Linearly dispersing plasmons in monolayer transition metal dichalcogenides

We describe the collective excitations of the electronic liquid in monolayer transition metal dichalcogenides such as MoS$_2$ in a strong Zeeman field. The combination of the Dresselhaus type spin-orbit coupling and the Zeeman field lifts the valley degeneracy, and this manifests in the appearance of an additional plasmon mode that has linear dispersion. There is a well-defined quasiparticle peak in the spectral function which corresponds to this second mode.