Coherent potential approximation for charge ordering in the extended Hubbard model

We study charge ordering (CO) in the extended Hubbard model with both on-site and nearest-neighbour Coulomb repulsion (U and V, respectively) within the coherent potential approximation. The phase boundary between the homogeneous and charge-ordered phases for the square lattice is obtained for different values of U. It is shown that at quarter-filling for all values of U the CO exists only if the inter-site Coulomb repulsion V exceeds certain a critical value which is of the order of the kinetic energy t. At finite temperature a re- entrant transition is found in some region of V