Optical properties and electronic structure of single crystals of EuAl2 and YbAl2

The optical conductivities of single crystals of LuAl2 and YbAl2 were measured by spectroscopic ellipsometry in the energy range of 1.4-5.5 eV for LuAl2 and 1.4-5.2 eV for YbAl2. The optical conductivity spectra of LuAl2 and YbAl2 show similar features except for a difference in magnitude. Both have peaks near 1.8-2.1 eV and broad shoulders between 3.0 and 4.0 eV. The shoulder is weaker in YbAl2. The band structure, density of states, and optical conductivity were calculated with the tight-binding linear muffin-tin orbital method in the atomic sphere approximation. The calculated optical conductivity with the inclusion of energy-dependent broadening agrees well with the experimental data. Oxidation effects on the surface of the sample were modeled using a three-phase model. The calculated optical conductivity of the clean surface is enhanced over that of the oxidized surface.