Eliashberg analysis of optical spectra reveals a strong coupling of charge carriers to spin fluctuations in doped iron-pnictide BaFe2As2 superconductors

The temperature and frequency dependences of the optical conductivity of Co-doped BaFe2As2 are analyzed and the electron-boson spectral density alpha F-2(omega) are extracted using Eliashberg's formalism. For the normal state at T=30 K there is a relatively sharp and large peak around 10 meV and a secondary smaller and broader peak centered around 50 meV with the spectrum extending to high energies beyond the maximum phonon energy. The electron-boson mass enhancement parameter is 4.4, a value more consistent with spin-fluctuation scattering rather than with phonons. In addition the spectrum is found to evolve with temperature toward a less structured background at higher energies as in the spin susceptibility.