  By means of one-step model calculations the strong in-plane anisotropy seen
in angle-resolved photoemission of the well-known iron pnictide prototype
compounds BaFe$_2$As$_2$ and Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ in their
low-temperature antiferromagnetic phases is investigated. The
fully-relativistic calculations are based on the Korringa-Kohn-Rostoker-Green
function approach combined with the coherent potential approximation alloy
theory to account for the disorder induced by Co substitution on Fe sites in a
reliable way. The results of the calculations can be compared directly to
experimental spectra of detwinned single crystals. One finds very good
agreement with experiment and can reveal all features of the electronic
structure contributing to the in-plane anisotropy. In particular the local
density approximation can capture most of the correlation effects for the
investigated system without the need for more advanced techniques. In addition,
the evolution of the anisotropy for increasing Co concentration $x$ in
Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ can be tracked almost continuously. The results
are also used to discuss surface effects and it is possible to identify clear
signatures to conclude about different types of surface termination.

Superconductivity