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

TL;DR

This study investigates the terahertz optical response of MgB$_2$, revealing two-band conductivity behavior in the superconducting state and how impurity levels influence the dominance of the $\sigma$- and $\pi$-bands.

Contribution

It demonstrates that MgB$_2$'s terahertz electrodynamics can be modeled by a two-band parallel conductivity, highlighting the impact of impurity levels on band dominance.

Findings

High-energy $\sigma$-gap affects reflectance ratio in low-impurity films.

Impurity level influences whether $\sigma$- or $\pi$-bands dominate the response.

Two-band model accurately describes MgB$_2$'s terahertz electrodynamics.

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 $\sigma$-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 electrodynamic of MgB$_2$ 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 ultra-clean sample.