Nmr study of superconductivity in κ-(BEDT-TTF)2X

The electron pairing symmetry in the superconductivity in κ-(BEDT-TTF)2Cu[N(CN)2]Br, a typical superconductor in a family of κ-(BEDT-TTF)2X, has been investigated by 13C and 1H NMR measurements. The 13C nuclear spin-lattice relaxation rate, 13T1−1, in a field parallel to the conducting layers shows an abrupt decrease below the transition temperature. There is no sign of the Hebel-Slichter coherence peak expected in the conventional superconductivity. Analyzing 1H NMR relaxation rate, which probes the vortex dynamics preferentially, we succeeded in separating the measured 13T1−1 into the vortex-dynamics contribution and the quasi-particle contribution. The latter is found to follow a T3-law, that is consistent with unconventional superconductivity with line nodes in gap parameter. The NMR measurements at the two nuclear sites with different hyperfine couplings within a molecule provide a useful method to extract nuclear relaxation due to quasi-particles.