Resonant Inelastic X-ray Scattering (RIXS) is a powerful probe of excitations from the electronic ground state of quantum materials involving lattice, charge, orbital and spin degrees of freedom. RIXS gives direct information as a function of momentum and energy transfer, on the elementary excitations within these degrees of freedom. In this talk we illustrate the scientific capabilities of momentum resolved soft X-ray RIXS on quasi-two-dimensional correlated quantum materials in investigations on the Fe-based superconductor FeSe and the ferromagnetic Kagome metal Fe3Sn2.
We will first show how RIXS at the Fe L3 edge is sensitive to collective magnetic excitations in iron-based superconductors that are persistent for under-, nearly optimal- and over-doping across the phase diagram of iron pnictide superconductors [1]. Furthermore, we use Fe L3-edge RIXS to probe the high-energy magnetic excitations of uniaxial-strain detwinned FeSe [2]. A prominent anisotropy between the magnetic excitations along the H and K directions is found to persist to ∼ 200 meV. This anisotropy decreases gradually with increasing temperature and finally vanishes at a temperature around the tetragonal-to-orthorhombic transition Ts. Our results reveal an unprecedented strong spin excitation anisotropy with a large energy scale well above the dxz/dyz orbital splitting, suggesting that the nematic phase transition is primarily spin-driven [2].
Using magnetic circular dichroism (MCD) in X-ray absorption and RIXS for the unambiguous isolation of magnetic signals, we report a nearly flat spin wave band and large (compared to elemental iron) orbital moment for the magnetic Kagome metal Fe3Sn2 [3]. As a function of out-of-plane momentum, the flat optical mode and the global rotation symmetry-restoring acoustic mode are out of phase, consistent with a bilayer exchange coupling that is larger than the already large in-plane couplings. Our results suggest the defining units of this very popular topological metal are therefore a triangular lattice of octahedral iron clusters rather than weakly coupled kagome planes. The spin waves are strongly damped when compared to elemental iron, opening the topic of topological interactions of topological bosons (spin waves) and fermions (electrons).