Role of doubleTiO2layers at the interface of FeSe/SrTiO3superconductors

We determine the surface reconstruction of $\mathrm{SrTi}{\mathrm{O}}_{3}$ used to achieve superconducting FeSe films in experiments, which is different from the $1\ifmmode\times\else\texttimes\fi{}1\mathrm{Ti}{\mathrm{O}}_{2}$-terminated $\mathrm{SrTi}{\mathrm{O}}_{3}$ assumed by most previous theoretical studies. In particular, we identify the existence of a double $\mathrm{Ti}{\mathrm{O}}_{2}$ layer at the FeSe/$\mathrm{SrTi}{\mathrm{O}}_{3}$ 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 $\mathrm{SrTi}{\mathrm{O}}_{3}$ surface to FeSe when the double $\mathrm{Ti}{\mathrm{O}}_{2}$ layer is present. The double layer helps to remove the hole pocket in the FeSe at the \ensuremath{\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 superconductor.