Abstract
Nonlinear conductivity effects are studied experimentally and theoretically for thin samples of disordered ionic conductors. Experimentally observed nonlinear effects can be expressed by a length scale, which, in the literature, has often been interpreted as an effective hopping distance. This interpretation implies that it is possible to map a disordered hopping model system on a coarse-grained regular model and to identify the effective hopping distance with the distance between neighboring sites of the coarse-grained model. With an improved experimental setup, using ac electric fields and higher-order harmonic current detection, the results of older dc field experiments are checked and any undesired effects arising from Joule heating are excluded. The values we obtain for the length scale are up to 43 Å and are thus higher than typical values extracted from dc field experiments. Additionally, we explore the influence of temperature and sample thickness on the nonlinearity. Studying a simple disordered hopping model analytically we find that a mapping of a disordered system on a regular system is, in general, not justified. This invalidates the standard interpretation of the nonlinear conductivity experiments in terms of effective hopping distances. A possible future extension of the disordered hopping model is suggested to closer recover properties of real ion conductors and thus to improve the understanding of the information content of nonlinear conductivity experiments.
- Received 2 March 2005
DOI:https://doi.org/10.1103/PhysRevB.72.174304
©2005 American Physical Society