Two-band conductivity of a FeSe$_{0.5}$Te$_{0.5}$ film by reflectance   measurements in the terahertz and infrared range
  We report an infrared spectroscopy study of a 200 nm thick FeSe$_{0.5}$Te$_{0.5}$ film grown on LaAlO$_3$ with T$_c$=13.7 K. We analyze the 20 K normal state absolute reflectance R$_N$ measured over a broad infrared range and the reflectance ratio R$_S$/R$_N$, R$_S$ being the superconducting state reflectance, measured at 6 K in the terahertz range down to 12 cm$^{-1}$. We show that the normal state model conductivity is given by two Drude components, one of which much broader and intense than the other. In the superconducting state, we find that a gap $\Delta$=37$\pm$3 cm$^{-1}$ opens up in the narrow Drude band only, while the broad Drude band results to be ungapped, at least in the explored spectral range. Our results show that only a two-band model can coherently describe both normal and superconducting state data. 
A. Perucchi, B. Joseph, S. Caramazza, M. Autore, E. Bellingeri, S.   Kawale, C. Ferdeghini, M. Putti, S. Lupi and P. Dore
cond-mat.supr-con