Half-Filled Layered Organic Superconductors and the Resonating-Valence-Bond Theory of the Hubbard-Heisenberg Model

We present a resonating-valence-bond theory of superconductivity for the Hubbard-Heisenberg model on an anisotropic triangular lattice. Our calculations are consistent with the observed phase diagram of the half-filled layered organic superconductors, such as the β, β^', κ, and λ phases of (BEDT-TTF)₂X [bis(ethylenedithio)tetrathiafulvalene] and (BETS)₂X [bis(ethylenedithio)tetraselenafulvalene]. We find a first order transition from a Mott insulator to a d_x²-y² superconductor with a small superfluid stiffness and a pseudogap with d_x²-y² symmetry.