Negative plasmon dispersion in the transition-metal dichalcogenide 2H-TaSe2

We report about an electron energy-loss study on the dispersion of the charge-carrier plasmon in the charge-density-wave system 2H-TaSe2 above and below the metal-insulator transition. In both cases we find a negative slope, which is in remarkable contrast to the traditional prediction for metals based on the random-phase approximation. Possible scenarios for this intriguing phenomenon are discussed.