Formation of Cooper pairs between conduction and localized electrons in heavy-fermion superconductors

Cooper pairing between a conduction electron (c electron) and an f electron, referred to as the "c-f pairing," is examined to explain s-wave superconductivity in heavy-fermion systems. We first apply the Schrieffer-Wolff transformation to the periodic Anderson model assuming deep f level and strong Coulomb repulsion. The resulting effective Hamiltonian contains direct and spin-exchange interactions between c and f electrons, which are responsible for the formation of the c-f Cooper pairs. The mean-field analysis shows that the fully gapped c-f pairing phase with anisotropic s-wave symmetry appears in a large region of the phase diagram. We also find two different types of exotic c-f pairing phases, the Fulde-Ferrell and breached pairing phases. The formation of the c-f Cooper pairs is attributed to the fact that the strong Coulomb repulsion makes a quasiparticle f band near the center of the conduction band.