Unscreened plasmon dispersion of2Htransition metal dichalcogenides

The plasmon dispersion in $2H$ transition metal dichalcogenides has been reported to be negative, which is a strong renormalization with respect to the expectations for a homogeneous electron gas. This has been interpreted to be either due to the interaction of the plasmon oscillations with charge density fluctuations or due to the particular band structure of these materials. We present a separation of the momentum dependence of the charge carrier plasmon from that of single particle excitations (band-band transitions). We demonstrate that the (momentum dependent) screening of the plasmon oscillations by the single particle excitations predominantly determines the observed negative plasmon dispersion. However, even without this screening, the plasmon dispersion deviates from that of a simple metal, which indicates the additional, but smaller, impact of the charge density wave fluctuations.