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.
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 / A., Perucchi; Baldassarre, Leonetta; B., Joseph; Lupi, Stefano; S., Lee; C. B., Eom; J., Jiang; J. D., Weiss; E. E., Hellstrom; Dore, Paolo. - In: EUROPEAN PHYSICAL JOURNAL. B, CONDENSED MATTER AND COMPLEX SYSTEMS. - ISSN 1434-6036. - 86:(2013), pp. 274-280. [10.1140/epjb/e2013-30964-y]