Origin of low-temperature tunneling states in glasses

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 Tg, 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 such centers form per atom, 1% of which contribute to the heat capacity below 1 K.