Intrinsic interfacial van der Waals monolayers and their effect on the high-temperature superconductor FeSe/SrTiO3

The sensitive dependence of monolayer materials on their environment often gives rise to unexpected properties. It was recently demonstrated that monolayer FeSe on a SrTiO3 substrate exhibits a much higher superconducting critical temperature T-c than the bulk material. Here, we examine the interfacial structure of FeSe/SrTiO3 and the effect of an interfacial Ti1+xO2 layer on the increased T-c using a combination of scanning transmission electron microscopy and density functional theory. We find Ti1+xO2 forms its own quasi-two-dimensional layer, bonding to both the substrate and the FeSe film by van der Waals interactions. The excess Ti in this layer can reconstruct the FeSe Fermi surface in a manner consistent with experimental observations. Moreover, the interfacial layer introduces symmetry-breaking distortions in the FeSe film that may favor a T-c increase. These results suggest that this common substrate may be functionalized to modify the electronic structure of a variety of thin films and monolayers.