The authors report ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ NMR spin-lattice relaxation rates 1/${\mathit{T}}_1$ and Knight shifts ${\mathit{K}}_{\mathit{S}}$ in the quasi-two-dimensional organic superconductor κ-(ET${)}_2$Cu[N(CN${)}_2$]Br (${\mathit{T}}_{\mathit{c}}$=11.6 K), for an aligned single crystal. The normal-state behavior is reminiscent of the high-${\mathit{T}}_{\mathit{c}}$ 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.

• Received 6 July 1995

DOI:https://doi.org/10.1103/PhysRevB.52.10364