Ferromagnetic Fluctuation and Possible Triplet Superconductivity inNaxCoO2·yH2O: Fluctuation-Exchange Study of the Multiorbital Hubbard Model

Masahito Mochizuki, Youichi Yanase, Masao Ogata
2005 Physical Review Letters  
Spin and charge fluctuations and superconductivity in a recently discovered superconductor Na_xCoO_2*yH_2O are studied based on a multi-orbital Hubbard model. Tight-binding parameters are determined to reproduce the LDA band dispersions with the Fermi surface, which consist of a large cylindrical one around the Gamma-point and six hole pockets near the K-points. By applying the fluctuation-exchange (FLEX) approximation, we show that the Hund's-rule coupling between the Co t2g orbitals causes
more » ... romagnetic (FM) spin fluctuation. Triplet f_y(y^2-3x^2)-wave and p-wave pairings are favored by this FM fluctuation on the hole-pocket band. We propose that, in Na_xCoO_2*yH_2O, the 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 in which the orbital degrees of freedom play substantial roles.
doi:10.1103/physrevlett.94.147005 pmid:15904098 fatcat:ymnb3szvcvakzc2b5riczngvme