Sign reversal of order parameter in single layer FeSe/SrTiO₃ film

Single layer FeSe film grown on SrTiO₃ substrate has drawn much interest for its novel interfacial effects, which have led to the highest superconducting temperature (T_C) to date amongst all Fe-based superconductors. While several paring symmetries, such as s₊₊-wave, nodeless d-wave, as well as s_±-wave have been suggested, the experimental determination of its superconducting paring symmetry remains elusive. Here we investigate the intrinsic impurity-induced in-gap bound states and quasiparticle interference (QPI) patterns in single layer FeSe/SrTiO₃ by scanning tunneling microscopy/spectroscopy. We observed bound states induced by nonmagnetic impurities, which strongly suggests a sign-changing order parameter. Through detailed analysis of the phase-sensitive QPI patterns, we further confirm that the order parameter indeed changes sign within the electron pockets. This identification of a sign change pairing symmetry in single layer FeSe/SrTiO₃ presents an important step towards the understanding of the mechanism of its high T_c superconductivity.