The authors report ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ NMR spin-lattice relaxation rates, ${\mathit{T}}_1^{\mathrm{-}1}$, in the quasi-2D organic superconductor κ-(ET${)}_2$Cu[N(CN${)}_2$]Br (${\mathit{T}}_{\mathit{c}}$=11.6 K), for an aligned single crystal. The relaxation in the normal-state obeys the Korringa law (${\mathit{T}}_1$T=const). In the superconducting state, for weak fields (${\mathit{H}}_0$=0.59 T), ${\mathit{T}}_1^{\mathrm{-}1}$ is greatly enhanced and displays strong orientation dependence for field directions nearly parallel to the superconducting layers. This behavior indicates that motion of the fluxoid system is the cause of the extra relaxation, and is evidence for a ‘‘lock-in’’ transition of the flux lattice.

• Received 21 January 1993

DOI:https://doi.org/10.1103/PhysRevLett.70.2956