Physica Scripta T66, 196 (1996)

Pablo Esquinazi

Abstract

We have investigated the relative change of sound velocity and intemal friction in amorphous and polycrystalline solids at frequencies 0.l kHz < v <20 kHz and in the temperature range 0.04mK < T < 10 K. We found that the temperature dependence of the acoustic properties at 0.1 K < T < 1 K can be well understood within the standard tunneling model assuming the existing of tunneling system with a broad spectrum of energy splitting and relaxation rate. AtT < 0.1 K the sound velocity is strongly strain amplitude dependent and the sound velocity and internal friction show substantial deviations from the standard tunneling model. The acoustic properties of polycrystalline metals resemble those of amorphous dielectrics with the same strain dependence. In contrast to amorphous superconductors no difference in the acoustic properties has been observed between the normal and superconducting states of polycrystalline superconductors. At T > 1 K the sound velocity decreases linearly with temperature. This dependence can be explained qantitatively by a new relaxation mechanism arising from overdamped tunneling systems.