Unveiling the hidden nematicity and spin subsystem in FeSe

The nematic order (nematicity) is considered as one of the essential ingredients to understand the mechanism of Fe-based superconductivity. In most Fe-based superconductors (pnictides), nematic order is reasonably close to the antiferromagnetic order. In FeSe, in contrast, a nematic order emerges below the structure phase transition at T s = 90 K with no magnetic order. The case of FeSe is of paramount importance to a universal picture of Fe-based superconductors. The polarized ultrafast spectroscopy provides a tool to probe simultaneously the electronic structure and the magnetic interactions through quasiparticle dynamics. Here we show that this approach reveals both the electronic and magnetic nematicity below and, surprisingly, its fluctuations far above T s to at least 200 K. The quantitative pump–probe data clearly identify a correlation between the topology of the Fermi surface and the magnetism in all temperature regimes, thus providing profound insight into the driving factors of nematicity in FeSe and the origin of its uniqueness. Ultrafast spectroscopy unveils hidden nematic fluctuations and a spin subsystem in the iron-based superconductor iron selenide. Layered iron-based materials recently emerged as a new class of high temperature superconductor. The mechanism of superconductivity in these materials, however, is a contentious issue. Nematic ordering is thought to be a key ingredient, but the apparent absence of magnetic ordering in iron selenide, which is the iron-based superconductor with the simplest structure, has caused confusion over what drives the nematicity. An international team of researchers led by Chih-Wei Luo and Jenh-Yih Juang from National Chiao Tung University use polarized ultrafast spectroscopy to unveil a hidden spin subsystem in FeSe, along with both nematic and magnetic fluctuations at relatively high temperatures, providing insights into the driving factors of nematicity in this fascinating material.