Possible Pairing Symmetry of Superconductor Na X Coo 2 ·yh 2 O

To discuss a possibility that the superconductivities in NaxCoO2·yH2O are induced by the electron correlation, we investigate the possible pairing symmetry based on the single-band Hubbard model whose dispersion of the bare energy band is obtained by using FLAPW-LDA 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 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 fy(y2−3x2)-wave (B1 representation of D6 group). It is found that λ max ≃ λ ST max and the superconducting transition temperatures are estimated to be too low compared with observed temperature within our simple model.