  Understanding the formation of Cooper pairs in iron-based superconductors is
one of the most important topics in condensed matter physics. In conventional
superconductors, the electron-phonon interaction leads to the formation of
Cooper pairs. In conventional strong-coupling superconductors like lead (Pb),
the features due to electron-phonon interaction are evident in the infrared
absorption spectra. Here we investigate the infrared absorption spectra of the
iron arsenide superconductor BaFe1.9Pt0.1As2. We find that this superconductor
has fully gapped (nodeless) Fermi surfaces, and we observe the strong-coupling
electron-boson interaction features in the infrared absorption spectra. Through
modeling with the Eliashberg function based on Eliashberg theory, we obtain a
good quantitative description of the energy gaps and the strong-coupling
features. The full Eliashberg equations are solved to check the
self-consistency of the electron-boson coupling spectrum, the largest energy
gap, and the transition temperature (Tc). Our experimental data and analysis
provide compelling evidence that superconductivity in BaFe1.9Pt0.1As2 is
induced by the coupling of electrons to a low-energy bosonic mode that does not
originate solely from phonons.
