13C-NMR study on κ-(BEDT-TTF)2X

The electronic state of the κ phase family of BEDT-TTF compounds, κ-(BEDT-TTF)2X [XCu(NCS)2, Cu[N(CN)2]Br and Cu[N(CN)2]Cl], has been investigated by 13C-NMR as well as 1H-NMR. The 13C isotope was substituted selectively into the central double bonded carbon sites of BEDT-TTF. The 13C nuclear spin-lattice relaxation rate and line spectra have been measured for the Cu(NCS)2 and Cu[N(CN)2]Br salts. The Knight shift evaluated from the the width of the spectra of the powdered samples is temperature-insensitive and scaled to the uniform susceptibility. However, the relaxation rate, T1−1, does not obey the Korringa law but exhibits anomalous temperature dependence with a peak formation around 50 K. The absolute value of T1−1 is in excess of the Korringa relation. These behaviors are considered to come from strong aniferromagnetic spin fluctuation with finite wave vector, which is suppressed below 50 K. On the other hand, the 1H NMR and magnetization measurements gave an unambiguous evidence for antiferromagnetic transition with spin canting at 25 K in κ-(BEDT-TTF)2Cu[N(CN)2]Cl. The κ phase family, situated in the vicinity of metal-insulator and magnetic transitions, is characterized by strong electron correlations.