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Fases moduladas em filmes magnéticos ultrafinos com anisotropia perpendicular : modelos e simulações

Nicolao, Lucas
Fonte: Universidade Federal do Rio Grande do Sul Publicador: Universidade Federal do Rio Grande do Sul
Tipo: Tese de Doutorado Formato: application/pdf
POR
Relevância na Pesquisa
26.96%
Neste trabalho estudamos os fenômenos observados em filmes magnéticos ultrafinos com anisotropia perpendicular, onde domínios razoavelmente regulares de faixas dominam o regime de baixas temperaturas a campo nulo. Esses domínios magnéticos de faixas constituem uma realização de fases moduladas presentes em inúmeros sistemas físicos, químicos e biológicos, e são resultados da presença de interaçães competindo em diferentes escalas espaciais. No caso de interesse, a competição entre as interações de troca e dipolar levam à estabilidade de uma estrutura de domínios de faixas que possuem ambas ordens translacional anisotrópica e orientacional, semelhantes às encontradas em filmes de cristais líquidos. Através de um modelo escalar de Landau Ginzburg que captura a formação dos domínios de faixas nos filmes magnéticos ultrafinos com anisotropia perpendicular, estudamos o efeito das flutuações térmicas atuando nas escalas de comprimento introduzidas pela competição entre as interações, que, aliadas à baixa dimensionalidade do problema, estabelecem fases ordenadas de baixa temperatura com ordem de quase longo alcance, onde defeitos topológicos exercem um papel fundamental. Introduzimos uma técnica de simulação de Langevin para o modelo de Landau Ginzburg...

Langevin simulation of scalar fields: Additive and multiplicative noises and lattice renormalization

Cassol-Seewald, N. C.; Farias, R. L. S.; Fraga, E. S.; Krein, Gastão Inácio; Ramos, Rudnei O.
Fonte: Elsevier B.V. Publicador: Elsevier B.V.
Tipo: Artigo de Revista Científica Formato: 4088-4099
ENG
Relevância na Pesquisa
36.87%
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES); Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq); Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP); We consider the Langevin lattice dynamics for a spontaneously broken 44 scalar field theory where both additive and multiplicative noise terms are incorporated. The lattice renormalization for the corresponding stochastic Ginzburg-Landau-Langevin and the subtleties related to the multiplicative noise are investigated. (C) 2012 Elsevier B.V. All rights reserved.

Hot Brownian carriers in the Langevin picture: Application to a simple model for the Gunn effect in GaAs

Bauke, F. C.; Lagos, R. E.
Fonte: Universidade Estadual Paulista Publicador: Universidade Estadual Paulista
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
36.74%
We consider a charged Brownian gas under the influence of external, static and uniform electric and magnetic fields, immersed in a uniform bath temperature. We obtain the solution for the associated Langevin equation, and thereafter the evolution of the nonequilibrium temperature towards a nonequilibrium (hot) steady state. We apply our results to a simple yet relevant Brownian model for carrier transport in GaAs. We obtain a negative differential conductivity regime (Gunn effect) and discuss and compare our results with the experimental results. © 2013.

Classes de soluções para a equação de Langevin generalizada

Santos, Fabiano Fortunato Teixeira dos
Fonte: Universidade de Brasília Publicador: Universidade de Brasília
Tipo: Tese
POR
Relevância na Pesquisa
37.03%
Tese (doutorado)—Universidade de Brasília, Departamento de Matemática, 2011.; Para a equação de Langevin generalizada (ELG) governada por um ruído de cauda pesada, determinamos duas classes de soluções. Neste caso, ao contrário da equação de Langevin clássica, o cálculo de Itô não pode ser aplicado para obter soluções em média quadrática. Nossa abordagem baseia-se nas propriedades da transformada de Laplace para processos estáveis e na identificação da ELG como uma equação de Volterra estocástica. Para o índice de estabilidade 1 < α ≤ 2 mostramos que a conjectura de A. V. Medino [24], é realmente uma classe de soluções em probabilidade. Além disso, mostramos que algumas séries de Fourier-Stieltjes aleatórias convergem para a solução da ELG e discutimos o papel do índice de estabilidade no modo de convergência. ______________________________________________________________________________ ABSTRACT; For the Generalized Langevin Equation (GLE) driven by heavy-tailed noise we derive several classes of solutions. In this case, unlike the classical Langevin Equation case, the Ito’s calculus cannot be applied to obtain mean square solutions. Our approach relies on the properties of Laplace transforms for stable processes and on the identification of GLE as Volterra stochastic integro-differential equation. For stability index 1 < α ≤ 2 we show that Medino’s conjecture [24] is indeed a class of solutions in probability. Moreover...

