We report on the superconducting properties of $A_x$Fe${}_{2-y}$Se${}_2$ ($A$$=$Rb, K) single crystals studied with the muon spin relaxation or rotation ($\mu$SR) technique. At low temperatures, close to 90$\%$ of the sample volumes exhibit large-moment magnetic order which impedes the investigation of their superconducting properties by $\mu$SR. On the other hand, about 10$\%$ of the sample volumes remain paramagnetic and clearly show a superconducting response. The temperature dependence of the superconducting carrier density was analyzed within the framework of a single $s$-wave gap scenario. The zero-temperature values of the in-plane magnetic penetration depths ${\lambda }_{\mathrm{ab}}\left(0\right)=258\left(2\right)$ and 225(2) nm and the superconducting gaps $\Delta \left(0\right)=7.7\left(2\right)$ and 6.3(2) meV have been determined for $A$ $=$ Rb and K, respectively. The microscopic coexistence and/or phase separation of superconductivity and magnetism is discussed.

• Received 22 November 2011

DOI:https://doi.org/10.1103/PhysRevB.85.100501