Intrinsic interfacial van der Waals monolayers and their effect on the high-temperature superconductor FeSe/SrTiO${}_3$

The sensitive dependence of monolayer materials on their environment often gives rise to unexpected properties. It was recently demonstrated that monolayer FeSe on a SrTiO${}_3$ substrate exhibits a much higher superconducting critical temperature T${}_C$ than the bulk material. Here, we examine the interfacial structure of FeSe / SrTiO${}_3$ and the effect of an interfacial Ti${}_{1+x}$O${}_2$ layer on the increased T${}_C$ using a combination of scanning transmission electron microscopy and density functional theory. We find Ti${}_{1+x}$O${}_2$ 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 electron-dopes the FeSe monolayer in agreement with experimental observations. Moreover, the interfacial layer introduces symmetry-breaking distortions in the FeSe film that 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.