Possible Pairing Symmetry of Superconductor Na x CoO

To discuss the possibility that the superconductivities in NaxCoO2·yH2O are induced by electron correlation, we investigate the possible pairing symmetry based on the single-band Hubbard model whose dispersion of the bare energy band is obtained using the band structure calculation of NaxCoO2·yH2O. The superconducting transition temperature is estimated by solving the Éliashberg equation. In this equation, both normal and anomalous self-energies are calculated up to the third-order terms with respect to the Coulomb repulsion. In the case of spin-singlet pairing, the candidate of pairing symmetry (the maximum eigen value λ max of Éliashberg’s equation) belongs to the d-wave(E2 representation of D6 group). In the case of spin-triplet pairing, the candidate of pairing symmetry (the maximum eigen value λ max of Éliashberg’s equation) belongs to the fy(y2−3x2)-wave (B1 representation of D6 group). It is found that λ max ≃ λ ST max and the transition temperatures of unconventional pairing state are estimated to be low compared with the observed temperature within our simple model.