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Physical Review E 2001年第5期64卷 051503-051503页

作者： Y. Gebremichael T. B. Schrøder F. W. Starr S. C. Glotzer Center for Theoretical and Computational Materials Science and Polymers Division National Institute of Standards and Technology Gaithersburg Maryland 20899 Chemical Physics Program Institute for Physical Sciences and Technology University of Maryland College Park Maryland 20742 Departments of Chemical Engineering and Materials Science and Engineering University of Michigan Ann Arbor Michigan 48109

In recent years, experimental and computational studies have demonstrated that the dynamics of glass-forming liquids are spatially heterogeneous, exhibiting regions of temporarily enhanced or diminished mobility. Here we present a detailed analysis of dynamical heterogeneity in a simulated “bead-spring” model of a low-molecular-weight polymer melt. We investigate the transient nature and size distribution of clusters of “mobile” chain segments (monomers) as the polymer melt is cooled toward its glass transition. We also explore the dependence of this clustering on the way in which the mobile subset is defined. We show that the mean cluster size is time dependent with a peak at intermediate time, and that the mean cluster size at the peak time grows with decreasing temperature T. We show that for each T a particular fraction of particles maximizes the mean cluster size at some characteristic time, and this fraction depends on T. The growing size of the clusters demonstrates the growing range of correlated motion, previously reported for this same system [C. Beneman et al. Nature (London) 399, 246 (1999)]. The distribution of cluster sizes approaches a power law near the mode-coupling temperature, similar to behavior reported for a simulated binary mixture and a dense colloidal suspension, but with a different exponent. We calculate the correlation length of the clusters, and show that it exhibits similar temperature- and time-dependent behavior as the mean cluster size, with a maximum at intermediate time. We show that the characteristic time of the maximum cluster size follows the scaling predicted by mode-coupling theory (MCT) for the β time scale, revealing a possible connection between spatially heterogeneous dynamics and MCT.

关键词： .size distribution clustering intermediate segments glass transition investigate toward glass-forming monomers Polymer Melt Dynamical Heterogeneity spatially heterogeneous mean cluster correlated dynamics mobile subset peak time

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Magnetic field effects on quantum ring excitons

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Physical Review B 2001年第12期63卷 125302-125302页

作者： Jakyoung Song Sergio E. Ulloa National CRI Center for Nano Particle Control School of Mechanical and Aerospace Engineering Seoul National University Seoul 151-742 Korea Department of Physics and Astronomy and Condensed Matter and Surface Sciences Program Ohio University Athens Ohio 45701–2979 Sektion Physik Ludwig-Maximilians-Universität Geschwister-Scholl-Platz 1 80539 Germany

We study the effect of magnetic field and geometric confinement on excitons confined to a quantum ring. We use analytical matrix elements of the Coulomb interaction and diagonalize numerically the effective-mass Hamiltonian of the problem. To explore the role of different boundary conditions, we investigate the quantum ring structure with a parabolic confinement potential, which allows the wave functions to be expressed in terms of center of mass and relative degrees of freedom of the exciton. On the other hand, wave functions expressed in terms of Bessel functions for electron and hole are used for a hard-wall confinement potential. The binding energy and electron–hole separation of the exciton are calculated as function of the width of the ring and the strength of an external magnetic field. The linear optical susceptibility as a function of magnetic fields is also discussed. We explore the Coulomb electron–hole correlation and magnetic confinement for several ring width and size combinations. The Aharanov–Bohm oscillations of exciton characteristics predicted for one-dimensional rings are found to not be present in these finite-width systems.

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Evaluation of rotating slant-hole SPECT mammography using Monte-Carlo simulation methods

Evaluation of rotating slant-hole SPECT mammography using Mo...

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IEEE Symposium on Nuclear Science (NSS/MIC)

作者： W.H. Baird E.C. Frey B.M.W. Tsui Yuchuan Wang D.E. Wessell Department of Physics College of Charleston Charleston SC USA Department of Biomedical Engineering University of North Carolina Chapel Hill Chapel Hill NC USA Internal Medicine Residency Program University of Pittsburgh Medical Center Pittsburgh PA USA University of North Carolina Chapel Hill Chapel Hill NC USA

