In Ca2RuO4, the competition of spin-orbit coupling ζ and tetragonal crystal field splitting ΔCF has been discussed controversially for many years. The orbital occupation depends on ΔCF/ζ, which allows us to address this ratio via the optical spectral weights of the lowest intersite Mott-Hubbard excitations. We study the optical conductivity of Ca2Ru0.99Ti0.01O4 in the range of 0.75 - 5 eV by ellipsometry, using the large single crystals that can be grown for small Ti concentrations. Based on a local multiplet calculation, our analysis results in 2.4≤ΔCF/ζ≤4 at 15 K. The dominant crystal field yields a ground state close to xy orbital order but spin-orbit coupling is essential for a quantitative description of the properties. Furthermore, we observe a pronounced decrease of ΔCF with increasing temperature, as expected based on the reduction of octahedral distortions.
@article{arxiv.2203.13651,
title = {Spin-orbit coupling and crystal-field splitting in Ti-doped Ca2RuO4 studied by ellipsometry},
author = {I. Vergara and M. Magnaterra and J. Attig and S. Kunkemöller and D. I. Khomskii and M. Braden and M. Hermanns and M. Grüninger},
journal= {arXiv preprint arXiv:2203.13651},
year = {2023}
}