Pairing symmetry and ferromagnetism in the t-J model on a triangular lattice

With NaxCoO2·yH2O in mind, a single-band t-J model on a triangular lattice is studied, using a variational Monte Carlo method. It is found that among various superconducting symmetries, a d+id wave is the most stable for any hole density δ, and irrespective of the sign of hopping integral t; a simple d wave, however, has a fairly close energy. Triplet superconductivity is not realized. For t<0, Nagaoka ferromagnetism is dominant in a wide range of δ and J/|t|, which eliminates the possibility of superconductivity for realistic model parameters.