Charge doping versus impurity scattering in chemically substituted iron-pnictides

To reveal the relative importance of charge doping and defect scattering in substitutionally modified 122 iron pnictides, we perform a systematic first principles study on selected bands at the Fermi level. Disorder effects are induced by various substitutions using an orbital based coherent potential approximation (CPA). Pronounced level shifts of individual bands suggest that transition metal substitutions introduce mobile charge carriers into the system. However, important deviations from such a rigid band scenario as well as spectral broadenings due to impurity scattering correlate with the band character. Finally a $T$-matrix analysis exhibits a larger intraband than interband scattering consistent with an $s^{+-}$ pairing state. Comparing different substitutions reveals an increase of pair-breaking along the transition metal series.