Pattern of charge ordering in quasi-one-dimensional organic charge-transfer solids

We examine two recently proposed models of charge ordering (CO) in the nominally 1/4-filled, quasi-one-dimensional (1D) organic charge-transfer solids (CTS). The two models are characterized by site charge density ``cartoons'' 1010 and 1100, respectively. We use the Peierls-extended Hubbard model to incorporate both electron-electron (e-e) and electron-phonon (e-ph) interactions. We first compare the results, for the purely electronic Hamiltonian, of exact many-body calculations with those of Hartree-Fock (HF) mean-field theory. We find that HF gives qualitatively and quantitatively incorrect values for the critical nearest-neighbor Coulomb repulsion (Vc) necessary for 1010 order to become the ground state. Second, we establish that spin-Peierls order can occur in either the 1100 and 1010 states and calculate the phase diagram including both on-site and intrasite e-ph interactions. Third, we discuss the expected temperature dependence of the CO and metal-insulator transitions for both 1010 and 1100 CO states. Finally, we show that experimental observations clearly indicate the 1100 CO in the 1:2 anionic CTS and the (TMTSF)₂X materials, while the results for (TMTTF)₂X with narrower one-electron bandwidths are more ambiguous, likely because the nearest-neighbor Coulomb interaction in these materials is near Vc.