Electron correlation, metal-insulator transition and superconductivity in quasi-2D organic systems, (ET)(2)X

A family of quasi-two-dimensional organic systems, (ET)(2)X, have been studied by NMR, susceptibility and specific heat measurements in the light of role of electron correlation. A systematic variation of antiferromagnetic spin fluctuations toward metal(superconductor)-to-insulator transition and the ordered spin structure in the insulating phase are uncovered. Based on the results, we propose a conceptual phase diagram for (ET)(2)X, where the kappa-type of family span a critical region of Mott transition. The superconductivity, which is enhanced near the Mott transition, is suggested to have line nodes in gap parameter by NMR relaxation and specific heat profiles.