Diana Csontosova - Hidden covalent insulator and spin excitations in SrRu2O6

This talk was part of the Workshop on "Spin-Orbit Entangled Quantum Magnetism" held at the ESI September 23 -- 27, 2024.
We study theoretically SrRu2O6 with a partially filled 4d shell, which is a high TN antiferromagnetic insulator based on ruthenium-oxide honeycomb layers. We use Density Functional Theory (DFT) combined with Dynamical Mean Field Theory (DMFT) [1]. The easy axis of magnetization is oriented out-of-plane (along the c-axis) due to the single-ion anisotropy caused by non-negligible spin-orbit coupling. Since the Hubbard interaction, together with Hund’s exchange, competes with electron hopping amplitudes, this kind of system is still not fully understood [2, 3]. One of the discussed questions is the nature of the paramagnetic state. On the one hand, the tendency to form a high-spin Mott insulator is maximal at half-filling. On the other hand, the Fermi level falls into the band gap between the molecular peaks at half-filling, which stabilizes the covalent insulator state. These two regimes can be realized by varying Hund’s exchange.
In 2019, the magnon spectrum of SrRu2O6 was measured using the Resonant Inelastic X-ray Scattering (RIXS) technique by Suzuki et al. [4], and the magnon was theoretically described by the S=3/2 Heisenberg model. By inverting the Bethe-Salpeter equation, we calculated two-particle correlation functions to reproduce measured low-energy magnon spectra and S=1/2 points to S = 3/2 excitations. We further study the qualitative change of magnon spectra by tuning the strength of Hund’s exchange. We find that the nature of the paramagnetic state has negligible influence on the charge and spin excitation spectra. We argue that antiferromagnetic correlations hide the covalent insulator state for realistic choices of the interaction parameters.
[1] Csontosová, D., Chaloupka, J., Shinaoka, H., Hariki, A. & Kuneš, J. Phys. Rev. B 108, 194137 (2023).
[2] Hariki, A., Hausoel, A., Sangiovanni, G. & Kuneš, J. Phys. Rev. B 96, 155135 (2017).
[3] Okamoto, S., Ochi, M., Arita, R., Yan, J. & Trivedi, N. Sci. Rep. 7, 11742 (2017).
[4] Suzuki, H., Gretarsson, H., Ishikawa, H. et al. Nat. Mater. 18, 563-567 (2019).