Exciton-activated effective phonon magnetic moment in monolayer MoS2

Optical excitation of chiral phonons plays a vital role in studying the\nphonon-driven magnetic phenomena in solids. Transition metal dichalcogenides\nhost chiral phonons at high symmetry points of the Brillouin zone, providing an\nideal platform to explore the interplay between chiral phonons and valley\ndegree of freedom. Here, we investigate the helicity-resolved magneto-Raman\nresponse of monolayer MoS2 and identify a doubly degenerate\nBrillouin-zone-center chiral phonon mode at ~270 cm-1. Our wavelength- and\ntemperature-dependent measurements show that this chiral phonon is activated\nthrough the resonant excitation of A exciton. Under an out-of-plane magnetic\nfield, the chiral phonon exhibits giant Zeeman splitting, which corresponds to\nan effective magnetic moment of ~2.5mu_B. Moreover, we carry out theoretical\ncalculations based on the morphic effects in nonmagnetic crystals, which\nreproduce the linear Zeeman splitting and Raman cross-section of the chiral\nphonon. Our study provides important insights into lifting the chiral phonon\ndegeneracy in an achiral covalent material, paving a new route to excite and\ncontrol chiral phonons.\n