Optical Conductivity of Colossal Magnetoresistance Manganites.

Abstract

The colossal magnetoresistance manganites are a group of materials whose unusual physical properties are a symptom of strongly interacting electrons and phonons. In order to elucidate some of these electronic and vibrational properties, an infrared optical investigation of manganites with a broad range of physical characteristics has been performed. Temperature-dependent normal incidence reflectivity measurements have been made on two samples of manganites, in the energy range of 60 cm-1 - 50000 cm-1, 1 for La0.9 Ca0.1 MnO3, an insulating ferromagnet, and 2 La0.735 Ca0.265 MnO3, a metallic ferromagnet. Temperature-dependent ellipsometric reflection measurements were performed in the energy range of 50 cm-1 - 5000 cm-1, on four faces of two samples of structurally anisotropic manganite, probing the 3. ab plane and c-axis of La1.2 Sr1.8 Mn2O7, a metallic ferromagnet, and 4. the ab plane and c-axis of PrSr2 Mn2O7, an insulating antiferromagnet. The optical conductivity for each of the first two samples has been deduced by a careful Kramers-Kronig analyis of the normal incidence reflectivity. For samples 3. and 4. the optical conductivity has been deduced by inversion of the ellipsometric constants, and a careful subsequent fitting to account for their anisotropy. The transition temperatures and types of magnetic order for all samples have also been characterised by magnetisation measurements. Treatment of the surface is shown to be critical in reflectivity measurements by the observation of hugely contrasting spectra, measured from a polished sample of metallic-like La0.735 Ca0.256 MnO3, before and after annealing. Several features observed in the measurements, especially for the layered materials, are consistent with the idea that a polaron, or electron-lattice interaction, is hugely important in a description of the electron dynamics of these materials. The correlation between spectral features and the structural and magnetic properties of the materials is investigated, finding that the cause of charge transport modification seen in the metallic-like materials could be explained by either a polaron or localisation due to disorder.