The temperature dependences of the plasma frequency, superfluid density and London penetration depth were determined from terahertz spectra of conductivity and dielectric permittivity of BaFe$_{1.91}$Ni$_{0.09}$As$_{2}$ film with critical temperature $T_c=19.6$~K. Part of 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 $\Omega_{0}$ scales with $T_{c}$ according to the empirical law $\Omega_{0} = 4.65k_{B}T_{c}$, and with a total electron-boson coupling strength $\lambda_{tot} = 2.17$.

中文翻译：

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BaFe1.91Ni0.09As2薄膜的太赫兹电动力学在多波段Eliashberg理论框架下分析

从具有临界温度的 BaFe$_{1.91}$Ni$_{0.09}$As$_{2}$ 薄膜的电导率和介电常数的太赫兹光谱确定等离子体频率、超流体密度和伦敦穿透深度的温度依赖性$T_c=19.6$~K。部分实验数据在一个简单的三波段 Eliashberg 模型中进行分析，其中超导耦合的机制是由反铁磁自旋波动介导的，其特征能量 $\Omega_{0}$ 根据经验定律与 $T_{c}$ 成比例$\Omega_{0} = 4.65k_{B}T_{c}$，总电子-玻色子耦合强度 $\lambda_{tot} = 2.17$。