Multi-segment slant-hole (MSSH) collimators provide much higher detection efficiency of breast lesions at a single view than conventional low-energy high-resolution (LEHR) parallel-hole collimators. The purpose of this study is to evaluate rotating MSSH (RMSSH) SPECT mammography using Monte Carlo Simulation methods. We modified the SIMIND Monte Carlo code to generate RMSSH projection data including the effects of attenuation, collimator-detector response and scatter from a realistic NCAT phantom with an attached breast and a lesion placed at locations at the center and at the near and far walls of the breast. The projection data were reconstructed using analytic and iterative image reconstruction methods. For comparison, we examine an LEHR collimator with a resolution of 1 cm at 15.2 cm, which is the same as that of the MSSH at the center of the common volume-of-view. Also, we simulated the inclusion of a lead shield that covers the body to keep photons emitted from the other organs from reaching the MSSH collimator. Our results indicated that RMSSH SPECT provides greater breast lesion contrast than that of a planar image taken using a conventional LEHR collimator at all lesion locations. Depending on the lesion location, the spatial resolution of the RMSSH SPECT images may be comparable or slightly poorer than that of the planar scintimammographic images obtained using a LEHR collimator. Shielding of photon emissions from the other organs substantially improves the RMSSH SPECT image quality. We conclude that the Monte Carlo simulation methods are useful in evaluating the application of RMSSH SPECT to breast imaging as compared to conventional planar scintimammography and that RMSSH SPECT may enhance lesion detectability.

关键词： Mammography Collimators Lesions Breast Image reconstruction Single photon emission computed tomography Monte Carlo methods Attenuation Electromagnetic scattering Particle scattering

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Ultra shallow Sb doped layer formation in Si (001) by the use of recoil implantation

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AIP Conference Proceedings 2001年第1期576卷

作者： K. E. Daley D. T. Vonk R. J. Culbertson Science and Engineering of Materials Program Arizona State University Tempe Arizona 85287-1704 Department of Physics and Astronomy Arizona State University Tempe Arizona 85287-1504

Recoil implantation was performed to create ultra shallow Sb doped layers in an Si (001) substrate. The technique consists of the initial deposition of thin (40 to 140 nm) Sb layers followed by high energy Ar+ ion irradiation and final chemical stripping of the residual Sb film. The resulting Sb atoms are recoil implanted into the underlying Si substrate. The results show a linear dependence of Sb concentration with Ar+ ion dose. High resolution Medium Energy Ion Scattering (MEIS) measurements have shown the projected range to be 3.0 nm with a doped layer width of 2.8 nm. The deposited Sb layer thickness is also shown to be a weakly dependent parameter in determining the Sb concentration for a given ion energy and dose.

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A New Combustion Synthesis Method for GaN:Eu3+and Ga2O3:Eu3+Luminescent Powders

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physica status solidi (a) 2001年第1期188卷

作者： G.A. Hirata F. Ramos R. Garcia E.J. Bosze J. McKittrick F.A. Ponce Centro de Ciencias de la Materia Condensada Universidad Nacional Autónoma de México Ensenada Baja California C.P. 22860 Mexico Department of Mechanical and Aerospace Engineering and Materials Science Program and Engineering University of California at San Diego La Jolla CA 92093-0411 USA Department of Physics and Astronomy Arizona State University Tempe AZ 85287-1504 USA

New low-temperature methods to produce GaN:Eu and Ga 2 O 3 :Eu (0 ≤ x ≤ 1) highly-luminescent powders are presented. These procedures yield finely divided powders through exothermic reactions between the precursors. The preparation of Eu-doped Ga 2 O 3 powders was achieved using a new combustion synthesis technique (hydrazine/metal-nitrate method). The process starts with aqueous solutions of Eu(NO 3 ) 3 and Ga(NO 3 ) 3 as the precursors and hydrazine as (non-carbonaceous) fuel. A spontaneous combustion reaction occurs by increasing the temperature to between 150 and 200 °C in a closed vessel filled with argon, and produces (Eu x Ga 1— x ) 2 O 3 directly. The preparation of Eu-doped GaN uses the ammonium hexafluoro-metal method. The powders present strong luminescence associated with the dopant. A sharp and strong GaN luminescence is observed, indicative of high purity and crystallinity as determined by low-temperature cathodoluminescence. The composition and powder morphology have been studied using energy dispersive spectroscopy and scanning electron microscopy.

关键词： 61.10. Ht 68.37.Hk 78.60.Hk 81.05.Gc S7.14

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Growth of ZnO Thin Film by Laser MBE: Lasing of Exciton at Room Temperature

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physica status solidi (b) 2001年第2期202卷

作者： Y. Segawa A. Ohtomo M. Kawasaki H. Koinuma Z. K. Tang P. Yu G. K. L. Wong Photodynamics Research Center Frontier Research Program The Institute of Physical and Chemical Research 19-1399 Koeji-Nagamachi Aoba Sendai 980 Japan Research Laboratory of Engineering Materials Tokyo Institute of Technology 4259 Nagatsuda Midori-Ku Yokohama 226 Japan Physics Department Hongkong University of Science and Technology Clear Water Bay Kowloon Hong Kong

High quality ZnO thin film was grown by Laser MBE. A pure ceramic ZnO target was ablated by the KrF laser pulses (248 nm, 10 Hz, 1 J/cm 2 ) in an ultra high vacuum to deposit ZnO film on sapphire (0001) substrate. The lateral grain size was about 50 nm for the sample with thickness of 55 nm. At room temperature, the peak of the exciton absorption and the photoluminescence have the same energy. Under high density excitation (355 nm, 35 ps, 10 Hz), an exciton–exciton collision process was observed as P 2 and P lines where 2S exciton and ionized exciton remain. From the edge of the sample, a very rapid increase of the P line was observed with the increase of the excitation power. A fine structure that comes from the cavity mode was also observed. These facts suggest that the lasing of the exciton was observed at room temperature.

