SUPERCONDUCTING STATE OF KAPPA-(ET)(2)CU[N(CN)(2)]BR STUDIED BY C-13 NMR - EVIDENCE FOR VORTEX-CORE-INDUCED NUCLEAR-RELAXATION AND UNCONVENTIONAL PAIRING

We present the first C-13 NMR measurements carried out in the supercon ducting state of a two-dimensional organic superconducting single crys tal. Spin lattice relaxation rate and Knight shift are reported for ma gnetic fields parallel and perpendicular to the conducting layers. For perpendicular fields, the relaxation is dominated by the electronic e xcitations in the vortex cores. From the field dependence of (T-1)(-1) we obtain the upper critical field [H-c2(0) = 10 T]. In parallel orie ntation, the absence of field dependence reveals the existence of a lo ck-in state; where only relaxation by superconducting excitations is e xpected. The (T-1)(-1) then exhibits a T-3 law suggesting an unconvent ional pairing with a very anisotropic gap.