Multi-gap superconductivity in a BaFe1.84Co0.16As2 film from optical measurements at terahertz frequencies

We measured the THz reflectance properties of a high quality epitaxial thin film of the Fe-based superconductor BaFe${}_{1.84}$Co${}_{0.16}$As${}_2$ with T${}_c$=22.5 K. The film was grown by pulsed laser deposition on a DyScO${}_3$ substrate with an epitaxial SrTiO${}_3$ intermediate layer. The measured $R_S\mathrm{/}{R}_N$ spectrum, i.e. the reflectivity ratio between the superconducting and normal state reflectance, provides clear evidence of a superconducting gap ${\mathrm{\Delta }}_A$ close to 15 cm${}^{-1}$. A detailed data analysis shows that a two-band, two-gap model is absolutely necessary to obtain a good description of the measured $R_S\mathrm{/}{R}_N$ spectrum. The low-energy ${\mathrm{\Delta }}_A$ gap results to be well determined (${\mathrm{\Delta }}_A$=15.5$\pm$0.5 cm${}^{-1}$), while the value of the high-energy gap ${\mathrm{\Delta }}_B$ is more uncertain (${\mathrm{\Delta }}_B$=55$\pm$7 cm${}^{-1}$). Our results provide evidence of a nodeless isotropic double-gap scenario, with the presence of two optical gaps corresponding to 2$\mathrm{\Delta }\mathrm{/}k{T}_c$ values close to 2 and 7.