Structure determination in thin film B a1-x L ax F e2 A s2: Relation between the FeA s4 geometry and superconductivity

A thin film superconductor Ba1-xLaxFe2As2 is the first electron-doped compound of BaFe2As2 obtained by the substitution of Ba with La within a blocking layer. By contrast, in conventional electron-doped Ba(Fe1-xCox)2As2, the dopant (Co) is inserted into the conduction layer. The different shapes, or geometries, of FeAs4 are expected in the two compounds above. However, the structure of Ba1-xLaxFe2As2 is investigated here because of being a thin film. To clarify the effect of the geometry, we make use of an up-to-date method to determine the atomic positions of the Ba1-xLaxFe2As2 thin film using synchrotron x-ray diffraction. We established that the FeAs4 geometry in Ba1-xLaxFe2As2 indicates a systematic variation upon doping, which is opposite to that in Ba(Fe1-xCox)2As2. However, the superconducting transition temperatures nearly coincide with each other for the same amount of dopings. The present result contrasts with a suggestion that the FeAs4 geometry strongly influences the superconductivity in iron pnictides. Hence, we propose that the carrier density is the more important parameter than the FeAs4 geometry and that the electronic correlation plays a significant role in the superconductivity for the electron-doped BaFe2As2.