Intra-gap absorption in superconducting Ba(Fe1-x Co x)2As2 thin films studied by a Fabry-Pérot resonant technique

For superconducting films, the optical intra-gap conductivity provides valuable information on the superconductivity mechanism, in particular on the gap symmetry, quasiparticles, and superconducting carrier dynamics. Experimentally, however, it is extracted with rather large uncertainty caused by the huge negative value of the dielectric constant. We have performed systematic numerical analysis of Fabry–Pérot resonators, which consist of two identical superconducting iron pnictide thin films on dielectric substrates that are positioned face-to-face to each other and are separated by a spacer. We demonstrate that such a Fabry–Pérot arrangement can significantly enhance the accuracy to the dynamical conductivity σ 1 of the films, as the efficiency of interaction of the probing radiation with the superconducting films is improved. Using a coherent source terahertz spectrometer (4–45 cm−1) with a Mach–Zehnder interferometer, we have measured the complex transmissivity of three Fabry–Pérot resonators composed by identical pairs of Ba(Fe0.9Co0.1)2As2 films with the thicknesses: 25, 30, and 50 nm. We show that the experimental spectra can be well described by a corresponding five-layer model based on Fresnel’s equations and analyze the advantages and challenges of the Fabry–Pérot resonant technique for iron pnictide films.