A ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ NMR study on $\kappa ={(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F})}_{2}X$ ($X=\mathrm{Cu}{\left(\mathrm{NCS}\right)}_2, \mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Br}, \mathrm{and} \mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Cl}$) is reported, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene. The superconducting salts of $X=\mathrm{Cu}{\left(\mathrm{NCS}\right)}_2 \mathrm{and} \mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Br}$ show anamalous enhancement in nuclear spin-lattice relaxation rate $T_1^{-1\mathrm{}}$ around 50 K, in spite of monotonic temperature dependence of the anisotropic Knight shift. On the other hand, the insulating salt of $X=\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Cl}$ exhibits a divergent peak in ${\left(T_{1}T\right)}^{-1\mathrm{}}$ at 26-27 K, which is confirmed as a manifestation of an antiferromagnetic transition by broadening of the NMR line. Above 27 K, $T_1^{-1\mathrm{}}$ shows a temperature dependence typical of critical magnetic fluctuations. The ${\left(T_{1}T\right)}^{-1\mathrm{}}$ and the line shape showed nearly the same profile for these three salts above 60 K, that gives experimental evidence for a correspondence between the anomalous enhancement of ${\left(T_{1}T\right)}^{-1\mathrm{}}$ in the superconducting salts and antiferromagnetic fluctuations established in the insulating salt of $\kappa$ type. The present results demonstrate the presence of strong electron correlation in both superconducting and insulating salts. Analytical expressions of $T_1$ and the Knight shift of the ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ NMR for $\kappa -{(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F})}_{2}X$ are given, which are general formulas applicable to other $p\pi$ electronic systems.

• Received 19 May 1995

DOI:https://doi.org/10.1103/PhysRevB.52.15522