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 BaFe1.84Co0.16As2 with T (c) = 22.5 K. The film was grown by pulsed laser deposition on a DyScO3 substrate with an epitaxial SrTiO3 intermediate layer. The measured R (S) /R (N) spectrum, i.e. the reflectivity ratio between the superconducting and normal state reflectance, suggests the presence of a superconducting gap Delta (A) close to 15 cm(-1). A detailed data analysis shows that a two-band, two-gap model is necessary to obtain a good description of the measured R (S) /R (N) spectrum. The low-energy Delta (A) gap results to be well determined (Delta (A) = 15.5 +/- 0.5 cm(-1)), while the value of the high-energy gap Delta (B) is more uncertain (Delta (B) = 55 +/- 7 cm(-1)). Our results provide evidence of two electronic contributions to the system conductivity with the presence of two optical gaps corresponding to 2 Delta/kT (c) values close to 2 and 7.