  Infrared reflectivity measurements on 122 iron-pnictides reveal the existence
of two electronic subsystems. The one gapped due to the spin-density-wave
transition in the parent materials, such as EuFe$_2$As$_{2}$, is responsible
for superconductivity in the doped compounds, like
Ba(Fe$_{0.92}$Co$_{0.08})_2$As$_{2}$ and Ba(Fe$_{0.95}$Ni$_{0.05})_2$As$_{2}$.
Analyzing the dc resistivity and scattering rate of this contribution, a hidden
$T^2$ dependence is found, indicating that superconductivity evolves out of a
Fermi-liquid state. The second subsystem gives rise to incoherent background,
present in all 122 compounds, which is basically temperature independent, but
affected by the superconducting transition.
