Image of a chiral phonon in the moire cell

Chiral valley phonons and flat phonon bands in moiré materials

I. Maity, A. A. Mostofi and J. Lischner, Chiral valley phonons and flat phonon bands in moiré materials, Phys. Rev. B Letters 105, L041408 (2022).

Chirality–the characteristic of an object that can be distinguished from its mirror image–plays a fundamental role in understanding many phenomena in physics, chemistry, and biology. Recently, the study of chirality in phonons has attracted significant interest. The interaction of chiral phonons with other quasiparticles present in a crystal is relevant for understanding and controlling many electronic and optical phenomena.

A particularly promising platform for observing and manipulating chiral phonons are twisted bilayers of two-dimensional (2D) materials. Since the discovery of flat electronic bands in twisted bilayer graphene, such moiré materials have served as a rich playground for investigating the properties of correlated electrons, excitons and phonons. In this work, we demonstrate that phonons in moiré materials can also be chiral. Focusing on twisted WSe2 at twist angles close to 60 degress, we find two sets of emergent chiral moiré valley phonons that originate from symmetry breaking at the moiré scale. We also discover flat chiral phonon bands in the energy gap between the acoustic and optical phonon modes, which, similar to the flat electronic bands in this system, occur over a range of twist angles and without the requirement of a magic angle.

Our findings, which are expected to be generic for moiré systems composed of two-dimensional materials that break inversion symmetry, are relevant for understanding electron-phonon and exciton-phonon scattering, and also for the design of phononic analogues of flat band electrons.