Dispersion and damping of a two-dimensional plasmon in a metallic surface-state band

We have studied, for the first time, the energy and the linewidth dispersio n of a plasmon in a dense two-dimensional electron system in a metallic sur face-state band on a silicon surface. As expected from the considerably hig h effective density and long Fermi wavelength of the system, the plasmon en ergy dispersion exhibited,an excellent agreement with the nearly,free-elect ron theory. However, in a small wave number region below the Landau edge,we have observed an anomalous linewidth dispersion which nearly free-electron theories do not predict.