Resilient nodeless d-wave superconductivity in monolayer FeSe

Monolayer FeSe exhibits the highest transition temperature among the iron based superconductors and appears to be fully gapped, seemingly consistent with s -wave superconductivity. Here, we develop a theory for the superconductivity based on coupling to fluctuations of checkerboard magnetic order (which has the same translation symmetry as the lattice) [1]. The electronic states are described by a symmetry based kp-like theory and naturally account for the states observed by angle resolved photoemission spectroscopy. We show that a prediction of this theory is that the resultant superconducting state is a fully gapped, nodeless, d -wave state. This state, which would usually have nodes, stays nodeless because, as seen experimentally, the relevant spin-orbit coupling term has an energy scale smaller than the superconducting gap [2].

[1] T. Shishidou et al., "Magnetic fluctuatons in single-layer FeSe", arXiv:1708.0786v2 (2017).
[2] D.F. Agterberg et al., "Resilient nodeless d-wave superconductivity in monolayer FeSe", arXiv:1706.01978v2 (2017).