Echoes from Anharmonic Normal Modes in Model Glasses

TL;DR

This paper demonstrates that classical models of glasses exhibit acoustic echoes caused by anharmonic vibrational modes, offering an alternative explanation to the traditional two-level systems theory for low-temperature glass behavior.

Contribution

It reveals that anharmonic vibrational modes in classical glass models can produce acoustic echoes, challenging the conventional two-level systems explanation.

Findings

Classical glass models show acoustic echoes due to anharmonic modes.

Echoes are observed in both repulsive and attractive particle models.

Results suggest an alternative mechanism for low-temperature glass echoes.

Abstract

Glasses display a wide array of nonlinear acoustic phenomena at temperatures $T\lesssim 1$ K. This behavior has traditionally been explained by an ensemble of weakly-coupled, two-level tunneling states, a theory that is also used to describe the thermodynamic properties of glasses at low temperatures. One of the most striking acoustic signatures in this regime is the existence of phonon echoes, a feature that has been associated with two-level systems with the same formalism as spin echoes in NMR. Here we report the existence of distinctly different type of acoustic echo in classical models of glassy materials. Our simulations consist of finite-ranged, repulsive spheres and also particles with attractive forces using Lennard-Jones interactions. We show that these echoes are due to anharmonic, weakly-coupled vibrational modes, and perhaps provide an alternative explanation for the phonon echoes observed in glasses at low temperatures.