Infrared reflectance measurements have been carried on highly $p$-type ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ crystals over the broad temperature range from 10 to 650 K and their response function investigated. An anomalous nonmonotonous temperature dependence of the plasma edge frequency is observed—with a pronounced redshift and blueshift that appear above and below room temperature, respectively. Such behavior is explained in terms of the temperature evolution of the free carrier optical weight $N/m^{*}$ given by particular temperature dependence of the total carrier concentration $N$ (determined by transport measurements) and characteristic (optical) mass $m^{*}$. The effective mass strongly varies from $m^{*}=0.1m_0$ ($m_0$ electron rest mass) at 10 K to $m^{*}=0.5m_0$ respective $m^{*}=0.9m_0$ at 650 K depending on the evaluating procedure. The interpretation is corroborated by complementary magnetotransport and magnetoreflectivity experiments.

• Received 2 August 2021
• Accepted 13 October 2021

DOI:https://doi.org/10.1103/PhysRevB.104.165203