Nonlinear response from the perspective of energy landscapes and beyond

The paper discusses the nonlinear response of disordered systems. In particular we show how the nonlinear response can be interpreted in terms of properties of the potential energy landscape. It is shown why the use of relatively small systems is very helpful for this approach. For a standard model system we check which system sizes are particular suited. In case of the driving of a single particle via an external force the concept of an effective temperature helps to scale the force dependence for different temperature on a single master curve. In all cases the mobility increases with increasing external force. These results are compared with a stochastic process described by a 1d Langevin equation where a similar scaling is observed. Furthermore it is shown that for different classes of disordered systems the mobility can also decrease with increasing force. The results can be related to the properties of the chosen potential energy landscape. Finally, results for the crossover from the linear to the nonlinear conductivity of ionic liquids are presented, inspired by recent experimental results in the Roling group. Apart from a standard imidazolium-based ionic liquid we study a system which is characterized by a low conductivity as compared to other ionic liquids and very small nonlinear effects. We show via a real space structural analysis that for this system a particularly strong pair formation is observed and that the strength of the pair formation is insensitive to the application of strong electric fields. Consequences of this observation are discussed.