Picoscale structural insight into superconductivity of monolayer FeSe/SrTiO₃

Remarkable enhancement of the superconducting transition temperature (T _c) has been observed for monolayer (ML) FeSe films grown on SrTiO₃ substrates. The atomic-scale structure of the FeSe/SrTiO₃ interface is an important determinant of both the magnetic and interfacial electron-phonon interactions and is a key ingredient to understanding its high-T _c superconductivity. We resolve the atomic-scale structure of the FeSe/SrTiO₃ interface through a complementary analysis of scanning transmission electron microscopy and in situ surface x-ray diffraction. We find that the interface is more strongly bonded for a particular registration, which leads to a coherently strained ML. We also determine structural parameters, such as the distance between ML FeSe and the oxide, Se─Fe─Se bond angles, layer-resolved distances between Fe─Se, and registry of the FeSe lattice relative to the oxide. This picoscale structure determination provides an explicit structural framework and constraint for theoretical approaches addressing the high-T _c mechanism in FeSe/SrTiO₃.