Identical effects of indirect and direct electron doping of superconducting BaFe2As2thin films

Abstract

Electron doping of a 122-type iron pnictide BaFe${}_{2}$As${}_{2}$ by substituting the Ba site with an aliovalent ion (indirect doping), which had been unsuccessful by conventional solid-state synthesis methods, was achieved by a nonequilibrium film growth process. The substitution with La was substantiated by a systematic shrinkage of the $c$-axis lattice parameter due to the smaller ionic radius of La${}^{3+}$ than that of Ba${}^{2+}$. A negative Hall coefficient indicated that the majority carriers were electrons, as is consistent with this aliovalent ion doping. The La substitution suppressed an antiferromagnetic transition and induced bulk superconductivity at a maximum onset critical temperature (${T}_{\mathrm{c}}$) of 22.4 K. The electronic phase diagram for (Ba${}_{1\ensuremath{-}x}$La${}_{x}$)Fe${}_{2}$As${}_{2}$ was built, which revealed that the indirect electron doping at the Ba site with La [(Ba${}_{1\ensuremath{-}x}$La${}_{x}$)Fe${}_{2}$As${}_{2}$] exhibits almost the same ${T}_{\mathrm{c}}$- doping level relation as that of the direct electron doping at the Fe site with Co [Ba(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$]. This finding clarified that ${T}_{\mathrm{c}}$ in 122-type compounds is not affected by a crystallographic doping site, which is in sharp contrast to the 1111-type compounds, $RE$FeAsO ($RE$ $=$ rare earth). It is tentatively attributed to the differences in their dimensionality of electronic structures and electron pairing symmetries.