Superconducting properties of single-crystalline A sub(x)Fe sub(2-y)Se sub(2)(A = Rb, K) studied using muon spin spectroscopy

We report on the superconducting properties of A sub(x)Fe sub(2-y)Se sub(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] sub(ab) (0) = 258 (2) and 225(2) nm and the superconducting gaps Delta (0) = 7.7(2) 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.