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

Abstract

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 $2\hat{I}{”}_0=3.7$ 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 $1.94\times 10^7{\mathrm{cm}}^{-2}$ corresponds to a London penetration depth $\hat{I}{»}_L=3600$ à .

4pages,3figures