Change of Electronic Structure inCa2RuO4Induced by Orbital Ordering

Optical conductivity spectra $\sigma (\omega )$ were used to investigate the effect of orbital ordering on the electronic structure of ${\mathrm{C}\mathrm{a}}_2{\mathrm{R}\mathrm{u}\mathrm{O}}_4$. Our $\mathrm{L}\mathrm{D}\mathrm{A}+U$ calculation predicts Ru $4d_{xy}$ ferro-orbital ordering at the ground state, and well explains the present $\sigma (\omega )$ as well as the reported O $1s$ x-ray absorption spectra. Variation of temperature ($T$) causes a large change of spectral weight over several eV as well as collapse of a charge gap accompanied by elongation of the $c$-axis Ru-O bond length. These results clearly indicate that the $d_{xy}$ orbital ordering plays a crucial role in the metal-insulator transition and the $T$-dependent electronic structure on a large energy scale.