Effect of Charge Order on the Plasmon Dispersion in Transition-Metal Dichalcogenides

Abstract

We investigate the dispersion of the charge carrier plasmon in the three prototypical charge-density wave bearing transition-metal dichalcogenides $2H\mathrm{\text{\ensuremath{-}}}{\mathrm{TaSe}}_{2}$, $2H\mathrm{\text{\ensuremath{-}}}{\mathrm{TaS}}_{2}$, and $2H\mathrm{\text{\ensuremath{-}}}{\mathrm{NbSe}}_{2}$ employing electron energy-loss spectroscopy. For all three compounds the plasmon dispersion is found to be negative for small momentum transfers. This is in contrast with the generic behavior observed in simple metals as well as the related system $2H\mathrm{\text{\ensuremath{-}}}{\mathrm{NbS}}_{2}$, which does not exhibit charge order. We present a semiclassical Ginzburg-Landau model which accounts for these observations, and argue that the vicinity to a charge ordered state is thus reflected in the properties of the collective excitations.