The Experts below are selected from a list of 324 Experts worldwide ranked by ideXlab platform
Gen Tatara - One of the best experts on this subject based on the ideXlab platform.
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thermal Vector Potential theory of magnon driven magnetization dynamics
Physical Review B, 2015Co-Authors: Gen TataraAbstract:Thermal Vector Potential formulation is applied to study the thermal dynamics of magnetic structures for insulating ferromagnets. By separating the variables of the magnetic structure and the magnons, the equation of motion for the structure, including spin-transfer effect because of thermal magnons, is derived for the cases of a domain wall and a vortex. The magnon current is evaluated based on the linear response theory with the thermal Vector Potential representing the temperature gradient. The velocity of a domain wall when driven by thermal magnons exhibits a strong temperature dependence unlike the case of an electrically driven domain wall in metals.
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thermal Vector Potential theory of transport induced by a temperature gradient
Physical Review Letters, 2015Co-Authors: Gen TataraAbstract:: A microscopic formalism to calculate thermal transport coefficients is presented based on a thermal Vector Potential, whose time derivative is related to a thermal force. The formalism is free from the unphysical divergences reported to arise when Luttinger's formalism is applied naively, because the equilibrium ("diamagnetic") currents are treated consistently. The mathematical structure for the thermal transport coefficients is shown to be identical with that for the electric ones if the electric charge is replaced by the energy. The results indicate that the thermal Vector Potential couples to the energy current via the minimal coupling.
Oszkar Biro - One of the best experts on this subject based on the ideXlab platform.
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A Transient Current Vector Potential to Consider the Rotor Excitation of Synchronous Machines Under Short Circuit Condition
IEEE Transactions on Magnetics, 2015Co-Authors: Stefan Riegler, Oszkar Biro, Gebhard WallingerAbstract:This paper presents a method to approximately take account of the rotor excitation field of a synchronous machine under maximum aperiodic short-circuit condition using an impressed current Vector Potential in a 3-D transient finite-element analysis. The method is applied to compute end-region phenomena like end-winding forces or eddy current losses in conducting machine parts where the effects of the rotor excitation field cannot be neglected. A complex 3-D geometry model, the numerical method, and the computation results like end-winding forces are presented. The transient analysis of the electromagnetic field has been performed using the T, Φ-Φ formulation solved with an in-house software.
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the coulomb gauged Vector Potential formulation for the eddy current problem in general geometry well posedness and numerical approximation
Computer Methods in Applied Mechanics and Engineering, 2007Co-Authors: Oszkar Biro, Alberto ValliAbstract:In this paper we prove that, in a general geometric situation, the Coulomb gauged Vector Potential formulation of the eddy-current problem for the time-harmonic Maxwell equations is well-posed, i.e., its solution exists and is unique. Moreover, a quasi-optimal error estimate for its finite element approximation with nodal elements is proved. To illustrate the performances of the finite element algorithm, some numerical results are also presented.
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on the use of the magnetic Vector Potential in the nodal and edge finite element analysis of 3d magnetostatic problems
IEEE Transactions on Magnetics, 1996Co-Authors: Oszkar Biro, Kurt Preis, K R RichterAbstract:An overview of various finite element techniques based on the magnetic Vector Potential for the solution of three-dimensional magnetostatic problems is presented. If nodal finite elements are used for the approximation of the Vector Potential, a lack of gauging results in an ill-conditioned system. The implicit enforcement of the Coulomb gauge dramatically improves the numerical stability, but the normal component of the Vector Potential must be allowed to be discontinuous on iron/air interfaces. If the Vector Potential is is interpolated with the aid of edge finite elements and no gauge is enforced, a singular system results. It can be solved efficiently by conjugate gradient methods, provided care is taken to ensure that the current density is divergence free. Finally, if a tree-cotree gauging of the Vector Potential is introduced, the numerical stability depends on how the tree is selected with no obvious optimal choice available.
