Charge order and superconductivity in two-dimensional triangular lattice at n=2/3

To investigate the possibility of charge order and superconductivity in a doped two-dimensional triangular lattice, we study an extended Hubbard model with the variational Monte Carlo method. At n = 2/3, a commensurate filling for a triangular lattice, it is shown that the nearest-neighbor Coulomb interaction V induces honeycomb-type charge order and antiferromagnetic spin order at U greater than or similar to 10t. We also discuss the possibility of superconductivity induced by charge fluctuation and the relation to the superconductivity in Na0.35CoO2 center dot 1.3H(2)O and theta-type organic condoctors.