Magnetic penetration depth in the organic superconductor κ-[BEDT-TTF${]}_2$Cu[NCS${]}_2$

We report the first direct measurement of the effective magnetic penetration depth in oriented single crystals of κ-[BEDT-TTF${]}_2$Cu[NCS${]}_2$, with ${\mathit{T}}_{\mathit{c}}$(5 G)≊9 K. Results yield an effective in-plane value of ${\lambda }_{\mathit{e}\mathit{f}\mathit{f}}^{\mathit{b}\mathit{c}}$(0)≊9800 Å (for ${\mathit{H}}_{\mathrm{ext}}$≊3 kG), and a temperature dependence consistent with conventional s-wave pairing. Comparison with the London penetration depth, ${\lambda }_{\mathit{L}}$(0) (estimated to be ≊5100 Å), indicates a tendency toward dirty-limit superconductivity, with the ratio of coherence length over mean free path of ${\xi }_0^{\mathit{b}\mathit{c}}$/${\mathit{l}}_{\mathit{b}\mathit{c}}$≊2.7. From our results, it appears unnecessary to invoke any unconventional pairing schemes to explain the superconductivity in this material.