Excitonic Magnetism at the intersection of Spin-orbit coupling and crystal-field splitting

Abstract: Excitonic magnetism involving superpositions of singlet and triplet states is expected to arise for two holes in strongly correlated and spin-orbit coupled $t_{2g}$ orbitals. However, uncontested material examples for its realization are rare. Applying the Variational Cluster Approach to the square lattice, we find conventional spin antiferromagnetism combined with orbital order at weak and excitonic order at strong spin-orbit coupling. We address the specific example of Ca$_2$RuO$_4$ using ab-initio modeling and conclude it to realize excitonic magnetism despite its pronounced orbital polarization.