Langevin simulations with colored noise and non-Markovian dissipation

Farias,R. L. S.; Ramos,R. O.; Silva,L. A. da
Fonte: Sociedade Brasileira de Física Publicador: Sociedade Brasileira de Física
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/09/2008 EN
Relevância na Pesquisa
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The nonequilibrium dynamics of an homogeneous scalar field is studied using Langevin equations. Microscopic derivations based on quantum field theory methods can lead to complicated nonlocal equations of motion. Here we study, numerically, the results obtained by appropriately approximating these equations in a local form (the Markovian approximation) and compare with results obtained with suitable prescriptions for accounting for the nonlocal terms, i.e. the non-Markovian form. We use a prescription for the nonlocal equations motivated by the results obtained from previous derivations using nonequilibrium quantum field theory methods.

Erratum to: Langevin SM, Houseman EA, Christensen BC, Wiencke JK, Nelson HH, Karagas MR, et al. The influence of aging, environmental exposures and local sequence features on the variation of DNA methylation in blood. Epigenetics 2011; 6

Langevin, Scott M.; Houseman, E. Andrés; Christensen, Brock C.; Wiencke, John K.; Nelson, Heather H.; Karagas, Margaret R.; Marsit, Carmen J.; Kelsey, Karl T.
Fonte: Landes Bioscience Publicador: Landes Bioscience
Tipo: Artigo de Revista Científica
Publicado em 01/06/2012 EN
Relevância na Pesquisa
36.87%
Authors Scott M. Langevin and E. Andres Houseman contributed equally and should be listed as joint first authors in Epigenetics Volume 6, Issue 7 in the manuscript: Langevin SM, Houseman EA, Christensen BC, Wiencke JK, Nelson HH, Karagas MR, et al. The influence of aging, environmental exposures and local sequence features on the variation of DNA methylation in blood. Epigenetics 2011; 6:908-19.

Langevin Type Equations in Nonequilibrium State: Derivation and Applications to Linear Response Theory and Scattering Cross Section

Furukawa, Hiroshi
Fonte: Oxford University Press Publicador: Oxford University Press
Tipo: Artigo de Revista Científica Formato: text/html
EN
Relevância na Pesquisa
26.96%
The generalizations of the Langevin equation and the fluctuation-dissipation theorem are discussed in nonequilibrium states. The existence of the general form of the fluctuation-dissipation theorem associated with the Langevin type equation in nonequilibrium states is shown on the basis of a kinematical consideration. The equation of motion for the average fluctuations of dynamical variables in the presence of external fields is discussed within the linear response theory and is reduced to a more useful form by means of the Langevin type equation. The scattering cross section of particles from a system of interacting particles in a nonequilibrium state is also considered in connection with the Langevin type equation.

