Flexural phonons and thermal transport in multilayer graphene and graphite

We present a theory for the lattice thermal conductivity κ_L of multilayer graphene (MLG) and graphite, which is based on an exact numerical solution of the Boltzmann equation for phonons. Dominant contributions to κ_L from out-of-plane or flexural phonons are found, which is consistent with previous findings for single-layer graphene (SLG). However, the interaction between graphene layers in MLG and graphite breaks a selection rule on phonon-phonon scattering, causing their κ_Ls to be much lower than that of SLG. C13 isotopes are shown to be an important scattering mechanism, accounting for an ∼15% additional drop in the κ_L of these systems. We demonstrate that the κ_L values converge to that of graphite after only about five layers, a consequence of weak interlayer coupling. These findings are qualitatively consistent with recent measurements of κ_L for MLG.