Charge ordering in organic conductors and transition metal oxides

Theoretical studies based on Hartree approximation on charge ordering phenomena induced by mutual Coulomb interaction are briefly reviewed for organic conductors and transition metal oxides, which share common features of the quarter-filling of the band. It has been indicated that there exists in general a competition between the formation of the Mott insulator due to the on-site Coulomb interaction together with the dimerization and the charge disproportionation as in the Wigner crystal due to intersite interaction. Especially, the ‘zigzag’ charge ordered state has been predicted for NaV2O5, which is consistent with the existing experimental results.