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. 
Takayoshi Katase, Soshi Iimura, Hidenori Hiramatsu, Toshio Kamiya and   Hideo Hosono
cond-mat.supr-con