Um estudo na teoria do movimento brownianno com viscosidade variável

Silva, João Maria da
Fonte: Universidade Federal do Rio Grande do Norte; BR; UFRN; Programa de Pós-Graduação em Física; Física da Matéria Condensada; Astrofísica e Cosmologia; Física da Ionosfera Publicador: Universidade Federal do Rio Grande do Norte; BR; UFRN; Programa de Pós-Graduação em Física; Física da Matéria Condensada; Astrofísica e Cosmologia; Física da Ionosfera
Tipo: Dissertação Formato: application/pdf
POR
Relevância na Pesquisa
27.08%
In this work we investigate the stochastic behavior of a large class of systems with variable damping which are described by a time-dependent Lagrangian. Our stochastic approach is based on the Langevin treatment describing the motion of a classical Brownian particle of mass m. Two situations of physical interest are considered. In the first one, we discuss in detail an application of the standard Langevin treatment (white noise) for the variable damping system. In the second one, a more general viewpoint is adopted by assuming a given expression to the so-called collored noise. For both cases, the basic diffententiaql equations are analytically solved and al the quantities physically relevant are explicitly determined. The results depend on an arbitrary q parameter measuring how the behavior of the system departs from the standard brownian particle with constant viscosity. Several types of sthocastic behavior (superdiffusive and subdiffusive) are obteinded when the free pamameter varies continuosly. However, all the results of the conventional Langevin approach with constant damping are recovered in the limit q = 1; Coordenação de Aperfeiçoamento de Pessoal de Nível Superior; Neste trabalho nós investigamos o comportamento estocástico de uma grande classe de sistemas com amortecimento variável descritos por uma lagraniana dependente do tempo. Nossa abordagem estocástica é baseada no formalismo de Langevin descrevendo o comportamento de uma partícula browniana clássica de massa m. Duas situações de interesse físico são consideradas. Inicialmente...

Notice biographique Mgr. Jean Langevin Sa mort et ses funerailles.

Langevin, Jean, Mgr.
Fonte: Quebec, C. Darveau, Publicador: Quebec, C. Darveau,
Tipo: Artigo de Revista Científica
FRE
Relevância na Pesquisa
36.74%

Langevin representation of laser heating in PIC simulations

Detering, Frank; Bychenknov, V.Y.; Rozmus, F; Capjack, C.E.; Huller, S; Masson-Laborde, P E; Pesme, D; Detering, F; Sydora, R; Maximov, A
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
36.87%
An algorithm for inverse bremsstrahlung heating based on a Langevin equation, suitable for particle-in-cell (PIC) codes, is presented. We consider a quasi-neutral plasma with laser heating as described by inverse bremsstrahlung. This enables the inclusion

Complex Langevin Dynamics for chiral Random Matrix Theory

Mollgaard, A.; Splittorff, K.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 17/09/2013
Relevância na Pesquisa
26.96%
We apply complex Langevin dynamics to chiral random matrix theory at nonzero chemical potential. At large quark mass the simulations agree with the analytical results while incorrect convergence is found for small quark masses. The region of quark masses for which the complex Langevin dynamics converges incorrectly is identified as the region where the fermion determinant frequently traces out a path surrounding the origin of the complex plane during the Langevin flow. This links the incorrect convergence to an ambiguity in the Langevin force due to the presence of the logarithm of the fermion determinant in the action.; Comment: 23 pages, 10 figures

To the methodology of low temperature technologies: mathematical analysis of Langevin forces in superconductor

Tolbatov, Iogann
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 17/10/2009
Relevância na Pesquisa
27.03%
Langevin forces are investigated in the framework of the phenomenological Ginzburg-Landau (GL) theory. These stochastic forces introducted in the time-dependent Ginzburg-Landau (TDGL) equation describing the superconductor transport properties above the critical temperature model the fluctuations action. We assume that there exists a profound connection between the fluctuation Cooper pair energy spectrum and the Langevin forces possible values spectrum. In investigation carried out on the basis of that hypothesis we obtain the analytical expression for Langevin forces defining them by means of order parameter, i.e., by means of the fluctuation Cooper pair wave function. Langevin forces properties are analyzed. The conlusion about their complex nature is done. Asymptotic analysis of Langevin forces is performed.; Comment: 8 pages, 0 figures

Comparison of Langevin and Markov channel noise models for neuronal signal generation

