Quantum effects on charge order

Charge ordering in strongly correlated 1/4-filled systems, such as charge transfer organic conductors and transition metal oxides, is discussed from a theoretical point of view. Strong electron-electron Coulomb interaction especially that between neighboring molecules induces Wigner crystal-type charge order, which competes with another insulating state due to strong correlation in these systems, i.e., the dimer Mott insulating state. Based on such idea, new aspects of the charge ordering phenomenon are pointed out, where full quantum mechanical treatments are required. One case is the interplay between electron-phonon interaction leading to a novel co-existence of charge order and spin-Peierls lattice distortion, and the other is the effect of geometrical frustration resulting in a quantum melting of the charge order.