STOCHASTIC MODELING OF ION DYNAMICS IN COMPLEX-SYSTEMS - DIPOLAR EFFECTS

The motion of charge carriers in many solid ionic conductors is determ ined both by long-range Coulomb interactions and by structural disorde r in the host matrix. While the dynamical behaviour at low frequencies is governed by long-range, thermally activated hopping processes, loc alized motions of the charge carriers with a much weaker temperature d ependence can dominate the relaxation spectra at higher frequencies. A fter a brief summary of the present status of stochastic models in exp laining the dynamical behaviour, we focus on the localized degrees of freedom, described as a random lattice of Brownian dipoles. From Brown ian dynamics simulations in two dimensions we find an ac response at i ntermediate frequencies which is of the power-law type, with exponents depending both on temperature and on the degree of randomness. Distin ct differences are observed in comparing the collective with the singl e-dipole response. In particular we discuss our results in relation to the high-frequency 'nearly-constant-loss regime' observed in ion-cond ucting glasses.