Electrodynamics of electron-doped iron pnictide superconductors: Normal-state properties

The electrodynamic properties of Ba(Fe0.92Co0.08)₂As₂ and Ba(Fe0.95Ni0.05)₂As₂ single crystals have been investigated by reflectivity measurements in a wide frequency range. In the metallic state, the optical conductivity consists of a broad incoherent background and a narrow Drude-type component which determines the transport properties; only the latter contribution strongly depends on the composition and temperature. This subsystem reveals a T² behavior in the dc resistivity and scattering rate disclosing a hidden Fermi-liquid behavior in the 122 iron pnictide family. An extended Drude analysis yields the frequency dependence of the effective mass (with m^/mb≈5 in the static limit) and scattering rate that does not disclose a simple power law. The spectral weight shifts to lower energies upon cooling; a significant fraction is not recovered within the infrared range of frequencies.