Unified description of superconducting pairing symmetry in electron-doped Fe-based-122 compounds

The pairing symmetry is examined in highly electron-doped Ba(Fe1-xCoxAs)2 and AyFe2Se2 (with A=K, Cs) compounds, with similar crystallographic and electronic band structures. Starting from a phenomenological two-orbital model, we consider nearest-neighbor and next-nearest-neighbor intraorbital pairing interactions on the Fe square lattice. In this model, we find a unified description of the evolution from s±-wave pairing (2.0<n 2.4) to d-wave pairing (2.4 n 2.5) as a function of electron filling. In the crossover region, a time-reversal symmetry breaking s±+id pairing state emerges. This minimal model offers an overall picture of the evolution of superconductivity with electron doping for both s±-wave and d-wave pairings, as long as the dopants only play the role of a charge reservoir. However, the situation is more complicated for Ba(Fe1-xCoxAs)2. A real-space study further shows that when the impurity scattering effects of Co dopants are taken into account, the superconductivity is completely suppressed for n>2.4. This preempts any observation of d-wave pairing in this compound, in contrast to AyFe2Se2 with 0.8<y<1.0.