Extensive impurity-scattering study on the pairing symmetry of monolayer FeSe films on SrTiO3

Determination of the pairing symmetry in monolayer FeSe films on SrTiO3 is a requisite for understanding the high superconducting transition temperature in this system, which has attracted intense theoretical and experimental studies but remains controversial. Here, by introducing several types of point defects in FeSe monolayer films, we conduct a systematic investigation on the impurity-induced electronic states by spatially resolved scanning tunneling spectroscopy. Ranging from surface adsorption, chemical substitution, to intrinsic structural modification, these defects generate a variety of scattering strengths, which renders insights on the pairing symmetry.