Two-band parallel conductivity at terahertz frequencies in the superconducting state of MgB${}_2$.

Abstract

The optical response of the two-band superconductor MgB${}_2$ has been studied in the 0.7-4 THz range on films with very low impurity level. The effect of the high-energy $\ddot{I}ƒ$-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 $\ddot{I}€$-gap is evident as reported in previous experiments. At terahertz frequencies, the electrodynamic of MgB${}_2$ can be well described by the two-band parallel conductivity model and is dominated by the $\ddot{I}€$-bands when the impurity level is only slightly higher than that of an ultra-clean sample.

4 pages, 3 figures