Weyl phonons in chiral crystals

TL;DR

This paper reveals the intrinsic connection between Weyl and chiral phonons in chiral crystals, demonstrating their entanglement through theoretical analysis and proposing Raman scattering to detect Weyl phonons and surface states.

Contribution

It unifies the concepts of Weyl and chiral phonons in chiral crystals and introduces Raman scattering as a method to observe Weyl phonons and surface states.

Findings

Weyl and chiral phonons are entangled in chiral crystals.

Raman scattering can detect Weyl phonons via energy splitting.

Obstructed phonon surface states are observed.

Abstract

Chirality is an indispensable concept that pervades fundamental science and nature, manifesting itself in diverse forms such as chiral quasiparticles and chiral structures. Of particular interest are Weyl phonons carrying specific Chern numbers and chiral phonons doing circular motions in crystals. Up to now, Weyl and chiral phonons have been studied independently and the interpretations of chirality seem to be different in these two concepts, impeding our understanding. Here, we demonstrate that Weyl and chiral phonons are entangled in chiral crystals. Employing a typical chiral crystal of elementary tellurium (Te) as a case study, we expound on the intrinsic relationship between Chern number of Weyl phonons and pseudo-angular momentum (PAM) of chiral phonons. In light of the mutual coupling, we propose Raman scattering as a new technique to demonstrate the existence of Weyl phonons in Te, by detecting the chirality-induced energy splitting between the two constituent chiral phonon branches for Weyl phonons. By using the same experimental approach, we also observe the obstructed phonon surface states for the first time.