Electron and orbital correlations in Ca2-xSrxRuO4 probed by optical spectroscopy.

The optical conductivity spectra of the quasi-two-dimensional Ca2-xSrxRuO4 (0.0< or =x< or =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.