Optical study of high-T-c superconductors and related oxides

Optical reflectivity measurements have been carried out on the two- and one-dimensional Cu and Ni oxides, La2-xSrxCuO4, La2-xSrxNiO4 and Y2-xCaxBaNiO5 with focus on the doping-induced changes. In the parent compounds, the optical spectra are dominated by the excitations from the O2p valence band to the upper Hubbard band. Upon doping holes, the transfer of the spectral weight into the low-energy excitations is commonly seen in these systems. However, the profile of the transferred spectral weight shows remarkable material dependence and anisotropy. We discuss the orbital character and the charge dynamics of the holes doped into these quantum spin fluids based on the experimental results.