Chiral valley phonons and flat phonon bands in moir\'e materials

TL;DR

This paper explores chiral valley phonons and flat phonon bands in twisted bilayer WSe2, revealing twist-angle-dependent chiral behaviors and emergent modes due to symmetry breaking, with implications for phonon-based device design.

Contribution

It uncovers twist-angle-dependent chiral phonon modes and emergent valley modes caused by symmetry breaking in moiré materials, and demonstrates flat phonon bands across various angles.

Findings

Chiral non-degenerate K/K' valley phonons near 60°

Emergent chiral valley modes from inversion symmetry breaking

Flat phonon bands observed between acoustic and optical modes

Abstract

We investigate the chirality of phonon modes in twisted bilayer WSe2 and demonstrate distinct chiral behavior of the $K/K^\prime$ valley phonons for twist angles close to $0^{\circ}$ and close to $60^{\circ}$. In particular, multiple chiral non-degenerate $K/K^\prime$ valley phonons are found for twist angles near $60^{\circ}$ whereas no non-degenerate chiral modes are found for twist angles close to $0^\circ$. Moreover, we discover two sets of emergent chiral valley modes that originate from an inversion symmetry breaking at the moir\'{e} scale and find similar modes in moir\'{e} patterns of strain-engineered bilayers WSe2 and MoSe2/WSe2 heterostructures. At the energy gap between acoustic and optical modes, the formation of flat phonon bands for a broad range of twist angles is observed in the twisted bilayer WSe2. Our findings are relevant for understanding electron-phonon and exciton-phonon scattering in moir\'e materials and also for the design of phononic analogues of flat band electrons.