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Metamagnetic Phase Transition of the Antiferromagnetic Heisenberg Icosahedron

Physical Review Letters 2005年第20期94卷 207203-207203页

作者： Christian Schröder Heinz-Jürgen Schmidt Jürgen Schnack Marshall Luban Department of Electrical Engineering and Computer Science University of Applied Sciences Bielefeld D-33602 Bielefeld Germany and Ames Laboratory Ames Iowa 50011 USA Universität Osnabrück Fachbereich Physik D-49069 Osnabrück Germany Ames Laboratory & Department of Physics and Astronomy Iowa State University Ames Iowa 50011 USA

The observation of hysteresis effects in single molecule magnets like Mn12-acetate has initiated ideas of future applications in storage technology. The appearance of a hysteresis loop in such compounds is an outcome of their magnetic anisotropy. In this Letter we report that magnetic hysteresis occurs in a spin system without any anisotropy, specifically where spins mounted on the vertices of an icosahedron are coupled by antiferromagnetic isotropic nearest-neighbor Heisenberg interaction giving rise to geometric frustration. At T=0 this system undergoes a first-order metamagnetic phase transition at a critical field Bc between two distinct families of ground state configurations. The metastable phase of the system is characterized by a temperature and field dependent survival probability distribution.

关键词： Antiferromagnetic materials

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Algebraic $H_infty$ Design of Higher-Order Iterative Learning Controllers

Algebraic $H_infty$ Design of Higher-Order Iterative Learnin...

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IEEE International Symposium on Intelligent Control (ISIC)

作者： K.L. Moore YangQuan Chen Hyo-Sung Ahn Research and Technology Development Center Johns Hopkins University-Applied Physics Laboratory Laurel MD USA Center for Self-Organizing and Intelligent Systems (CSOIS) Department of Electrical and Computer Engineering Utah State University Logan UT USA

This paper presents an H infin -based design technique for synthesis of higher-order iterative learning controllers (ILC) for plants subject to iteration-domain input/output disturbances and plant model uncertainty. Using a formulation of the higher-order ILC problem into a high-dimensional MIMO discrete-time system, it is shown how the addition of input/output disturbances and plant model uncertainty to the ILC problem can be cast into a standard H infin framework. An algebraic approach to solving the problem in this framework is presented, resulting in a sub-optimal controller that can achieve both stability and robust performance. The key observation is that H infin synthesis can be used for higher-order ILC design to achieve reliable performance in the presence of iteration-varying external disturbances and model uncertainty

关键词： Uncertainty Robust stability Robustness MIMO Robust control Delay Paper technology physics Laboratories Intelligent systems

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Schur stability radius bounds for robust iterative learning controller design

Schur stability radius bounds for robust iterative learning ...

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American Control Conference (ACC)

作者： Hyo-Sung Ahn K.L. Moore YangQuan Chen Center for Self-Organizing and Intelligent Systems (CSOIS) Department of Electrical and Computer Engineering Utah State University Logan UT USA Research and Technology Development Center Johns Hopkins University-Applied Physics Laboratory Laurel MD USA

This paper computes bounds on the Schur stability radius for use in designing robust iterative learning controllers. The discrete Lyapunov equation is used to compute the Schur stability radius of the ILC system for the case of parametric or interval perturbations in the system Markov parameters. This paper is distinct from the existing ILC works in that the interval robustness concepts are integrated with asymptotic stability and monotonic convergence conditions to suggest design of the learning gain matrix that makes the system robust against interval model uncertainties up to a computable bound. After derivations of the analytical stability radius, optimization schemes are suggested to design the learning gain matrix. The proposed approach allows design of causal/noncausal time-varying learning matrix gains.

关键词： Robust stability Robust control Convergence Uncertainty Asymptotic stability Linear matrix inequalities Intelligent systems physics computing Equations Stability analysis

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Universal statistics of the scattering coefficient of chaotic microwave cavities

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Physical Review E 2005年第5期71卷 056215-056215页

作者： Sameer Hemmady Xing Zheng Thomas M. Antonsen, Jr. Edward Ott Steven M. Anlage Department of Physics University of Maryland College Park Maryland 20742 U.S.A. Department of Electrical and Computer Engineering University of Maryland College Park Maryland 20742 U.S.A. Institute for Research in Electronics and Applied Physics University of Maryland College Park Maryland 20742 U.S.A. Center for Superconductivity Research University of Maryland College Park Maryland 20742 U.S.A.

