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

We report the relationship between the surface superstructure of SrTiO3(001) (STO) substrates and the super-conducting 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 x 1 and root 2 x root 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 x 1, 12-17 meV on root 2 x root 2, and 9-13 meV for 2 x 1 domains on the root 2 x root 2 surface). Our results strongly suggest the important role of the film/substrate interface on high-temperature superconductivity.