While 4d metals play an eminent role in many areas of condensed matter physics, ranging from unconventional superconductivity to oxide electronics, many fundamental questions regarding the interplay between electronic correlations and spin-orbit coupling at the atomic scale remain unanswered. IRIXS represents the establishment of a new condensed-matter platform to study these physics, giving comprehensive information about the interaction parameters specifying electronic Hamiltonians, providing unique insight into the spin-orbital composition of their excited-state wavefunctions, and a definitive test of proposals such as the realization of Kitaev models with quantum spin-liquid states or soft-moment pseudospin magnetism in the two-dimensional antiferromagnet Ca2RuO4 [1,2].
 A. Jain et al., Nature Physics 13, 633 (2017).
 M. Souliou et al., Phys. Rev. Lett. 119, 067201 (2017).