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作者： Schröder, C. Schnack, J. Mentrup, D. Luban, M. Department of Electrical Engineering and Computer Science University of Applied Sciences of Bielefeld 33602 Bielefeld Germany Department of Physics and Astronomy Iowa State University Ames IA 50011 United States Department of Physics University of Osnabrück 49069 Osnabrück Germany Philips Research Laboratories Hamburg 22335 Hamburg Germany

Two-spin equilibrium time correlation functions (TCF) are of fundamental importance for studying the spin dynamics of magnetic molecules since these quantities appear in formulas for inelastic neutron scattering and magnetic resonance experiments. For systems of classical and quantum spins (s > 1) interacting via antiferromagnetic Heisenberg exchange, we find the general result that the TCF exhibit critical slowing-down effects for T &rarr 0 K.: the Fourier time transform of the TCF exhibits a growing peak that shifts towards zero frequency on cooling. © 2004 Published by Elsevier B.V.

关键词： Molecular dynamics

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Nanophysics of Electron Dynamics near Surfaces in High Power Microwave Devices and Systems

NanoPhysics of Electron Dynamics near Surfaces in High Power...

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The 5th International Vacuum Electron Sources Conference(IVESC2004)

作者： N.C.Luhmann G..Caryotakis J.P.Heritage R.Umstadd Department of Applied Science University of California Electrical and Computer Engineering University of California Department of Physics Naval Postgraduate School

Recently,a Consortium comprising the above indicated universities have been awarded a five year Multidisciplinary University Research Initiative (MURI) program aimed at providing a fundamental understanding of RF breakdown phenomena and developing novel, high current density cathodes. The start date is June 1,2004 and this talk will focus on both the initial activities and long term directions. The above goals will be accomplished by addressing two underlying issues: "pulse shortening" caused by RF breakdown and the lack of suitable, robust high current density cathodes. RF breakdown is a phenomenon that is poorly understood with respect to both the "nano-physics" of its inception as well the techniques to minimize its effects. Understanding it and preventing it require coordinated experimental and simulation programs. The cathodes required for so-called high power microwave (HPM) sources must provide in excess of 100 A/cm2 and conventional thermionic cathode technology falls short of that current density by about a factor of 5.

关键词： Nanophysics of Electron Dynamics near Surfaces in High Power Microwave Devices and Systems

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Design of space-charge-limited (SCL) magnetron injection guns (MIGs) for gyro-devices

Design of space-charge-limited (SCL) magnetron injection gun...

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International Conference on Infrared and Millimeter Waves

作者： W. Lawson Department of Electrical and Computer Engineering and Institute for Research Electronics and Applied Physics University of Maryland College Park MD USA

To eliminate the problem of non-uniform azimuthal current density experienced by some temperature-limited (TL) MIGs, we designed a number of SCL MIGs for gyro-devices with accelerator and fusion applications. We present the simulated capabilities of guns with and without control grids and compare these SCL guns to comparable TL MIGs.

关键词： Guns Cathodes Electrodes Lead Azimuthal current Magnetic devices Voltage physics Educational institutions Electron accelerators

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The design of space-charge limited MIGs with control grids for gyroklystron applications

The design of space-charge limited MIGs with control grids f...

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IEEE International Conference on Plasma Science (ICOPS)

作者： W. Lawson H. Raghunathan Department of Electrical and Computer Engineering and Institute of Research in Electronics and Applied Physics University of Maryland College Park MD USA

Summary form only given. At the University of Maryland we have been investigating the designs of space-charge-limited (SCL) Magnetron Injection Guns (MIGs) as potential replacements for temperature-limited (TL) MIGs, which can suffer from non-uniform azimuthal current density distributions due to thermal and work-function variations. Preliminary designs indicate that the same beam-quality can be achieved with only a slight cost in terms of required peak electric fields and cathode loading densities. One objection to these devices has been that you lose some control of the beam current which is available with TL MIGs. In this paper we address this issue, in part with parameter studies of the SCL MIG designs, and in part with the design of SCL MIGs with control grids. Two different designs with non-intercepting control grids are presented. The first design has small grids with low capacitance that can be adjusted by a few percent of the main beam voltage to adjust the current by +/-20%. The second design has larger control grids that can be biased sufficiently to actually cut off the beam flow even when the full beam voltage is applied.

关键词： Klystrons Cyclotrons Frequency Application software physics Educational institutions Stochastic resonance Plasma accelerators Electron guns Azimuthal current

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Design of space-charge-limited magnetron injection guns

Design of space-charge-limited magnetron injection guns

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International Conference on Vacuum Electronics

作者： W. Lawson H. Raghunathan Department of Electrical and Computer Engineering and Institute for Research in Electronics and Applied Physics University of Maryland College Park MD USA

We present the results of several space-charge-limited (SCL) magnetron injection gun (MIG) designs which are intended for use with a 500 kV, 500 A gyroklystron. The design performances are compared to that of a temperature-limited (TL) gun that was constructed for the same application. The SCL designs yield similar values for beam quality, namely an axial velocity spread under 3% for an average perpendicular-to-parallel velocity ratio of 1.5. The peak electric fields and the cathode loadings of the SCL designs are somewhat higher than for the TL design.

