NMR relaxation rate in the superconducting state of the organic conductor kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br

The C-13 nuclear spin-lattice relaxation rate, T-13(1)-1 has been measured in the superconducting state of kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br in a field parallel to the conducting layers. The T-13(1)-1 exhibits an abrupt decrease just below the transition temperature without any indication of the Hebel-Slichter coherence peak. By separation of the vortex-dynamics contribution to T-13(1)-1 with the use of H-1-NMR, the quasiparticle contribution to T-13(1)-1 is found to follow a T-3 law that is consistent with unconventional superconductivity with line nodes in gap parameter on the Fermi surface or highly anisotropic superconductivity.