Role of anisotropy and spin-orbit interaction in the optical and dielectric properties of BiTeI and BiTeCl compounds

The dielectric and optical properties of semiconductor compounds BiTeI and BiTeCl have been studied theoretically within time dependent density functional theory and experimentally by spectral ellipsometry. Anisotropy of dielectric constants in the long-wavelength limit and dispersion of the $\sigma$ and $\sigma + \pi$ bulk plasmons in the longitudinal and transverse directions of the crystals have been discovered. It has been shown that the inclusion of the spin-orbit interaction is necessary for agreement between the theory and the results of optical measurements.