Scaling and universality of AC conduction in disordered solids

Recent scaling results for the AC conductivity of ionic glasses by Roling et al. [Phys. Rev. Lett. vol 78, 2160 (1997)] and Sidebottom [Phys. Rev. Lett. vol 82, 3653 (1999)] are discussed. It is shown that Sidebottom's version of scaling is completely general. A new analytical approximation to the universal AC conductivity of hopping in the extreme disorder limit, the ``diffusion cluster approximation,'' is presented and compared to simulations and experiments.