Cooper pairing of fermions with unequal masses in heavy-fermion systems

We investigate the Cooper pairing between a conduction-band electron (c electron) and an f electron as a possible mechanism of the s-wave superconductivity in heavy-fermion materials. We first obtain an effective Hamiltonian of the system by applying the Schrieffer- Wolff transformation to the periodic Anderson model. The effective Hamiltonian includes spin-exchange and direct interactions between c and f electrons, which play the role of a glue for the s-wave c-f pairing. This pairing may arise from the fact that the strong Hubbard interaction forms the quasiparticle f band near the center of the c band.