Symmetry origin of lattice vibration modes in twisted multilayer graphene: Phasons versus moiré phonons

Abstract

Lattice dynamics play a crucial role in the physics of Moir\'e systems. In twisted bilayer graphene (TBG), it was shown that, in addition to the graphene phonons, there is another set of gapless excitations termed Moir\'e Phonons [Phys. Rev. B, 075416, 2019] reflecting the lattice dynamics at the Moire superlattice level. These modes were later suggested to be phasons due to the incommensurate stacking of the two graphene layers [Phys. Rev. B, 155426, 2019]. In this work, we elucidate the equivalence of these two seemingly distinct perspectives by identifying an underlying symmetry, which we dub mismatch symmetry, that exists for any twist angle. For commensurate angles, this is a discrete symmetry whereas for incommensurate angles, it is equivalent to a continuous phase symmetry giving rise to phason modes. In the small angle limit, such symmetry becomes a continuous local symmetry whose spontaneous breaking gives rise to Moir\'e phonons as its Goldstone mode. We derive an effective field theory for these collective modes in TBG in precise agreement with the full model and discuss their different properties. Our analysis is then generalized to twisted multilayer graphene (TMG) where we identify higher order mismatch and deduce the count of gapless modes including graphene phonons, Moir\'e phonons and phasons. Especially, we study twisted mirror-symmetric trilayer graphene with an alternating twist angle $\theta$ and find that it can be mapped to a TBG with the re-scaled twist angle $\sqrt{2/3}\theta$, hosting the same Moir\'e phonon modes in the even mirror sector with an additional set of gapped modes in the odd sector. Our work presents a systematic study of lattice symmetries in TMG providing insights into its unique lattice dynamics.