Symmetry analysis of possible superconducting states in KxFeySe2 superconductors

A newly discovered family of 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. If indeed superconductivity in this system is nodeless, the possible choice of admissible symmetries is severely 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.