title: Ferromagnetic fluctuation and possible triplet superconductivity in NaxCoO2.yH2O: fluctuation-exchange study of the multiorbital Hubbard model.
abstract: Spin and charge fluctuations and superconductivity in NaxCoO2.yH(2)O are studied based on a multiorbital Hubbard model. Tight-binding parameters are determined to reproduce the results of band calculations. By applying the fluctuation-exchange approximation, we show that the Hund's-rule coupling between the Co t(2g) orbitals causes ferromagnetic (FM) spin fluctuation. Triplet fy((y(2)-3x(2)))-wave and p-wave pairings are favored by this FM fluctuation on the hole-pocket band. We propose that, in NaxCoO2.yH(2)O, the Co t(2g) orbitals and interorbital 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.
authors.affiliation: Department of Physics, University of Tokyo, Hongo Bunkyo-ku Tokyo 113-0033, Japan.
authors.forename: Masahito
authors.initials: M
authors.lastname: Mochizuki
authors.forename: Youichi
authors.initials: Y
authors.lastname: Yanase
authors.forename: Masao
authors.initials: M
authors.lastname: Ogata