Understanding the interplay between superconductivity and atomic-scale interface structure of multilayer FeSe /SrTiO₃

The discovery of greatly enhanced high-temperature superconductivity in FeSe /SrTiO₃ has made this system an attractive platform to explore the science of high-T_c materials. While the exact mechanism of T_c enhancement remains not fully understood, interface structure and interfacial effects are believed to play a critical role. A better understanding of the atomic interface structure of FeSe/SrTiO₃ and its correlation with T_c would clarify the role of the interface and constrain theoretical approaches for explaining T_c enhancement. Here, using high-angle annual dark-field scanning transmission electron microscopy and electron energy loss spectroscopy, we directly compare the interface structure of superconducting and non-superconducting FeSe/SrTiO₃ films prepared on a range of SrTiO₃ surfaces. We find that while a uniform SrTiO₃ surface reconstruction tends to improve the quality of growth, superconductivity is not affected by mixed surface termination. We report on the statistical analysis of the distance of FeSe and surface TiO₂ for both superconducting and non-superconducting states. We further discuss the role of interface Se on the superconductivity of FeSe in the context of interfacial charge transfer.