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

With NaXCoO2 center dot yH(2)O 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 6, 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 delta and J/vertical bar t vertical bar, which eliminates the possibility of superconductivity for realistic model parameters. (c) 2005 Elsevier Ltd. All rights reserved.