Optical properties and electronic structures of single crystalline $R{\mathrm{Al}}_3(R=\mathrm{Sc},$ Yb, and Lu)

The optical conductivities of single crystals of ${\mathrm{ScAl}}_3,$ ${\mathrm{LuAl}}_3,$ and ${\mathrm{YbAl}}_3$ were measured by spectroscopic ellipsometry in the energy range of 1.5–5.5 eV. The conductivity spectra of all show peaks between 1.5 and 2.0 eV originating from interband transitions. Two additional peaks around at 2.9 and 4.2 eV are observed for ${\mathrm{ScAl}}_3,$ and a weak shoulder is observed around at 2.8 eV for ${\mathrm{LuAl}}_3.$ Band structure, density of states, and optical conductivity were calculated using the full-potential linear augmented plane wave method. The calculated conductivity spectrum for ${\mathrm{ScAl}}_3$ shows peaks around at 2.7 and 4.3 eV. The distinctive feature of the optical conductivity for ${\mathrm{ScAl}}_3$ comes from the extraordinary large Sc d unoccupied density of states compared to those of Yb d and Lu d. Oxidation effects on the optical properties of the sample were studied using a three-phase model. The calculated optical conductivity of the clean surface using the three-phase model is larger than that of the oxidized surface.