Potassium fluoride doped LaOFeAs multi-band superconductor: Evidence of extremely high upper critical field

The recently discovered superconductors based on oxypnictides have rekindled the search for new non-copper based high-T_c superconductors. After the initial report by Kamihara et al. of superconductivity in (La-O/F-Fe-As)-based compounds, there have been several reports on these new superconductors but the synthesis of pure phases of these oxypnictides has remained a challenge. Here we describe a new methodology of synthesizing these oxypnictide superconductors with the commonly available potassium fluoride (KF) as a source of fluorine instead of the expensive LaF₃. This route also allows the substitution of potassium at lanthanum sites which leads to an increase in the upper critical field as would be expected for a multi-band superconductor. We also report the highest-T_c (onset) of 28.50 K and highest upper critical field at ambient pressure in the family of La-based oxypnictides.