We consider the statistics of the scattering coefficient S of a chaotic microwave cavity coupled to a single port. We remove the nonuniversal effects of the coupling from the experimental S data using the radiation impedance obtained directly from the experiments. We thus obtain the normalized scattering coefficient whose probability density function (PDF) is predicted to be universal in that it depends only on the loss (quality factor) of the cavity. We compare experimental PDFs of the normalized scattering coefficients with those obtained from random matrix theory (RMT), and find excellent agreement. The results apply to scattering measurements on any wave chaotic system.

关键词： TIME-REVERSAL SYMMETRY CHARACTERISTIC VECTORS INFINITE DIMENSIONS BORDEREDMATRICES ENERGY LEVELS PARTICLES NMR

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Relative output factor measurements of a 5 mm diameter radiosurgical photon beam using polymer gel dosimetry

Relative output factor measurements of a 5 mm diameter radio...

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作者： Pappas, E. Petrokokkinos, L. Angelopoulos, A. Maris, T.G. Kozicki, M. Dalezios, I. Kouloulias, V. Nuclear and Particle Physics Section Physics Department University of Athens Ilisia 157 71 Athens Greece Medical Physics Department MRI Unit University Hospital of Heraklion 711 10 Stavrakia-Heraklion Crete Greece Faculty of Textile Engineering Technical University of Lodz Zeromskiego St 116 90-543 Lodz Poland Institute of Applied Radiation Chemistry Technical University of Lodz Wroblewskiego Street 15 93-590 Lodz Poland Department of Biomedical Engineering University General Hospital Attikon 1 Rimini Street 124 62 Chaidari Athens Greece Department of Electrical Engineering and Computer Science National Technical University of Athens 9 Hroon Polytechniou Street 157 73 Athens Greece Medical Physics Department Anticancer-Oncologic Hospital of Athens Saint Savvas 171 Alexandras Ave. 115 22 Athens Greece Faculty of Medicine University Hospital of Heraklion University of Crete 711 10 Stavrakia-Heraklion Crete Greece

Besides the fine spatial resolution inherent in polymer gel-magnetic resonance imaging (MRI) dosimetry, the method also features the potential for multiple measurements of varying sensitive volume in a single experiment by integrating results in MRI voxels of finite dimensions (i.e., in plane resolution by slice thickness). This work exploits this feature of polymer gel dosimetry to propose an experimental technique for relative output factor (OF) measurements of small radiosurgical beams. Two gel vials were irradiated with a 5 and 30 mm diameter 6 MV radiosurgery beam and MR scanned with the same slice thickness and three different in plane resolutions. Using this experimental data set, 5 mm OF measurements with the PinPoint ion chamber are simulated by integrating results over a sensitive volume equal to that of the chamber. Results are found in agreement within experimental uncertainties with actual PinPoint measurements verifying the validity of the proposed experimental procedure. The polymer gel data set is subsequently utilized for OF measurements of the 5 mm beam with varying sensitive volume to discuss the magnitude of detector volume averaging effects. Seeking to correct for volume averaging, results are extrapolated to zero sensitive volume yielding a 5 mm OF measurement of (0.665%). This result compares reasonably with corresponding ionometric and radiographic film measurements of this work and corresponding, limited, data in the literature. Overall, results suggest that polymer gel dosimetry coupled with the proposed experimental procedure helps overcome not only tissue-equivalence and beam perturbation implications but also volume averaging and positioning uncertainties which constitute the main drawback in small radiosurgical beam dosimetry. © 2005 American Association of Physicists in Medicine.

关键词： Dosimetry

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Full Wave Modeling of Substrate Doping Effects and Nonideal Conductors in Integrated Circuit Interconnects

Full Wave Modeling of Substrate Doping Effects and Nonideal ...

