Extended Drude model and role of interband transitions in the   mid-infrared spectra of pnictides
  We analyze the outcomes of an extended-Drude-model approach to the optical spectra of pnictides, where the multiband nature of the electronic excitations requires a careful analysis of the role of interband processes in the optical conductivity.Through a direct comparison between model calculations of the intraband optical spectra and experimental data, we show that interband transitions,whose relevance is shown by first-principle calculations,give a non negligible contribution already in the infrared region. This leads to a substantial failure of the extended-Drude-model analysis on the measured optical data without subtraction of interband contributions. 
L. Benfatto, E. Cappelluti, L. Ortenzi and L. Boeri
cond-mat.supr-con