Two-magnon Raman scattering in A${}_{0.8}$Fe${}_{1.6}$Se${}_2$ systems: competition between superconductivity and antiferromagnetic order

Abstract

We have performed Raman-scattering measurements on high-quality single crystals of A${}_{0.8}$Fe${}_{1.6}$Se${}_2$ superconductors of several compositions. We find 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 KFe${}_{1.5}$Se${}_2$, requires a strong mutual exclusion between antiferromagnetism and superconductivity arising from proximity effects within regions of microscale phase separation.

8 pages, 5 figures