Substrate-induced broken C4 symmetry and gap variation in superconducting single-layer FeSe/SrTiO3−(13×13)

Abstract

We performed scanning tunneling microscopy/spectroscopy (STM/STS) measurements to investigate the superconducting properties of single-layer FeSe grown on ${\text{SrTiO}}_{3}$ (STO) $(001)\text{\ensuremath{-}}(\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13})$. We found ``$+$'' and ``z'' patterns arranged in $\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13}$ periodicity in the STM images. Surprisingly, they show ${C}_{2}$ symmetry, although free-standing FeSe itself has ${C}_{4}$ symmetry. STS spectra and quasiparticle interference patterns near the Fermi level also indicate the broken ${C}_{4}$ symmetry with the superconducting gap showing a local variation within the STO surface unit cell because of the short coherence length. This clearly shows the non-negligible role of the substrate surface in the superconductivity of the FeSe/STO system.