Superconductivity of single unit cell FeSe/SrTiO3 (001): Substrate-surface superstructure dependence

We report the relationship between the surface superstructure of ${\mathrm{SrTiO}}₃$(001) (STO) substrates and the superconducting properties of single unit cell (1-UC) FeSe films using scanning tunneling microscopy/spectroscopy (STM/STS). Under reflection high-energy-electron diffraction observation, we controlled the periodicity of the STO surface to $2\ifmmode\times\else\texttimes\fi{}1$ and $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ and grew high-quality FeSe films. We found that the substrate periodicity can be imaged through the 1-UC FeSe from high-resolution STM observations and obtained clear evidence that the superconducting gap size depends on the surface periodicity (10--15 meV on $2\ifmmode\times\else\texttimes\fi{}1$, 12--17 meV on $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$, and 9--13 meV for $2\ifmmode\times\else\texttimes\fi{}1$ domains on the $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ surface). Our results strongly suggest the important role of the film/substrate interface on high-temperature superconductivity.