Transmittance and reflectance measurements at terahertz frequencies on a superconducting BaFe

Here we report an optical investigation in the terahertz region of a 40 nm ultrathin BaFe1.84Co0.16As2 superconducting film with superconducting transition temperature Tc = 17.5 K. A detailed analysis of the combined reflectance and transmittance measurements showed that the optical properties of the superconducting system can be described in terms of a two-band, two-gap model. The zero temperature value of the large gap ΔB, which seems to follow a BCS-like behavior, results to be ΔB(0) = 17 cm-1. For the small gap, for which ΔA(0) = 8 cm-1, the temperature dependence cannot be clearly established. These gap values and those reported in the literature for the BaFe2−xCoxAs2 system by using infrared spectroscopy, when put together as a function of Tc, show a tendency to cluster along two main curves, providing a unified perspective of the measured optical gaps. Below a temperature around 20 K, the gap-sizes as a function of Tc seem to have a BCS-like linear behavior, but with different slopes. Above this temperature, both gaps show different supra-linear behaviors