Moir\'e phonons in graphene/hexagonal boron nitride moir\'e superlattice

TL;DR

This paper theoretically investigates in-plane acoustic phonons in graphene/hBN moiré superlattices, revealing hybridized phonon bands, flat modes, and angular momentum effects, with implications for understanding moiré phonon properties.

Contribution

It introduces a continuum and spring-mass network model to describe moiré phonon bands and compares flat-band origins in G/hBN and twisted bilayer graphene.

Findings

Hybridized moiré phonon bands with flat and dispersive modes

Flat-band modes interpreted as vibrations of decoupled strings

Moiré phonons exhibit angular momentum due to symmetry breaking

Abstract

We theoretically study in-plane acoustic phonons of graphene/hexagonal boron nitride moir\'e superlattice by using a continuum model. We demonstrate that the original phonon bands of individual layers are strongly hybridized and reconstructed into moir\'e phonon bands consisting of dispersive bands and flat bands. The phonon band structure can be effectively described by a spring-mass network model to simulate the motion of moir\'e domain walls, where the flat-band modes are interpreted as vibrations of independent, decoupled strings. We also show that the moir\'e phonon has angular momentum due to the inversion symmetry breaking by hBN, with high amplitudes concentrated near narrow gap region. Finally, we apply the same approach to twisted bilayer graphene, and we find a notable difference between the origins of the flat-band modes in G/hBN and TBG, reflecting distinct geometric structures of domain pattern.