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Sep 22, 2013
09/13

by
S. M. Tang; S. N. Zhang

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The amplification caused by gravitational light bending by compact objects in a foreground galaxy can affect the apparent number density of background QSOs, as well as their distribution in the fields of galactic halos. In this work we investigate the number enhancement of QSOs in the fields of galactic halos caused by point mass lensing effect and singular isothermal lensing effect, and apply the microlensing effect due to dark compact objects in the halo to NGC 3628. NGC 3628 is a...

Source: http://arxiv.org/abs/astro-ph/0409009v3

2
2.0

Jun 30, 2018
06/18

by
Jonathan Squire; Hong Qin; William M. Tang

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Variational symplectic algorithms have recently been developed for carrying out long-time simulation of charged particles in magnetic fields. As a direct consequence of their derivation from a discrete variational principle, these algorithms have very good long-time energy conservation, as well as exactly preserving discrete momenta. We present stability results for these algorithms, focusing on understanding how explicit variational integrators can be designed for this type of system. It is...

Topics: Physics, Plasma Physics, Computational Physics

Source: http://arxiv.org/abs/1401.6725

4
4.0

Jun 30, 2018
06/18

by
Jonathan Squire; Hong Qin; William M. Tang

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A fully variational, unstructured, electromagnetic particle-in-cell integrator is developed for integration of the Vlasov-Maxwell equations. Using the formalism of Discrete Exterior Calculus, the field solver, interpolation scheme and particle advance algorithm are derived through minimization of a single discrete field theory action. As a consequence of ensuring that the action is invariant under discrete electromagnetic gauge transformations, the integrator exactly conserves Gauss's law.

Topics: Physics, Accelerator Physics, Plasma Physics, Computational Physics

Source: http://arxiv.org/abs/1401.6723

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63

Sep 21, 2013
09/13

by
J. Squire; H. Qin; W. M. Tang; C. Chandre

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We present a new variational principle for the gyrokinetic system, similar to the Maxwell-Vlasov action presented in Ref. 1. The variational principle is in the Eulerian frame and based on constrained variations of the phase space fluid velocity and particle distribution function. Using a Legendre transform, we explicitly derive the field theoretic Hamiltonian structure of the system. This is carried out with a modified Dirac theory of constraints, which is used to construct meaningful brackets...

Source: http://arxiv.org/abs/1301.6066v2

2
2.0

Jun 29, 2018
06/18

by
X. L. Chen; C. Yang; L. F. Zhong; M. Tang

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A first-order percolation transition, called explosive percolation, was recently discovered in evolution networks with random edge selection under a certain restriction. However, the network percolation with more realistic evolution mechanisms such as preferential attachment has not yet been concerned. We propose a tunable network percolation model by introducing a hybrid mechanism of edge selection into the Bohman-Frieze-Wormald model, in which a parameter adjusts the relative weights between...

Topics: Social and Information Networks, Computing Research Repository, Physics, Physics and Society

Source: http://arxiv.org/abs/1604.00110

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32

Sep 18, 2013
09/13

by
Pierre Carrier; Jok M. Tang; Yousef Saad; James K. Freericks

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Inhomogeneous dynamical mean-field theory has been employed to solve many interesting strongly interacting problems from transport in multilayered devices to the properties of ultracold atoms in a trap. The main computational step, especially for large systems, is the problem of calculating the inverse of a large sparse matrix to solve Dyson's equation and determine the local Green's function at each lattice site from the corresponding local self-energy. We present a new efficient algorithm,...

Source: http://arxiv.org/abs/1102.3137v2

2
2.0

Jun 30, 2018
06/18

by
C. L. Ellison; J. W. Burby; J. M. Finn; H. Qin; W. M. Tang

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Backward error initialization and parasitic mode control are well-suited for use in algorithms that arise from a discrete variational principle on phase-space dynamics. Dynamical systems described by degenerate Lagrangians, such as those occurring in phase-space action principles, lead to variational multistep algorithms for the integration of first-order differential equations. As multistep algorithms, an initialization procedure must be chosen and the stability of parasitic modes assessed....