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computation of 3 d current driven skin effect problems using a current Vector Potential
IEEE Conference on Electromagnetic Field Computation, 1992Co-Authors: Oszkar Biro, Kurt Preis, Werner Renhart, G Vrisk, K R RichterAbstract:A finite element formulation of current-driven eddy current problems in terms of a current Vector Potential and a magnetic scalar Potential is developed. Since the traditional T- Omega method enforces zero net current in conductors, an impressed current Vector Potential T/sub 0/ is introduced in both conducting and nonconducting regions, describing an arbitrary current distribution with the prescribed net current in each conductor. The function T/sub 0/ is represented by edge elements, while nodal elements are used to approximate the current Vector Potential and the magnetic scalar Potential. The tangential component of T is set to zero on the conductor-nonconductor interfaces. The method is validated by computing the solution to an axisymmetric problem. Problems involving a coil with several turns wound around an iron core are solved. >
Gongbo Zhao - One of the best experts on this subject based on the ideXlab platform.
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f r gravity on non linear scales the post friedmann expansion and the Vector Potential
Journal of Cosmology and Astroparticle Physics, 2015Co-Authors: D Thomas, Marco Bruni, Kazuya Koyama, Gongbo ZhaoAbstract:Many modified gravity theories are under consideration in cosmology as the source of the accelerated expansion of the universe and linear perturbation theory, valid on the largest scales, has been examined in many of these models. However, smaller non-linear scales offer a richer phenomenology with which to constrain modified gravity theories. Here, we consider the Hu-Sawicki form of f(R) gravity and apply the post-Friedmann approach to derive the leading order equations for non-linear scales, i.e. the equations valid in the Newtonian-like regime. We reproduce the standard equations for the scalar field, gravitational slip and the modified Poisson equation in a coherent framework. In addition, we derive the equation for the leading order correction to the Newtonian regime, the Vector Potential. We measure this Vector Potential from f(R) N-body simulations at redshift zero and one, for two values of the fR0 parameter. We find that the Vector Potential at redshift zero in f(R) gravity can be close to 50% larger than in GR on small scales for |fR0|=1.289 × 10−5, although this is less for larger scales, earlier times and smaller values of the fR0 parameter. Similarly to in GR, the small amplitude of this Vector Potential suggests that the Newtonian approximation is highly accurate for f(R) gravity, and also that the non-linear cosmological behaviour of f(R) gravity can be completely described by just the scalar Potentials and the f(R) field.
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f r gravity on non linear scales the post friedmann expansion and the Vector Potential
arXiv: General Relativity and Quantum Cosmology, 2015Co-Authors: D Thomas, Marco Bruni, Kazuya Koyama, Gongbo ZhaoAbstract:Many modified gravity theories are under consideration in cosmology as the source of the accelerated expansion of the universe and linear perturbation theory, valid on the largest scales, has been examined in many of these models. However, smaller non-linear scales offer a richer phenomenology with which to constrain modified gravity theories. Here, we consider the Hu-Sawicki form of $f(R)$ gravity and apply the post-Friedmann approach to derive the leading order equations for non-linear scales, i.e. the equations valid in the Newtonian-like regime. We reproduce the standard equations for the scalar field, gravitational slip and the modified Poisson equation in a coherent framework. In addition, we derive the equation for the leading order correction to the Newtonian regime, the Vector Potential. We measure this Vector Potential from $f(R)$ N-body simulations at redshift zero and one, for two values of the $f_{R_0}$ parameter. We find that the Vector Potential at redshift zero in $f(R)$ gravity can be close to 50\% larger than in GR on small scales for $|f_{R_0}|=1.289\times10^{-5}$, although this is less for larger scales, earlier times and smaller values of the $f_{R_0}$ parameter. Similarly to in GR, the small amplitude of this Vector Potential suggests that the Newtonian approximation is highly accurate for $f(R)$ gravity, and also that the non-linear cosmological behaviour of $f(R)$ gravity can be completely described by just the scalar Potentials and the $f(R)$ field.
Alberto Valli - One of the best experts on this subject based on the ideXlab platform.
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the coulomb gauged Vector Potential formulation for the eddy current problem in general geometry well posedness and numerical approximation
Computer Methods in Applied Mechanics and Engineering, 2007Co-Authors: Oszkar Biro, Alberto ValliAbstract:In this paper we prove that, in a general geometric situation, the Coulomb gauged Vector Potential formulation of the eddy-current problem for the time-harmonic Maxwell equations is well-posed, i.e., its solution exists and is unique. Moreover, a quasi-optimal error estimate for its finite element approximation with nodal elements is proved. To illustrate the performances of the finite element algorithm, some numerical results are also presented.
