Unified picture for the order parameter and mechanism of iron based superconductors

Iron-based superconductors are the only family of high-temperature unconventional superconductors besides cuprates, and their pairing symmetry has attracted considerable attention since their discovery. The widely accepted pairing model is S^±, which requires both hole and electron pockets. However, this picture was questioned after the discovery of the intercalated or monolayer form of FeSe-based systems which have no hole pockets.We have conducted a series of experiments to study the characteristics of the gap structure or order parameter in various iron-based superconductors with different Fermi surface topologies. In two typical FeAs-based superconductors, namely NaFe_1-xCo_xAs and Ba_1-xK_xFe₂As₂, by measuring the scanning tunneling spectra or detecting non-magnetic impurity induced bound states, we found clear evidence supporting the S^± pairing. Concerning the systems without hole pockets, we took the (Li_1−xFe_x)OHFeSe system as a platform and conducted extensive investigations. Two anisotropic superconducting gaps were observed in (Li_1−xFe_x)OHFeSe by our scanning tunneling microscopy measurements. Detailed experimental data based on the Fourier transformed quasiparticle interference (FT-QPI) allowed us for the first time to assign two superconducting gaps to the outer and inner electron Fermi pockets after folding or hybridization. In addition, a sign reversal of the superconducting order parameter in (Li_{1 − x}Fe_x)OHFe_1-yZn_ySe is evidenced according to the further data and analysis based on the in-gap bound state from a non-magnetic impurity as well as the phase-sensitive analysis of FT-QPI data. These results provide a unified picture for the gap structure and pairing mechanism of iron-based superconductors with or without Fermi hole pockets and support the conclusion that repulsive interaction plays the key role in electron pairing.