Antiferromagnetic Fluctuations in the Organic Superconductor κ-(BEDT-TTF)2Cu(NCS)2 under Pressure

We measured the C-13-NMR spectrum and T-1 of the quasi-two-dimensional organic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2) under pressure. This material was thought to show a relationship between T-c and the effective cyclotron mass m(c)(*), obtained from the Shubnikov-de Haas (SdH) effect. We found that kappa-(BEDT-TTF)(2)Cu(NCS)(2) behaved as a Fermi liquid at low temperature under all pressures, and antiferromagnetic fluctuations were expected. The pressure dependence of the Korringa factor is similar to that of the effective cyclotron mass m(c)(*), suggesting that antiferromagnetic fluctuations contribute to the superconductivity of this material. We also found that, under pressure, T-* was shifted to 150 K, the temperature characteristic of the shift from bad metal to good metal.