Superconductivity in the hole-doped oxy-arsenide RE1−xSrxFeAsO (RE = La, Pr)

Bulk superconductivity was observed in the FeAs-based RE1−xSrxFeAsO (RE = La, Pr) when the di-valence element Sr was substituted to the site of the tri-valence element La and Pr. The maximum superconducting transition temperatures Tc for the two systems are 26 K and 16.3 K, respectively. The doping dependence of the electrical properties and structure of these two systems were investigated systematically. A roughly monotonic increase of Tc and the lattice constants (a-axis and c-axis) with Sr concentration and a saturation behavior in the high doping levels were found. We confirmed that conduction in this type of materials is dominated by hole-like charge carriers by the Hall effect measurements. Also the resistive measurements revealed possible higher upper critical field in these systems comparing with the electron-doped ones.