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_2As_2 pnictide

摘要

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_xAs_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.