Variational Monte Carlo studies of pairing symmetry for the t-J model on a triangular lattice

As a model of a novel superconductor Na _xCoO ₂· yH ₂O, a single-band t-J model on a triangular lattice is studied, using a variational Monte Carlo method. We calculate the energies of various superconducting (SC) states, changing the doping rate δ and sign of t for small J/|t|. Symmetries of s, d, and d+id (p+ip and f) waves are taken up as candidates for singlet (triplet) pairing. In addition, the possibility of Nagaoka ferromagnetism and phase separation is considered. It is revealed that, among the SC states, the d+id wave always has the lowest energy, which result supports previous mean-field studies. There is no possibility of triplet pairing, although the f-wave state becomes stable relative to a normal state in a special case (δ = 0.5 and t < 0). For t < 0, the complete ferromagnetic state is dominant in a wide range of δ and J/|t|, which covers the realistic parameter region of superconductivity.