Negative plasmon dispersion in 2 H -NbS 2 beyond the charge-density-wave interpretation

We examine the experimental and theoretical electron-energy loss spectra in 2 H - $\#$$\#$IMG$\#$$\#$ [http://ej.iop.org/images/1367-2630/18/10/103050/njpaa44fbieqn1.gif] $\mathrmCu_0.2$ NbS 2 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 $\#$$\#$IMG$\#$$\#$ [http://ej.iop.org/images/1367-2630/18/10/103050/njpaa44fbieqn2.gif] $q\lt 0.35$ A -1 . We argue that for a stoichiometric pure 2 H -NbS 2 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.