Atomic structure of single-layer FeSe/SrTiO3 interface studied by synchrotron X-ray diffraction

The superconducting transition temperature of single unit cell epitaxial FeSe/SrTiO$_{\mathrm{3}}$ interface (60-109 K) is significantly enhanced compared to the transition temperature in bulk FeSe (8K). It is now widely accepted that the oxide-FeSe interface plays a crucial role in enhancing the superconductivity of FeSe. One key prerequisite to understanding the role of the interface is to resolve its atomic structure and chemistry. Here we report synchrotron X-ray diffraction studies on single-layer FeSe/SrTiO$_{\mathrm{3}}$ grown and measured in ultra-high vacuum chamber. Crystal truncation rod analysis reveals a $\surd $13$\times$ $\surd $13 reconstructed SrTiO$_{\mathrm{3}}$ surface with double TiO$_{\mathrm{2}}$ termination and rumpling of SrTiO$_{\mathrm{3}}$ atoms near the interface. The details of the interface structure provide an experimental foundation towards acquiring a theoretical understanding of the interface-enhanced superconductivity in single-layer FeSe/SrTiO$_{\mathrm{3}}$.