Superconductivity at 31×3 K in Yb-doped La(O/F)FeAs superconductors

Abstract

The effect of ytterbium substitution at the lanthanum site on the superconducting properties of La_1−x Yb _x O_0·8F_0·2FeAs (x = 0·10, 0·20 and 0·30) oxypnictides has been investigated. Powder X-ray diffraction studies show the presence of Yb₂O₃ and LaOF as secondary phases. The superconducting transition temperature (T _c ) of 31·3 (± 0·05) K has been observed in x = 0·1 composition which is the maximum T _c so far in the La(O/F)FeAs superconductor family at ambient pressure. Further increase in x leads to suppression and broadening of superconducting transition. The resistive transition curves under different magnetic fields were investigated, leading to determination of upper critical field H _c2 (T) of this new superconductor. The value of H _c2 at zero temperature is estimated to be about 46 T corresponding to coherence length ~27 Å.