Understanding phonon thermal transport in twisted bilayer graphene

The phonon thermal transport properties of twisted bilayer graphene are investigated using lattice dynamics and the Boltzmann transport equation. The thermal conductivities of $13.{2}^{\ensuremath{\circ}}$ and $21.{8}^{\ensuremath{\circ}}$ twisted configurations are 56 and 36% lower than the untwisted configuration, which has a room temperature thermal conductivity of 2260 W/m K. The reason for this lower thermal conductivity is unraveled from phonon mode-level analysis made possible through the untwisting of layers. Due to a large commensurate unit cell of twisted configuration, the Brillouin zone is folded but this folding has no bearing on the phonon scattering phase space. The major impact of twisting is felt by flexural phonons with out-of-plane vibrations via the change in the strength of flexural interatomic interactions, and since these flexural phonons carry the majority of the heat in bilayer graphene (65% at room temperature) the thermal conductivity is sensitive to layer twisting. Our paper suggests that twisting will affect thermal transport only for those materials that have a major contribution from flexural phonon modes.