Topics: Physics, Mathematics, Numerical Analysis, Computational Physics

Source: http://arxiv.org/abs/1403.0890

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Sep 19, 2013
09/13

by
B. L. Sheu; R. C. Myers; J. -M. Tang; N. Samarth; D. D. Awschalom; P. Schiffer; M. E. Flatté

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We develop a quantitatively predictive theory for impurity-band ferromagnetism in the low-doping regime of GaMnAs and compare with experimental measurements of a series of samples whose compositions span the transition from paramagnetic insulating to ferromagnetic conducting behavior. The theoretical Curie temperatures depend sensitively on the local fluctuations in the Mn-hole binding energy, which originates from disorder in the Mn distribution as well as the presence of As antisite defects....

Source: http://arxiv.org/abs/0708.1063v1

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49

Sep 23, 2013
09/13

by
J. A. Baumgaertel; E. A. Belli; W. Dorland; W. Guttenfelder; G. W. Hammett; D. R. Mikkelsen; G. Rewoldt; W. M. Tang; P. Xanthopoulos

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The nonlinear gyrokinetic code GS2 has been extended to treat non-axisymmetric stellarator geometry. Electromagnetic perturbations and multiple trapped particle regions are allowed. Here, linear, collisionless, electrostatic simulations of the quasi-axisymmetric, three-field period National Compact Stellarator Experiment (NCSX) design QAS3-C82 have been successfully benchmarked against the eigenvalue code FULL. Quantitatively, the linear stability calculations of GS2 and FULL agree to within...

Source: http://arxiv.org/abs/1109.4558v1

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Sep 18, 2013
09/13

by
A. M. Yakunin; A. Yu. Silov; P. M. Koenraad; J. -M. Tang; M. E. Flatté; W. Van Roy; J. De Boeck; J. H. Wolter

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The local density of states of Mn-Mn pairs in GaAs is mapped with cross-sectional scanning tunneling microscopy and compared with theoretical calculations based on envelope-function and tight-binding models. These measurements and calculations show that the crosslike shape of the Mn-acceptor wavefunction in GaAs persists even at very short Mn-Mn spatial separations. The resilience of the Mn-acceptor wave-function to high doping levels suggests that ferromagnetism in GaMnAs is strongly...

Source: http://arxiv.org/abs/cond-mat/0505536v2

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46

Sep 21, 2013
09/13

by
A. M. Yakunin; A. Yu. Silov; P. M. Koenraad; J. H. Wolter; W. Van Roy; J. De Boeck; J. -M. Tang; M. E. Flatte

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The wave function of a hole bound to an individual Mn acceptor in GaAs is spatially mapped by scanning tunneling microscopy at room temperature and an anisotropic, cross-like shape is observed. The spatial structure is compared with that from an envelope-function, effective mass model, and from a tight-binding model. This demonstrates that anisotropy arising from the cubic symmetry of the GaAs crystal produces the cross-like shape for the hole wave-function. Thus the coupling between Mn dopants...

Source: http://arxiv.org/abs/cond-mat/0402019v1

4
4.0

Jun 29, 2018
06/18

by
H. L. Tian; J. R. Zhang; L. L. Yan; M. Tang; L. Hu; D. X. Zhao; Y. X. Qiu; H. Y. Zhang; J. Zhuang; R. Du

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China Spallation Neutron Source (CSNS) is the first high-performance pulsed neutron source in China, which will meet the increasing fundamental research and technique applications demands domestically and overseas. A new distributed data processing and analysis environment has been developed, which has generic functionalities for neutron scattering experiments. The environment consists of three parts, an object-oriented data processing framework adopting a data centered architecture, a...

Topics: Data Analysis, Statistics and Probability, Physics

Source: http://arxiv.org/abs/1605.04053

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51

Jul 20, 2013
07/13

by
M. Bozkurt; M. R. Mahani; P. Studer; J. -M. Tang; S. R. Schofield; N. J. Curson; M. E Flatte; A. Yu. Silov; C. F. Hirjibehedin; C. M. Canali; P. M. Koenraad

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We investigate the effect of an external magnetic field on the physical properties of the acceptor hole states associated with single Mn acceptors placed near the (110) surface of GaAs. Crosssectional scanning tunneling microscopy images of the acceptor local density of states (LDOS) show that the strongly anisotropic hole wavefunction is not significantly affected by a magnetic field up to 6 T. These experimental results are supported by theoretical calculations based on a tightbinding model...

Source: http://arxiv.org/abs/1304.3303v2