Pseudogap formation in rare-earth compounds

The generalized periodic Anderson model with plane-wave conduction-band states is considered as a model to describe the properties of Ce, Sm, Eu, Tm and Yb compounds showing a variety of valence fluctuation phenomena. The mixing matrix elements between localized 4f-electron states and the conduction-electron states, which are antisotropic in k space, are derived, and it is shown how pseudogap or real-but-anisotropic gap is formed in this model. The structures of gap or pseudogap to be derived here are considered to bear some of the characteristics of those of rare-earth compounds in such as heavy-fermion, Kondo-insulator and intermediate-valence states.