Some facts, old and new, about the low temperature thermal properties of noncrystalline solids

Abstract

The low energy vibrational excitations of perfect crystals seen in specific heat and thermal conductivity measurements can be described in terms of Debye phonons. In amorphous solids, however, the specific heat is dominated by another kind of excitations, which also couple to the phonons and thus apparently determine the universal low temperature thermal conductivity of this class of solids. A phenomenological model of tunneling states with a uniform density of states has been very successful in describing a wide range of investigations of these excitations. Of particular importance was the discovery of a logarithmic time dependence of the specific heat, which was predicted by this model. It now appears well established that thermal equilibrium in an amorphous solid at low temperatures cannot be reached on any time scale. A physical model of the tunneling states is still lacking. The major hurdle is that of finding an entity which is common to all amorphous solids, and which leads to a thermal conductivity of the same magnitude to within a factor of ten. The symposium following this lecture will review recent efforts to create the same excitations in appropriately disordered crystalline solids, in which the microscopic structures and the excitations would be easier to understand.