The role of double TiO2 layers at the interface of FeSe/SrTiO3 superconductors

TL;DR

This study reveals that a double TiO2 layer at the SrTiO3-FeSe interface is crucial for superconductivity, aiding epitaxial growth and electron transfer, thus explaining key electronic properties.

Contribution

It identifies and characterizes a double TiO2 layer at the interface, providing new insights into the interface structure and its role in FeSe/SrTiO3 superconductors.

Findings

Double TiO2 layer facilitates FeSe growth

Enhanced electron transfer from SrTiO3 to FeSe

Band structure aligns with superconducting behavior

Abstract

We determine the surface reconstruction of SrTiO3 used to achieve superconducting FeSe films in experiments, which is different from the 1x1 TiO2 terminated SrTiO3 assumed by most previous theoretical studies. In particular, we identify the existence of a double TiO2 layer at the SrTiO3-FeSe interface that plays two important roles. First, it facilitates the epitaxial growth of FeSe. Second, ab initio calculations reveal a strong tendency for electrons to transfer from an oxygen deficient SrTiO3 surface to FeSe when the double TiO2 layer is present. As a better electron donor than previously proposed interfacial structures, the double layer helps to remove the hole pocket in the FeSe at the {\Gamma} point of the Brillouin zone and leads to a band structure characteristic of superconducting samples. The characterization of the interface structure presented here is a key step towards the resolution of many open questions about this novel superconductor.