Distribution of tunneling-level barrier heights in (KBr)1−x(KCN)x: Comparison of dielectric response and specific heat

Abstract

We have measured the frequency-dependent dielectric response of the glassy crystal (KBr${)}_{1\mathrm{-}x}$(KCN${)}_x$ for x=0.25, 0.5, 0.7, and 1.0. From the data on the pure crystal, KCN, we cal- culate the asymmetry energy of the two-level states which form the tunneling centers in the glassy compositions. Using the dielectric data we calculate the low-temperature specific heat and compare with experimental data. We find this calculated specific heat to have the same magnitude, composition dependence, and time dependence as seen in those experiments. This gives strong support to a microscopic theory of the glassy dynamics of this system. We also find the data to be consistent with a mean-field theory of the quadrupolar ordering transition in (KBr${)}_{1\mathrm{-}x}$(KCN${)}_x$. We relate the number of low-temperature tunneling levels to the quadrupolar transition temperature. The relevance of this microscopic model for understanding the glassy behavior in real structural glasses is discussed.