Recent progress in NMR studies on organic conductors

Recent C-13-NMR studies on a family of quasi-two-dimensional organic conductors based on BEDT-TTF molecules are reviewed in the light of the role of electron-electron correlation in a variety of electronic states and of the symmetry of electron pairing in the superconducting state. Comparison of the nuclear spin-lattice relaxation;ate, conducting properly and molecular arrangement indicates a close relationship between the molecular arrangement and manifestation of electron correlation. The metal/nonmetal phases in the isostructural kappa-(BEDT-TTF)(2)X systems, which are in the strong dimeric regime, are understood as highly correlated metals and insulators crossing the Mott transition. For the 10-K superconducting phase situated near the Mott transition, the absence of Hebel-Slichter coherence peak and a low-temperature T-3-dependence of the nuclear spin-lattice relaxation rate suggest unconventional superconductivity with line nodes in gap parameter or highly anisotropic one.