Microscopic origin of low-energy excitations in superionic glasses.

Abstract

A low-temperature ultrasonic study of AgI-${\mathrm{Ag}}_{2}$O-${\mathrm{B}}_{2}$${\mathrm{O}}_{3}$ superionic glasses is presented. We show that for T<10 K the acoustic behavior is governed by the two-level-system (TLS) phonon-assisted relaxation. The analysis of the sound-velocity data seems to suggest that TLS-phonon interactions are essentially governed by a two-phonon or first-order Raman relaxation process for T>3 K. The TLS density of states P\ifmmode\bar\else\textasciimacron\fi{} does not follow the recently proposed empirical law, that P\ifmmode\bar\else\textasciimacron\fi{} should be an exponentially increasing function of ${T}_{G}^{\mathrm{\ensuremath{-}}1}$, ${T}_{G}$ being the glass transition temperature. By calculating the tunneling frequency for the silver ions, which are subjected to thermally activated jumps at high temperatures (T>77 K), values have been obtained which exclude them as a microscopic origin of TLS's, and some alternative hypotheses are proposed.