We examine two recently proposed models of charge ordering (CO) in the nominally 1 4 -filled, quasione-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 (SP) order can occur in either the ...1100... and ...1010... states and calculate the phase diagram including both on-site and intra-site e-ph interactions. Third, we discuss the expected temperature dependence of the CO and metal-insulator (MI) 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.

We present a detailed numerical study of the Hubbard-Holstein model in one dimension at half filling, including full finite-frequency quantum phonons. At half filling, the effects of the electronphonon and electron-electron interactions compete, with the Holstein phonon coupling acting as an effective negative Hubbard onsite interaction U that promotes on-site electron pairs and a Peierls charge-density wave state. Most previous work on this model has assumed that only Peierls or U > 0 Mott insulator phases are possible at half filling. However, there has been speculation that a third metallic phase exists between the Peierls and Mott phases. We present results confirming the intermediate metallic phase, and show that the Luttinger liquid correlation exponent Kρ > 1 in this region, indicating dominant superconducting pair correlations. We explore the full phase diagram as a function of onsite Hubbard U , phonon coupling constant, and phonon frequency. Electron-phonon (e-ph) interactions can give rise to a number of interesting effects in low-dimensional materials, including superconductivity as well as charge-density wave and insulating phenomena. Frequently these materials feature strong electron-electron (e-e) interactions as well, leading to very rich phase diagrams that combine lattice, charge, and spin (magnetic) orderings. We will focus specifically on materials where the electrons are coupled to localized vibrational modes, which may be of relatively high frequency. This type of e-ph interaction is most studied in molecular crystal materials, including the quasi-one-and quasi-two-dimensional organic superconductors[1] and fullerene superconductors [2]. In all of these materials, a fundamental question is whether the effects of e-e and e-ph interactions compete or cooperate with each other. In this Letter, we examine this issue within one of the most basic models. We find that despite the two interactions each separately favoring insulating states, together they can mediate an unexpected metallic phase with superconducting (SC) pair correlations.The model we consider is the one-dimensional (1D) Hubbard-Holstein model (HHM), with Hamiltonian