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

TL;DR

This paper suggests that spin-triplet f-wave pairing, facilitated by disconnected Fermi surfaces, could explain superconductivity in Na$_x$CoO$_2$·yH$_2$O, highlighting a novel pairing mechanism with potential finite critical temperature.

Contribution

It introduces a single-band model with disconnected Fermi surfaces and demonstrates that f-wave spin-triplet pairing is the dominant superconducting state.

Findings

Dominant pairing symmetry is spin-triplet f-wave.

Gap nodes do not intersect the Fermi surfaces.

Finite T_c is possible despite gap nodes.

Abstract

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.