MAGNETIC PENETRATION DEPTH AND FLUXON-LINE DYNAMICS IN THE ORGANIC SUPERCONDUCTOR KAPPA-[BEDT-TTF]2CU[NCS]2

The muon-spin-relaxation (muSR) technique has been used to study the internal field modulation dynamics of fluxons in crystals of kappa-[BEDT-TTF]2Cu[NCS]2 (T(c) almost-equal-to 10. 5 K). The data exhibit a strong field dependence for applied fields in the range 129 Oe less-than-or-equal-to \H(ext)\ less-than-or-equal-to 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) approximately 5 K, as evidenced by a peak in the second moment of the local field distribution at approximately 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, lambda(L)(bc)(0)=7680+/-700 angstrom.