  Single-layer FeSe film on SrTiO3(001) was recently found to be the champion
of interfacial superconducting systems, with a much enhanced superconductivity
than the bulk iron-based superconductors. Its superconducting mechanism is of
great interest. Although the film has a simple Fermi surface topology, its
pairing symmetry is unsettled. Here by using low-temperature scanning tunneling
microscopy (STM), we systematically investigated the superconductivity of
single-layer FeSe/SrTiO3(001) films. We observed fully gapped tunneling
spectrum and magnetic vortex lattice in the film. Quasi-particle interference
(QPI) patterns reveal scatterings between and within the electron pockets, and
put constraints on possible pairing symmetries. By introducing impurity atoms
onto the sample, we show that the magnetic impurities (Cr, Mn) can locally
suppress the superconductivity but the non-magnetic impurities (Zn, Ag and K)
cannot. Our results indicate that single-layer FeSe/SrTiO3 has a plain s-wave
paring symmetry whose order parameter has the same phase on all Fermi surface
sections.
