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

TL;DR

This study uses Raman scattering to explore the interplay between magnetism and superconductivity in A$_{0.8}$Fe$_{1.6}$Se$_2$ crystals, revealing a mutual exclusion likely due to phase separation.

Contribution

It provides direct experimental evidence of the competition between antiferromagnetic order and superconductivity in these materials.

Findings

Two-magnon Raman peak observed at 1600 cm$^{-1}$.

Peak intensity decreases sharply in the superconducting phase.

No two-magnon peak in non-superconducting KFe$_{1.5}$Se$_2$.

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.