Sengupta, B.; Laughlin, S. B.; Niven, J. E.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 16/12/2009
Relevância na Pesquisa
27.08%
The stochastic opening and closing of voltage-gated ion channels produces noise in neurons. The effect of this noise on the neuronal performance has been modelled using either approximate or Langevin model, based on stochastic differential equations or an exact model, based on a Markov process model of channel gating. Yet whether the Langevin model accurately reproduces the channel noise produced by the Markov model remains unclear. Here we present a comparison between Langevin and Markov models of channel noise in neurons using single compartment Hodgkin-Huxley models containing either $Na^{+}$ and $K^{+}$, or only $K^{+}$ voltage-gated ion channels. The performance of the Langevin and Markov models was quantified over a range of stimulus statistics, membrane areas and channel numbers. We find that in comparison to the Markov model, the Langevin model underestimates the noise contributed by voltage-gated ion channels, overestimating information rates for both spiking and non-spiking membranes. Even with increasing numbers of channels the difference between the two models persists. This suggests that the Langevin model may not be suitable for accurately simulating channel noise in neurons, even in simulations with large numbers of ion channels.

Langevin power curve analysis for numerical WEC models with new insights on high frequency power performance

Mücke, Tanja A.; Wächter, Matthias; Milan, Patrick; Peinke, Joachim
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 05/11/2015
Relevância na Pesquisa
27.03%
Based on the Langevin equation it has been proposed to obtain power curves for wind turbines from high frequency data of wind speed measurements u(t) and power output P (t). The two parts of the Langevin approach, power curve and drift field, give a comprehensive description of the conversion dynamic over the whole operating range of the wind turbine. The method deals with high frequent data instead of 10 min means. It is therefore possible to gain a reliable power curve already from a small amount of data per wind speed. Furthermore, the method is able to visualize multiple fixed points, which is e.g. characteristic for the transition from partial to full load or in case the conversion process deviates from the standard procedures. In order to gain a deeper knowledge it is essential that the method works not only for measured data but also for numerical wind turbine models and synthetic wind fields. Here, we characterize the dynamics of a detailed numerical wind turbine model and calculate the Langevin power curve for different data samplings. We show, how to get reliable results from synthetic data and verify the applicability of the method for field measurements with ultra-sonic, cup and Lidar measurements. The independence of the fixed points on site specific turbulence effects is also confirmed with the numerical model. Furthermore...

New Langevin and Gradient Thermostats for Rigid Body Dynamics

Davidchack, R. L.; Ouldridge, T. E.; Tretyakov, M. V.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
27.17%
We introduce two new thermostats, one of Langevin type and one of gradient (Brownian) type, for rigid body dynamics. We formulate rotation using the quaternion representation of angular coordinates; both thermostats preserve the unit length of quaternions. The Langevin thermostat also ensures that the conjugate angular momenta stay within the tangent space of the quaternion coordinates, as required by the Hamiltonian dynamics of rigid bodies. We have constructed three geometric numerical integrators for the Langevin thermostat and one for the gradient thermostat. The numerical integrators reflect key properties of the thermostats themselves. Namely, they all preserve the unit length of quaternions, automatically, without the need of a projection onto the unit sphere. The Langevin integrators also ensure that the angular momenta remain within the tangent space of the quaternion coordinates. The Langevin integrators are quasi-symplectic and of weak order two. The numerical method for the gradient thermostat is of weak order one. Its construction exploits ideas of Lie-group type integrators for differential equations on manifolds. We numerically compare the discretization errors of the Langevin integrators, as well as the efficiency of the gradient integrator compared to the Langevin ones when used in the simulation of rigid TIP4P water model with smoothly truncated electrostatic interactions. We observe that the gradient integrator is computationally less efficient than the Langevin integrators. We also compare the relative accuracy of the Langevin integrators in evaluating various static quantities and give recommendations as to the choice of an appropriate integrator.; Comment: 16 pages...

