  The phonon thermal transport properties of twisted bi-layer graphene are
investigated using lattice dynamics and the Boltzmann transport equation. The
thermal conductivities of 13.2 and 21.8 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 unitcell 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 strength of flexural
interatomic interactions and since these flexural phonons carry majority of the
heat in bilayer graphene (65% at room temperature), the thermal conductivity is
sensitive to layer twisting. Our study suggests that twisting will affect
thermal transport only for those materials that have a major contribution from
flexural phonon modes.
