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Frequency-selected enhancement of high-order-harmonic generation by interference of degenerate Rydberg states in a few-cycle laser pulse

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Physical Review A 2012年第3期86卷 033417-033417页

作者： Jigen Chen Ruquang Wang Zhen Zhai Jing Chen Panming Fu Bingbing Wang Wu-Ming Liu Department of Physics and Materials Engineering Taizhou University Taizhou 318000 China Beijing National Laboratory for Condensed Matter Physics Institute of Physics Chinese Academy of Sciences Beijing 100190 China Key Laboratory of High Energy Density Physics Simulation CAPT Peking University Beijing 100084 China Institute of Applied Physics and Computational Mathematics P. O. Box 8009 Beijing 100088 China

We demonstrate that the frequency-selected enhancement of high-order-harmonic generation (HHG) can be achieved by a few-cycle laser pulse interacting with a coherent superposition state, which is prepared by the ground state and two degenerate Rydberg states. The degenerate states have the same orbital radius and hence have a large overlap in the electronic density distribution. By controlling the relative phase between the two degenerate states, the constructive or destructive interference of them can markedly change the initial density distribution of the Rydberg electron, thereby we can manipulate the characteristics and the conversion efficiency of HHG. Specifically, a significant enhancement in the continuous harmonics near HHG cutoff can be obtained, hence an intense isolated pulse with a duration less than 100 attoseconds is straightforwardly generated. On the other hand, since there exists a specific dependence of the harmonic efficiency on the relative phase of the two degenerate states, one can expect that the relative phase may be probed by examining the corresponding harmonic intensity. In practice, we may apply a weak static electric field in the whole dynamic process to obtain an asymmetry electron density distribution at a large radius; hence similar HHG results can be obtained.

关键词： Ground state

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Results of an implementation of the dual surface method to treat the non-uniqueness in solving acoustic exterior problems using the boundary element method

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Proceedings of Meetings on Acoustics 2013年第1期19卷

作者： Ralf Burgschweiger Ingo Schäfer Adel Mohsen Rafael Piscoya Martin Ochmann Bodo Nolte Department II Project Group Computational Acoustics Beuth Hochschule für Technik Berlin University of Applied Sciences Luxemburger Str. 10 Berlin Berlin 13353 Germany Maritime Technology and Research (WTD71/FWG) Federal Armed Forces Underwater Acoustics and Marine Geophysics Research Institute Klausdorfer Weg 2-24 Kiel 24148 Germany Engineering Mathematics & Physics Department Engineering Faculty Cairo University Giza 12613 Egypt Department II Project Group Computational Acoustics Beuth Hochschule für Technik Berlin University of Applied Sciences Luxemburger Str. 10 Berlin 13353 Germany

The problem of non-uniqueness (NU) of the solution of exterior acoustic problems when using the boundary element method (BEM) is well known. Methods like the Burton-Miller technique or the CHIEF method are used to solve this challenge at the expense of more complex procedures for handling hypersingular integrals and/or higher computing times due to higher complexity of the algorithm or additional equations. The dual surface method, commonly used for electromagnetic problems, was adapted for acoustic radiation and scattering problems. The basic principles of methods to solve the NU problem are outlined and results for different models and solution procedures are presented, taking into account quality, solution time and the numerical advantages when using iterative solvers.

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Model reduction for agent-based social simulation: Coarse-graining a civil violence model

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Physical Review E 2012年第6期85卷 066106-066106页

作者： Yu Zou Vladimir A. Fonoberov Maria Fonoberova Igor Mezic Ioannis G. Kevrekidis Department of Chemical and Biological Engineering and Program in Applied and Computational Mathematics Princeton University Princeton New Jersey 08544 USA Aimdyn Inc. 1919 State Street Suite 207 Santa Barbara California 93101 USA Department of Mechanical Engineering University of California Santa Barbara Santa Barbara California 93106 USA

Agent-based modeling (ABM) constitutes a powerful computational tool for the exploration of phenomena involving emergent dynamic behavior in the social sciences. This paper demonstrates a computer-assisted approach that bridges the significant gap between the single-agent microscopic level and the macroscopic (coarse-grained population) level, where fundamental questions must be rationally answered and policies guiding the emergent dynamics devised. Our approach will be illustrated through an agent-based model of civil violence. This spatiotemporally varying ABM incorporates interactions between a heterogeneous population of citizens [active (insurgent), inactive, or jailed] and a population of police officers. Detailed simulations exhibit an equilibrium punctuated by periods of social upheavals. We show how to effectively reduce the agent-based dynamics to a stochastic model with only two coarse-grained degrees of freedom: the number of jailed citizens and the number of active ones. The coarse-grained model captures the ABM dynamics while drastically reducing the computation time (by a factor of approximately 20).

