Surface dielectric response of uniaxial crystals: Application to graphite

The model of a semi-infinite uniaxial crystal with a flat surface is reexamined. The surface-energy-loss function for uniaxial crystals, which generalizes the conventional expression Im-1/[1+(ω)], is derived. We demonstrate that in uniaxial crystals the three-dimensional charge-density oscillations can be excited by incident electrons, which do not penetrate into the bulk but are reflected at or above the surface plane. This effect is altogether due to the system anisotropy and it is inhibited in cubic crystals or homogeneous isotropic media. The implications of the theory are compared with the electron-energy-loss data for graphite.