Hallmarks of Hund's coupling in the Mott insulator Ca$_2$RuO$_4$

TL;DR

This study uses photoemission spectroscopy and theoretical calculations to reveal how Hund's coupling and multiband effects drive the Mott insulating state in Ca$_2$RuO$_4$, highlighting the importance of Hund's coupling in such materials.

Contribution

It provides the first detailed experimental band structure of Ca$_2$RuO$_4$'s insulating phase and quantifies Hund's coupling, integrating experimental data with dynamical mean-field theory calculations.

Findings

Identification of multiple energy scales in the band structure.

Quantitative estimate of Hund's coupling J=0.4 eV.

Agreement between experimental spectra and DMFT calculations.

Abstract

A paradigmatic case of multi-band Mott physics including spin-orbit and Hund's coupling is realised 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 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$_2$RuO$_4$. These results underscore the importance of Hund's coupling in the ruthenates and related multiband materials.