Complex optical properties of BaFe$_2$As$_2$

The complex optical properties of BaFe$_2$As$_2$, the parent material for numerous iron-based superconductors, have been examined over a wide frequency range above and below the structural and magnetic transition at $T_N \simeq 140$~K. The minimal description of this multiband material requires the use of the two-Drude model, revealing a strong, broad component, and a weaker, much narrower component. Above $T_N$ both components display a weak temperature dependence; however, at $T_N$ both scattering rates decrease dramatically. Below $T_N$ the broad component shows little temperature dependence, while for the narrow component $1/\tau \simeq 30 \rightarrow 3$~cm$^{-1}$ for $T\simeq T_N \rightarrow 5$~K. The transition results in a reconstruction of the Fermi surface, revealing Dirac-like cones in the electronic dispersion\footnote{Z. P. Yin {\it et al.}, Nat. Phys. {\bf 7}, 294 (2011)}. Dirac and Weyl materials display very small scattering rates at low temperature; we speculate that the collapse of $1/\tau$ in the coherent component is a consequence of these non-parabolic bands.