INTERNAL-FRICTION AND SOUND-VELOCITY OF POLYCRYSTALLINE METALS AT VERY-LOW TEMPERATURES

We have investigated the acoustical properties of polycrystalline Ag, Al, Cu, Nb, Nb with a Cu-matrix, Nb48Ti52, Pt and Ta at frequencies of 0.1 kHz less than or equal to v less than or equal to 8 kHz and in th e temperature range 0.04 mK less than or equal to T less than or equal to 1 K. The polycrystalline samples Al, Nb, Nb with a Cu-matrix, NbTi and Ta in the superconducting state as well as normal conducting Ag, Cu and Pt show qualitatively the same acoustical behaviour (with simil ar strain dependence) as observed for amorphous, dielectric materials. The glass-like ''anomalies'' can be understood assuming the existence of tunneling systems with a broad spectrum of energy splitting and re laxation rates. When the acoustical intensity is of the order of the t hermal energy, non-linear effects are observed. We show and discuss; a n anomalous time dependence of the sound velocity in Pt as well as a s train-dependent and temperature-independent internal friction in Cu.