples. Regardless of the change in InAlAs spacer thickness of different samples, (i) the nanowires of one InAs layer are positioned above the nanowire spacing in the previous InAs layer and (ii) the layer-ordering orientation angle of nanowires is fixed. The results were explained from the viewpoint of the growth kinetics. The effect of InAlAs spacers is suggested to play an important role on the spatial ordering of the nanowire arrays.
Comparision of neoclassical rotation theory with experiment under a variety of conditions in DIII-D
Authors: Stacey W. M. , Mandrekas J.
Published: July 2002

A neoclassical theory of gyroviscous radial momentum transport and poloidal and toroidal rotation has been compared with experiment in DIII-D [Luxon, Anderson, Batty et al., Plasma Physics and Controlled Nuclear Fusion Research 1986 (IAEA, Vienna, 1987), Vol. 1, p. 159] discharges in different confinement regimes, with a range of neutral beam powers and with co- and counter-injection, and with various types of dominant impurity species present. Calculated central toroidal rotation velocities and momentum confinement times agreed with experiment over a wide range of these conditions, with one notable exception in which a drift correction may be needed to reduce the gyroviscous toroidal force. Radial distributions of toroidal and poloidal rotation velocities and radial electric field, calculated using the radial distribution of toroidal angular momentum input density, agreed with measured distributions for the one time in an L-mode discharge that was examined in detail.
Electromechanical properties of Pb(Mg1/3Nb2/3)O3–7%PbTiO3 thin films made by pulsed laser deposition
Authors: Donnelly N. J. , Catalan G. , Morros C. , Bowman R. M. , Gregg J. M.
Published: July 2002

Thin film capacitors of Au/Pb(Mg1/3Nb2/3)O3–7%PbTiO3 [PMN–(0.07)PT]/(La1/2Sr1/2)CoO3 were made by pulsed laser deposition on single crystal {001} MgO substrates. The PMN–(0.07)PT dielectric was perovskite dominated, and demonstrated functional behavior typical of relaxors. Electrostrictive behavior was observed at fields up to 200 kV cm−1, however, the maximum strain was disappointingly low at only ∼0.14%. The macroscopic electromechanical d33 and Q33 coefficients were determined using piezo-response atomic force microscopy. At 100 kV cm−1 the macroscopic Q33 was found to be (2.6±0.2)×10−2 C−2 m4. The crystallographic electrostrictive coefficient was determined using in-situ x-ray diffraction and at the same field found to be significantly higher: (4.9±0.2)×10−2 C−2 m4. Since these electrostrictive coefficients are of the same order of magnitude as found in single crystal experiments (2.5–3.8×10−2 C−2 m4), it appears that the low out-of-plane strain is simply the result of poor polarizability in the thin films. An effective Q13 component of the electrostrictive tensor was also determined, and found to be ∼−0.32 ×10−2 C−2 m4.
Thermomechanical model and temperature measurements for shocked ammonium perchlorate single crystals
Authors: Winey J. M. , Gruzdkov Y. A. , Dreger Z. A. , Jensen B. J. , Gupta Y. M.
Published: July 2002

A consistent thermomechanical material model was developed for unreacted ammonium perchlorate (AP) single crystals for shock compression normal to the (210) and (001) crystal planes. Building on previous work, the mechanical response for both orientations was described using a single isotropic elastic-plastic model and an overstress model to describe rate-dependent yielding. Velocity interferometer measurements to 12 GPa were performed to extend the AP Hugoniot curve to higher stresses. The specific heat cv, the coefficient of thermal pressure (∂P/∂T)V, and the isothermal bulk modulus BT were determined from Hugoniot and isothermal compression curves, along with available data at atmospheric pressure. Time-resolved Raman spectroscopy experiments were carried out under stepwise loading to obtain temperatures in the shocked state. Calculated temperatures using our material model are in good agreement with the temperatures obtained from our experiments, thus providing validation for our modeling approach.
Carbon incorporation pathways and lattice sites in Si1−yCy alloys grown on Si(001) by molecular-beam epitaxy
Authors: Park S. Y. , D’Arcy-Gall J. , Gall D. , Soares J. A. N. T , Kim Y.-W. , Kim H. , Desjardins P. , Greene J. E. , Bishop S. G.
Published: July 2002

