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