Transition Temperature and Upper Critical Field in $ \hbox{SmFeAsO}_{1 - x}\hbox{F}_{x}$ 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 SmFeAsO 1-x F x with nominal x content varying in a wide range of x = 0-0.35 synthesized at 900 ° C. This synthesis temperature is around 300 ° C lower than the conventional synthesis temperature. The variation in the lattice parameters and transition temperature ( T c ) of various F-doped samples indicates that reduction of the unit cell volume ( V ) seems to be the main reason for the rise of T c up to 57.8 K. Magnetoresistance measurements showed that the upper critical field slope ( dH c2 / 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 (ξ GL ) of 10 A. At still higher F doping levels, dH c2 / dT and the low field J c decreased; above 0.5 T, however, J c had almost the same value. Compared with previous reports, the present synthesis route with low synthesis temperatures and commonly available FeF 2 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.