We use a combination of in situ and postdeposition experimental probes together with ab initio calculations of strain coefficients and formation energies associated with specific C configurations in the Si lattice to determine C incorporation pathways and lattice site distributions in fully coherent Si1−yCy alloy layers grown by molecular-beam epitaxy on Si(001) as a function of deposition temperature Ts (380 °C–680 °C) and C fraction y (0–0.026). Lattice strain and Raman spectroscopy measurements demonstrate that all C, irrespective of y, is incorporated into substitutional lattice sites in Si1−yCy(001) layers grown at Ts⩽480 °C. Increasing Ts⩾580 °C leads to strong C surface segregation, as shown by in situ angle-resolved x-ray photoelectron spectroscopy, yielding additional pathways for C incorporation. Photoluminescence measurements indicate that an increasing fraction of the incorporated C in the higher-temperature layers resides in dicarbon complexes. Reflection high-energy electron diffraction and cross sectional transmission electron microscopy reveal surface roughening at Ts⩾580 °C with the formation of bulk planar structures, interconnected by {113} segments, that are periodic along [001] with a periodicity which decreases with increasing Ts. We interpret the planar structures as layers of C-rich Si1−yCy which form in the presence of excess surface C resulting from segregation. Our ab initio density functional calculations show that substitutional C arranged in an ordered Si4C phase is 0.34 eV per C atom more stable than isolated substitutional C atoms.
Proximity magnetoresistance in Au80Fe20 and Au70Fe30 below the ordering temperature
Authors: Allia P. , Coisson M. , Durin G. F. , Moya J. , Selvaggini V. , Tiberto P. , Vinai F.
Published: July 2002

The magnetization and magnetoresistance of Au80Fe20 and Au70Fe30 melt-spun ribbons have been measured between 2 and 300 K. The solid solution Au80Fe20, paramagnetic at room temperature, exhibits a transition to a frustrated ferromagnetic state on lowering T. The solid solution Au70Fe30 is in a similar ordered state over the entire temperature interval. A negative magnetoresistance is observed in both systems. In Au80Fe20, its functional dependence on magnetization gradually changes from parabolic (typical of the paramagnetic phase) to box-like with lowering T. In Au70Fe30, the shape of the curves is always box-like. In both alloys, this effect exhibits a nonmonotonic behavior with temperature. The magnetoresistance of these alloys arises from the spin-dependent scattering of conduction electrons at the interfaces between adjacent regions of magnetic coherence of size comparable to the electron mean free path. This effect is therefore referred to as the “proximity” magnetoresistance. Two magnetization mechanisms independently acting in these systems are disentangled by combining magnetic and magnetoresistance measurements. In this way the spin patterns of these magnetic systems, characterized by a magnetic coherence range of the order of few nanometers, can be effectively studied.
Calculating approximate quantum mechanical rates without an a priori reaction coordinate
Authors: Brumer Yisroel , Golosov Andrei A. , Chen Zong Da , Reichman David R.
Published: July 2002

Two distinct reaction path methods are combined with the imaginary time centroid formalism to yield an approximate means of computing effective quantum rates without a preconceived notion of a reaction coordinate or transition state. The first method, which combines the imaginary time centroid formalism with the determination of minimum energy pathways, is appropriate for use when energetic factors dominate the rate process. The second utilizes an approximate determination of an effective centroid potential and the transition path sampling method of Chandler and co-workers, an approach designed for reactions that occur on more complex landscapes. The two methods are applied to the isomerization of a seven-atom argon cluster at 5 K where quantum effects are relevant.