We consider the competition between superconducting, charge ordered, and metallic phases in layered molecular crystals with the $\theta$ and ${\beta }^{\prime \prime }$ structures. Applying slave-boson theory to the relevant extended Hubbard model, we show that the superconductivity is mediated by charge fluctuations and the Cooper pairs have $d_{\mathrm{xy}}$ symmetry. This is in contrast to the $\kappa -(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F}{)}_{2}X$ family, for which theoretical calculations give superconductivity mediated by spin fluctuations and with $d_{x^2{-y}^2}$ symmetry. We predict several materials that should become superconducting under pressure.

• Received 5 July 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.237002