Optical conductivity of oxides

Optical conductivity spectra of conducting oxides, cuprates, including high-Tc superconductors, nickelates, manganites, doped titanates, barium–lead–bismuth oxides, barium–potassium–bismuth–lead oxides, oxides showing metal–insulator phase transitions, and conducting polymers for the sake of comparison, are reviewed in the spectral range from the milli-electron-volt up to a few electron-volts. The optical conductivity profiles evolve from Drude-like to incoherent scattering and even pseudo-gap-like, depending on doping and temperature. When Drude or Drude-like models do not apply, the screening of phonons by charge carrier is generally incomplete and the polaron concept is considered. Spectra are fairly well modeled within a general approach compatible with the Drude framework and extended by factorizing over all complex poles and zeroes of the dielectric response, allowing discrimination of trapped and mobile polarons. Other models are reviewed: extended Drude with either frequency-dependent relaxation time or non-persistent electron velocity, marginal Fermi liquid, Luttinger liquid, localization, Anderson disordered metal, and those implying opening of gaps or pseudo-gaps in the conductivity. The signatures of stripes resulting from ordering of charges and spins—commensurate or incommensurate with the period of the crystal lattice—observed in certain nickelates, cuprates and manganites are discussed.