Electronic, magneto-optical, and magnetotransport properties of the topological insulator <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi mathvariant="normal">Sb</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:msu

The structural, magneto-optical, and magnetotransport properties as well as the electronic band structure of the bulk crystalline topological insulator (TI) ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{2}\mathrm{Se}$, grown by the vertical Bridgman technique, were studied. The high structural quality of the grown crystals was established by x-ray diffraction and Raman spectroscopy. Angular resolved photoelectron spectroscopy revealed a single Dirac cone with the Dirac point, away from the valence as well as conduction bands, 0.22 eV above the Fermi level. The magnetotransport data exhibited a distinct single-frequency Shubnikov-de Haas oscillation in the magnetic fields above $B$ = 10 T. The Lifshitz-Kosevich analysis of the data suggests that this oscillation originates from the Dirac-type states. A sharp fundamental absorption edge in the mid-infrared transmission spectra measured at 4.2 K demonstrated a direct band gap of 0.377 eV located at a momentum of 0.1 ${\AA{}}^{\ensuremath{-}1}$ along the $\overline{\mathrm{\ensuremath{\Gamma}}}\text{\ensuremath{-}}\overline{K}$ directions of the Brillouin zone. A two-band model developed for massive Dirac electrons in the bulk of topological insulators with the direct band gap at a non-\cyrchar\CYRG{} point suggests hyperbolic dispersion relations for the conduction and valence bands displaying the full electron-hole symmetry. Equal magnitude of the electron and hole effective masses ${m}_{e}\ensuremath{\approx}{m}_{h}\ensuremath{\approx}0.21{m}_{0}$ and $g$ factors ${g}_{e}\ensuremath{\approx}{g}_{h}\ensuremath{\approx}10$ were estimated.