Temperature dependence of reflectivity spectra of oxide conductors and superconductors

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Abstract
The infrared-visible-near-ultraviolet spectra of a number of conducting oxides TiO2-5, NbO2, La2NiO4 and related nickelates, BaBiO3, (Ba, K)2(Pb, Bi)O4, Nb-doped SrTiO3, YBa2Cu3O7-5, and their temperature dependence, are reviewed. A common model consisting of a modified Drude-type contribution for mobile charge carriers extended to two fluids in the superconducting phase, added to a multi-oscillator term valid for phonons, trapped polarons or optical gaps, is able to fit the spectra of insulating, conducting and superconducting oxides. It is emphasised that the model accounts for the coupling of the excitations. Ignoring such couplings and extracting information by simple deconvolution of added contributions may give rise to incorrect frequencies and intensities as is shown in the examples of mid-infrared bands and superconducting condensate.