The nature of the insulating state in organic superconductors

We review our recent proposal for a unified, microscopic, many-body theory for the 2:1 organic charge transfer solids (CTS), which range from nearly 1 D to strongly 2D in electronic character. Our theory predicts an insulating bond-charge density wave (BCDW) ground state in the limit of exact 1/4-fil ling for all anisotropies, consistent with experimental observations of coe xisting density waves in the real materials. For weakly 2D materials, we fi nd that a spin-density wave (SDW) coexists with the BCDW; the resulting BCS DW is again consistent with experiments. In contrast, both in the 1D limit and in the strongly 2D regime the systems can be in the nonmagnetic singlet state. We suggest that slightly away from exact 1/4-filling, commensurabil ity defects can form pairs leading to superconductivity.