Fluctuations of nematicity and the spin subsystem in FeSe and FeSeS

In most Fe-based superconductors (pnictides), nematic order is reasonably close to the antiferromagnetic order. In FeSe and FeSeS, in contrast, a nematic order emerges below the structure phase transition at T_s = 90 K (70 K for FeSeS) 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 novel approach reveals both the electronic and magnetic nematicity below and, surprisingly, far above Ts. The quantitative pump-probe data clearly identify a correlation between the topology of the Fermi surface (FS) and the magnetism in all temperature regimes, thus providing profound insight into the driving factors of nematicity in both FeSe and FeSeS and the origin of its uniqueness. The effects of S doping on nematicity will be discussed.

[1] Chih-Wei Luo, P. C. Cheng, S.-H. Wang, J.-C. Chiang, Jiunn-Yuan Lin, Kaung-Hsiung Wu, Jenh Yih Juang, Dmitriy Chareev, Olga Volkova, and Alexander Vasiliev, npj Quantum Materials, 2, 32.