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(Li0.8Fe0.2)OHFeSe is a newly-discovered intercalated iron-selenidesuperconductor with a Tc above 40 K, which is much higher than the Tc of bulkFeSe (8 K). Here we report a systematic study of (Li0.8Fe0.2)OHFeSe by lowtemperature scanning tunneling microscopy (STM). We observed two kinds ofsurface terminations, namely FeSe and (Li0.8Fe0.2)OH surfaces. On the FeSesurface, the superconducting state is fully gapped with double coherence peaks,and a vortex core state with split peaks near EF is observed. Throughquasi-particle interference (QPI) measurements, we clearly observed intra- andinter-pocket scatterings in between the electron pockets at the M point, aswell as some evidence of scattering that connects gamma and M points. Uponapplying magnetic field, the QPI intensity of all the scattering channels arefound to behave similarly. Furthermore, we studied impurity effects on thesuperconductivity by investigating intentionally introduced impurities andintrinsic defects. We observed that magnetic impurities such as Cr adatoms caninduce in-gap states and suppress superconductivity. However, nonmagneticimpurities such as Zn adatoms do not induce visible in-gap states. Meanwhile,we show that Zn adatoms can induce in-gap states in thick FeSe films, which isbelieved to have an (s+-)wave pairing symmetry. Our experimental resultssuggest it is likely that (Li0.8Fe0.2)OHFeSe is a plain s-wave superconductor,whose order parameter has the same sign on all Fermi surface sections.

Surface electronic structure and evidence of plain s-wave superconductivity in (Li0.8Fe0.2)OHFeSe