Metal-insulator transition in double cobaltites RBaCo$_2$O$_{5.5}$ (R = Eu, Gd): Specific features of their optical properties

The temperature dependence of the optical properties of GdBaCo$_2$O$_{5.5}$ in the region of the metal-insulator transition was studied in the spectral range $\lambda$ = 4.0–0.258 $\mu$m ($E$ = 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 GdBaCo$_2$O$_{5.5}$ and EuBaCo$_2$O$_{5.5}$ 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 GdBaCo$_2$O$_{5.5}$ is experimentally determined to be $E_g$ = 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.