Sound absorption in glasses

TL;DR

This paper models sound wave absorption in glasses across temperature ranges using phenomenological models, revealing universal features and the influence of the energy landscape on relaxation behaviors.

Contribution

It extends existing models to describe sound absorption in glasses, incorporating the energy landscape and barrier density, and validates the approach with literature data.

Findings

Universal features of sound absorption in glasses are described by the tunneling and soft potential models.

Barrier density at the glass transition shows an exponential rise, reflecting the frozen viscous flow.

Mechanical relaxation indicates a temperature-independent barrier density, contrasting dielectric data.

Abstract

The paper presents a description of the sound wave absorption in glasses, from the lowest temperatures up to the glass transition, in terms of three compatible phenomenological models. Resonant tunneling, the rise of the relaxational tunneling to the tunneling plateau and the crossover to classical relaxation are universal features of glasses and are well described by the tunneling model and its extension to include soft vibrations and low barrier relaxations, the soft potential model. Its further extension to non-universal features at higher temperatures is the very flexible Gilroy-Phillips model, which allows to determine the barrier density of the energy landscape of the specific glass from the frequency and temperature dependence of the sound wave absorption in the classical relaxation domain. To apply it properly at elevated temperatures, one needs its formulation in terms of the shear compliance. As one approaches the glass transition, universality sets in again with an exponential rise of the barrier density reflecting the frozen fast Kohlrausch t^beta-tail (in time t, with beta close to 1/2) of the viscous flow at the glass temperature. The validity of the scheme is checked for literature data of several glasses and polymers with and without secondary relaxation peaks. The frozen Kohlrausch tail of the mechanical relaxation shows no indication of the strongly temperature-dependent barrier density observed in dielectric data of molecular glasses with hydrogen bonds. Instead, the mechanical relaxation data indicate an energy landscape describable with a frozen temperature-independent barrier density for any glass.