Splitting of the absorption edge in the topological insulator Bi1.1Sb0.9Te2S: 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 0.152m(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.