On the ground state of mixed-valence semiconductors

A new variational procedure is proposed for calculating the ground-state energy of mixed-valence (MV) semiconductors such as SmS and SmB₆. This procedure uses the localised basis involving one- and two-electron states of a dielectric Anderson lattice with electron-hole attraction. The mixed-valence state can be obtained only beyond the Hartree-Fock approximation. The non-orthogonality catastrophe arising due to the overlap of the exciton wavefunctions is solved by means of a modified Lowdin-Carr method. Some physical consequences of the model accepted are discussed; it is shown in particular that the model resolves the controversy between the semiconducting behaviour of the static dielectric permittivity and its metallic high-frequency properties responsible for the golden colour of SmS in the high-pressure phase.