Nodal d+id pairing and topological phases on the triangular lattice of NaxCoO2 • yH2O:: Evidence for an unconventional superconducting state

We show that finite angular momentum pairing chiral superconductors on the triangular lattice have point zeroes in the complex gap function. A topological quantum phase transition takes place through a nodal superconducting state at a specific carrier density x(c) where the normal state Fermi surface crosses the isolated zeros. For spin-singlet pairing, we show that the second-nearest-neighbor (d + id)-wave pairing can be the dominant pairing channel. The gapless critical state at x(c) similar or equal to 0.25 has six Dirac points and is topologically nontrivial with a T(3) spin relaxation rate below T(c). This picture provides a possible explanation for the unconventional superconducting state of Na(x)CoO(2) center dot yH(2)O. Analyzing a pairing model with strong correlation using the Gutzwiller projection and symmetry arguments, we study these topological phases and phase transitions as a function of Na doping.