Microwave study of the superconducting state of h$_{8}$ and d$_{8}$ $\kappa $-(BEDT-TTF)$_{2}$Cu[ N(CN)$_{2}$] Cl organic conductors

At ambient pressure, the hydrogenated (h8) and deuterated (d8) $\kappa $-(BEDT-TTF)$_{2}$Cu[ N(CN)$_{2}$] Cl organic compounds are Mott insulators. Under a pressure of a few hundreds bars, an unconventional metallic state is induced and a superconducting one is observed below 13 K. In the insulating state, dc resistivity measurements have always indicated traces of superconductivity at low pressures ($\sim $1 bar) although the most recent pressure-temperature phase diagrams are surprisingly silent on that issue. We have studied this unusual superconducting phase in both h8 and d8 single crystals by measuring the parallel and perpendicular microwave resistivities and dielectric constants as a function of temperature, pressure and magnetic field. For all pressure values, anomalies are always detected below 11 K; their behavior in a magnetic field is consistent with superconductivity. Key words. Organic - superconductivity- microwave