s-wave superconductivity due to Suhl-Kondo mechanism in NaxCoO2•yH2O:: Effect of Coulomb interaction and trigonal distortion

To study the electron-phonon mechanism of superconductivity in NaxCoO(2)center dot yH(2)O, we perform semiquantitative analysis of the electron-phonon interaction (EPI) between relevant optical phonons (breathing and shear phonons) and t(2g) electrons (a(1g) and e'(g) electrons) in the presence of trigonal distortion. We consider two kinds of contributions to the EPI: the EPI originating from the Coulomb potential of O ions and that originating from the d-p transfer integral between Co and O in CoO(6) octahedron. We find that the EPI for shear phonons, which induces the interorbital hopping of electrons, is large in Na(x)CoO(2)center dot yH(2)O because of the trigonal distortion of CoO(2) layer. For this reason, T(c) for s-wave pairing is prominently enlarged owing to interorbital hopping of Cooper pairs induced by shear phonons, even if the top of e, electron band is close to but below the Fermi level as suggested experimentally. This mechanism of superconductivity is referred to as the valence-band Suhl-Kondo (SK) mechanism. Since the SK mechanism is seldom damaged by the Coulomb repulsion, s-wave superconductivity is realized irrespective of large Coulomb interaction U similar to 5 eV at Co sites. We also study the oxygen isotope effect on T(c) and find that it becomes very small due to strong Coulomb interaction. Finally, we discuss the possible mechanism of anisotropic s-wave superconducting state in NaCoO(2)center dot yH(2)O, resulting from the coexistence of strong EPI and the antiferromagnetic fluctuations.