Two-dimensional plasmon in a surface-state band

By use of electron-energy-loss spectrometer with hi.-h-momentum resolution. we have measured a collective electronic excitation in a surface-state ban d on Si(1 1 1)=root3 x root3-Ag. We have clarified that the energy dispersi on is isotropic with respect to the azimuthal orientation and shows no depe ndence on the electron probing depth. The obtained energy dispersion agreed very well with the plasmon dispersion calculated from two-dimensional (2D) nearly free-electron theory. As hallmarked from these observations, we ide ntify the measured loss as a longitudinal intraband 2D plasmon in the surfa ce-state band. (C) 2001 Published by Elsevier Science B.V.