Ferromagnetic fluctuation and superconductivity in Na_0.35CoO_2*1.3H_2O: FLEX study of multi-orbital Hubbard model

TL;DR

This study uses the FLEX approximation on a multi-orbital Hubbard model to show that ferromagnetic fluctuations driven by Hund's-rule coupling promote triplet superconductivity in Na_0.35CoO_2*1.3H_2O, highlighting the role of inter-orbital interactions.

Contribution

It demonstrates that inter-orbital Hund's-rule coupling induces ferromagnetic fluctuations that favor triplet pairing in a specific cobaltate superconductor, a novel mechanism.

Findings

Ferromagnetic spin fluctuations are caused by Hund's-rule coupling.

Triplet pairing is favored on the hole-pocket band.

Na_0.35CoO_2*1.3H_2O may be the first triplet superconductor mediated by inter-orbital interactions.

Abstract

Spin and charge fluctuations and superconductivity in Na_0.35CoO_2*1.3H_2O are studied based on a multi-orbital Hubbard model. By applying the fluctuation exchange (FLEX) approximation, we show that the Hund's-rule coupling between the Co t2g orbitals causes ferromagnetic spin fluctuation. Triplet pairing is favored by this ferromagnetic fluctuation on the hole-pocket band. We propose that, in Na_0.35CoO_2*1.3H_2O, Co t2g orbitals and inter-orbital Hund's-rule coupling play important roles on the triplet pairing, and this compound can be a first example of the triplet superconductor mediated by inter-orbital-interaction-induced ferromagnetic fluctuation.