Nonlinear response theory for Markov processes: Simple models for glassy relaxation

Abstract

The theory of nonlinear response for Markov processes obeying a master equation is formulated in terms of time-dependent perturbation theory for the Green's functions and general expressions for the response functions up to third order in the external field are given. The nonlinear response is calculated for a model of dipole reorientations in an asymmetric double well potential, a standard model in the field of dielectric spectroscopy. The static nonlinear response is finite with the exception of a certain temperature T_{0} determined by the value of the asymmetry. In a narrow temperature range around T_{0}, the modulus of the frequency-dependent cubic response shows a peak at a frequency on the order of the relaxation rate and it vanishes for both low frequencies and high frequencies. At temperatures at which the static response is finite (lower and higher than T_{0}), the modulus is found to decay monotonously from the static limit to zero at high frequencies. In addition, results of calculations for a trap model with a Gaussian density of states are presented. In this case, the cubic response depends on the specific dynamical variable considered and also on the way the external field is coupled to the kinetics of the model. In particular, a set of different dynamical variables that gives rise to identical shapes of the linear susceptibility and only to different temperature dependencies of the relaxation times is considered. It is found that the frequency dependence of the nonlinear response functions, however, strongly depends on the particular choice of the variables. The results are discussed in the context of recent theoretical and experimental findings regarding the nonlinear response of supercooled liquids and glasses.

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Cooperativity and Heterogeneity in Plastic Crystals Studied by Nonlinear Dielectric Spectroscopy

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Nonlinear Response of Inertial Tracers in Steady Laminar Flows: Differential and Absolute Negative Mobility

“…For weak perturbations of an equilibrium state, the fluctuation-dissipation theorem (FDT) solves the problem, expressing the system response in terms of correlation functions [1]. Generalizations of this result have been recently derived, to address the much more complex issue of predicting the response in nonequilibrium conditions [2][3][4][5][6], when detailed balance does not hold and currents cross the system, or in the nonlinear response regime, where higher order response functions have to be taken into account [7][8][9]. All these approaches point out the role played by the coupling among degrees of freedom which emerges out of equilibrium, adding extra-terms to the standard FDT [10][11][12][13][14].…”