Optical properties and electronic structure of single crystals of ${\mathrm{LuAl}}_2$ and ${\mathrm{YbAl}}_2$

The optical conductivities of single crystals of ${\mathrm{LuAl}}_2$ and ${\mathrm{YbAl}}_2$ were measured by spectroscopic ellipsometry in the energy range of $1.4–5.5\mathrm{eV}$ for ${\mathrm{LuAl}}_2$ and $1.4–5.2\mathrm{eV}$ for ${\mathrm{YbAl}}_2.$ The optical conductivity spectra of ${\mathrm{LuAl}}_2$ and ${\mathrm{YbAl}}_2$ show similar features except for a difference in magnitude. Both have peaks near $1.8–2.1\mathrm{eV}$ and broad shoulders between $3.0\hspace{0.5em}\mathrm{and}\hspace{0.5em}4.0\mathrm{eV}.$ The shoulder is weaker in ${\mathrm{YbAl}}_2.$ 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.