Electron correlation in theκ-phase family of BEDT-TTF compounds studied byC13NMR, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene

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Abstract
A C13 NMR study on κ=(BEDT−TTF)2X (X=Cu(NCS)2, Cu[N(CN)2]Br, and Cu[N(CN)2]Cl) is reported, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene. The superconducting salts of X=Cu(NCS)2 and Cu[N(CN)2]Br show anamalous enhancement in nuclear spin-lattice relaxation rate T1−1 around 50 K, in spite of monotonic temperature dependence of the anisotropic Knight shift. On the other hand, the insulating salt of X=Cu[N(CN)2]Cl exhibits a divergent peak in (T1T)−1 at 26-27 K, which is confirmed as a manifestation of an antiferromagnetic transition by broadening of the NMR line. Above 27 K, T1−1 shows a temperature dependence typical of critical magnetic fluctuations. The (T1T)−1 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 (T1T)−1 in the superconducting salts and antiferromagnetic fluctuations established in the insulating salt of κ type. The present results demonstrate the presence of strong electron correlation in both superconducting and insulating salts. Analytical expressions of T1 and the Knight shift of the C13 NMR for κ−(BEDT−TTF)2X are given, which are general formulas applicable to other pπ electronic systems.