Paramagnon dispersion in ${\hat{I}}^2$-FeSe observed by Fe $L$-edge resonant inelastic x-ray scattering

Abstract

We report an Fe $L$-edge resonant inelastic x-ray scattering (RIXS) study of the unusual superconductor ${\hat{I}}^2$-FeSe. The high energy resolution of this RIXS experiment ($\approx$55meV FWHM) made it possible to resolve low-energy excitations of the Fe $3d$ manifold. These include a broad peak which shows dispersive trends between 100-200meV along the $(\ddot{I}€,0)$ and $(\ddot{I}€,\ddot{I}€)$ 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.