Electronic correlations and unconventional spectral weight transfer in BaFe${}_{2-x}$Co${}_x$As${}_2$

We report an infrared optical study of the pnictide high-temperature superconductor BaFe${}_{1.84}$Co${}_{0.16}$As${}_2$ and its parent compound BaFe${}_2$As${}_2$. We demonstrate that electronic correlations are moderately strong and do not change across the spin-density wave transition or with doping. By examining the energy scale and direction of spectral weight transfer, we argue that Hund’s coupling \emph{J} is the primary mechanism that gives rise to correlations.