Superconducting dome revealed by surface structure dependence in single unit cell FeSe on SrTiO3(001)

We report the superconducting properties of single unit cell FeSe prepared on ${\mathrm{SrTiO}}₃\phantom{\rule{0.28em}{0ex}}(\mathrm{STO})\phantom{\rule{0.28em}{0ex}}(001)\text{\ensuremath{-}}c(6\ifmmode\times\else\texttimes\fi{}2)$ using scanning tunneling microscopy/spectroscopy (STM/STS). A surface superstructure of $c(6\ifmmode\times\else\texttimes\fi{}2)$ was obtained by annealing the substrate under an oxygen atmosphere. A $3\ifmmode\times\else\texttimes\fi{}1$ periodicity, which corresponds to the topmost Ti atoms of the substrate, was observed in high-resolution STM images, and the superconducting gap size was $11.5\ifmmode\pm\else\textpm\fi{}2.5\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$. A nonsuperconducting region with additional $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ periodicity was also found, and this originates from the difference in the arrangement of the Sr and Ti atoms at the surface. We conclude that there is a universal superconducting dome structure in the (electron-doping level)---(superconducting gap size) phase diagram by comparing with previous results that controlled the surface of the substrate/film and the gap size can be maximized/minimized by selecting the appropriate STO surface superstructure.