We have performed Raman-scattering measurements on high-quality single crystals of A0.8Fe1.6Se2 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 KFe1.5Se2, requires a strong mutual exclusion between antiferromagnetism and superconductivity arising from proximity effects within regions of microscale phase separation.
@article{arxiv.1106.2706,
title = {Two-magnon Raman scattering in A$_{0.8}$Fe$_{1.6}$Se$_2$ systems: competition between superconductivity and antiferromagnetic order},
author = {A. M. Zhang and J. H. Xiao and Y. S. Li and J. B. He and D. M. Wang and G. F. Chen and B. Normand and Q. M. Zhang and T. Xiang},
journal= {arXiv preprint arXiv:1106.2706},
year = {2013}
}