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

Bo Li; Lihua Pan; Yuan-Yen Tai; Matthias J. Graf; Jian-Xin Zhu; Kevin E. Bassler; C. S. Ting

paper

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

arXiv:1503.06706

Abstract

The pairing symmetry is examined in highly electron-doped Ba(Fe $_{1 - x}$ Co $_{x}$ As) $_{2}$ and A $_{y}$ Fe $_{2}$ Se $_{2}$ (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 $s_{\pm}$ -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 $s_{\pm} + i d$ pairing symmetry emerges. This minimal model offers an overall picture of the evolution of superconductivity with electron doping for both $s_{\pm}$ -wave [Ba(Fe $_{1 - x}$ Co $_{x}$ As) $_{2}$ ] and $d$ -wave [A $_{y}$ Fe $_{2}$ Se $_{2}$ ] pairing, 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 $_{y}$ Fe $_{2}$ Se $_{2}$ .

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Phase diagram of the isovalent phosphorous-substituted 122-type iron pnictides