Potassium-doped BaFe2 As2 superconducting thin films with a transition temperature of 40 K

We report the growth of potassium-doped BaFe₂As₂ thin films, where the major charge carriers are holes, on Al₂O ₃(0001) and LaAlO₃(001) substrates by using an ex situ pulsed laser deposition technique. The measured superconducting transition temperatures are 40 K and 39 K for the films grown on Al₂O ₃ and LaAlO₃, respectively, and diamagnetism indicates that the films have good bulk superconducting properties below 36 K and 30 K, respectively. The X-ray diffraction patterns for both films indicated a preferred c-axis orientation, regardless of the substrate structures of LaAlO₃ and Al₂O₃. The upper critical field at zero temperature was estimated to be about 155 T.