D Thomas - One of the best experts on this subject based on the ideXlab platform.
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f r gravity on non linear scales the post friedmann expansion and the Vector Potential
Journal of Cosmology and Astroparticle Physics, 2015Co-Authors: D Thomas, Marco Bruni, Kazuya Koyama, Gongbo ZhaoAbstract:Many modified gravity theories are under consideration in cosmology as the source of the accelerated expansion of the universe and linear perturbation theory, valid on the largest scales, has been examined in many of these models. However, smaller non-linear scales offer a richer phenomenology with which to constrain modified gravity theories. Here, we consider the Hu-Sawicki form of f(R) gravity and apply the post-Friedmann approach to derive the leading order equations for non-linear scales, i.e. the equations valid in the Newtonian-like regime. We reproduce the standard equations for the scalar field, gravitational slip and the modified Poisson equation in a coherent framework. In addition, we derive the equation for the leading order correction to the Newtonian regime, the Vector Potential. We measure this Vector Potential from f(R) N-body simulations at redshift zero and one, for two values of the fR0 parameter. We find that the Vector Potential at redshift zero in f(R) gravity can be close to 50% larger than in GR on small scales for |fR0|=1.289 × 10−5, although this is less for larger scales, earlier times and smaller values of the fR0 parameter. Similarly to in GR, the small amplitude of this Vector Potential suggests that the Newtonian approximation is highly accurate for f(R) gravity, and also that the non-linear cosmological behaviour of f(R) gravity can be completely described by just the scalar Potentials and the f(R) field.
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f r gravity on non linear scales the post friedmann expansion and the Vector Potential
arXiv: General Relativity and Quantum Cosmology, 2015Co-Authors: D Thomas, Marco Bruni, Kazuya Koyama, Gongbo ZhaoAbstract:Many modified gravity theories are under consideration in cosmology as the source of the accelerated expansion of the universe and linear perturbation theory, valid on the largest scales, has been examined in many of these models. However, smaller non-linear scales offer a richer phenomenology with which to constrain modified gravity theories. Here, we consider the Hu-Sawicki form of $f(R)$ gravity and apply the post-Friedmann approach to derive the leading order equations for non-linear scales, i.e. the equations valid in the Newtonian-like regime. We reproduce the standard equations for the scalar field, gravitational slip and the modified Poisson equation in a coherent framework. In addition, we derive the equation for the leading order correction to the Newtonian regime, the Vector Potential. We measure this Vector Potential from $f(R)$ N-body simulations at redshift zero and one, for two values of the $f_{R_0}$ parameter. We find that the Vector Potential at redshift zero in $f(R)$ gravity can be close to 50\% larger than in GR on small scales for $|f_{R_0}|=1.289\times10^{-5}$, although this is less for larger scales, earlier times and smaller values of the $f_{R_0}$ parameter. Similarly to in GR, the small amplitude of this Vector Potential suggests that the Newtonian approximation is highly accurate for $f(R)$ gravity, and also that the non-linear cosmological behaviour of $f(R)$ gravity can be completely described by just the scalar Potentials and the $f(R)$ field.
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computing general relativistic effects from newtonian n body simulations frame dragging in the post friedmann approach
Physical Review D, 2014Co-Authors: Marco Bruni, D Thomas, David WandsAbstract:We present the first calculation of an intrinsically relativistic quantity, the leading-order correction to Newtonian theory, in fully nonlinear cosmological large-scale structure studies. Traditionally, nonlinear structure formation in standard ΛCDM cosmology is studied using N-body simulations, based on Newtonian gravitational dynamics on an expanding background. When one derives the Newtonian regime in a way that is a consistent approximation to the Einstein equations, the first relativistic correction to the usual Newtonian scalar Potential is a gravitomagnetic Vector Potential, giving rise to frame dragging. At leading order, this Vector Potential does not affect the matter dynamics, thus it can be computed from Newtonian N-body simulations. We explain how we compute the Vector Potential from simulations in ΛCDM and examine its magnitude relative to the scalar Potential, finding that the power spectrum of the Vector Potential is of the order 10 −5 times the scalar power spectrum over the range of nonlinear scales we consider. On these scales the Vector Potential is up to two orders of magnitudes larger than the value predicted by second-order perturbation theory extrapolated to the same scales. We also discuss some possible observable effects and future developments.
