Superconducting (Ba,K)Fe₂As₂ epitaxial films on single and bicrystal SrTiO₃ substrates

The realization of single crystal and bicrystal films of superconducting materials is of great interest because they allow the investigation of the intragrain performance as well as the understanding of potential limitations in the grain boundary transparency. For many years, the realization of a high-quality (Ba,K)Fe₂As₂ film has been challenging. Here, the realization of (Ba,K)Fe₂As₂ epitaxial thin films on single crystal SrTiO₃(001) and [001]-tilt-type SrTiO₃ bicrystal substrates with high superconducting properties is demonstrated. The epitaxial growth of (Ba,K)Fe₂As₂ was enabled by implementing an undoped BaFe₂As₂ buffer layer between the SrTiO₃ substrate and (Ba,K)Fe₂As₂ film. The film exhibits a high T_c of 38.0 K and an extremely high J_c of 14.3 MA/cm² at 4.2 K. Artificial grain boundaries of (Ba,K)Fe₂As₂ were also successfully achieved on bicrystals with misorientation angles up to 36.8° by the same preparation methods. The artificial grain boundaries exhibited an identical T_c of 38.0 K and an excellent transfer of the grain orientation from the bicrystal substrates with high crystallinity comparable to that of the high-quality Ba(Fe,Co)₂As₂ films. This enables the investigation of the intrinsic (Ba,K)Fe₂As₂ grain boundary nature, which will clarify its potential for superconducting applications, like Josephson junctions, wires, and magnets.