Magnetic penetration depth and fluxon-line dynamics in the organic superconductor [kappa]-[BEDT-TTF][sub 2]Cu[NCS][sub 2]

The muon-spin-relaxation ([mu]SR) technique has been used to study the internal field modulation dynamics of fluxons in crystals of [kappa]-[BEDT-TTF][sub 2]Cu[NCS][sub 2] ([ital T][sub [ital c]][approx]10.5 K). The data exhibit a strong field dependence for applied fields in the range 129 Oe[le][vert bar][bold H][sub ext][vert bar][le]4 kOe, which is attributed to a combination of vortex motion, finite core size effects, and pinning. In low fields ([vert bar][bold H][sub ext][vert bar]=129 Oe), the data exhibit a flux-pinning transition at [ital T][sub [ital x]][similar to]5 K, as evidenced by a peak in the second moment of the local field distribution at [similar to]5 K accompanied by an enhanced minimum in the first moment at the same temperature. In higher fields ([vert bar][bold H][sub ext][vert bar]=750 Oe, 3 kOe, and 4 kOe), where the effects of motion and pinning are minimized, the symmetry of the underlying gap function is confirmed as [ital s] wave. Corrections for finite fluxon core, finite mean free path, and flux-lattice disorder were applied in determining the London penetration depth, [lambda][sub [ital L]][sup ([ital b][ital c])](0)=7680[plus minus]700 A.