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Electron-Phonon Coupling in Superconducting Alkali-Metal Doped Fullerenes

MRS Online Proceedings Library 1992年第1期270卷 161-166页

作者： R.A. Jishi M.S. Dresselhaus Department of Physics California State University at Los Angeles Los Angeles Department of Electrical Engineering and Computer Science and Department of Physics MIT Cambridge

The dimensionless electron-phonon coupling parameter in alkali metal-doped fullerenes isevaluated in a model whereby the electrons are treated within a tight binding formalism. The phonon mode frequencies and eigenvectors are obtained from a lattice dynamical modelwhich accurately fits all available experimental data on these modes. It is shown that the electrormphonon interaction can account for the relatively high values of the superconducting transition temperatures in alkali-metal fullerenes.

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A Model Calculation for the Vibrational Modes in C 60

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MRS Online Proceedings Library 1992年第1期270卷 197-202页

作者： R.A. Jishi R.M. Mirie M.S. Dresselhaus Department of Physics California State University at Los Angeles Los Angeles Department of Mathematics University of Massachusetts at Lowell Lowell Department of Electrical Engineering and Computer Science and Department of Physics MIT Cambridge

A force constant model for the vibrational modes in C60, based on bond-stretching and anglebending interactions, is presented. The results of this model are compared with the experimental data available from Raman, infrared, and high resolution electron energy loss spectroscopies, as well as neutron inelastic scattering measurements. Excellent agreement is obtained between the calculated and experimental mode frequencies. The pressure dependence of the Ramanand infrared-active mode frequencies is calculated within a simple model and is compared to available experimental data.

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VARIANCE OF PHASE FLUCTUATIONS OF WAVES PROPAGATING THROUGH A RANDOM MEDIUM

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JOURNAL OF ELECTROMAGNETIC WAVES AND APPLICATIONS 1992年第2期6卷 169-197页

作者： CHU, NC KONG, JA YUEH, HA NGHIEM, SV FLEISCHMAN, JG AYASLI, S SHIN, RT Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics Massachusetts Institute of Technology Cambridge MA 02139 United States MIT Lincoln Laboratory Lexington MA 02173 United States

As an electromagnetic wave propagates through a random scattering medium, such as a forest, its energy is attenuated and random phase fluctuations are induced. The magnitude of the random phase fluctuations induced is important in estimating how well a Synthetic Aperture Radar (SAR) can image objects within the scattering medium. The two-layer random medium model, consisting of a scattering layer between free space and ground, is used to calculate the variance of the phase fluctuations induced between a transmitter located above the random medium and a receiver located below the random medium. The scattering properties of the random medium are characterized by a correlation function of the random permittivity fluctuations. The effective permittivity of the random medium is first calculated using the strong fluctuation theory, which accounts for large permittivity fluctuations of the scatterers. The distorted Born approximation is used to calculate the first-order scattered field. A perturbation series for the phase of the received field in the Rytov approximation is then introduced and the variance of the phase fluctuations is solved to first order in the permittivity fluctuations. The variance of the phase fluctuations is also calculated assuming that the transmitter and receiver are in the paraxial limit of the random medium, which allows an analytic solution to be obtained. Results axe compared using the paraxial approximation, scalar Green's function formulation, and dyadic Green's function formulation. The effects studied are the dependence of the variance of the phase fluctuations on receiver location in lossy and lossless regions, medium thickness, correlation length and fractional volume of scatterers, depolarization of the incident wave, ground layer permittivity, angle of incidence, and polarization.

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A multiscale approach to solving one dimensional inverse problems

A multiscale approach to solving one dimensional inverse pro...

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IEEE-SP International Symposium on Time-Frequency and Time-Scale Analysis

作者： E. Miller A. Willsky Dept. of Electr. Eng. & Comput. Sci. MIT Cambridge MA USA Department of Electrical Engineering and Computer Science Massachusetts Institute of Technology Cambridge MA USA

A multiresolution approach to solving one dimensional inverse problems is explored. Inverse problems described by that class of operators which are made sparse under the action of the wavelet transform are considered. Statistically based inversion procedures utilizing multiscale a priori stochastic models are also considered. As a concrete example, a deconvolution problem arising in wellbore induction measurement of conductivity is examined.< >

