Chiral valley phonons and flat phonon bands in moiré materials

A chiral object is not the same as its mirror image. The chirality of quasiparticles is a critical concept behind many modern developments of condensed matter physics. Some recent examples include Klein tunneling in graphene and the chiral magnetic effect due to chiral fermions in three-dimensional semi-metals. Here, we show that phonons, bosonic collective excitations, in moiré patterns created with WSe2 can be chiral. Furthermore, we observe the flattening of bands near the phononic band-gap edges in these systems. Our findings, which are expected to be generic for moiré materials created with two-dimensional materials that break inversion symmetry, are important in understanding electron-phonon, exciton-phonon scattering, and in designing efficient phononic crystals.