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Abstract

Motivated by recent experiments, we perform a microscopic analysis of the dynamical charge response of
layered transition-metal dichalcogenides that display a low-temperature charge-density wave (CDW) order. In
agreement with measurements, our parameter-free results show a negative in-plane plasmon dispersion that
switches to positive slope upon electron (or hole) doping. This finding is explained by the peculiar behavior of
the intraband transitions, which are partially suppressed under doping, and it is not linked to the CDW order.
Finally, in the direction perpendicular to the layers, we predict the reappearance around the Bragg reflections of
the spectra of the first Brillouin zone, a clear effect of the crystal local-field impact. Our results give a general
picture of the collective excitations in these materials suggesting a simpler reinterpretation of the experiments.