Measurements of Multiple Gap Substructure in MgB$_2$ Below 1 Kelvin

The two superconducting energy gaps of magnesium diboride (MgB$_2$) are well established. First-principles calculations have also predicted substructure within the sigma- and pi-band gaps. However, due to anticipated intraband impurity scattering, there is controversy as to whether these finer structures can be observed in real samples. Prior experimental evidence above 1 Kelvin has provided evidence supportive of these features. We have performed tunneling spectroscopy experiments on MgB$_2$/insulator/Pb Josephson junctions on SiC substrates, at temperatures as low as 20 mK. By measuring differential conductance at low temperatures, and by using extremely clean MgB$_2$ thin films, we have resolved features within the energy gaps to under 1 meV. We report results of these experiments, which are in remarkable agreement with theoretical predictions for this substructure.