Pairing Mechanism of the FeSe-monolayer and related Systems: Dynamical Tuning of Pairing Cutoff Energy

There are a group of FeSe systems: FeSe/SrTiO3 monolayer system (Tc~60-100K) and other heavily electron-doped iron selenide (HEDIS) compounds such as AxFe2-ySe2 (A=K, Rb, Cs, Tl, etc.) (Tc~30-40K), (Li1-xFexOH)FeSe (Tc~40K), etc. These systems have all very high Tc (30K -100K) despite having only the electron Fermi surfaces (FSs) but no hole FS. Here we propose a unifying pairing mechanism [1] based on a new concept: dynamical tuning of pairing cutoff energy. With this mechanism, I show how the incipient band without a Fermi surface can participate pairing interaction through RG process and the system forms the s++-wave state only with the electron pockets. In this way, the HEDIS system can achieve the maximum Tc, stored in the system, and yet avoid the detrimental impurity pair-breaking scattering. Our theory not only provides a unifying pairing mechanism for all HEDIS system, but also naturally explains the puzzling double dome structure of the phase diagram of Tc versus tuning parameter (doping, pressure, etc) [2, 3].1