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

TL;DR

This study systematically investigates how rare earth elements (La-Gd) influence the superconducting properties of Co-doped (Ca,RE)FeAs2, revealing trends in critical temperature and effects of Eu valence states.

Contribution

It provides new insights into the dependence of superconducting properties on RE elements and co-doping effects in (Ca,RE)FeAs2 compounds.

Findings

Superconducting Tc decreases with smaller RE ionic radii.

Co-doping improves superconducting properties across all RE elements.

Eu co-doped samples show unusually low Tc due to Eu valence states.

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.