Coexistence of distinct charge fluctuations in θ-(BEDT-TTF)₂X

Using the Lanczos exact-diagonalization and density-matrix renormalization group methods, we study the extended Hubbard model at quarter filling defined on the anisotropic triangular lattice. We focus on charge ordering (CO) phenomena induced by onsite and intersite Coulomb interactions. We determine the ground-state phase diagram including three CO phases, i.e., diagonal, vertical, and three-fold CO phases, based on the calculated results of the hole density and double occupancy. We also calculate the dynamical density-density correlation functions and find possible coexistence of the diagonal and three-fold charge fluctuations in a certain parameter region where the onsite and intersite interactions compete. Furthermore, the characteristic features of the optical conductivity for each CO phase are discussed.