  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 strength, which renders new insights on the pairing symmetry.
