NMR study of superconductivity in kappa-(BEDT-TTF)(2)X

The electron pairing symmetry in the superconductivity in kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br, a typical superconductor in a family of kappa-(BEDT-TTF)(2)X, has been investigated by C-13 and H-1 NMR measurements. The C-13 nuclear spin-lattice relaxation rate, T-13(1)-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 H-1 NMR relaxation rate, which probes the vortex dynamics preferentially, we succeeded in separating the measured T-13(1)-1 into the vortex-dynamics contribution and the quasi-particle contribution. The latter is found to follow a T-3-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.