Nanoscale inhomogeneities and optical properties of doped cuprates

We propose a new approach to theoretical description of doped HTSC cuprate, assuming phase separation and treating it as inhomogeneous composite material, containing the dielectric and metallic nanoparticles. The formalism of effective medium theory is applied for calculation of dielectric permittivity and optical spectra of Sr-doped La214 system for a broad range of doping concentrations. The model is able to reproduce all essential features of optical conductivity, electron energy-loss function at zero wave vector and transmittance both for thin films and bulk single-crystalline samples. Substantial difference in spectral and doping dependence of optical absorption for the thin-film and bulk samples is easily explained by assuming different shape of metallic and dielectric regions in both materials. New peaks of optical conductivity, that emerge in the infrared range near 0.5 and 1.5 eV upon doping, are attributed to surface plasmon resonances.