Spin-Orbit Excitons in a Correlated Metal

 

Multi-band metals are currently of major interest in the quantum materials community, as epitomized by unconventional superconductors such as FeSe and Sr₂RuO₄. Since the valence electrons are spread out over several energy bands and Fermi surfaces, capturing their mutual interactions and their influence on the macroscopic phase behavior is a major theoretical challenge. We have carried out a combined Raman scattering and theoretical study of spin-orbit excitations in the disorder-free multi-band metal Sr₂RhO₄, where prior experiments had revealed sharp Fermi surfaces. We observe unusual but well-defined excitations around 230 meV, and identify them as excitonic transitions between the spin-orbit multiplets of the Rh ions, analogous to those recently observed in the Mott insulators Sr₂IrO₄ and RuCl₃. Our discovery of atomic-like electronic excitations in a clean d-electron metal opens up a rich source of information on electronic correlations in multi-band metals.