Valley contrasting chiral phonons in monolayer hexagonal lattices

TL;DR

This paper explores valley contrasting chiral phonons in monolayer hexagonal lattices, revealing their properties, selection rules, and potential for valley-based electronic and phononic applications.

Contribution

It introduces the concept of valley contrasting chiral phonons with quantized pseudo angular momentum and predicts their role in valley phonon Hall effects in monolayer materials.

Findings

Chiral phonons with valley contrasting properties are identified.

Selection rules for intervalley electron-phonon scattering are derived.

Valley phonon Hall effect is predicted and characterized.

Abstract

In monolayer hexagonal lattices, two inequivalent valleys appear in the Brillouin zone. With inversion symmetry breaking, we find chiral phonons with valley contrasting circular polarization and ionic magnetic moment. At valley centers, there is a three-fold rotational symmetry endowing phonons with a quantized pseudo angular momentum, which includes spin and orbital parts. From conservation of the pseudo angular momentum, crystal momentum and energy, selection rules in intervalley scattering of electrons by phonons are obtained. The chiral valley phonons are verified and the selection rules are predicted in monolayer Molybdenum disulfide. Due to valley contrasting phonon Berry curvature, one can also detect a valley phonon Hall effect. The valley-contrasting chiral phonon, together with phonon circular polarization, ionic magnetic moment, phonon pseudo angular momentum, valley phonon Hall effect, will form the basis for valley-based electronics and phononics applications in the future.