Abstract
We study the renormalized Coulomb interactions due to retardation effect in Na x CoO 2 . Although the Morel Anderson's pseudo-potential for a 1 g orbital μ a 1 g * is relatively large because the direct Coulomb repulsion U is large, that for interband transition between a 1 g and e g ′ orbitals μ a 1 g , e g ′ * is very small since the renormalization factor for pair hopping J is square of that for U. Therefore, the s-wave superconductivity due to valence-band Suhl–Kondo mechanism will survive against strong Coulomb interactions. The interband hopping of Cooper pairs due to shear phonons is essential to understand the superconductivity in Na x CoO 2 .
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