Theories on 1/4-filled one-dimensional systems—effects of frustration and electron–lattice coupling

The interplay between the charge ordering due to inter-site Coulomb interaction and additional effects, such as the geometrical frustration and the coupling to the lattice, are studied for 1/4-filled one-dimensional systems. We have found that in the transition metal oxide PrBa2Cu4O8 the frustration effect can give rise to a “melting” of the charge ordered state resulting in a peculiar metallic state, while in the organic conductors (TMTTF)2X (X=PF6, AsF6), the electron–phonon interaction stabilizes a new coexistent state of charge order (CO) and spin-Peierls lattice distortion.