Electron renormalization of sound interaction with two-level systems in superconducting metallic glasses

Abstract

The crossing of the temperature dependences of sound velocity in the normal and the superconducting state of metallic glasses indicates renormalization of the intensity of sound interaction with two-level systems (TLS’s) caused by their coupling with electrons. In this paper we examine different theoretical approaches to a quantitative description of the renormalization, using the results of a low-temperature ultrasonic investigation of Zr41.2Ti13.8Cu12.5Ni10Be22.5 amorphous alloy. It is shown that the adiabatic renormalization of the coherent tunneling amplitude can explain only part of the whole effect observed in the experiment. There exists another mechanism of the renormalization affecting only nearly symmetric TLS’s, which may be associated with the effect of electron density fluctuations on the interwell potential.