Origin of superconductivity in 2D-organic compounds and high-Tc cuprates

A phase diagram is drawn in a parameter space of the nearly half-filled single band two-dimensional Hubbard model with U/t, t'/t and n as the parameters, U, t, t' and n being the on-site interaction, the nearest and second nearest neighbor transfer matrices and the number of electrons per site, respectively. The parameter space is chosen so as to include the models for the high-T-c cuprates as well as 2D-organic superconductors. The superconducting and antiferromagnetic instability surfaces are drawn from the results of calculations by using the spin fluctuation theory within the fluctuation exchange approximation. The metal-insulator transition surface is also indicated in the diagram. From this phase diagram we discuss the possibility of the spin fluctuation mechanism being the common origin of the superconductivity in the high-T-c cuprates and 2D-organic compounds.