Propagative and diffusive regimes of acoustic damping in bulk amorphous material

TL;DR

This paper investigates how acoustic waves propagate and attenuate in amorphous silicon, revealing propagative, diffusive, and mixed regimes, and discusses implications for thermal transport modeling in disordered solids.

Contribution

It provides a detailed analysis of wave packet dynamics in amorphous silicon, identifying different regimes and the transition between them, enhancing understanding of thermal energy transport in glasses.

Findings

DHO fits estimate mean-free path below Ioffe-Regel limit

Diffusive regime dominates above Ioffe-Regel limit

Mixed propagative and diffusive regimes occur at intermediate frequencies

Abstract

In amorphous solids, a non-negligible part of thermal conductivity results from phonon scattering on the structural disorder. The conversion of acoustic energy into thermal energy is often measured by the Dynamical Structure Factor (DSF) thanks to inelastic neutron or X-Ray scattering. The DSF is used to quantify the dispersion relation of phonons, together with their damping. However, the connection of the dynamical structure factor with dynamical attenuation of wave packets in glasses is still a matter of debate. We focus here on the analysis of wave packets propagation in numerical models of amorphous silicon. We show that the DHO fits (Damped Harmonic Oscillator model) of the dynamical structure factors give a good estimate of the wave packets mean-free path, only below the Ioffe-Regel limit. Above the Ioffe-Regel limit and below the mobility edge, a pure diffusive regime without a definite mean free path is observed. The high-frequency mobility edge is characteristic of a transition to localized vibrations. Below the Ioffe-Regel criterion, a mixed regime is evidenced at intermediate frequencies, with a coexistence of propagative and diffusive wave fronts. The transition between these different regimes is analyzed in details and reveals a complex dynamics for energy transportation, thus raising the question of the correct modeling of thermal transport in amorphous materials.