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

We report the first direct measurement of the effective magnetic penetration depth in oriented single crystals of κ-[BEDT-TTF]2Cu[NCS]2, with Tc(5 G)≊9 K. Results yield an effective in-plane value of λeffbc(0)≊9800 Å (for Hext≊3 kG), and a temperature dependence consistent with conventional s-wave pairing. Comparison with the London penetration depth, λL(0) (estimated to be ≊5100 Å), indicates a tendency toward dirty-limit superconductivity, with the ratio of coherence length over mean free path of ξ0bc/lbc≊2.7. From our results, it appears unnecessary to invoke any unconventional pairing schemes to explain the superconductivity in this material.