关键词： Klystrons Cathodes Azimuthal current Temperature Pulsed power supplies Current density physics Educational institutions Electron guns Electron beams

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Growing networks with geographical attachment preference: Emergence of small worlds

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Physical Review E 2004年第2期69卷 026108-026108页

作者： Jonathan Ozik Brian R. Hunt Edward Ott Department of Physics and Institute for Research in Electronics and Applied Physics University of Maryland College Park Maryland 20742 USA Department of Mathematics and Institute for Physical Science and Technology University of Maryland College Park Maryland 20742 USA Department of Physics Department of Electrical and Computer Engineering and Institute for Research in Electronics and Applied Physics University of Maryland College Park Maryland 20742 USA

We introduce a simple mechanism for the evolution of small world networks. Our model is a growing network in which all connections are made locally to geographically nearby sites. Although connections are made purely locally, network growth leads to stretching of old connections and to high clustering. Our results suggest that the abundance of small world networks in geographically constrained systems is a natural consequence of system growth and local interactions.

关键词： computer networks

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A high-frequency equivalent circuit for a gap source on a microstrip line

A high-frequency equivalent circuit for a gap source on a mi...

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Microwave, MTT-S International Symposium

作者： F. Mesa D.R. Jackson R. Rodriguez-Berral Department of Applied Physics 1 University of Seville Seville Spain Department of Electrical and Computer Engineering University of Houston Houston Texas USA

An equivalent circuit is derived for a voltage gap source on an infinite microstrip line. The circuit model is valid for all frequencies, including high frequencies where simple transmission-line theory is not accurate. The circuit model accounts for the fact that different definitions of characteristic impedance diverge at high frequency, as well as the fact that the continuous-spectrum current excited by the gap source becomes appreciable at high frequency. The continuous-spectrum current accounts for leaky modes that may get excited at high frequency, as well as the fact that radiation effects become important at high frequency.

关键词： Equivalent circuits Microstrip Frequency Voltage Impedance Radiation effects Fourier transforms Strips physics Transmission line theory

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Polarization Surface-Charge Density of Single Semiconductor Quantum Rods

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Physical Review Letters 2004年第21期92卷 216803-216803页

作者： Rishikesh Krishnan Megan A. Hahn Zhiheng Yu John Silcox Philippe M. Fauchet Todd D. Krauss Department of Electrical and Computer Engineering University of Rochester Rochester New York 14627 Department of Chemistry University of Rochester Rochester New York 14627 School of Applied and Engineering Physics Cornell University Ithaca New York 14853

Electrostatic force microscopy was used to determine that single CdSe quantum rods (QRs) have a permanent polarization surface-charge density, an unexpected observation for supposedly well-shaped particles. The surface charge results from a slight angle between the QR sides and the direction of internal electric polarization. By contrast, despite the large dipole moment expected for CdSe QRs, none was observed. The unavoidable presence of permanently charged surfaces on CdSe QRs has the potential to impede the development of novel devices incorporating these materials.

关键词： Semiconductor quantum dots

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Silicon on sapphire CMOS architectures for interferometric array readout

Silicon on sapphire CMOS architectures for interferometric a...

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IEEE International Symposium on Circuits and Systems (ISCAS)

作者： F. Tejada A.G. Andreou J.A. Miragliotta R. Osiander D. Wesolek Department of Electrical and Computer Engineering Johns Hopkins University Baltimore MD USA Applied Physics Laboratory Laurel MD USA

The performance of microelectromechanical sensor systems relies critically on the transduction method employed to convert the mechanical displacement into an electrical signal. Optical readout techniques have distinct advantages over more traditional capacitive and piezoelectricity transduction methods. They are employed in applications where atomic resolution with a small sensing area is necessary, for example, scanning probe microscopes. In this paper, we present two architectures for optoelectronic sensing based on silicon on sapphire CMOS (SOS-CMOS) technology. We show how to heterogeneously integrate photodetectors, analog CMOS signal processing circuits, and VCSELs for an array based optical readout system. The optical transparency of the sapphire substrate allows for the design of both Michelson and Fabry-Perot type interferometers that are amenable to 2D array sensing. We present preliminary experimental data demonstrating standing wave detection in the 100 nm thin silicon PIN photodiodes available in the SOS-CMOS technology for the Fabry-Perot interferometer.

关键词： Silicon Optical interferometry Optical signal processing Optical arrays Optical sensors Atom optics CMOS technology Sensor systems Piezoelectricity Signal resolution

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Surface micromachining in Silicon on Sapphire CMOS technology

Surface micromachining in Silicon on Sapphire CMOS technolog...

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IEEE International Symposium on Circuits and Systems (ISCAS)

作者： F. Tejada A.G. Andreou D.K. Wickenden A.S. Francomacaro Department of Electrical and Computer Engineering Johns Hopkins University Baltimore MD USA Applied Physics Laboratory Laurel MD USA

We report on the design and fabrication of surface micromachined microelectromechanical structures (MEMS) in an ultra thin silicon (UTSi) on sapphire CMOS process [Peregrine Semiconductor (PE) Silicon on Sapphire (SOS) process]. This is the first demonstration of surface micromachined MEMS structures in a CMOS process fabricated on a sapphire substrate.

关键词： CMOS technology Micromachining Silicon Micromechanical devices Fabrication physics Laboratories CMOS process Semiconductor device measurement Capacitors

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