Universal Decomposition of the Low-Frequency Conductivity Spectra of Iron-Pnictides Uncovering Fermi-Liquid Behavior

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.