to the optical conductivity from the transition between the bands (37,38) $\rightarrow$ (39,40) from different parts of the Fermi surface (fig. inset) (b) Linear fit of the inter-band transition peaked at 950cm$^{-1}$.[]{data-label="fig:dirac"}](vf.png "fig:"){width="48.00000%"}

We can isolate the low frequency features of this particular inter-band contribution from the conductivity measured experimentally by fitting the peak at 200 cm$^{-1}$ to a Lorentz oscillator and subtracting it from the previously calculated inter-band conductivity. The resultant shows a roughly linear regime between 300 cm$^{-1}$ and 950 cm$^{-1}$ that can be fitted to a straight line with zero intercept (see figure \[fig:dirac\](b)). With the assumption that this linearity is due to the Dirac dispersion, we can calculate the Fermi velocity using equation . There are four Dirac points in each Brillouin zone of YbMnBi$_2$ [@borisenko2015time] and thus the calculated Fermi velocity is (1.1$\pm$0.4)$\times$10$^5$ cm/s \[or 0.045$\pm$0.01 eV$\cdot$Å\]. This estimated Fermi velocity is within the range expected from ARPES which varies between 9 eV$\cdot$Å(perpendicular to the $k_xk_y$ plane) and 0.043 eV$\cdot$Å(along $k_z$). As discussed before, anisotropic Dirac cones are a hallmark of AMnBi$_2$ compounds and our observations here on YbMnBi$_2$ fits well into this description. The existence of a Dirac fermionic dispersion is also in good 