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

TL;DR

This study investigates how an interfacial Ti$_{1+x}$O$_2$ layer in FeSe/SrTiO$_3$ enhances superconductivity by electron doping and symmetry-breaking, revealing potential for substrate functionalization to modify thin film properties.

Contribution

It uncovers the formation and role of a Ti$_{1+x}$O$_2$ interfacial layer in boosting T$_C$ in FeSe monolayers through combined microscopy and theoretical analysis.

Findings

Ti$_{1+x}$O$_2$ forms a quasi-2D interfacial layer bonding via van der Waals forces.

The interfacial layer electron-dopes the FeSe monolayer, increasing T$_C$.

Symmetry-breaking distortions induced by the interfacial layer favor higher superconducting transition temperatures.

Abstract

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.