Two-magnon Raman scattering in A <inf>0.8</inf>Fe <inf>1.6</inf>Se <inf>2</inf> systems (A=K, Rb, Cs, and Tl): Competition between superconductivity and antiferromagnetic order

Abstract

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 KFe 1.5 Se 2 , requires a strong mutual exclusion between antiferromagnetism and superconductivity arising from proximity effects within regions of microscale phase separation. © 2012 American Physical Society.