Novel charge order and superconductivity in two-dimensional frustrated lattice at quarter filling

Motivated by the various physical properties observed in θ-(BEDT-TTF)₂X, we study the ground state of an extended Hubbard model on two-dimensional anisotropic triangular lattice at 1/4-filling with the variational Monte Carlo method. It is shown that the nearest-neighbor Coulomb interaction enhances charge fluctuation and it induces anomalous states such as the charge-ordered metallic state and triplet next-nearest-neighbor f-wave superconductivity. We consider that these results can explain the coexistence of different charge orders observed in the case of X = CsCo(SCN)₄ and superconductivity in the case of X = I₃. We then propose a unified view of the family of organic conductors θ-(BEDT-TTF)₂X with the value of t_c/t_p.