C-13-NMR STUDY ON KAPPA-(BEDT-TTF)(2)X

The electronic state of the kappa phase family of BEDT-TTF compounds, kappa-(BEDT-TTF)(2)X [X=Cu(NCS)(2), Cu[N(CN)(2)]Br and Cu[N(CN)(2)]Cl] , has been investigated by C-13-NMR as well as H-1-NMR. The C-13 isoto pe was substituted selectively into the central double bonded carbon s ites of BEDT-TTF. The C-13 nuclear spin-lattice relaxation rate and li ne spectra have been measured for the Cu(NCS)(2) and Cu[N(CN)(2)]Br sa lts. The Knight shift evaluated from the the width of the spectra of t he powdered samples is temperature-insensitive and scaled to the unifo rm susceptibility. However, the relaxation rate, T-1(-1), does not obe y the Korringa law but exhibits anomalous temperature dependence with a peak formation around 50 K. The absolute value of T-1(-1) is in exce ss of the Korringa relation. These behaviors are considered to come fr om strong antiferromagnetic spin fluctuation with finite wave vector, which is suppressed below 50 K. On the other hand, the H-1 NMR and mag netization measurements gave an unambiguous evidence for antiferromagn etic transition with spin canting at 25 K in kappa-(BEDT-TTF)(2)Cu[N(C N)(2)]Cl. The kappa phase family, situated in the vicinity of metal-in sulator and magnetic transitions, is characterized by strong electron correlations.