Langevin dynamics with constraints and computation of free energy differences

Lelievre, Tony; Rousset, Mathias; Stoltz, Gabriel
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
27.03%
In this paper, we consider Langevin processes with mechanical constraints. The latter are a fundamental tool in molecular dynamics simulation for sampling purposes and for the computation of free energy differences. The results of this paper can be divided into three parts. (i) We propose a simple discretization of the constrained Langevin process based on a standard splitting strategy. We show how to correct the scheme so that it samples {\em exactly} the canonical measure restricted on a submanifold, using a Metropolis rule in the spirit of the Generalized Hybrid Monte Carlo (GHMC) algorithm. Moreover, we obtain, in some limiting regime, a consistent discretization of the overdamped Langevin (Brownian) dynamics on a submanifold, also sampling exactly the correct canonical measure with constraints. The corresponding numerical methods can be used to sample (without any bias) a probability measure supported by a submanifold. (ii) For free energy computation using thermodynamic integration, we rigorously prove that the longtime average of the Lagrange multipliers of the constrained Langevin dynamics yields the gradient of a rigid version of the free energy associated with the constraints. A second order time discretization using the Lagrange multipliers is proposed. (iii) The Jarzynski-Crooks fluctuation relation is proved for Langevin processes with mechanical constraints evolving in time. An original numerical discretization without time-step error is proposed. Numerical illustrations are provided for (ii) and (iii).

Geometric Langevin equations on submanifolds and applications to the stochastic melt-spinning process of nonwovens and biology

Grothaus, Martin; Stilgenbauer, Patrik
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 25/04/2012
Relevância na Pesquisa
27.17%
In this article we develop geometric versions of the classical Langevin equation on regular submanifolds in euclidean space in an easy, natural way and combine them with a bunch of applications. The equations are formulated as Stratonovich stochastic differential equations on manifolds. The first version of the geometric Langevin equation has already been detected before by Leli\`evre, Rousset and Stoltz with a different derivation. We propose an additional extension of the models, the geometric Langevin equations with velocity of constant absolute value. The latters are seemingly new and provide a galaxy of new, beautiful and powerful mathematical models. Up to the authors best knowledge there are not many mathematical papers available dealing with geometric Langevin processes. We connect the first version of the geometric Langevin equation via proving that its generator coincides with the generalized Langevin operator proposed by Soloveitchik, Jorgensen and Kolokoltsov. All our studies are strongly motivated by industrial applications in modeling the fiber lay-down dynamics in the production process of nonwovens. We light up the geometry occuring in these models and show up the connection with the spherical velocity version of the geometric Langevin process. Moreover...

A quantum Langevin formulation of risk-sensitive optimal control

James, Matthew
Fonte: Institute of Physics Publishing Publicador: Institute of Physics Publishing
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
36.87%
In this paper we formulate a risk-sensitive optimal control problem for continuously monitored open quantum systems modelled by quantum Langevin equations. The optimal controller is expressed in terms of a modified conditional state, which we call a risk-sensitive state, that represents measurement knowledge tempered by the control purpose. One of the two components of the optimal controller is dynamic, a filter that computes the risk-sensitive state. The second component is an optimal control feedback function that is found by solving the dynamic programming equation. The optimal controller can be implemented using classical electronics. The ideas are illustrated using an example of feedback control of a two-level atom.

Generalised Langevin equation for nonequilibrium systems

McPhie, M G; Daivis, P; Snook, Ian K; Ennis-King, Jonathan Patrick; Evans, Denis
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
37.03%
We derive a generalized Langevin Equation (GLE) for the evolution of an arbitrary phase variable in a classical fluid that is initially at equilibrium and is subsequently subjected to an external field and a thermostat of the type used in nonequilibrium molecular dynamics computer simulations. The nonequilibrium steady state GLE is shown to have the same form as the equilibrium GLE at long times. We use our generalized Langevin equation to derive a simple Langevin equation for the motion of a single heavy Brownian particle in a solvent under steady state planar shear.

Stretching a Single Diblock Copolymer in a Selective Solvent: Langevin Dynamics Simulations

Edwards, Scott; Williams, David
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
36.74%
Using the Langevin dynamics technique, we have carried out simulations of a single-chain flexible diblock copolymer. The polymer consists of two blocks of equal length, one very poorly solvated and the other close to θ-conditions. We study what happens w