Energy gap substructures in conductance measurements of MgB₂-based Josephson junctions: beyond the two-gap model

Several theoretical analyses of the two superconducting energy gaps of magnesium diboride, ${\Delta }_{\pi }$ and ${\Delta }_{\sigma }$, predict substructures within each energy gap. We report additional experimental evidence for these features, in tunneling conductance data using multiple samples measured at very low temperatures. The absence of these features in c-axis tunneling, and a sharp peak in the subgap (associated with the counterelectrode material), support the conclusion that these features are intrinsic to MgB₂. By demonstrating the inadequacy of a simple two-gap model in fitting the data, we illustrate that distinctions between theoretical models of energy gap substructures are experimentally accessible. (paper)