Optical and transport properties of Ba(Fe1−xNix)2As2 films

The broad-band optical spectroscopy was used to study the optical and the hidden transport properties of the Ba(Fe${}_{1-x}$Ni${}_x$)${}_2$As${}_2$ superconducting films with different Ni contents. The normal state data were analyzed using a Drude–Lorentz model with two Drude components: narrow and broad ones. In the superconducting state, two gaps with $2{\Delta }_0^{\left(1\right)}/k_B{T}_c=1.57$ and $2{\Delta }_0^{\left(2\right)}/k_B{T}_c=3.48$ are formed for the Ba(Fe_0.965Ni_0.035)${}_2$As${}_2$ films, while for the Ba(Fe_0.95Ni_0.05)${}_2$As${}_2$ films these characteristic ratios are 1.88–2.08 and 3.66–4.13. Both gaps are formed from the narrow Drude component, whereas the broad Drude component remains ungapped. The calculated from infrared data total dc resistivity of the films with Ni contents $x=0.05$ and $x=0.08$ as well as the low-temperature scattering rate for the narrow Drude component show a hidden Fermi-liquid behavior. On the contrary, the films with $x=0.035$ manifest a hidden non-Fermi-liquid behavior. The Allen theory generalized to a multiband systems was applied to the analysis of the temperature dependences of a resistivity of the Ba(Fe${}_{1-x}$Ni${}_x$)${}_2$As${}_2$ films. The change of total electron-boson coupling and representative energy in the normal state versus the superconducting state for this system was shown to be typical of other iron-based superconducting materials as well as high-temperature superconducting cuprates.