Origin of Low-Temperature Tunneling States in Glasses

Abstract

The low-temperature tunneling levels observed in glasses are discussed in terms of the free-volume model. As the system falls out of thermodynamic equilibrium near its glass transition temperature $T_g$, liquid clusters are frozen in. This Letter proposes that voids are formed within the liquid clusters as they in turn freeze at lower temperatures. Approximately ${10}^{-4\mathrm{}}$ such centers form per atom, 1% of which contribute to the heat capacity below 1 K.