Unified ab initio thermal transport theory for insulators*

In insulators, heat is carried by phonons. Standard ab initio methods to get the thermal properties of solids rely on the phonon quasiparticles being well-defined, and assume that the lowest order 3-phonon scattering sufficiently describes thermal transport. Here we show that this is not the case for weakly bonded solids, where phonon scattering is so strong that the standard phonon quasiparticle picture can break down, and 3-phonon scattering is insufficient to explain the experiments. To address this issue, we present a novel ab initio method that features an anharmonic many-body renormalization scheme to create well-defined phonon quasiparticles with weakened interactions, and includes both 3-phonon and 4-phonon scattering to obtain thermal transport properties [1]. With this approach, we accurately capture the measured phonon dispersions, thermal expansion and thermal conductivity of both weakly bonded solids like sodium chloride and strongly bonded solids like diamond. Our work presents a unified ab initio framework to accurately predict the thermal properties of solids with varying bond strengths.

[1] N. K. Ravichandran and D. Broido, Phys. Rev. B. 98, 085205 (2018).