We investigate the chirality of phonon modes in twisted bilayer WSe2 and demonstrate distinct chiral behavior of the K/K′ valley phonons for twist angles close to 0∘ and close to 60∘. In particular, multiple chiral non-degenerate K/K′ valley phonons are found for twist angles near 60∘ whereas no non-degenerate chiral modes are found for twist angles close to 0∘. 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.
Cite
@article{arxiv.2108.03965,
title = {Chiral valley phonons and flat phonon bands in moir\'e materials},
author = {Indrajit Maity and Arash A. Mostofi and Johannes Lischner},
journal= {arXiv preprint arXiv:2108.03965},
year = {2022}
}