2016, Cudazzo, Pierluigi, Müller, Eric, Habenicht, Carsten, Gatti, Matteo, Berger, Helmuth, Knupfer, Martin, Rubio, Angel, Huotari, Simo

We examine the experimental and theoretical electron-energy loss spectra in 2H-${\mathrm{Cu}}_{0.2}$NbS2 and find that the 1 eV plasmon in this material does not exhibit the regular positive quadratic plasmon dispersion that would be expected for a normal broad-parabolic-band system. Instead we find a nearly non-dispersing plasmon in the momentum-transfer range $q\lt 0.35$ Å−1. We argue that for a stoichiometric pure 2H-NbS2 the dispersion relation is expected to have a negative slope as is the case for other transition-metal dichalcogenides. The presence of Cu impurities, required to stabilize the crystal growth, tends to shift the negative plasmon dispersion into a positive one, but the doping level in the current system is small enough to result in a nearly-non-dispersing plasmon. We conclude that a negative-slope plasmon dispersion is not connected with the existence of a charge-density-wave order in transition metal dichalcogenides.