Spin-orbit coupling and crystal-field splitting in Ti-doped Ca2RuO4 studied by ellipsometry

In Ca2RuO4, the competition of spin-orbit coupling zeta and tetragonal crystal-field splitting Delta(CF) has been discussed controversially for many years. The orbital occupation depends on Delta(CF)/zeta , 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 <= Delta(CF)/zeta less than or similar to 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 Delta(CF) with increasing temperature, as expected based on the reduction of octahedral distortions.

Automated summary

This article discusses the competition between spin-orbit coupling and crystal field splitting in Ca2RuO4, and explains the orbital occupation ratio using optical spectral weights. The study shows that Δ(CF)/ζ is between 2.4 and 4 at 15 K. The crystal field dominates the orbital order of the ground state, but spin-orbit coupling is necessary for a quantitative description of the properties. Furthermore, the crystal field splitting decreases with increasing temperature.