Robustness of s-wave Pairing in Electron-Overdoped $\text{A}_{1-y}\text{Fe}_{2-x}\text{Se}_2$

TL;DR

This study demonstrates that s-wave pairing symmetry remains stable in heavily electron-doped iron chalcogenides due to their unique magnetic exchange interactions, contrasting with other iron-based superconductors.

Contribution

The paper reveals that s-wave pairing is robust in $( ext{K, Cs}) ext{Fe}_{2-x} ext{Se}_2$ owing to dominant NNN AFM exchange, differing from previous theories emphasizing non-local interactions.

Findings

s-wave pairing is robust in electron-overdoped iron chalcogenides

NNN AFM exchange dominates over NN FM in these materials

Contrasts with iron pnictides where NN AFM exchange leads to pairing competition

Abstract

Using self consistent mean field and functional renormalization group approaches we show that s-wave pairing symmetry is robust in the heavily electron-doped iron chalcogenides $(\text{K, Cs}) \text{Fe}_{2-x}\text{Se}_2 $. This is because in these materials the leading antiferromagnetic (AFM) exchange coupling is between next-nearest-neighbor (NNN) sites while the nearest neighbor (NN) magnetic exchange coupling is ferromagnetic (FM). This is different from the iron pnictides, where the NN magnetic exchange coupling is AFM and leads to strong competition between s-wave and d-wave pairing in the electron overdoped region. Our finding of a robust s-wave pairing in $(\text{K, Cs}) \text{Fe}_{2-x}\text{Se}_2$ differs from the d-wave pairing result obtained by other theories where non-local bare interaction terms and the NNN $J_2$ term are underestimated. Detecting the pairing symmetry in $(\text{K, Cs}) \text{Fe}_{2-x}\text{Se}_2 $ may hence provide important insights regarding the mechanism of superconducting pairing in iron based superconductors.