Electronic states structure of Gd$_5$Sb$_3$ and Gd$_5$Ge$_2$Sb compounds according to band calculations and optical spectroscopy

Results of investigations of the electronic structure and optical properties of Gd$_5$Sb$_3$ and Gd$_5$Ge$_2$Sb compounds are presented. Calculations of band spectra were carried out in frame of local density approximation with a correction for strong correlation effects in 4$f$ shell of rare-earth ion (DFT + U + SO method). Optical constants of these materials were measured by ellipsometric technique in wide wavelength interval. Energy dependencies of a number of spectral parameters were determined. The nature of quantum light absorption is discussed on the base of comparative analysis of the experimental and calculated spectra of optical conductivity.