Two-band conductivity of a FeSe0.5Te0.5 film by reflectance measurements in the terahertz and infrared range

We report an infrared spectroscopy study of a 200 nm thick FeSe0.5Te0.5 film grown on LaAlO3 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 +/- 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.