Direct observation of the superconducting energy gap in the optical conductivity of the iron pnictide superconductor Ba(Fe_0.9Co_0.1)₂As₂

The temperature-dependent optical reflectivity and complex transmissivity of an epitaxially grown Ba(Fe_0.9Co_0.1)₂As₂ thin film were measured over a wide frequency range (4–35 000  cm⁻¹). The opening of the superconducting gap 2₀=(3.7±0.3)  meV is directly observed by vanishing optical conductivity at 30  cm⁻¹ for T<T_c=20  K. While in this range the measured temperature- and frequency-dependent electrodynamic properties agree well with the BCS predictions of a nodeless order parameter, unexpectedly a strong quasiparticle absorption shows up below 1.5 meV. The spectral weight of the condensate 1.94×10⁷  cm⁻² corresponds to a penetration depth =3600  Å. ©2010 The American Physical Society