Paramagnon dispersion in β -FeSe observed by Fe L -edge resonant inelastic x-ray scattering

We report an Fe $L$-edge resonant inelastic x-ray scattering (RIXS) study of the unusual superconductor $\ensuremath{\beta}$-FeSe. The high-energy resolution of this RIXS experiment ($\ensuremath{\approx}55\phantom{\rule{3.33333pt}{0ex}}\mathrm{meV}$ 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 and 200 meV along the $(\ensuremath{\pi},0)$ and $(\ensuremath{\pi},\ensuremath{\pi})$ directions of the one-Fe square reciprocal lattice, and which can be attributed to paramagnon excitations. The multiband valence state of FeSe is among the most metallic in which such excitations have been discerned by soft x-ray RIXS.