Orbitally degenerate spin-fluctuation model for heavy-fermion superconductivity

In this paper, a generalization of standard spin-fluctuation theory is considered by replacing the simple Hubbard interaction by the screened Hartree-Fock interaction for f electrons. This model is then used in both an LS and a JJ coupling scheme to construct the particle-particle scattering vertex. In an on-site approximation, this vertex is shown to lead to an instability for a superconducting pair state which obeys Hunds rules, with L=5, S=1, and J=4, whereas for near-neighbor pairing, the most attractive instability is for J=0. The degeneracy of these multiplets is broken by anisotropy of the quasiparticle wave functions. Detailed calculations are presented for the case of UPt₃.