Enhancement in superconducting transition temperature (T_c) and upper critical field (H_c2) in new Yb-doped Ce_1−xYb_xO_0.9F_0.1FeAs superconductors

We report the synthesis and characterization of new oxypnictides of the type Ce_1−xYb_xO_0.9F_0.1FeAs (‘x’ = 0.3, 0.4 and 0.5) for the first time. All these compounds crystallize in the tetragonal ZrCuSiAs type structure (space group: P4/nmm). Reduction in both the lattice parameters (a and c) was observed on substitution of smaller Yb ions at Ce sites in O_0.9F_0.1FeAs. All these compounds were found to be superconducting with maximum T_c of 48.7 K for ‘x’ = 0.5 composition which is highest among Ce(O/F)FeAs based superconductors. Temperature dependence of resistivity under magnetic field has been studied to evaluate the upper critical field (H_c2 (0)) of these superconductors. Enhancement in upper critical field (H_c2) was observed for these Yb doped Ce_1−xYb_xO_0.9F_0.1FeAs superconductors (max H_c2 (0) = ∼142 T for ‘x’ = 0.3). The average coherence length of all Yb doped samples was estimated to be ∼23 Å