Metal-insulator transition in double cobaltites RBaCo2O5.5 (R = Eu, Gd): Specific features of their optical properties

The temperature dependence of the optical properties of GdBaCo2O5.5 in the region of the metal-insulator transition was studied in the spectral range λ = 4.0–0.258 μm (E = 0.3–4.8 eV) by the spectroscopic ellipsometry method. The electronic structure of the crystal at T = 300 K was calculated by the linearized muffin-tin orbital method in the local-density approximation with inclusion of on-site Coulomb interaction (LDA + U). The general features of the behavior of the optical properties of GdBaCo2O5.5 and EuBaCo2O5.5 are discussed. It is shown that the optical response on the metal side of the metal-insulator transition is determined by the redistribution of the optical-conductivity spectral weight in going from high to low energies and is significantly incoherent in character. The optical band gap width of the GdBaCo2O5.5 is experimentally determined to be Eg = 0.26 eV, which is close to a theoretical estimate. The type of interband transitions is determined from analyzing the temperature dependence of the optical functions for absorption bands.