Dielectric constants of α- Sn in the infrared region

α-Sn is a group-IV, zero-gap semiconductor with potential use in infrared
devices. We report the first-principles calculations of the band structure
and dielectric function of α-Sn using density functional theory, emphasizing
the effects of strain, spin-orbit interaction, and pseudopotentials on the
electronic and optical properties of α-Sn in the infrared region (photon
energy <1 eV). In α-Sn, spin-orbit coupling greatly influences the
electronic band structure that leads to unusual optical behavior. We explain
an apparently anomalous absorption at 0.45 eV caused by interband
transitions within the valence band. Infrared spectroscopic ellipsometry on
several α-Sn films grown by molecular beam epitaxy validate our
band-structure calculations. Our computational and experimental methods and
results are discussed in details.