Enhancement in superconducting transition temperature and upper critical field of LaO_0.8F_0.2FeAs with antimony doping

Abstract

We report the synthesis and characterization of the antimony-doped oxypnictide superconductor, LaO_0.8F_0.2FeAs_1-xSb_x (x = 0.05 and 0.10). The parent compound LaOFeAs with fluorine doping exhibits superconductivity at the maximum transition temperature ~28.5 K (at ambient pressure). Here we partially substitute As by Sb (LaO_0.8F_0.2FeAs_1-xSb_x) and observe enhancement of the transition temperature to 30.1 K. This is one rare case where T_c increases with doping in the conducting layer (FeAs) and this leads to the highest transition temperature in any La-based oxypnictide. X-ray diffraction and energy-dispersive x-ray analysis measurements confirm the phase purity of the samples and the presence of Sb. The magneto-resistance measurements show that the value of the upper critical field H_c2(0) is about 73 T, corresponding to a coherence length (ξ_GL) of 22 Å. The Seebeck coefficient measurements indicate electron transport with a strong contribution from electron-electron correlation. These results provide interesting insight into the origin of superconductivity in this novel series of compounds.