Robustness of s-wave superconductivity against Coulomb interactions in NaxCoO2

We study the depairing effect due to Coulomb interactions in NaxCoO2. We consider the electron-phonon coupling and the Coulomb interactions, and determine T-c for s-wave superconductivity by solving the linearized Eliashberg equation. When we consider shear phonons as well as breathing phonons, T-c is enhanced by Suhl-Kondo (SK) mechanism. Since SK mechanism is strong against Coulomb interactions, T-c remains finite even if strong Coulomb interactions cancel out the attractive force due to breathing phonons. The orbital degree of freedom is important to understand the mechanism of superconductivity in NaxCoO2. (C) 2007 Elsevier B.V. All rights reserved.