Infrared Optical Conductivity of Bulk Bi₂Te₂Se

id- and near-infrared measurements reveal that the optical conductivity of the three-dimensional topological insulator, Bi₂Te₂Se, is dominated by bulk carriers and shows a linear-in-frequency increase at 0.5 to 0.8 eV. This linearity might be interpreted as a signature of three-dimensional (bulk) Dirac bands; however, band-structure calculations show that transitions between bands with complex dispersion contribute instead to the inter-band optical conductivity at these frequencies and, hence, the observed linearity is accidental. These results warn against the oversimplified interpretations of optical-conductivity measurements in different Dirac materials.