Optical investigations of the normal and superconducting states reveal two electronic subsystems in iron pnictides

Infrared reflectivity measurements on several 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 EuFe₂As₂, is responsible for superconductivity in the doped compounds, such as Ba(Fe0.92Co0.08)₂As₂ and Ba(Fe0.95Ni0.05)₂As₂. Analyzing the dc resistivity and scattering rate of this contribution, a hidden T² dependence is found in the normal 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.