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International Symposium Semiconductor Device Research (ISDRS)

作者： Bo Yang Xi Shao N. Goldsman O. Ramahi P.N. Guzdar Electrical and Computer Engineering Department University of Maryland College Park MD USA Space Physics Data Facility Goddard Space Flight Center NASA Greenbelt MD USA Mechanical Engineering Department University of Maryland College Park MD USA Institute for Research in Electronic and Applied Physics University of Maryland College Park MD USA

On-chip interconnects form complex networks composed of coupled metal lines on oxide and substrate layers. Substrate parasitic effects (substrate currents and propagation loss) are becoming limiting factors to the performance of modern high-speed digital and analog integrated circuits (ICs). Signals traveling along the metal-insulator-silicon-metal (MISM) structure (a fundamental on-chip interconnect unit) will have different attenuation and dispersion when changing operation frequencies and substrate doping densities. These are categorized into three fundamental propagation modes: the slow-wave mode, the dielectric quasi-TEM mode, and the skin-effect mode (Hasegawa et al., 1971). In high-speed radio frequency (RF) circuits, interconnects are likely to operate in the skin-effect mode. In this mode, the nonideal substrate functions as a current return path of the signal, located very close to the oxide layer. Accurate modeling of interconnects in this mode is difficult, because the Courant's stability condition limits most explicit time-domain full-wave Maxwell solvers. To overcome this problem, we apply the alternating-direction-implicit finite-difference time-domain (ADI-FDTD) method (Namiki, 1999) to solve Maxwell's equations. In spite of the conductor loss, we have found considerable substrate current and loss, which depend on the substrate doping and operation frequency

关键词： Integrated circuit modeling Semiconductor process modeling Doping Conductors Integrated circuit interconnections Dielectric substrates Time domain analysis Metal-insulator structures Radio frequency Finite difference methods

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Complete pose determination for low altitude unmanned aerial vehicle using stereo vision

Complete pose determination for low altitude unmanned aerial...

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2005 IEEE/RSJ International Conference on Intelligent Robots and Systems

作者： L.K. Wang S.-C. Hsieh E.C.-W. Hsueh Fei-Bin Hsaio Kou-Yuan Huang Department of Electrical Engineering National Kaohsiung University of Applied Sciences Kaohsiung Taiwan National Space Program Office Hsinchu Taiwan Institute of Aeronautics and Astronautics National Cheng Kung University Tainan County Taiwan Department of Computer and Information Science National Chiao Tung University Hsinchu Taiwan

A well-developed pose estimation scenario suitable for low altitude unmanned aerial vehicle (UAV) is proposed. By employing dual CCD cameras onboard, the instant pose of UAV can be determined without any use of expensive sensor like gyro. The unscented Kalman filter (UKF) is hereafter introduced to resolve the highly nonlinear system dynamics as well as the measurement process of the pose estimation system. The only measurements recorded are those snapshots of ground targets/landmarks taken by two CCD cameras. The proposed scenario can also detect large angle rotation of UAV. Simulation is conducted via a simple case, both UAV and ground targets are stationary, to show the feasibility and applicability of the proposed scheme. Actual GPS measurement data of ground targets coordinates was recorded for UKF processing. A highlight phenomenon, implied by simulation, reveals that a sudden transition of estimation errors arises at the epoch when the UAV is experiencing a large angle maneuvering up to 180/spl deg/ per second.

关键词： Unmanned aerial vehicles Stereo vision Charge coupled devices Charge-coupled image sensors Nonlinear systems Nonlinear dynamical systems Vehicle dynamics Global Positioning System Coordinate measuring machines Estimation error

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Molecular dynamics simulations of the mechanical strength of Si∕Si3N4 interfaces

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Physical Review B 2005年第9期72卷 094115-094115页

