A new paradigm for the low-T glassy-like thermal properties of solids

TL;DR

This paper proposes a universal framework linking low-temperature thermal properties of solids, including glasses and ordered structures, to diffusive and optical-like modes, challenging the notion that these features are solely due to disorder.

Contribution

It introduces a new paradigm connecting boson peaks and linear specific heat to diffusive and optical modes, extending explanations beyond disordered systems.

Findings

Linear in T specific heat linked to low-energy diffusive modes

Boson peak excess can be caused by gapped optical-like modes

Ordered and minimally disordered structures exhibit similar thermal features

Abstract

Glasses and disordered materials are known to display anomalous features in the density of states, in the specific heat and in thermal transport. Nevertheless, in recent years, the question whether these properties are really anomalous (and peculiar of disordered systems) or rather more universal than previously thought, has emerged. New experimental and theoretical observations have questioned the origin of the boson peak and the linear in T specific heat exclusively from disorder and TLS. The same properties have been indeed observed in ordered or minimally disordered compounds and in incommensurate structures for which the standard explanations are not applicable. Using the formal analogy between phason modes (e.g. in quasicrystals and incommensurate lattices) and diffusons, and between amplitude modes and optical phonons, we suggest the existence of a more universal physics behind these properties. In particular, we strengthen the idea that linear in T specific heat is linked to low energy diffusive modes and that a BP excess can be simply induced by gapped optical-like modes.