Two-magnon Raman scattering inA0.8Fe1.6Se2…

We performed Raman-scattering measurements on high-quality single crystals of ${A}_{0.8}$Fe${}_{1.6}$Se${}_{2}$ superconductors of several compositions. We found a broad, asymmetric peak around 1600 cm${}^{\ensuremath{-}1}$ (200 meV), which we identify as a two-magnon process involving optical magnons. The intensity of the two-magnon peak falls sharply on entering the superconducting phase. This effect, which is entirely absent in the non-superconducting system KFe${}_{1.5}$Se${}_{2}$, requires a strong mutual exclusion between antiferromagnetism and superconductivity arising from proximity effects within regions of microscale phase separation.