作者： Martina E. Bachlechner Jennifer Zhang Ye Wang Jarrod Schiffbauer Steven R. Knudsen Dimitris Korakakis Physics Department West Virginia University Morgantown West Virginia 26506 USA NASA Independent Verification and Validation Facility Science and Engineering Apprenticeship Program—Morgantown High School Morgantown West Virginia 26501 USA Institute for Scientific Research Inc. Fairmont West Virginia 26555 USA Lane Department for Computer Science and Electrical Engineering West Virginia University Morgantown West Virginia 26506 USA

Molecular dynamics simulations are performed on parallel computers to investigate the crystalline Si(111)∕Si3N4(0001) interface that is modeled as an eight-component system. The average total energy per particle and the average kinetic energy per particle of the subsystems are monitored during the preparation of the system. The Young’s modulus of the interface is compared with that of the silicon part alone and that of the silicon-nitride film, respectively. The results for one extended simulation feature a crack in the silicon-nitride film and dislocated atoms in silicon below the crack. Simulations at rates of strain ranging from 0.00125to0.05ps−1 show that for lower strain rates, the systems stretched faster reach their ultimate strength at a higher strain value than those that were stretched more slowly. At the highest strain rates, however, the failure mechanisms change qualitatively indicative of a more ductile behavior.

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Carbon nanotubes: Anomalous photo-galvanic effect, electromechanical instability, and IR detection

Carbon nanotubes: Anomalous photo-galvanic effect, electrome...

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Frontiers in Optics, FiO 2005

作者： Rakitin, A. Meĺnikov, I.V. Haus, J.W. Aitchison, J.S. Department of Physics and Astronomy York University 4700 Keele Street North York Ontario M3P 1P3 Canada Comtex Consulting Inc 162 Princess Margaret Boulevard Etobicoke Ontario M9B 2Z5 Canada Electro-Optics Program School of Engineering University of Dayton 300 College Park Dayton Ohio 45469-0245 United States The E. S. Rogers Sr. Department of Electrical and Computer Engineering University of Toronto 10 King's College Road Toronto. Ontario M5S 3G4 Canada

ISBN: (纸本)1557527970

Carbon nanotubes (CNTs) are hollow cylinders composed of one or more concentric layers of carbon atoms arranged in a honeycomb lattice. CNTs are widely considered as an elemental material for a future nanoelectronic technology. This is not simply due to their small size but rather to their overall properties: CNTs are known to exhibit either metallic or semiconducting behaviour, with the band gap being dependent on the nanotube diameter. Both active devices (transistors) and interconnects can be made out of semiconducting and metallic nanotubes, respectively. The key nanotube dimension, their diameter, is controlled by chemistry, not conventional fabrication. The strong covalent bonding gives the CNTs high mechanical and thermal stability and ability to sustain current densities as high as 10 A/cm2.

关键词： Atoms Carbon nanotubes Chemistry Curvature Infrared detection Scattering

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Field test results for low power bearing estimator sensor nodes

Field test results for low power bearing estimator sensor no...

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IEEE International Symposium on Circuits and Systems (ISCAS)

作者： P. Julian A.G. Andreou G. Cauwenberghs M. Stanacevic H. Goldberg P.S. Mandolesi L. Riddle S. Shamma Department de Ing. Eléctrica y Computadoras Universidad Nacional del Sur Bahia Blanca Argentina Consejo Nacional de Investigaciones Científicas y Técnicas Capital Federal Argentina Department of Electrical and Computer Engineering Johns Hopkins University-Applied Physics Laboratory Baltimore MD USA Cornell University New York NY USA CIC Comision de Investigaciones Cientificas de la Provincia Buenos Aires Argentina Signals Systems Corporation Severna Park MD USA Department of Electrical Engineering University of Maryland College Park MD USA

The paper describes experimental results of low power sensor nodes designed to perform bearing estimation. The nodes are intended to form a wireless sensor network able to locate an audio source. Two different nodes are tested: one is based on a cross-correlation derivative integrated circuit (IC), and the other on a gradient flow IC. Implementation details and experimental results of both systems working in a natural environment are presented.

关键词： Microphones Charge coupled devices Circuit testing Sensor phenomena and characterization Direction of arrival estimation Wireless sensor networks Energy consumption Algorithm design and analysis Educational institutions Integrated circuit testing

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