Truly chiral phonons in {\alpha}-HgS

TL;DR

This paper reports the discovery of truly chiral phonons in three-dimensional chiral crystals using circularly polarized Raman scattering and first-principles calculations, revealing their propagation and conservation of pseudo-angular momentum.

Contribution

It introduces the identification of propagating chiral phonons in bulk materials and demonstrates their conservation law, advancing understanding of phonon chirality in 3D systems.

Findings

Truly chiral phonons propagate in { extalpha}-HgS crystals.

The conservation of pseudo-angular momentum is confirmed.

Chiral phonons can be identified non-destructively via Raman scattering.

Abstract

Chirality is a manifestation of the asymmetry inherent in nature. It has been defined as the symmetry breaking of the parity of static objects, and the definition was extended to dynamic motion such that true and false chiralities were distinguished. Recently, rotating, yet not propagating, atomic motions were predicted and observed in two-dimensional materials, and they were referred to as "chiral phonons" . A natural development would be the discovery of truly chiral phonons that propagate while rotating in three-dimensional materials. Here, we used circularly polarised Raman scattering and first-principles calculations to identify truly chiral phonons in chiral bulk crystals. This approach enabled us to determine the chirality of a crystal in a non-contact and non-destructive manner. In addition, we demonstrated that the law of the conservation of pseudo-angular momentum holds between circularly polarised photons and chiral phonons. These findings are expected to help develop ways for transferring the pseudo-angular momentum from photons to electron spins via the propagating chiral phonons in opto-phononic-spintronic devices.