Diagrammatic approach to the intermediate-valence compounds

A model for intermediate-valence compounds is considered that is based on correlated ionic states with $n_0$ and $n_0+1\mathrm{} f$ electrons and includes the hybridization with band electrons as a perturbation. The expansion is formulated diagrammatically with Goldstone diagrams for the strongly correlated on-site processes and Feynman propagators between different sites. Suitable infinite-order resummations of the on-site processes lead to Brillouin-Wigner-type self-consistency equations for the $f$ quasiparticle energy shifts. The imaginary part of the single-site $f$-level Green's function exhibits a spike above a continuum which vanishes at the Fermi level. Deviations from the Ruderman-Kittel-Kasuya-Yosida behavior are found for the intersite terms. The effect of the shift of the chemical potential due to the fixed number of electrons is discussed.