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

The temperature-dependent optical reflectivity and complex transmissivity of an epitaxially grown Ba(Fe${}_{0.9}$Co${}_{0.1}$)${}_2$As${}_2$ thin film were measured and the optical conductivity and permittivity evaluated over a wide frequency range. The opening of the superconducting gap $2{\mathrm{\Delta }}_0=3.7$ meV below $T_c\approx 20$ K is {\em directly} observed by a completely vanishing optical conductivity. The temperature and frequency dependent electrodynamic properties of Ba(Fe${}_{0.9}$Co${}_{0.1}$)${}_2$As${}_2$ in the superconducting state agree well with the BCS predictions with no nodes in the order parameter. The spectral weight of the condensate $1.94\times {10}^7{\mathrm{c}\mathrm{m}}^{-2}$ corresponds to a London penetration depth ${\lambda }_L=3600$ \AA.