Charge order in organic conductors

It is pointed out that there exists in general a competition between dimerization and inter-site Coulomb interaction for the onset of insulating states in the quarter-filled band. In the presence of the strong dimerization the on-site Coulomb interaction can give rise to a dimer Mott insulating state, while the inter-site Coulomb interaction can stabilize a charge ordered state as the Wigner crystal, even when weak dimerization is present. Such possibility of charge ordering is theoretically examined systematically in detail for quasi-one-dimensional TM2X and then quasi-two-dimensional ET2X, whose results are compared with various experiments. The effects of geometrical frustration and electron–phonon interaction on the charge order are also pointed out. The extensions of these ideas to transition metal oxides are also discussed.