Electron localization in the strongly correlated organic system κ-(BEDT-TTF)2X probed with nuclear magnetic resonance 13C-NMR -: art. no. 060510

kappa-(BEDT-TTF)(2)X salts exhibit a wide range of properties from superconductivity to magnetic ordering. The phase diagram of this system has been understood as the competition between anti-ferromagnetic insulating behavior and superconductivity. The kappa-(BEDT-TTF)(2)X system has been well explained using a parameter, U/W, as well as the high T-c cuprates. kappa-(BEDT-TTF)(2)Cu-2(CN)(3), however, behaved as an insulator without magnetic ordering. This salt cannot be described by the phase diagram described by the competition between superconductivity and magnetism. It is interesting to explore the nature of this insulating phase with an eye toward understanding the universal phase diagram of the kappa-(BEDT-TTF)(2)X system. Magnetic frustration due to the triangular spin lattice has recently been reported for kappa-(BEDT-TTF)(2)Cu-2(CN)(3) from H-1-nuclear magnetic resonance (NMR) study. To investigate the insulating properties of this salt, we characterized it using C-13-NMR. In contrast to previous H-1-NMR studies, at low temperatures, we observed line broadenings that were proportional to the hyperfine coupling constants, which suggested inhomogeous electron localization. We also observed antiferromagnetic fluctuations which, as is the case with other salts, intensified with decreasing temperature. To understand the electronic properties of kappa-(BEDT-TTF)(2)X system, in addition to the U/W, the electron localization effect from the relatively narrow bandwidth is important.