Symmetry analysis of possible superconducting states in KxFe2Se2 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 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 state, different from the one believed to be present in other Fe-based superconductors.