s-wave Superconductivity due to Suhl-Kondo Mechanism in Na_xCoO₂cdot yH₂O: Effect of Coulomb Interaction and Trigonal Distortion

To study the electron-phonon mechanism of superconductivity in NaxCoO2, 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 NaxCoO_2 because of the trigonal distortion of CoO_2 layer. For this reason, Tc for s-wave pairing is prominently enlarged owing to interorbital hopping of Cooper pairs induced by shear phonons, even if the top of e_g' 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~5 eV at Co sites. We also study the oxygen isotope effect on Tc, 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 NaxCoO2, resulting from the coexistence of strong EPI and the antiferromagnetic fluctuations.