Optical spectroscopy was used to study the electrodynamics and hidden transport properties of a BaFe$_{ 1.91}$Ni$_{ 0.09}$As$_{ 2}$ thin superconducting film. We analyzed the normal state data using a Drude-Lorentz model with two Drude components: one narrow ($D_1$) and another broad one ($D_2$). In the superconducting state, two gaps with $2\Delta _{ 0}^{ (2)}/k_BT_c=1.9$--2.0 and $2\Delta _{ 0}^{ (1)}/k_BT_c=4.0$--4.3 are formed from the narrow component $D_1$ while the broad component $D_2$ remains 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 ($\lambda_{ tot}$) and representative energy ($\Omega_{ 0}$) in the normal state with respect to the superconducting state is typical of other iron-based materials as well as high-temperature superconducting (HTSC) cuprates.
Categories: cond-mat.supr-con