Transition Temperature and Upper Critical Field in SmFeAsO1-xFx Synthesized at Low Heating Temperatures

Low-temperature synthesis is a promising and potentially effective method for improving superconducting properties. We report on the fabrication of polycrystalline samples of SmFeAsO1-xFx with nominal x content varying in a wide range of x = 0-0.35 synthesized at 900 deg C. This synthesis temperature is around 300 deg C lower than the conventional synthesis temperature. The variation in the lattice parameters and transition temperature (Tc) of various F-doped samples indicates that reduction of the unit cell volume (V) seems to be the main reason for the rise of Tc up to 57.8 K. Magnetoresistance measurements showed that the upper critical field slope (dHc2/dT) increased with increasing F concentration up to x = 0.2, where it reached a maximum value of -8 T/K corresponding to a coherence length of 10 angstrom. At still higher F doping levels, dHc2/dT and the low field Jc decreased; above 0.5 T, however, Jc had almost the same value. Compared with previous reports, the present synthesis route with low synthesis temperatures and commonly available FeF2 as the source of F is more effective at introducing F into the SmFeAsO system and thereby resulting in improved superconducting properties for the system. In addition, this new sample preparation method also reduces unnecessary problems such as the evaporation of F and reaction between the crucible and superconductor during the solid-state reaction.