Transition from population- to coherence-dominated nondiffusive thermal transport

TL;DR

This paper introduces a Wigner Transport Equation-based method to analyze non-diffusive thermal transport, revealing size-dependent effects in specific materials at nanometer to micron scales.

Contribution

It develops a novel scheme using the Wigner Transport Equation to predict size effects and thermal conductivities in complex crystalline insulators.

Findings

Significant deviations from bulk thermal conductivity at nanometer to micron scales.

Application to CsPbBr3 and La2Zr2O7 predicts observable size effects.

Method incorporates phonon coherences and tunnelling effects.

Abstract

Deviations from diffusive heat transport in high thermal conductivity crystalline insulators are generally understood within the framework of the phonon Boltzmann Transport Equation. However, for low thermal conductivity materials with large primitive cells or strong anharmonicity, the recently developed Wigner Transport Equation is more appropriate as it includes tunnelling between overlapping phonon bands. In this work, via solutions to the Wigner Transport Equation, we develop a scheme to obtain the dynamics of the phonon populations and coherences as a function of an arbitrary heat source. The approach is applied to predict size effects and dynamical thermal conductivities in CsPbBr$_\text{3}$ and La$_\text{2}$Zr$_\text{2}$O$_\text{7}$ using first-principles data as input. We predict significant deviations from the bulk thermal conductivity in these materials at length scales on the order of hundreds of nanometers to a few microns at room temperature, well within the reach of direct observation using current experimental techniques.