Low-temperature microwave surface impedance of the conventional organic superconductor κ-(BEDT-TTF)2Cu(NCS)2

We have performed measurements of the microwave surface impedance ${\mathit{Z}}_{\mathit{S}}$=${\mathit{R}}_{\mathit{S}}$+${\mathit{iX}}_{\mathit{S}}$ of the organic superconductor κ-(BEDT-TTF${)}_2$Cu(NCS${)}_2$ down to T=0.8 K. The reactance ${\mathit{X}}_{\mathit{S}}$, and therefore the penetration depth λ, exhibits no temperature dependence for T≪${\mathit{T}}_{\mathit{c}}$. This result is in full agreement with calculations of the BCS ground state and gives no indication of unconventional pairing. Furthermore, we have evaluated the complex conductivity σ^=${\mathrm{\sigma }}_1$+i${\mathrm{\sigma }}_2$ and find a pronounced peak in ${\mathrm{\sigma }}_1$ below ${\mathit{T}}_{\mathit{c}}$ which can be identified as a coherence peak.