Hallmarks of Hunds coupling in the Mott insulator Ca 2 RuO 4

A paradigmatic case of multi-band Mott physics including spin-orbit and Hund's coupling is realized in Ca₂RuO₄. Progress in understanding the nature of this Mott insulating phase has been impeded by the lack of knowledge about the low-energy electronic structure. Here we provide - using angle-resolved photoemission electron spectroscopy - the band structure of the paramagnetic insulating phase of Ca₂ RuO₄ and show how it features several distinct energy scales. Comparison to a simple analysis of atomic multiplets provides a quantitative estimate of the Hund's coupling J=0.4 eV. Furthermore, the experimental spectra are in good agreement with electronic structure calculations performed with Dynamical Mean-Field Theory. The crystal field stabilization of the d xy orbital due to c-axis contraction is shown to be essential to explain the insulating phase. These results underscore the importance of multi-band physics, Coulomb interaction and Hund's coupling that together generate the Mott insulating state of Ca₂RuO₄.