C-13 NMR-STUDIES OF THE NORMAL AND SUPERCONDUCTING STATES OF THE ORGANIC SUPERCONDUCTOR KAPPA-(ET)(2)CU[N(CN)(2)]BR

The authors report C-13 NMR spin-lattice relaxation rates 1/T-1 and Knight shifts K-S in the quasi-two-dimensional organic superconductor kappa-(ET)(2)Cu[N(CN)(2)]Br (T-c=11.6 K), for an aligned single crystal. The normal-state behavior is reminiscent of the high-T-c cuprates, in which antiferromagnetic fluctuations and spin-gap behavior dominate. In the superconducting state, the data rule out the BCS electron-phonon mechanism as the source of the superconductivity, but support an unconventional pairing state with possible nodes in the gap function.