Study of the structure of a superconducting state of Co-doped BaFe$_2$As$_2$ multiband compounds

The terahertz and infrared spectra of the complex dynamic conductivity, as well as the temperature dependences of the density of a superconducting condensate and the electronic specific heat of superconducting BaFe$_{1-x}$Co$_x$As$_2$ compounds, have been analyzed within a Bardeen–Cooper–Schrieffer-like model of a multiband superconductor with strong coupling. It has been shown that the superconducting state of these compounds is determined by three (one electronic and two hole) weakly interacting condensates. The order parameters of the condensates are: $\Delta_1\approx15\,$cm$^{-1}$, $\Delta_2\approx21\,$cm$^{-1}$, and $\Delta_3\approx(30{-}35)\,$cm$^{-1}$. The results significantly refine the existing notions on the structure of the superconducting state of Co-doped BaFe$_2$As$_2$ multiband compounds.