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

The muon-spin-relaxation (μSR) technique has been used to study the internal field modulation dynamics of fluxons in crystals of κ-[BEDT-TTF]₂Cu[NCS]₂ (T_c~=10.5 K). The data exhibit a strong field dependence for applied fields in the range 129 Oe<=||H_ext||<=4 kOe, which is attributed to a combination of vortex motion, finite core size effects, and pinning. In low fields (||H_ext||=129 Oe), the data exhibit a flux-pinning transition at T_x~5 K, as evidenced by a peak in the second moment of the local field distribution at ~5 K accompanied by an enhanced minimum in the first moment at the same temperature. In higher fields (||H_ext||=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 s wave. Corrections for finite fluxon core, finite mean free path, and flux-lattice disorder were applied in determining the London penetration depth, λ^(bc)_L(0)=7680+/-700 Å.