Two-magnon Raman scattering in A0.8Fe1.6Se2 systems (A = K, Rb, Cs, and Tl): Competition between superconductivity and antiferromagnetic order

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(-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 KFe1.5Se2, requires a strong mutual exclusion between antiferromagnetism and superconductivity arising from proximity effects within regions of microscale phase separation.