A unified description of superconducting pairing symmetry in electron-doped Fe-based-122 compounds
Abstract
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 novel time-reversal symmetry breaking state with s+id pairing symmetry emerges. This minimal model offers an overall picture of the evolution of superconductivity with electron doping for both s-wave [Ba(Fe1-xCoxAs)2] and d-wave [AyFe2Se2] pairing, 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.