We analyze the effect of a strong Zeeman field on the spectrum of collective excitations of monolayer transition metal dichalcogenides. The combination of the Dresselhaus-type spin-orbit coupling and an external Zeeman field results in the lifting of the valley degeneracy in the valence band of these crystals. We show that this lifting of the valley degeneracy manifests in the appearance of an additional plasmon mode with linear in wave-number dispersion along with the standard square root in wave-number mode. Despite this novel mode being subject to Landau damping, it corresponds to a well-defined quasiparticle peak in the spectral function of the electron gas.
Physical Review B