Possible Spin-Tripletf-Wave Pairing Due to Disconnected Fermi Surfaces In NaxCoO2·yH2O

We propose that the spin-triplet pairing mechanism due to disconnected Fermi surfaces proposed in our previous study [ Phys. Rev. B 63, 174507 (2001)] may be at work in a recently discovered superconductor ${\mathrm{N}\mathrm{a}}_x{\mathrm{C}\mathrm{o}\mathrm{O}}_2·y{\mathrm{H}}_2\mathrm{O}$. We introduce a single band effective model that takes into account the pocketlike Fermi surfaces along with the van Hove singularity near the K point found in the band calculation results. Applying the fluctuation exchange method and solving the linearized Éliashberg equation, the most dominant pairing is found to have spin-triplet $f$-wave symmetry, where the nodes of the gap function do not intersect the pocket Fermi surfaces. The presence of finite $T_c$ is suggested in sharp contrast to cases when the gap nodes intersect the Fermi surface.