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

Abstract

The electrodynamic properties of $\text{Ba}{({\text{Fe}}_{0.92}{\text{Co}}_{0.08})}_{2}{\text{As}}_{2}$ and $\text{Ba}{({\text{Fe}}_{0.95}{\text{Ni}}_{0.05})}_{2}{\text{As}}_{2}$ 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 ${T}^{2}$ 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 ${m}^{\ensuremath{\ast}}/{m}_{b}\ensuremath{\approx}5$ 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.