Two-dimensional plasmon in a surface-state band

By use of electron-energy-loss spectrometer with high-momentum resolution, we have measured a collective electronic excitation in a surface-state band on Si(1 1 1)–√3×√3-Ag. We have clarified that the energy dispersion is isotropic with respect to the azimuthal orientation and shows no dependence 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 identify the measured loss as a longitudinal intraband 2D plasmon in the surface-state band.