Direct observation of a nodeless superconducting energy gap in the optical conductivity of iron-pnictides

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The temperature-dependent optical reflectivity and complex transmissivity of an epitaxially grown Ba(Fe₀.₉Co₀.₁)₂As₂ thin film were measured and the optical conductivity and permittivity evaluated over a wide frequency range. The opening of the superconducting gap 2Δ₀ = 3.7 meV below Tc≈ 20 K is directly observed by a completely vanishing optical conductivity. The temperature and frequency dependent electrodynamic properties of Ba(Fe₀.₉Co₀.₁)₂As₂ in the superconducting state agree well with the BCS predictions with no nodes in the order parameter. The spectral weight of the condensate 1.94× 10⁷ cm⁻² corresponds to a London penetration depth λₗ=3600 Å.