Functional renormalization group study of the pairing symmetry and pairing mechanism in iron-selenide superconductors

In iron-selenide superconductors only electron-like Fermi pockets survive, challenging the $s_{\pm }$ pairing based on the quasi-nesting between the electron and hole pockets(as in iron arsenides). By functional renormalization group study we show that an in-phase $s$-wave pairing on the electron pockets is realized. The pairing mechanism involve two competing driving forces: the strong C-type spin fluctuations cause attractive pair scattering between and within electron pockets via Cooperon excitations on the virtual hole-like pockets, while the G-type spin fluctuations cause repulsive pairing scattering. The latter effect is however weakened by the hybridization splitting of the electron-like pockets. The in phase $s$-wave pairing symmetry is consistent with the existing experiments.