Control of Epitaxial BaFe₂As₂ Atomic Configurations with Substrate Surface Terminations

Atomic layer controlled growth of epitaxial thin films of unconventional superconductors opens the opportunity to discover novel high temperature superconductors. For instance, the interfacial atomic configurations may play an important role in superconducting behavior of monolayer FeSe on SrTiO₃ and other Fe-based superconducting thin films. Here, we demonstrate a selective control of the atomic configurations in Co-doped BaFe₂As₂ epitaxial thin films and its strong influence on superconducting transition temperatures by manipulating surface termination of (001) SrTiO₃ substrates. In a combination of first-principles calculations and high-resolution scanning transmission electron microscopy imaging, we show that Co-doped BaFe₂As₂ on TiO₂-terminated SrTiO₃ is a tetragonal structure with an atomically sharp interface and with an initial Ba layer. In contrast, Co-doped BaFe₂As₂ on SrO-terminated SrTiO₃ has a monoclinic distortion and a BaFeO_{3- x} initial layer. Furthermore, the superconducting transition temperature of Co-doped BaFe₂As₂ ultrathin films on TiO₂-terminated SrTiO₃ is significantly higher than that on SrO-terminated SrTiO₃, which we attribute to shaper interfaces with no lattice distortions. This study allows the design of the interfacial atomic configurations and the effects of the interface on superconductivity in Fe-based superconductors.