Doping dependent plasmon dispersion in 2H-transition metal dichalcogenides

We report the behavior of the charge carrier plasmon of $2H$-transition metal dichalcogenides (TMDs) as a function of intercalation with alkali metals. Intercalation and concurrent doping of the TMD layers have a substantial impact on plasmon energy and dispersion. While the plasmon energy shifts are related to the intercalation level as expected within a simple homogeneous electron gas picture, the plasmon dispersion changes in a peculiar manner independent of the intercalant and the TMD materials. Starting from a negative dispersion, the slope of the plasmon dispersion changes sign and grows monotonously upon doping. Quantitatively, the increase of this slope depends on the orbital character $(4d$ or $5d)$ of the conduction bands, which indicates a decisive role of band structure effects on the plasmon behavior.