Two-dimensional plasmon in a metallic monolayer on a semiconductor surface: Exchange-correlation effects

Taking account of exchange-correlation (XC) effects, we investigate two-dimensional (2D) plasmons (PL's) in a metallic monolayer on a semiconductor surface. The energy dispersion and the energy-loss intensity of the 2D PL are calculated in close relation to a recent experiment by high-resolution electron energy-loss spectroscopy. We evaluate the XC effects by using the local-field-correction theory and by comparing the calculated results among (i) the random-phase approximation, (ii) the Hartree-Fock approximation, and (iii) the approximation originally formulated by Singwi, Tosi, Land, and Sjolander. We determine the electron density n(0) and the electron effective mass m* so that the results in (iii) accord with the experimental ones. Our calculations give a good description of the energy dispersion and the energy-loss intensity of the 2D PL and the PL decay due to single-particle excitations in the experiment. With an increase in wave number q, the exchange and correlation begin to lower the dispersion curve and make the 2D PL decay at a smaller q value. Our electron system has a high effective density, because it lies on a semi-infinite dielectric medium. However, owing to low dimensionality, the XC effects start to appear remarkably in the 2D PL with an increase in q.