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

Abstract

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 ${\text{EuFe}}_{2}{\text{As}}_{2}$, is responsible for superconductivity in the doped compounds, such as $\text{Ba}{({\text{Fe}}_{0.92}{\text{Co}}_{0.08})}_{2}{\text{As}}_{2}$ and $\text{Ba}{({\text{Fe}}_{0.95}{\text{Ni}}_{0.05})}_{2}{\text{As}}_{2}$. Analyzing the dc resistivity and scattering rate of this contribution, a hidden ${T}^{2}$ 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.