Paramagnon dispersion in $\beta$-FeSe observed by Fe $L$-edge resonant inelastic x-ray scattering

TL;DR

This study uses high-resolution Fe L-edge RIXS to observe dispersive paramagnon excitations in the superconductor $eta$-FeSe, revealing low-energy magnetic fluctuations in a highly metallic state.

Contribution

First high-resolution RIXS observation of dispersive paramagnons in $eta$-FeSe, demonstrating magnetic excitations in a highly metallic superconductor.

Findings

Dispersive paramagnon peak observed between 100-200 meV.

Paramagnon excitations show dispersion along specific reciprocal lattice directions.

FeSe exhibits magnetic excitations despite its high metallicity.

Abstract

We report an Fe $L$-edge resonant inelastic x-ray scattering (RIXS) study of the unusual superconductor $\beta$-FeSe. The high energy resolution of this RIXS experiment ($\approx\,$55$\,$meV FWHM) made it possible to resolve low-energy excitations of the Fe $3d$ manifold. These include a broad peak which shows dispersive trends between 100-200$\,$meV along the $(\pi,0)$ and $(\pi,\pi)$ directions of the one-Fe square reciprocal lattice, and which can be attributed to paramagnon excitations. The multi-band valence state of FeSe is among the most metallic in which such excitations have been discerned by soft x-ray RIXS.