Resonant Inelastic X-ray Scattering Applications in Quantum Materials

Abstract

The essence of quantum materials lies in the intricate coupling among charge, spin, orbital and lattice degrees of freedom. Although X-ray photoemission spectroscopy and inelastic neutron scattering are advantegous in detecting fermionic single-particle spectral function and bosonic spin excitations in quantum materials, respectively, probing other bosonic collective excitations especially their coupling is not possible until the establishment of the advanced resonant inelastic X-ray scattering (RIXS). In the past decades, RIXS has flourished with continuously improved energy resolution which made a paradigm shift from measuring crystal-field splitting and the charge-transfer excitation, to probe collective excitations and the order parameters of all degrees of freedom. This review article summarises the most recent progress made by the soft X-ray RIXS in the field of quantum materials. For instance, three-dimensional collective charge excitations, plasmons, were discovered experimentally by RIXS in both electron and hole doped cuprate superconductors. The collective orbital excitations and excitons were found in copper and nickel based quantum materials. For the newly discovered nickelate superconductors, RIXS has made substantial contributions in characterising their electronic and magnetic excitations and the related ordering phenomena critical for an in-depth understanding of the underlying superconducting mechanicsm. RIXS is a unique tool in probing the higher-order spin excitations in quantum materials thanks to the strong spin-orbit coupling and the core-valence exchange interaction. RIXS is also found superior in probing the Stoner magnetic excitations in magnetic metals and topological magnetic materials. Finally, a short perspective is provided in terms of the development of RIXS technique in Chinese large-scale research facilities.