Electron and Orbital Correlations in ${\mathrm{C}\mathrm{a}}_{2-x}{\mathrm{S}\mathrm{r}}_x{\mathrm{R}\mathrm{u}\mathrm{O}}_4$ Probed by Optical Spectroscopy

The optical conductivity spectra of the quasi-two-dimensional ${\mathrm{C}\mathrm{a}}_{2-x}{\mathrm{S}\mathrm{r}}_x{\mathrm{R}\mathrm{u}\mathrm{O}}_4$ ($0.0\le x\le 2.0$) system were investigated. In the Mott insulating state, two electron correlation-induced peaks were observed about 1.0 and 1.9 eV, which could be explained using the 3-orbital Hubbard model. The low frequency peak showed a shift toward higher frequency as temperature was lowered, indicating that electron-phonon interactions play an important role in the orbital arrangements. From the systematic analysis, it was suggested that the antiferro-orbital and the ferro-orbital ordering states could coexist.