of YbMnBi$_2$ measured using FTIR spectroscopy. (b) Raw reflectivity data at 300 K along with reflectivity curve as obtained from VDF fitting. (c) Resistivity of the sample measured using a 4-probe technique along with the Drude scattering rates.](res_sc_rt.png){width="53.00000%"}

Optical properties were measured on a cleaved (001) surface ($\sim 3$ mm $\times$ 3 mm) of a high quality YbMnBi$_2$ crystal using Fourier transform infrared (FTIR) spectroscopy. For details of the sample preparation please refer to Ref.[@borisenko2015time]. The as-grown (001) surface of another slightly smaller crystal was also measured and had identical results. Fourier transform spectroscopy in known to offer excellent signal to noise and frequency resolution. Reflectivity of YbMnBi$_2$ single crystals were measured using a commercial FTIR spectrometer (Bruker, Vertex 80V, Source: Globar/Hg Arc Lamp, Detectors: DLaTGS/MCT/Bolometer) across far and mid infrared spectral ranges spanning from 50 cm$^{-1}$ to 8500 cm$^{-1}$ (i.e. 1.5-2500 THz) for several temperatures between 5 K and 300 K. The reflection spectra at each temperature were referenced to that of elemental gold, deposited on the sample in situ by thermal evaporation. To extend the measurement across a broader spectral range, the infrared spectra was supplemented by visible reflection spectrum between 11150 cm$^{-1}$ and 29000 cm$^{-1}$ measured at 