Enhancement in superconducting transition temperature and upper critical   field of LaO0.8F0.2FeAs with antimony doping
  We report the synthesis and characterization of antimony doped oxypnictide superconductor, LaO0.8F0.2FeAs1-xSbx (x = 0.05 and 0.10). The parent compound LaOFeAs with fluorine doping exhibits superconductivity at maximum transition temperature ~ 28.5 K [11]. Here we partially substitute As by Sb (LaO0.8F0.2FeAs1-xSbx) and observe enhancement of the transition temperature to 30.1 K. This is the only instance so far where Tc increases with doping in the conducting layer (FeAs) and this leads to the highest transition temperature in any La-based oxypnictide. XRD and EDAX measurements confirm phase purity of the samples and the presence of Sb. The magneto-resistance measurements show that the value of upper critical field Hc2(0) to be about 73 T corresponding to a coherence length of 22 angstrom. The Seebeck coefficient measurements indicate electron transport with strong contribution from electron-electron correlation. These results provide interesting insight to the origin of superconductivity in these novel series of compounds. 
S. J. Singh, J. Prakash, S. Patnaik and A. K. Ganguli