Contribution of carriers to optical conductivity spectra of lanthanum manganites

To study the band structure and carriers in lanthanum manganites, measurements have been made of the reflectance spectra of single crystals and polycrystals in the 0.04–1.6-eV range and of the optical conductivity σopt calculated by the Kramers-Kronig method as functions of the concentration and species of divalent ions in the paramagnetic (PM) and ferromagnetic (FM) regions. The optical gap for single-crystal La0.9Sr0.1MnO3 is ∼0.17 eV, and the polaronband energy is 0.12 eV. In the PM region, σopt spectra do not indicate a band-carrier contribution, and conduction is dominated by polaron hopping and activation to the mobility edge. In the FM region, the variation in the σopt and absorption spectra of La0.7Sr0.3MnO3 epitaxial films indicate the appearance of band carriers and a red shift of the absorption edge. The two band-carrier contributions, with weak and strong dependences on photon energy, are related to conduction in the antiferromagnetic matrix and the ferromagnetic regions.