Spin-Orbit Coupling in the Mott Insulator ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$

O $1s$ x-ray absorption study of the Mott insulator ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$ shows that the orbital population of the $4d$ $t_{2g}$ band dramatically changes with temperature. In addition, spin-resolved circularly polarized photoemission study of ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$ shows that a substantial orbital angular momentum is induced in the Ru $4d$ $t_{2g}$ band. Based on the experimental results and model Hartree-Fock calculations, we argue that the cooperation between the strong spin-orbit coupling in the Ru $4d$ $t_{2g}$ band and the small distortion of the ${\mathrm{RuO}}_6$ octahedra causes the interesting changeover of the spin and orbital anisotropy as a function of temperature.