Anisotropy of plasma reflection of solid solutions (Bi2 ? xSbx)Te3 (0 < x < 1) in the Temperature Range 78–293 K

We study IR polarized reflection spectra of solid solutions (Bi_2−x Sb _x )Te₃ (0 < x < 1) in the temperature range 78–293 K and in the excitation range of plasma oscillations of free charge carriers. We reveal that, as the content of Sb₂Te₃ in the solid solution increases from 0 to 50%, the anisotropy of both plasma frequencies and magnetic susceptibility vary insignificantly. This indicates that, at 293 K, these crystals do not show significant changes in the composition of groups of free carriers near the chemical potential level. We have found that to describe the observed anisotropy of plasma frequencies, it is necessary to take into account the effect of heavy holes of the valence band. This is also confirmed by the results of the study of the temperature behavior of plasma reflection spectra, which exhibit regularities that are similar to anomalous temperature changes of the Hall coefficient. We show that a decrease in plasma frequencies with increasing temperature cannot be described by an increase in the effective mass of charge carriers in accordance with the expression m* ∼ T ^0.17, which was obtained upon interpretation of temperature dependences of a number of kinetic coefficients. We use the empirical Moss rule ɛ_∞² E _g to estimate the rate of decrease of the thermal band gap dE _g /dT = −1.6 × 10⁻⁴ eV/deg in a crystal that contains 25% Sb₂Te₃.