Universality of ac conduction in disordered solids

The striking similarity of ac conduction in quite different disordered soli ds is discussed in terms of experimental results, modeling, and computer si mulations. After giving an overview of experiment, a macroscopic and a micr oscopic model are reviewed. For both models the normalized ac conductivity as a function of a suitably scaled frequency becomes independent of details of the disorder in the extreme disorder limit, i.e., when the local random ly varying mobilities cover many orders of magnitude. The two universal ac conductivities are similar, but not identical; both are examples of unusual non-power-law universalities. It is argued that ac universality reflects a n underlying percolation determining de as well as ac conductivity in the e xtreme disorder limit. Three analytical approximations to the universal ac conductivities are presented and compared to computer simulations. Finally, model predictions are briefly compared to experiment.