Microwave surface impedance of κ-(BEDT-TTF)2Cu(NCS)2, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene: Evidence for unconventional superconductivity

High-sensitivity microwave-impedance measurements performed on the quasi-two-dimensional organic superconductor κ-(BEDT-TTF${)}_2$Cu(NCS${)}_2$ give sound evidence for an unconventional type of superconductivity. The penetration depth λ(T) is in excellent agreement with recent muon-spin-relaxation measurements which are consistent with anisotropic pairings with lines of nodes in the energy gap. The real part of the conductivity ${\mathrm{\sigma }}_1$ shows a peaked structure which is free of coherence factor effects and results from a competition between two temperature dependences, the penetration depth and the inelastic relaxation time of the normal fluid.