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Signal Analysis of the Singing Voice: Low-Order Representations of Singer Identity 26

Signal Analysis of the Singing Voice: Low-Order Representati...

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26th International Computer Music Conference, ICMC 2000

作者： Mellody, Maureen Wakefield, Gregory H. Applied Physics Program University of Michigan United States Department of Electrical Engineering and Computer Science University of Michigan United States

Signals from 12 soprano and mezzo-soprano singers are analyzed with the modal distribution, a high-resolution time-frequency analysis method, to obtain measures of instantaneous amplitude and frequency of the signals’ partial components. These estimates are used to construct singer-specific transfer functions for each vowel and pitch analyzed. An AR approximation to a residual representation of these transfer functions is determined. This low-order residual approximation is used in a hierarchical clustering experiment. Using this representation, functions from each singer for a given vowel tend to cluster into a single class before classes collapse across singers. This result suggests that the low-order residual approximation captures singer-specific features in the sung signals. © 2000 ICMC. All Rights Reserved.

关键词： Transfer functions

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Evaluation of water-moderated lattices of UO2-2.0 wt. % PuO2 rods

Evaluation of water-moderated lattices of UO2-2.0 wt. % PuO2...

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2000 ANS International Topical Meeting on Advances in Reactor physics and Mathematics and Computation into the Next Millennium, PHYSOR 2000

作者： Campbell, Jim Rahnema, Farzad Bartholomay, Roger White, Travis L. Georgia Institute of Technology Nuclear Engineering and Health Physics Program George Woodruff School of Mechanical Engineering AtlantaGA30332-0405 United States Westinghouse Safety Management Solutions LLC 1993 South Centennial Drive AikenSC29803 United States

ISBN: (纸本)0894486551

In the mid 1960’s, three sets of experiments with lattices of mixed oxide (2 wt. % PuO2) fuel rods were performed at the Critical Approach Facility at Hanford Operations. These experiments have been evaluated as part of the International Criticality Safety Benchmark Evaluation Project for use by criticality safety specialists. This paper presents a description of the experimental configuration and discusses the benchmark model developed for the experiments. A sensitivity analysis determining the effects of data uncertainties on the keff value calculated from the benchmark model is described. The keff values calculated using the benchmark model with both MCNP, with continuous-energy ENDF/B-V cross section data, and KENO-Va, with 27 group ENDF/B-IV cross sections, are given. Trends observed in the calculated keff values and possible conclusions, which may be drawn from these trends, are discussed. © 2000 Proceedings of the PHYSOR 2000 - ANS International Topical Meeting on Advances in Reactor physics and Mathematics and Computation into the Next Millennium. All rights reserved.

关键词： Uranium dioxide

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Deformations in Extreme Matter

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Science 2000年第5473期288卷 1976-1977页

作者： Roderic Lakes The author is in the Department of Engineering Physics Engineering Mechanics Program Biomedical Engineering Department University of Wisconsin-Madison 147 Engineering Research Building 1500 Engineering Drive Madison WI 53706-1687 USA.

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Twirling and Whirling: Viscous Dynamics of Rotating Elastic Filaments

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Physical Review Letters 2000年第7期84卷 1623-1623页

作者： Charles W. Wolgemuth Thomas R. Powers Raymond E. Goldstein Department of Physics University of Arizona Tucson Arizona 85721 Division of Engineering and Applied Sciences Harvard University Cambridge Massachusetts 02138 Program in Applied Mathematics University of Arizona Tucson Arizona 85721

Motivated by diverse phenomena in cellular biophysics, including bacterial flagellar motion and DNA transcription and replication, we study the overdamped nonlinear dynamics of a rotationally forced filament with twist and bend elasticity. Competition between twist injection, twist diffusion, and writhing instabilities is described by coupled PDEs for twist and bend evolution. Analytical and numerical methods elucidate the twist/bend coupling and reveal two regimes separated by a Hopf bifurcation: (i) diffusion-dominated axial rotation, or twirling, and (ii) steady-state crankshafting motion, or whirling. The consequences of these phenomena for self-propulsion are investigated, and experimental tests proposed.

关键词：细菌生理学 DNA/化学 DNA/超微结构扩散弹性鞭毛/化学鞭毛/生理学鞭毛/超微结构行走/生理学旋转黏度

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