Heat conduction theory including phonon coherence

TL;DR

This paper develops a new heat conduction theory that incorporates phonon coherence, combining wave and particle perspectives, supported by atomic simulations, to better understand thermal transport in solids.

Contribution

It introduces a novel formalism that accounts for phonon coherence effects in thermal conductivity, extending beyond the traditional phonon gas model.

Findings

Identification of two types of phonon coherence: intrinsic and mutual.

Derivation of a thermal conductivity expression including coherence times.

Simulations confirm the impact of coherence on thermal transport at different temperatures.

Abstract

Understanding and quantifying the fundamental physical property of coherence of thermal excitations is a long-standing and general problem in physics. The conventional theory, i.e. the phonon gas model, fails to describe coherence and its impact on thermal transport. In this letter, we propose a general heat conduction formalism supported by theoretical arguments and direct atomic simulations, which takes into account both the conventional phonon gas model and the wave nature of thermal phonons. By naturally introducing wavepackets in the heat flux from fundamental concepts, we derive an original thermal conductivity expression including coherence times and lifetimes. Our theory and simulations reveal two distinct types of coherence, i.e., intrinsic and mutual, appearing in two different temperature ranges. This contribution establishes a fundamental frame for understanding and quantifying the coherence of thermal phonons, which should have a general impact on the estimation of the thermal properties of solids.