关键词： Inverse problems Conductivity measurement Geologic measurements Transmitters Fractals Wavelet transforms Spatial resolution Magnetic field measurement Telephony Stochastic processes

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Simulation of electromagnetic radiation and scattering using a finite difference-time domain technique

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computer Applications in engineering Education 1992年第1期1卷 45-45页

作者： Li, K. Tassoudji, M.A. Shin, R.T. Kong, J.A. Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics Massachusetts Institute of Technology Cambridge Massachusetts 02139 Kevin Li:received his BS and SM degrees (1990) in Electrical Engineering and EE (Electrical Engineer's) degree (1991) from the Massachusetts Institute of Technology where he is currently pursuing his PhD in Electrical Engineering. Since 1990 he has been the recipient of a United States Air Force Laboratory Graduate Fellowship. His main research interests include electromagnetic scattering radar cross section prediction and numerical techniques. Mr. Li is a member of Tau Beta Pi and a student member of IEEE. M. Ali Tassoudji:received his BS (1987) and MS (1989) degrees in Electrical Engineering from the University of Michigan Ann Arbor. Since 1989 he has been a research assistant at the Massachusetts Institute of Technology where he is currently working towards his PhD in Electrical Engineering. His research interests include electromagnetic scattering and propagation modeling of microwave circuits and numerical techniques. Mr. Tassoudji is a member of IEEE and Eta Kappa Nu. Robert T. Shin:received his SB (1977) SM (1980) and PhD (1984) in Electrical Engineering from the Massachusetts Institute of Technology. Since 1984 he has been a member of the Air Defense Techniques Group at MIT Lincoln Laboratory as a Research Staff member and as a Senior Staff member from 1989. His research interests are in the areas of electromagnetic wave scattering and propagation theoretical model development and data interpretation for microwave remote sensing. He is the coauthor ofTheory of Microwave Remote Sensing(Wiley 1985). Dr. Shin is a member of The Electromagnetics Academy IEEE American Geophysical Union Tau Beta Pi Eta Kappa Nu and Commission F of the International Union of Radio Science. Since 1987 he has served on the Editorial Board of theJournal of Electromagnetic Waves and Applications(JEWA). Jin Au Kong:is Professor of Electrical Enginee

The finite difference-time domain (FD-TD) technique is applied to the solution of Maxwell's equations. A computer program, which can be used to simulate and study numerous electromagnetic phenomena, is developed and implemented on an IBM 386 compatible personal computer. The FD-TD technique is a useful tool for students in electromagnetics. The technique is flexible and can be applied to many basic EM scattering and radiation problems. Because field solutions are found as a function of time, visualization of the propagation of the EM fields is possible. The FD-TD technique is implemented for a two-dimensional rectangular grid in conjunction with a second-order absorbing boundary condition. Both E- and H-field polarizations are analyzed. Finite objects consisting of dielectric, magnetic and conducting materials, and perfectly conducting infinite ground planes are modeled. Plane wave and line current sources are implemented. In addition to the capability of animating the propagation of the EM fields, radiation and scattering patterns can be generated.

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Electronic Structure of Carbon Fibers based on C 60

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MRS Online Proceedings Library 1991年第1期247卷 333-338页

作者： Riichiro Saito mitsutaka Fujita G. Dresselhaus M. S. Dresselhaus Department of Physics Massachusetts Institute of Technology Cambridge USA Francis Bitter National Magnet Laboratory MIT Cambridge USA Department of Electrical Engineering and Computer Science MIT Cambridge USA

The electronic structures of some possible carbon fibers nucleated from the hemisphere of a bucky ball are presented. A one-dimensional electronic energy band structure model of such carbon fibers, having not only a principal rotational axis but also fibers with screw axes, can be derived by folding the two-dimensional energy bands of graphite. By considering the variation of the fiber diameter and helicity within a simple tight binding model calculation, we show that 1/3 of the fibers are metallic and 2/3 are semiconducting. The effect of adding multiple tubule layers is considered.

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Low-complexity multi-mode VXC using multi-stage optimization and mode selection (speech coding)

Low-complexity multi-mode VXC using multi-stage optimization...

