  An unexpected insensitivity of the Fermi surface to impurity scattering is
found in Ru substituted BaFe$_2$As$_2$ from first-principles theory, offering a
natural explanation of the unusual resilience of transport and
superconductivity to a high level of disordered substitution in this material.
This robustness is shown to originate from a coherent interference of
correlated on-site and inter-site impurity scattering, similar in spirit to the
microscopic mechanism of superdiffusion in one dimension. Our result also
demonstrates a strong substitution dependence of the Fermi surface and carrier
concentration, and provides a natural resolution to current discrepancies in
recent photoelectron spectroscopy. These effects offer a natural explanation of
the diminishing long-range magnetic, orbital and superconducting order with
high substitution.
