Identical effects of indirect and direct electron doping of superconducting BaFe2As2 thin films

Electron doping of a 122-type iron pnictide BaFe2As2 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 non-equilibrium 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 La3+ than that of Ba2+. 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 (Tc) of 22.4 K. The electronic phase diagram for (Ba1-xLax)Fe2As2 was built, which revealed that the indirect electron doping at the Ba site with La [(Ba1-xLax)Fe2As2] exhibits almost the same Tc - doping level relation as that of the direct electron-doping at the Fe site with Co [Ba(Fe1-xCox)2As2]. This finding clarified that Tc in 122-type compounds is not affected by a crystallographic doping site, which is in sharp contrast to the 1111-type compounds, REFeAsO (RE = rare earth). It is tentatively attributed to the differences in their dimensionality of electronic structures and electron pairing symmetries.

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