The pairing symmetry is examined in highly electron-doped $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\mathrm{As}\right)}_2$ and $A_y{\mathrm{Fe}}_2{\mathrm{Se}}_2$ (with $A=\text{K}$, 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 $s_{\pm }$-wave pairing $\left(2.0<n\lesssim 2.4\right)$ to $d$-wave pairing $\left(2.4\lesssim n\lesssim 2.5\right)$ as a function of electron filling. In the crossover region, a time-reversal symmetry breaking $s_{\pm }+id$ pairing state emerges. This minimal model offers an overall picture of the evolution of superconductivity with electron doping for both $s_{\pm }$-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 $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\mathrm{As}\right)}_2$. 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 $A_y{\mathrm{Fe}}_2{\mathrm{Se}}_2$ with $0.8<y<1.0$.

• Received 23 March 2015
• Revised 2 June 2015

DOI:https://doi.org/10.1103/PhysRevB.91.220509