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-Tc 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-Tc (onset) of 28.50 K and highest upper critical field at ambient pressure in the family of La-based oxypnictides.