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

TL;DR

This paper investigates how electron interactions renormalize sound-TLS coupling in superconducting metallic glasses, revealing that multiple mechanisms contribute to the observed effects at low temperatures.

Contribution

It provides a comparative analysis of renormalization approaches and identifies an additional mechanism affecting symmetric TLS.

Findings

Adiabatic renormalization explains part of the effect

Another mechanism influences nearly symmetric TLS

Ultrasonic measurements support the proposed models

Abstract

The crossing of temperature dependencies 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) caused by their coupling with electrons. In this paper we have analyzed different approaches to a quantitative description of renormalization using the results of low-temperature ultrasonic investigation of ZrTiCuNiBe 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 renormalization affecting only nearly symmetric TLS.