Theory of sound attenuation in amorphous solids from nonaffine motions

TL;DR

This paper develops a theoretical framework for understanding sound attenuation in amorphous solids, revealing the roles of diffusive and Rayleigh damping mechanisms and their dependence on microscopic and macroscopic properties.

Contribution

It introduces a novel analytical theory incorporating nonaffine displacements to predict both diffusive and Rayleigh damping in amorphous solids, aligning with experimental and simulation results.

Findings

Diffusive damping scales as k^2 and depends on microscopic viscous damping and shear modulus corrections.

Rayleigh damping scales as k^4, arising from nonaffine motions, not elastic heterogeneity.

The theory explains the crossover between damping regimes observed experimentally.

Abstract

We present a theoretical derivation of acoustic phonon damping in amorphous solids based on the nonaffine response formalism for the viscoelasticity of amorphous solids. The analytical theory takes into account the nonaffine displacements in transverse waves and is able to predict both the ubiquitous low-energy diffusive damping $\sim k^{2}$, as well as a novel contribution to the Rayleigh damping $\sim k^{4}$ at higher wavevectors and the crossover between the two regimes observed experimentally. The coefficient of the diffusive term is proportional to the microscopic viscous (Langevin-type) damping in particle motion (which arises from anharmonicity), and to the nonaffine correction to the static shear modulus, whereas the Rayleigh damping emerges in the limit of low anharmonicity, consistent with previous observations and macroscopic models. Importantly, the $k^4$ Rayleigh contribution derived here does not arise from harmonic disorder or elastic heterogeneity effects and it is the dominant mechanism for sound attenuation in amorphous solids as recently suggested by molecular simulations.