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-onedimensional (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 (V_c) 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 V_c.