Electronic state of κ-(BMDT-TTF)2Au(CN)2 studied by NMR and thermoelectric measurements

The electronic state of κ-(BMDT-TTF)2Au(CN)2 was investigated by measurements of 1H-NMR and thermoelectric power. In 1H-NMR, the profile of the nuclear relaxation curve changed drastically at 75 K from exponential time-dependence to non-single exponential one reflecting inhomogeneous relaxation. This change evidences a transition in the electronic state at this temperature. However, the longest Tl below the transition temperature behaves like a Korringa law, suggesting that the low-temperature state is another metallic state. The thermoelectric power shows a strong anomaly around 75 K, being consistent with the NMR results. Above that, the temperature dependence is similar to that found in the κ phase of BEDT-TTF compounds, suggesting a similar Fermi-surface topology. These results suggest that the transition at 75 K is not a conventional metal-to-insulator transition but a metal-to-metal transition.