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

Abstract

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 Ca${}_2$Ru${}_{0.99}$Ti${}_{0.01}$O${}_4$ 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 $2.4\le {\Delta }_{CF}/\zeta \le 4$ 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.

arXiv comment12 pages, 10 figures

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• Spin-orbit coupling in a half-filled $t_{2g}$ shell: the case of $5d^3$ K${}_2$ReCl${}_6$
• Spin-orbital-lattice entanglement in the ideal j=1/2 compound K${}_2$IrCl${}_6$
• Emerging new phases in correlated Mott insulator Ca2RuO4