Investigation of the nematic phase of FeSe by resonant X-ray diffraction

We propose to study the nematic phase transition of FeSe by measuring Resonant Elastic X-ray Scattering (REXS) at selected “forbidden” reflections at the Fe K edge. This nematic phase transition is not accompanied by magnetic ordering, unlike in most other Fe-based superconductors, which makes this crystal an excellent opportunity to study the former without the latter and to deepen our understanding of nematic phase transitions in the vast class of Fe-based superconductors. The hk0 forbidden reflections should be extremely sensitive to any change of the local environment of the Fe atoms, due to the high symmetry of the crystallographic sites of Fe. In addition, we will pay attention to other anomalies reported in the physical properties at other temperatures, and we will look for possible intra-unit-cell magnetism at low temperature.