Unveiling the hidden nematicity and spin subsystem in FeSe

TL;DR

This study uses polarized ultrafast spectroscopy to reveal electronic and magnetic nematicity in FeSe, showing fluctuations above the structural transition and linking Fermi surface topology to magnetism, advancing understanding of Fe-based superconductors.

Contribution

It demonstrates the presence of electronic and magnetic nematic fluctuations in FeSe above the structural transition using ultrafast spectroscopy, offering new insights into its nematicity mechanism.

Findings

Nematic fluctuations exist above the structural transition temperature.

Electronic and magnetic nematicity are correlated with Fermi surface topology.

Nematicity in FeSe persists without magnetic order, unlike other Fe-based superconductors.

Abstract

The nematic order (nematicity) is considered 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 Ts to at least 200 K. 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 FeSe and the origin of its uniqueness.