Ultrasonic attenuation and velocity studies of amorphous PdSiCu at low temperatures

Abstract

Measurements of ultrasonic attenuation and velocity changes were carried out on the metallic glass ${\mathrm{Pd}}_{0.775}$ ${\mathrm{Si}}_{0.165}$ ${\mathrm{Cu}}_{0.06}$ as a function of amplitude, in the frequency range 10 to 90 MHz, for $0.3<~T<~10$ K. A comparison of the experimental results with the predictions of the current two-level-system (TLS) tunneling theory is presented. The amplitude-dependent attentuation changes observed in these studies are larger by a factor of 100 to 1000 than the values obtained from current TLS theory, with the use of parameters determined experimentally at higher frequencies by other workers. Below ∼1 K these attenuation changes have a linear (rather than quadratic) dependence on frequency and very weak temperature dependence (rather than $\frac{1}{T}$). The saturated (amplitude-independent) attenuation was found to have a very weak temperature dependence, rather than $T^3$ dependence. A significant amplitude dependence of the slope of the sound velocity change as a function of $\ln T$ was also observed.