The optical response of the two-band superconductor $\mathrm{Mg}{\mathrm{B}}_2$ has been studied in the $0.7–4\mathrm{THz}$ 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 $\pi$ gap is evident, as reported in previous experiments. At terahertz frequencies, the electrodynamics of $\mathrm{Mg}{\mathrm{B}}_2$ 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.

• Received 21 January 2008

DOI:https://doi.org/10.1103/PhysRevB.77.100507