Specific heat of twisted bilayer graphene: Engineering phonons by atomic plane rotations

We have studied the phonon specific heat in single-layer, bilayer, and twisted bilayer graphene. The calculations were performed using the Born-von Karman model of lattice dynamics for intralayer atomic interactions and spherically symmetric interatomic potential for interlayer interactions. We found that at temperature T< 15 K, specific heat varies with temperature as T-n, where n=1 for graphene, n=1.6 for bilayer graphene, and n=1.3 for the twisted bilayer graphene. The phonon specific heat reveals an intriguing dependence on the twist angle in bilayer graphene, which is particularly pronounced at low temperature. The results suggest a possibility of phonon engineering of thermal properties of layered materials by twisting the atomic planes. (C) 2014 AIP Publishing LLC.