Magneto-Raman signature of exciton-activated chiral phonon in MoS₂

The concept of chirality has been extended to the description of circularly polarized phonons in which the atomic circular motions produced nonzero angular momentum. In monolayer transition metal dichalcogenides, the noncentrosymmetric hexagonal lattice allows chiral phonons at the high symmetry points of the Brillouin zone. In this work, we investigate the helicity-resolved Raman response in monolayer MoS₂ grown via chemical vapor deposition. We identify a doubly degenerate Brillouin-zone-center chiral phonon mode at ~270 cm^-1. We establish the selection rule of this chiral phonon and show that this mode is activated by the resonant transition of A exciton. By applying an external magnetic field, we lift both the energy and intensity degeneracy of the chiral phonon. Remarkably, the phonon Zeeman splitting reveals a giant chiral phonon magnetic moment. Our study paves the route to excite and control chiral phonons in an achiral material.