Ion dynamics and correlations

In solid electrolytes the mobile ions tend to perform a highly correlated kind of hopping motion. Experimentally, detection of the correlation requires time resolved techniques, as, e.g., measurement of the frequency dependent electrical conductivity. Empirically, two universalities have been reported; the universal dielectric response and the new universality. Conceptually, two different kinds of correlated hopping can be discerned and visualized in terms of time dependent single particle potentials. One is translational, yielding the universal-dielectric-response behaviour. The other is localized (reorientational), often dominating the conductivity spectra at high frequencies and low temperatures. Examples will be given and the power-law behaviour of both effects will be discussed. In particular, the new universality will be shown to arise by superposition of the two phenomena.