13C NMR Study of Layered Organic Superconductors Based on BEDT-TTF Molecules

The normal state of the layered organic superconductors $\kappa -(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{TF}{)}_{2}X$ ( $X=\mathrm{Cu}(\mathrm{NCS}{)}_2$ and Cu[N(CN${)}_2$]Br) has been studied by ${}^{13}$C NMR for the first time. The nuclear spin-lattice relaxation rate, $T_1^{-1\mathrm{}}$, is greatly enhanced over the value expected from the Korringa relation and shows an anomalous peak around 50 K for both compounds. This is the first experimental evidence of strong antiferromagnetic spin fluctuations in these materials. The depression of the spin fluctuations observed below 50 K coincides with an appearance of Fermi liquid behavior in the charge transport.