Temperature-Dependence in Optical Properties of Topological Insulator Bi$_{1.5}$Sb$_{0.5}$Te$_{1.8}$Se$_{1.2}$

Topological Insulators are materials with insulating bulk states and conducting states on their edge or surface. Using Terahertz Time-Domain Spectroscopy (THz-TDS), we studied the temperature-dependence of the optical properties of Bi$_{1.5}$Sb$_{0.5}$Te$_{1.8}$Se$_{1.2}$ single-crystals from 5~K to 300~K in the terahertz regime ($0.4$ THz to $3.0$ THz). We observed a spectral weight shift from low frequencies to frequencies above 1.0 THz in the real conductivity at temperatures below $\sim$100 K.