Tunneling in MgB2 - Evidence for Two-Band Superconductivity

Superconductor-insulator-supercondcutor (SIS) break junctions on MgB₂ are utilized to go beyond initial tunneling reports on two-gap superconductivity, which were analyzed using weighted sums of two BCS conductances. We use the analysis of distinct features in the density of states of the small gap in terms of a model that treats pair exchange and the quasiparticle self-energy on an equal footing to present compelling evidence that MgB₂ is a two-band superconductor. The SIS geometry allows us to track the small gap, that remains visible in the raw data, to high temperatures where it smoothly closes near the bulk T_c. Finally and importantly, using unique features of SIS junctions, we can rule out proximity effects which otherwise can mimic the temperature dependence of the small, induced gap and the quasiparticle self-energy of two-band superconductivity. All the above features occur in the band with the induced gap. Fortuitously most of our junctions probe only this band.