A Model for Cooper Pairing in Heavy Fermion Superconductor

Cooper pair fonnation in the heavy fennion system is discussed on the basis of a tight-binding model for the Kondo lattice system. The attractive interaction between heavy fennions stems from the coupling with phonons, which arises through the modulations of the transfer to the nearest neighbor site and the single-particle level of the heavy fennion owing to the lattice vibrations. The interaction relevant to the Cooper pair fcinnation is written in the fonn of the superposition of separable fonns with SO, d- and p-like symmetries. A comparison among the transition temperatures for various types of Cooper pairing shows that the singlet pairing is always favorable compared to the triplet one, and that, of singlet pairings, the d-like one is most favorable in the weak coupling case and so is the s-like one in the strong coupling case. The manner in which the d- and s-components mix together below Tc is discussed in the Ginzburg-Landau region, from which it is shown that the possible type of pairing is purely s-like one or d-like one with s-like admixtures, depending whether the highest Tc occurs for s-like or d-like pairing. This implies that the low lying excitations can be nearly gapless for appropriate values of parameters characterizing the model.