Dependences on RE of Superconducting Properties of Transition Metal co-doped (Ca,RE)FeAs2 with RE = La-Gd

Abstract

Dependence of superconducting properties of (Ca,RE)(Fe,TM)As2 [(Ca,RE)112, TM: Co, Ni)] on RE elements (RE = La-Gd) was systematically investigated. Improvement of superconducting properties by Co or Ni co-doping was observed for all (Ca,RE)112, which is similar to Co-co-doped (Ca,La)112 or (Ca,Pr)112. Tc of Co-co-doped samples decreased from 38 K for RE = La to 29 K for RE = Gd with decreasing ionic radii of RE3+. However, Co-co-doped (Ca,Eu)112 showed exceptionally low Tc = 21 K probably due to the co-existence of Eu3+ and Eu2+ suggested by longer interlayer distance dFe-Fe of (Ca,Eu)112 than other (Ca,RE)112.