Electrodynamics of the organic superconductorsinebreak κ-(BEDT-TTF${)}_2$Cu(NCS${)}_2$ and κ-(BEDT-TTF${)}_2$Cu[N(CN${)}_2$]Br

We have performed measurements of the surface impedance in the normal and superconducting states of the title compounds in the millimeter wave-frequency range (1–3 ${\mathrm{cm}}^{\mathrm{-}1}$), and have evaluated the complex conductivity for different crystallographic orientations. Above the transition temperature, the materials behave like metals with a scattering rate of approximately 20 ${\mathrm{cm}}^{\mathrm{-}1}$. In the superconducting state the electrodynamics is in good agreement with calculations based on a BCS ground state: while the penetration depth and the coherence length are anisotropic, the superconducting energy gap shows no indications of line nodes.