Dependences on RE of superconducting properties of transition metal co-doped (Ca, RE)FeAs2 with RE = La-Gd

After the discovery of superconductivity in LaFeAs(O,F)[1],exploration of iron-based superconductors has been attemptedextensively and various type of new superconductors, such as 11[2], 111[3-5]and 122[6]systems, were discovered. Searches fornew iron-based superconductors have been still continuing,resulted in a recent discovery of new series of iron-based super-conductors (Ca,RE)FeAs2((Ca,RE)112)[7-9]. (Ca,RE)112 has severalunique properties, such as monoclinic structure with space groupofP21orP21/m, and co-existence of As1and As3in the crystallattice[7,8]. Moreover, anisotropy of (Ca,La)112 was reported tobe 2.0-4.2[10], which is smaller than that of 1111 type supercon-ductors, while interlayer distance between Fe layer (dFe-Fe)islonger in (Ca,La)112. More than 15% substitution of La for Ca siteis needed to form (Ca,La)112 phase, andTcdecreases with increas-ing La content[11]. It suggests that (Ca,La)112 is basically in theoverdoped state. Sala et al. reportedREdependence onTcin(Ca,RE)112 with nominalREcomposition of 15% for Ca. A decreaseinTcwith a decrease in ionic radii ofRE3+was observed except for(Ca,Ce)112 which did not show superconductivity. Kudo et al.reported theTcof 47 K in Sb co-doped (Ca,La)112[12].Enhancement ofTcin Sb co-doped (Ca,RE)112 is achieved byoptimization of As-Fe-As bond angle. In addition, we have recentlyreported Mn or Co-co-doping effects in (Ca,La)112 and (Ca,Pr)112[13]. Analyzed substitution levels of Co in Co-co-doped (Ca,RE)112were close to the nominal composition, thoughREcompositionwere higher than those of nominal compositions. Sharp supercon-ducting transition in (Ca,La)112 and enhancement ofTcinCa,Pr)112 was induced by Co-co-doping, while Mn co-dopingstrongly suppressed superconductivity. These results indicatesco-doping of transition metals (TM), such as Co, is promising wayto improve superconducting properties of (Ca,RE)112.In this paper, we reportTM(Co,Ni) co-doping effect for(Ca,RE)112 (RE= La-Gd) with lowREdoping level.REelementdependence on structural parameters as well as superconductingproperties ofTMco-doped (Ca,RE)112 are systematicallyinvestigated