Symmetry analysis of possible superconducting states in KxFe2Se2 superconductors

TL;DR

This paper analyzes the possible superconducting symmetries in KxFe2Se2, revealing that only certain s-wave states are compatible with its crystallography, challenging previous assumptions about its pairing symmetry.

Contribution

It provides a symmetry-based analysis showing that KxFe2Se2 can only host specific s-wave superconducting states, excluding d-wave and other symmetries.

Findings

Superconductivity in KxFe2Se2 is limited to certain s-wave states.

d-wave symmetry is incompatible with the crystal structure.

The possible pairing states differ from other Fe-based superconductors.

Abstract

A newly discovered family of the Fe-based superconductors is isostructural with the so-called 122 family of Fe pnictides, but has a qualitatively different doping state. Early experiments indicate that superconductivity is nodeless, yet prerequisites for the $s_\pm$ nodeless state (generally believed to be realized in Fe superconductors) are missing. It is tempting to assign a $d-$ wave symmetry to the new materials, and it does seem at first glance that such a state may be nodeless. Yet a more careful analysis shows that it is not possible, given the particular 122 crystallography, and that the possible choice of admissible symmetries is severly limited: it is either a conventional single-sign $s_{+}$ state, or another $s_\pm$ state, different from the one believed to be present in other Fe-based superconductors.