Splitting of the absorption edge in the topological insulator Bi_1.1Sb_0.9Te₂S: mid-infrared magneto-optical study

Abstract External magnetic fields can be used to control the spin properties of charge carriers in topological insulators (TIs). Thin p-type layers of the TI Bi 1.1 Sb 0.9 Te 2 S were studied using mid-infrared Fourier transform magneto-transmission spectroscopy in magnetic fields up to 11 T. Zero field spectra, measured at 4.2 K and 300 K, demonstrated a sharp absorption edge, used to determine an optical bandgap E g opt of 0.31 and 0.22 eV, respectively, as well as to establish the direct character of the bandgap. Fabry–Perot oscillations were used to estimate a refractive index of 6.4. A difference of E g opt from the bandgap, determined earlier by angular resolved photoelectron spectroscopy, was attributed to the formation of band tails generated by high concentrations of randomly distributed charged defects. Equal electron m e and hole m h effective masses of 0.152 m 0 were determined using a theoretical model employing simplified Dirac-type Hamiltonian. Magnetic fields split the absorption edge and the non-linear character of this splitting energy resulted in a strong decrease of the g -factor with increasing field.