Comparison of the phase diagram of the half-filled layered organic superconductors with the phase diagram of the RVB theory of the Hubbard-Heisenberg model

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We present an resonating valence bond (RVB) theory of superconductivity for the Hubbard--Heisenberg model on an anisotropic triangular lattice. We show that these calculations are consistent with the observed phase diagram of the half-filled layered organic superconductors, such as the beta, beta', kappa and lambda 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 dₓ₂₋y₂ superconductor with a small superfluid stiffness and a pseudogap with dₓ₂₋y₂ symmetry. The Mott--Hubbard transition can be driven either by increasing the on-site Coulomb repulsion, U, or by changing the anisotropy of the two hopping integrals, t'/t. Our results suggest that the ratio t'/t plays an important role in determining the phase diagram of the organic superconductors.