Hallmarks of Hunds coupling in the Mott insulator Ca2RuO4

TLDR

Ca₂RuO₄ exemplifies a multi‑band Mott insulator where spin‑orbit and Hund’s coupling play key roles, yet its low‑energy electronic structure remains poorly understood. We aim to map the band structure of paramagnetic Ca₂RuO₄ using ARPES to identify distinct energy scales. Angle‑resolved photoemission spectroscopy was employed to probe the paramagnetic insulating phase and resolve its electronic bands. The ARPES spectra reveal a Hund’s coupling of 0.4 eV, agree with DMFT calculations, and show that c‑axis–induced crystal‑field stabilization of the d_xy orbital is essential for the insulating state, highlighting the combined role of multi‑band physics, Coulomb interactions, and Hund’s coupling in Ca₂RuO₄.

Abstract

Abstract A paradigmatic case of multi-band Mott physics including spin-orbit and Hund’s coupling is realized in Ca 2 RuO 4 . 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 Ca 2 RuO 4 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 Ca 2 RuO 4 .