Long-Wavelength Intervalley Plasmons in Transition-Metal Dichalcogenide Monolayers

The spectrum of plasma oscillations in a transition-metal dichalcogenide monolayer strip (planar quantum wire) has been found. In the long-wavelength limit, in addition to the usual one-dimensional plasmon branch, a second branch with a linear dispersion relation appears due to the intervalley coupling ensured by the electric field of the wave rather than to virtual transitions of electrons between the valleys. In the infinite plane such a branch is absent.