THz electrodynamics of BaFe1.91Ni0.09As2 film analyzed in the framework of multiband Eliashberg theory

The temperature dependences of the plasma frequency, superfluid density and London penetration depth were determined from the terahertz spectra of conductivity and the dielectric permittivity of BaFe1.91Ni0.09As2 film with critical temperature Tc = 19.6 K. Part of the experimental data were analyzed within a simple three-band Eliashberg model where the mechanism of superconducting coupling is mediated by antiferromagnetic spin fluctuations, whose characteristic energy Ω0 scales with Tc according to the empirical law Ω0=4.65kBTc, and with a total electron-boson coupling strength λtot = 2.17.