Acoustic properties of amorphous PdSiCu at very low temperatures

We have studied in detail the temperature and strain dependence of the acou stic properties of the metallic glass PdSiCu in the temperature range 0.2 m K less than or equal to T less than or equal to 200 mK with the vibrating r eed technique at 980 Hz. At very low temperatures the sound velocity variat ion Delta nu/nu deviates from the expected logarithmic dependence with incr easing acoustic intensity. Strikingly, the internal friction Q(-1) shows al so a logarithmic decrease with decreasing temperature, instead of the plate au predicted in the kHz range by the standard tunneling model. We show that the extrapolated data to zero strain unveil the behaviour expected within the tunneling model. (C) 2000 Elsevier Science B.V. All rights reserved.