| Summary: | The effect of electron doping by trivalent charge state rare-earth-ion (RE = La, Ce, Pr and Nd) substitutions on the superconductivity in BaFe _2 As _2 was examined using epitaxial films. Each of the RE substitutions suppressed the resistivity anomaly associated with magnetic/structural phase transitions, leading to resistivity drops and superconductivity transitions. Bulk superconductivity was observed at the maximum onset critical temperature ( T _c ^onset ) of 22.4 K for La doping and 13.4 K for Ce doping, while only broad resistivity drops were observed at 6.2 K for Pr doping and 5.8 K for Nd doping although zero resistivity and the distinct Meissner effect were not observed at least down to 2 K. The decrease in T _c ^onset with increasing the number of RE 4f electrons cannot be explained in terms of the crystalline quality or crystallographic structure parameters of BaFe _2 As _2 films. It was clarified, based on resistivity–temperature analyses, that magnetic scattering became increasingly significant in the above order of RE dopants. The negative magnetoresistance was enhanced by Ce and Pr doping, implying that the decrease in T _c originates from magnetic pair breaking by interaction of the localized 4f orbitals in the RE dopants with the itinerant Fe 3d orbitals.
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