Thermal conductivity of amorphous germanium at low temperatures.

The amorphous tetrahedral semiconductors a-Ge and a-Si are currently under intense investigation from a fundamental as well as practical point of view. Many of the properties of these materials depend sensitively on the inhomogeneous microstructure, which consists of a mixture of high- and low- density regions. In this article we address the question of the existence in these amorphous semiconductors of the two-level tunneling systems which nearly universally dominate the properties of glasses at very low temperatures. We report measurements of the thermal conductivity of a-Ge films in the temperature range 0.003-5K under conditions which overcome problems in previous measurements. The data can be accounted for by scattering of the thermal phonons from two-level tunneling systems, cylindrical inhomogeneities, and the boundaries of the sample.