OPTICAL CONDUCTIVITY OF OXIDES

Optical conductivity spectra of conducting oxides including high-TC cuprates - compared to non-superconducting nickelates in particular - deduced from infrared reflectivity spectroscopy in polarized light and their doping and temperature dependence, are discussed with emphasis on the signatures of intrinsic inhomogeneities of charge carrier concentration. The model of disordered metal is shown to be compatible with the general loss of spectral weight in the optical conductivity upon decreasing frequency, instead of a maximum conductivity at zero frequency as found within a Drude description. In high-TC multilayer cuprates, the discussion focuses upon the c-axis response and the inhomogeneity of the electronic concentration related to the conducting planes alternating with insulating sheets. The onset of stripes that are intrinsic inhomogeneities of the charge carrier distribution is discussed in the example of nickelates.