High Upper Critical Field in Potassium Fluoride Doped LaOFeAs Superconductor

We report a new methodology for synthesizing oxypnictide superconductors with the commonly available potassium fluoride (KF) as a source of fluorine instead of expensive LaF₃. This results in simultaneous doping of potassium at lanthanum sites and leads to a three-fold increase in the upper critical field. We report the Tc (onset) of 28.50 K by F-doping and highest upper critical field (~122 T) at ambient pressure in the family of La-based oxypnictides. To study the contribution from inter and intra-granular current density, we compare remanent magnetization measurements on La_1.03O_0.9F_0.2FeAs and La_0.8K_0.2O_0.8F_0.2FeAs superconductors that show only one major peak indicating substantial electromagnetic granularity. The Seebeck effect (S) with respect to temperature has negative sign and exhibits unconventional temperature dependence. Analysis of resistivity transition broadening under magnetic field reveals the signatures of Arrhenius behavior and the calculated activation energy follows power-law dependence with respect to magnetic field.