Optical and hidden transport properties of BaFe_1.91Ni_0.09As₂film

Optical spectroscopy was used to study the electrodynamics and hidden transport properties of a BaFe_1.91Ni_0.09As₂thin superconducting (SC) film. We analyzed the normal state data using a Drude-Lorentz model with two Drude components: one narrow (D₁) and another broad one (D₂). In the SC state, two gaps with2Δ0(2)/kBTc=1.9-2.0 and2Δ0(1)/kBTc=4.0-4.3 are formed from the narrow component D₁while the broad component D₂remains ungapped. The calculated total DC resistivity of the film and the low-temperature scattering rate for the narrow Drude component show a hidden Fermi-liquid behavior. The change of total electron-boson coupling (λ_tot) and representative energy (Ω₀) in the normal state with respect to the SC state is typical of other iron-based materials as well as high-temperature superconducting (HTSC) cuprates.