Upper critical field of the 122-type iron pnictide superconductors

Abstract The upper critical fields ( H c 2 ) of the single crystals (Sr,Na)Fe 2 As 2 and Ba 0.55 K 0.45 Fe 2 As 2 were determined by means of measuring the electrical resistivity, ρ xx ( μ 0 H ) , using the facilities of pulsed magnetic field at Los Alamos. In general, these compounds possess a very large upper critical field ( H c 2 (0)) with a weak anisotropic effect. The detailed curvature of H c 2 ( T c ) may depend on the magnetic field orientation and the sample compositions. We argue that such a difference mainly results from the multi-band effect, which might be modified via doping.