IS THERE A UNIVERSAL EXPLANATION FOR THE UNIVERSAL DYNAMIC-RESPONSE

Structurally disordered solid electrolytes, both crystalline and glassy, as well as ionic melts, exhibit a set of spectroscopic peculiarities that are at variance with the predictions of simple random-hopping models. Typically, stretched exponential functions are encountered where ordinary exponential functions are expected. The effect has come to be called "universal dynamic response". In this paper, it is traced back to a mean square displacement which, in a broad time regime, varies as time to the power of beta, with 0 < beta < 1. Computer simulations as well as model considerations show that this particular shape of the mean square displacement is caused by the frequent occurrence of correlated forward-backward hopping sequences. The latter processes result from the mutual repulsive Coulomb interaction of the mobile ions.