The mechanism of the Mott phase in Ca₂RuO₄ has puzzled researchers for decades. This material is a paradigmatic case of multi-band Mott physics including spin-orbit and Hund's coupling. Several theoretical proposals have been put forward, however progress has been impeded by the lack of knowledge about the low-energy electronic structure. Using angle-resolved photoemission electron spectroscopy the group of Prof. Johan Chang recently provided the band structure of the paramagnetic insulating phase of Ca₂RuO₄ and showed 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 stabilisation of the d_xy orbital due to c-axis contraction is shown to be important in explaining the nature of the insulating state. It is thus a combination of multiband physics, Coulomb interaction and Hund's coupling that generates the Mott insulating state of Ca ₂RuO₄. These results underscore the importance of Hund's coupling in the ruthenates. Our findings suggest an orbital differentiated picture where the d_xy orbitals are band insulating whereas the resulting half-filled d_xz/yz band undergoes a conventional Mott transition.

D. Sutter et al., Hallmarks of Hunds coupling in the Mott insulator Ca ₂RuO₄
Nature Communications 8, Article number: 15176 (2017)