Stability of the superconducting state in the two-band Hubbard model on a triangular lattice

As a model of the novel superconductor NaxCoO2 · yH2O, we study a triangular Hubbard model with doubly degenerate atomic orbitals, using an optimization variational Monte Carlo method. It is found that when the isolated Fermi surface overlaps the van Hove singularity points, dx2-y2dx2-y2 and d + id-wave superconducting states, among various pairing symmetries, are appreciably stabilized by large Coulomb interactions. The pairing is originating from the potential energy gain induced dominantly by the enhancement of antiferromagnetic spin correlations. The possibility of triplet pairing is very slight in this model.