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International Conference on Acoustics, Speech, and Signal Processing (ICASSP)

作者： M. Johnson T. Taniguchi Department of Electrical Engineering and Computer Science MIT Cambridge MA USA Speech Signal Processing Fujitsu Laboratories Limited Kawasaki Japan

The authors propose an algorithm in which multi-mode coding is a transparent adaptation of multi-stage VXC (vector excitation coding) requiring almost zero additional computation. In the process, it is shown how the cost of the jointly optimized multi-stage VXC can be minimized using a backward-transform implementation of the perceptually orthogonal VXC (POVXC) algorithm previously proposed by the authors. It is shown that the performance of such a coder, while not as good as that of a normal multi-mode coder, is noticeably better than that of a normal POVXC.< >

关键词： Speech coding Linear predictive coding Speech processing Adaptive filters Bit rate Cost function Vocoders Computational complexity Vector quantization Code standards

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Pitch sharpening for perceptually improved CELP, and the sparse-delta codebook for reduced computation

Pitch sharpening for perceptually improved CELP, and the spa...

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International Conference on Acoustics, Speech, and Signal Processing (ICASSP)

作者： T. Taniguchi M. Johnson Y. Ohta Department of Electrical Engineering and Computer Science MIT Cambridge MA USA Speech Signal Processing Fujitsu Laboratories Limited Kawasaki Japan

The authors introduce two techniques for improving low-bit-rate CELP (code excited linear prediction) speech coders. The sparse-delta stochastic codebook is a recursive codebook design which can be searched using roughly 5% of the computational load required to search a full Gaussian codebook. Pitch sharpening is a class of algorithms which attempt to improve the perceptual quality of CELP by limiting the feedback of low-amplitude noiselike information to the adaptive codebook. Simulation results are presented for sparse-delta, ternary sparse-delta, and simplified-search sparse-delta coders, and for reduced-gain and sparse-adaptive-codebook pitch sharpening algorithms.< >

关键词： Stochastic resonance Books Feedback Stochastic processes Bit rate Finite impulse response filter Equations Speech synthesis Code standards Linear predictive coding

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GREEDY PACKET SCHEDULING 4th

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4TH INTERNATIONAL WORKSHOP ON DISTRIBUTED ALGORITHMS

作者： CIDON, I KUTTEN, S MANSOUR, Y PELEG, D IBM T.J. Watson Research Center P.O. Box 704 Yorktown Heights 10598 NY United States Faculty of Electrical Engineering The Technion Haifa 32000 Israel Laboratory for Computer Science MIT Cambridge 02139 MA United States Department of Applied Mathematics The Weizmann Institute Rehovot 76100 Israel

ISBN: (纸本)3540540997

Scheduling packets to be forwarded over a link is an important subtask of the routing process both in parallel computing and in communication networks. This paper investigates the simple class of greedy scheduling algorithms, namely, algorithms that always forward a packet if they can. It is first proved that for various “natural” classes of routes, the time required to complete the transmission of a set of packets is bounded by the sum of the number of packets and the maximal route length, for any greedy algorithm (including the arbitrary scheduling policy). Next, tight time bounds of Θ(n) are proved for a specific greedy algorithm on the class of shortest paths in n-vertex networks. Finally it is shown that when the routes are arbitrary, the time achieved by various “natural” greedy algorithms can be as bad as Ω(n^1.5), when O(n) packets have to be forwarded on an n-vertex network. © 1991, Springer Verlag. All rights reserved.

关键词： Scheduling algorithms

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Improving the performance of CELP-based speech coding at low bit rates

Improving the performance of CELP-based speech coding at low...

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IEEE International Symposium on Circuits and Systems (ISCAS)

作者： T. Taniguchi F. Amano M. Johnson Radio Communication Processing Technology Section Fujitsu Laboratories Limited Kawasaki Japan Department of Electrical Engineering and Computer Science MIT Cambridge MA USA

The authors review the CELP (code excited linear prediction) coding algorithm, and the efforts recently made for improving the performance and reducing the complexity. The efforts for improving the coding performance have been aimed not only at making the quality better, but also at further reducing the bit rate without degrading the quality. A sparse-delta codebook is introduced as an example of a codebook with a computationally efficient structure. Multimode coding is then described for improving the perceptual quality of CELP at low bit rates.< >

关键词： Speech coding Bit rate Speech synthesis Speech processing Vector quantization Speech analysis Computational complexity Search methods Degradation Signal synthesis

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