Acoustic experiments on amorphous Pd30Zr70 and Cu30Zr70 in the superconducting and the normal state

Measurements of internal friction and sound velocity of amorphous Pd₃₀Zr₇₀ and Cu₃₀Zr₇₀ around 1 kHz and of Pd₃₀Zr₇₀ around 1 GHz are presented for both the superconducting and the normal state. Our experiments reveal several serious discrepancics with the predictions of the Tunneling Model and its extension to metallic glasses. Particularly from internal friction we conclude that the density of states of the tunneling systems is affected by the presence of conduction electrons. We discuss our results on the basis of recent theories which consider a slowing down of the tunneling motion due to the interaction with conduction electrons.Dedicated to K. Dransfeld on the occasion of his 60th birthday