Origin of large effective phonon magnetic moments in monolayer MoS₂

Circularly polarized lattice vibrations or chiral phonons carry finite angular momentum that leads to phonon Zeeman splitting in external magnetic fields. Recent helicity-resolved magneto-Raman spectroscopy measurements demonstrate giant effective magnetic moments of ~2.5μ_B in monolayer MoS₂, while a microscopic picture of the intriguing phenomena remains lacking. In this work, we show that the orbital transition between the split conduction bands (△₀=4 meV) of MoS₂ couples to the doubly degenerate E’’ phonon mode (ω₀=33 meV), forming two hybridized states. The one with predominantly chiral phonon contribution appears at ~270 cm^-1 in the helicity-switched channels, while the one with primarily orbital contribution appears at ~30 cm^-1 in the helicity-conserved channels. Based on this model, we reproduce the giant effective magnetic moments of chiral phonon in MoS₂ and explain its thermal dynamics properties. Our results generalize the orbital-phonon coupling model of phonon magnetic moments to a new material system beyond the paramagnets and magnets.