Flux creep and scaling of the critical current with field in the quantum regime for a κ-(BEDT-TTF)2Cu(NCN)2Br single crystal

The critical current and the relaxation rate were measured for temperatures down to 150 mK and fields up to 40 kOe for the organic superconductor κ-(BEDT-TTF${)}_2$Cu(NCN${)}_2$Br, where BEDT-TTF stands for bis(ethylenedithio)tetrathiafulvalene. At low temperatures, the critical current is found to scale with ${\mathit{B}}^{\mathrm{-}1/2}$ while the tunneling rate is found roughly constant above 5 kOe. This disagrees with theories considering bundles of flux lines pinned collectively. Plastic shear of the flux-line lattice might provide an alternative mechanism in this case.