Chiral valley phonons and flat phonon bands in moire materials

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 degrees and close to 60 degrees. In particular, multiple chiral nondegenerate K/K' valley phonons are found for twist angles near 60 degrees whereas no nondegenerate chiral modes are found for twist angles close to 0 degrees. Moreover, we discover two sets of emergent chiral valley modes that originate from an inversion symmetry breaking at the moire scale and find similar modes in moire 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 twisted bilayer WSe2. Our findings are relevant for understanding electron-phonon and exciton-phonon scattering in moire materials and also for the design of phononic analogues of flat band electrons.