# Unified theory of thermal transport in crystals and disordered solids

## TL;DR

This paper develops a unified theoretical framework for thermal transport in both crystals and disordered solids, bridging the gap between existing models and accurately predicting thermal properties across different regimes.

## Contribution

It introduces a comprehensive transport equation that encompasses both Peierls and Allen-Feldman limits, accounting for anharmonicity and disorder effects simultaneously.

## Key findings

- The derived equation reduces to known limits in respective regimes.
- Successfully predicts thermal conductivity of ultralow-conductivity thermoelectric materials.
- Provides a unified understanding of heat conduction mechanisms in diverse solids.

## Abstract

Crystals and glasses exhibit fundamentally different heat conduction mechanisms: the periodicity of crystals allows for the excitation of propagating vibrational waves that carry heat, as first discussed by Peierls; in glasses, the lack of periodicity breaks Peierls' picture and heat is mainly carried by the coupling of vibrational modes, often described by a harmonic theory introduced by Allen and Feldman. Anharmonicity or disorder are thus the limiting factors for thermal conductivity in crystals or glasses; hitherto, no transport equation has been able to account for both. Here, we derive such equation, resulting in a thermal conductivity that reduces to the Peierls and Allen-Feldman limits, respectively, in anharmonic-and-ordered or harmonic-and-disordered solids, while also covering the intermediate regimes where both effects are relevant. This approach also solves the long-standing problem of accurately predicting the thermal properties of crystals with ultralow or glass-like thermal conductivity, as we show with an application to a thermoelectric material representative of this class.