Microscopic origin of low-energy excitations in superionic glasses

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¯ does not follow the recently proposed empirical law, that P¯ should be an exponentially increasing function of $T_G^{\mathrm{-}1}$, $T_G$ 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.