Possible spin-triplet $f$-wave pairing due to disconnected Fermi surfaces in Na${}_x$CoO${}_2\cdot y$H${}_2$O

We propose that spin-triplet pairing mechanism due to disconnected Fermi surfaces proposed in our previous study [Phys. Rev. B {\bf 63} 174507 (2001)] may be at work in a recently discovered superconductor Na${}_x$CoO${}_2$ $\cdot y$ H${}_2$O. We introduce a single band effective model that takes into account the pocket-like Fermi surfaces along with the van Hove singularity near the K point found in the band calculation results. Applying fluctuation exchange method and solving the linearized {'E}liashberg 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. Presence of finite $T_c$ is suggested in sharp contrast with cases when the gap nodes intersect the Fermi surface.