关键词： Stochastic models

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Efficient coarse simulation of a growing avascular tumor

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Physical Review E 2012年第3期85卷 031912-031912页

作者： Michail E. Kavousanakis Ping Liu Andreas G. Boudouvis John Lowengrub Ioannis G. Kevrekidis School of Chemical Engineering National Technical University of Athens 9 Heroon Polytechniou St. Zographos Athens GR-15780 Greece Department of Chemical and Biological Engineering Princeton University Princeton New Jersey 08544 USA Department of Mathematics University of California 103 MSTB Irvine California 92697 USA Program in Applied and Computational Mathematics Princeton University Princeton New Jersey 08544 USA

The subject of this work is the development and implementation of algorithms which accelerate the simulation of early stage tumor growth models. Among the different computational approaches used for the simulation of tumor progression, discrete stochastic models (e.g., cellular automata) have been widely used to describe processes occurring at the cell and subcell scales (e.g., cell-cell interactions and signaling processes). To describe macroscopic characteristics (e.g., morphology) of growing tumors, large numbers of interacting cells must be simulated. However, the high computational demands of stochastic models make the simulation of large-scale systems impractical. Alternatively, continuum models, which can describe behavior at the tumor scale, often rely on phenomenological assumptions in place of rigorous upscaling of microscopic models. This limits their predictive power. In this work, we circumvent the derivation of closed macroscopic equations for the growing cancer cell populations; instead, we construct, based on the so-called “equation-free” framework, a computational superstructure, which wraps around the individual-based cell-level simulator and accelerates the computations required for the study of the long-time behavior of systems involving many interacting cells. The microscopic model, e.g., a cellular automaton, which simulates the evolution of cancer cell populations, is executed for relatively short time intervals, at the end of which coarse-scale information is obtained. These coarse variables evolve on slower time scales than each individual cell in the population, enabling the application of forward projection schemes, which extrapolate their values at later times. This technique is referred to as coarse projective integration. Increasing the ratio of projection times to microscopic simulator execution times enhances the computational savings. Crucial accuracy issues arising for growing tumors with radial symmetry are addressed by applying th

关键词： Tumors

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Nonequilibrium static diverging length scales on approaching a prototypical model glassy state

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Physical Review E 2012年第2期86卷 021505-021505页

作者： Adam B. Hopkins Frank H. Stillinger Salvatore Torquato Department of Chemistry Princeton University Princeton New Jersey 08544 USA Princeton Institute for the Science and Technology of Materials Princeton University Princeton New Jersey 08544 USA Department of Physics Princeton University Princeton New Jersey 08544 USA Princeton Center for Theoretical Science Princeton University Princeton New Jersey 08544 USA Program in Applied and Computational Mathematics Princeton University Princeton New Jersey 08544 USA

Maximally random jammed states of hard spheres are prototypical glasses. We study the small wavenumber k behavior of the structure factor S(k) of overcompressed million-sphere packings as a function of density up to the jammed state. We find both a precursor to the glassy jammed state evident long before the jamming density is reached and two associated growing length scales, one extracted from the volume integral of the direct correlation function c(r) and the other from the small-k behavior of the structure factor S(k), that can diverge at the “critical” jammed state. We also define a nonequilibrium index X and use it to demonstrate that the packings studied are intrinsically nonequilibrium in nature well before the critical state is reached. The extension of the results reported in the present work to supercooled atomic-liquid models in which the atoms interact with both repulsive and attractive forces is also discussed.

关键词： Glass

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Spectral Method for Nonlinear Stochastic Partial Differential Equations of Elliptic Type

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Numerical mathematics(Theory,Methods and Applications) 2011年第1期4卷 38-52页

作者： Yanzhao Cao Li Yin Department of Mathematics Auburn UniversityAuburnAL 36849USA LCP Institute of Applied Physics and Computational MathematicsBeijing 100088China

This paper is concerned with the numerical approximations of semi-linear stochastic partial differential equations of elliptic type in ***gence analysis and error estimates are presented for the numerical solutions b... 详细信息

关键词： Stochastic differential equation spectral method error estimates white noise

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Effects of size polydispersity on the extinction spectra of colloidal nanoparticle aggregates

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Physical Review B 2012年第4期85卷 045421-045421页

作者： Alexander E. Ershov Ivan L. Isaev Polina N. Semina Vadim A. Markel Sergei V. Karpov L. V. Kirenskiy Institute of Physics Russian Academy of Sciences Kransoyarsk 660036 Russian Federation Siberian Federal University Krasnoyarsk 660041 Russian Federation Department of Radiology Department of Bioengineering and the Graduate Group in Applied Mathematics and Computational Science University of Pennsylvania Philadelphia Pennsylvania 19104 USA

We investigate the effect of particle polydispersity on the optical extinction spectra of colloidal aggregates of spherical metallic (silver) nanoparticles, taking into account the realistic interparticle gaps caused by layers of stabilizing polymer adsorbed on the metal surface (adlayers). The spectra of computer-generated aggregates are computed using two different methods. The coupled-multipole method is used in the quasistatic approximation and the coupled-dipole method beyond the quasistatics. The latter approach is applicable if the interparticle gaps are sufficiently wide relative to the particle radii. Simulations are performed for two different particle size distribution functions (bimodal and Gaussian), varying the number of particles per aggregate, and different distribution functions of the interparticle gap width. The strong influence of the latter factor on the spectra is demonstrated and investigated in detail.

