Extended s-wave pairing originating from the $a_{1g}$ band in Na$_x$CoO$_2\cdot y$ H$_2$O

TL;DR

This paper explores the potential for s-wave superconductivity originating from the $a_{1g}$ band in Na$_x$CoO$_2ullet y$H$_2$O, emphasizing the role of spin and charge fluctuations in pairing mechanisms.

Contribution

It introduces a single band $U$-$V$ model considering the $a_{1g}$ band dispersion and demonstrates s-wave pairing with sign change between Fermi surfaces using the fluctuation exchange method.

Findings

S-wave pairing is enhanced by spin and charge fluctuations.

The gap changes sign between inner and outer Fermi surfaces.

The model aligns with some experimental observations.

Abstract

Motivated by recent experiments and band calculation results, we investigate the possibility of superconductivity originating from the $a_{1g}$ band of Na$_x$CoO$_2\cdot y$H$_2$O assuming the absence of $e_g'$ pockets. We adopt a single band $U$-$V$ model that takes into account the local minimum of the $a_{1g}$ band dispersion at the $\Gamma$ point. Using the fluctuation exchange method, we find that an s-wave pairing with a gap that changes sign between the inner and the outer Fermi surfaces is enhanced due to the coexistence of spin fluctuations at wave vectors bridging the two Fermi surfaces and the charge fluctuations near the K point. Possible relevance to the experimental results is discussed.