Electron and orbital correlations in Ca_{2-x}Sr_{x}RuO_{4} probed by   optical spectroscopy
  The doping and temperature dependent optical conductivity spectra of the quasi-two-dimensional Ca_{2-x}Sr_xRuO_4 (0.0=<x=<2.0) system were investigated. In the Mott insulating state, two electron correlation-induced peaks were observed around 1.0 and 1.9 eV, which could be understood in terms of the 3-orbital Hubbard model. The low frequency peak showed a shift toward higher frequency as temperature was lowered, which indicated 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. 
J.S. Lee, Y.S. Lee, T.W. Noh, S.-J. Oh, Jaejun Yu, S. Nakatsuji, H.   Fukazawa, and Y. Maeno