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

Here, the pairing symmetry is examined in highly electron-doped Ba (Fe_1-x Co_x As)₂ and A_y Fe₂ Se₂ (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 (Fe_1-x Co_x As)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 A_y Fe₂ Se₂ with 0.8 < y < 1.0.