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

Motivated by the various physical properties observed in theta-(BEDT-TTF)(2)X, 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 = I-3. We then propose a unified view of the family of organic conductors theta-(BEDT-TTF)2X with the value of t(c)/t(p).