Large effective phonon magnetic moment revealed in magneto-Raman spectroscopy in monolayer MoS₂

Helicity-resolved magneto-Raman measurements reveal unusually large effective phonon magnetic moments (~2.5 μ_B) in monolayer MoS₂ when the excitation is resonant with the A exciton, providing a new way to drive Γ point circularly polarized phonons in transition metal dichalcogenides. The microscopic origin of this effect arises from an orbital transition between the spin-orbit split conduction bands (Δ₀ ≈ 4 meV) that couples strongly to the E″ chiral phonon mode (Ω₀ ≈ 33 meV), forming two hybridized states. Helicity-resolved Raman studies identify these as a phonon dominant mode in helicity-switched channels and an orbital dominant mode in helicity-conserved channels. Depositing a thin Ni overlayer on monolayer MoS₂ aligns the A exciton with the 633 nm laser, enhancing the intensity of the orbital transition mode. These findings demonstrate that large phonon magnetic moments can emerge in nonmagnetic semiconductors, establishing a pathway to engineer spin valley coupled and magneto-optically active states in 2D materials.