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

TL;DR

This paper develops a nonlinear response theory for Markov processes and applies it to models of glassy relaxation, revealing temperature-dependent behaviors and variable-dependent responses relevant to supercooled liquids and glasses.

Contribution

It formulates a general nonlinear response theory for Markov processes and applies it to simple glassy models, highlighting variable dependence and temperature effects.

Findings

Finite static nonlinear response except at a specific temperature T0

Peak in cubic response modulus near T0 at certain frequencies

Nonlinear response varies significantly with the choice of dynamical variables

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 is considered that gives rise to identical shapes of the linear susceptibility and only to different temperature dependencies of the relaxation times. 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.