Transmittance and reflectance measurements at terahertz frequencies on a superconducting BaFe(1.84)Co(0.16)As(2) ultrathin film: an analysis of the optical gaps in the Co-doped BaFe(2)As(2) pnictide

Here we report an optical investigation in the terahertz region of a 40 nm ultrathin BaFe 1.84 Co 0.16 As 2 superconducting film with superconducting transition temperature T c = 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 Δ B , which seems to follow a BCS-like behavior, results to be Δ B (0) = 17 cm− 1. For the small gap, for which Δ A (0) = 8 cm− 1, the temperature dependence cannot be clearly established. These gap values and those reported in the literature for the BaFe 2 − x Co x As 2 system by using infrared spectroscopy, when put together as a function of T c , 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 T c seem to have a BCS-like linear behavior, but with different slopes. Above this temperature, both gaps show different supra-linear behaviors.