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Seismic Probes of Solar Interior Magnetic Structure

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Physical Review Letters 2012年第10期109卷 101101-101101页

作者： Shravan Hanasoge Aaron Birch Laurent Gizon Jeroen Tromp Department of Geosciences Princeton University New Jersey 08544 USA Max-Planck Institut für Sonnensystemforschung 37191 Kalenburg-Lindau Germany Georg-August-Universität Institut für Astrophysik D-37077 Göttingen Germany Program for Applied and Computational Mathematics Princeton University New Jersey 08544 USA

Sun spots are prominent manifestations of solar magnetoconvection, and imaging their subsurface structure is an outstanding problem of wide physical importance. Travel times of seismic waves that propagate through these structures are typically used as inputs to inversions. Despite the presence of strongly anisotropic magnetic waveguides, these measurements have always been interpreted in terms of changes to isotropic wave speeds and flow-advection-related Doppler shifts. Here, we employ partial-differential-equation-constrained optimization to determine the appropriate parametrization of the structural properties of the magnetic interior. Seven different wave speeds fully characterize helioseismic wave propagation: the isotropic sound speed, a Doppler-shifting flow-advection velocity, and an anisotropic magnetic velocity. The structure of magnetic media is sensed by magnetoacoustic slow and fast modes and Alfvén waves, each of which propagates at a different wave speed. We show that even in the case of weak magnetic fields, significant errors may be incurred if these anisotropies are not accounted for in inversions. Translation invariance is demonstrably lost. These developments render plausible the accurate seismic imaging of magnetoconvection in the Sun.

关键词： Advection

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Frequency-domain theory of nonsequential double ionization in intense laser fields based on nonperturbative QED

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Physical Review A 2012年第2期85卷 023402-023402页

作者： Bingbing Wang Yingchun Guo Jing Chen Zong-Chao Yan Panming Fu Laboratory of Optical Physics Beijing National Laboratory for Condensed Matter Physics Institute of Physics Chinese Academy of Sciences Beijing 100190 China State Key Laboratory of Precision Spectroscopy Department of Physics East China Normal University Shanghai 200062 China Key Laboratory of High Energy Density Physics Simulation CAPT Peking University Beijing 100084 China Institute of Applied Physics and Computational Mathematics P.O. Box 8009 Beijing 100088 China Department of Physics University of New Brunswick Fredericton New Brunswick Canada E3B 5A3

We study nonsequential double ionization (NSDI) processes of an atom by applying the frequency-domain theory based on the nonperturbative quantum electrodynamics. We obtain the transition formulas that describe the NSDI processes caused by the collision ionization (CI) and the collision-excitation ionization (CEI) mechanisms. By analyzing the NSDI results of each above-threshold ionization (ATI) channel, we investigate the contributions to the NSDI from the backward and forward collisions. In particular, for the CI process, the backward collision makes a major contribution to the NSDI probability, whereas for the CEI process, it depends on the characteristics of the laser-atom system: if the energy that the recolliding electron needs to excite a bound electron is much larger than the laser photon energy, such as for the case of helium in this work, the backward collision dominates the contribution; otherwise, the forward collision dominates the contribution. We also discuss the source of interference fringes in the NSDI momentum spectra due to the CI mechanism and find that the fringes can be predicted by using a simple cosine function. This work can be regarded as a development of the frequency-domain theory, which may shed light on the study of multiparticle dynamics in intense laser fields.

关键词： Atom lasers

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FORMATION AND DESTRUCTION OF JETS IN X-RAY BINARIES

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International Journal of Modern physics: Conference Series 2012年第8期ijmpcs卷 287-292页

作者： IOANNIS CONTOPOULOS NIKOLAOS D. KYLAFIS DEMOSTHENES KAZANAS DIMITRIS M. CHRISTODOULOU Research Center for Astronomy and Applied Mathematics Academy of Athens Athens 11527 Greece University of Crete Physics Department & Institute of Theoretical & Computational Physics Heraklion 71003 Crete Greece Foundation for Research and Technology-Hellas Heraklion 71110 Crete Greece NASA/GSFC Code 663 Greenbelt MD 20771 USA Department of Mathematical Sciences University of Massachusetts Lowell Lowell MA 01854 USA

Neutron-star and black hole X-ray binaries (XRBs) exhibit radio jets, whose properties depend on the X-ray spectral state and history of the source. There is general agreement about the type of the accretion disk around the compact object in the various spectral states. What is missing is a physical explanation for the appearance, disappearance, and re-appearance of jets. We will demonstrate that by invoking a simple physical mechanism proposed more than ten years ago, the so-called Poynting-Robertson Cosmic Battery (PRCB), we can explain in a natural way the disk–jet connection in XRBs.

关键词： accretion disks X-ray binaries black holes magnetic fields

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