Two-band parallel conductivity at terahertz frequencies in the superconducting state of MgB2

The optical response of the two-band superconductor MgB2 has been studied in the 0.7-4 THz range on films with very low impurity level. The effect of the high-energy sigma gap is observed in the ratio R-S/R-N between the normal and superconducting state reflectance, while in a neutron irradiated film with a slightly higher impurity level, mainly the effect of the sigma gap is evident, as reported in previous experiments. At terahertz frequencies, the electrodynamics of MgB2 can be well described by the two-band parallel conductivity model and is dominated by the pi bands when the impurity level is only slightly higher than that of an ultraclean sample.