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 ${\mathrm{SrTiO}}₃$ substrate exhibits a much higher superconducting critical temperature ${T}_c$ than the bulk material. Here, we examine the interfacial structure of FeSe/${\mathrm{SrTiO}}₃$ and the effect of an interfacial ${\mathrm{Ti}}_1+x{\mathrm{O}}₂$ layer on the increased ${T}_c$ using a combination of scanning transmission electron microscopy and density functional theory. We find ${\mathrm{Ti}}_1+x{\mathrm{O}}₂$ 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 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.