Optical properties of BiTeI crystals

Some optical properties of BiTeI single crystals prepared from the melt or from the vapour phase are compared. From the spectral dependence of the reflectivity in the region of the plasma resonance frequency the high-frequency dielectric constant epsilon infinite(E perpendicular c) is determined to be 19 +- 2; this value is verified from the experimental interference pattern in the transmission spectrum of thin crystals. The short-wavelength absorption edge of crystals grown from the melt is distinctly shifted toward the region of shorther wavelength contrary to that of crystals grown from the vapour phase. The shape of the absorption edge in the long-wavelength region can be described by the relation K approximately lambdasup(α), the value α for the crystals grown from the vapour phase ranges from 2.3 to 2.5 while for the crystals grown from the melt from 2.8 to 3.8. The crystals grown from the vapour phase contain a smaller concentration of free electrons and thus also of defects Isub(Te) than the crystals grown from the melt. (author)