Break–junction Tunneling on Mgb 2

Tunneling data on magnesium diboride, MgB2, are reviewed with a particular focus on superconductor–insulator–superconductor (SIS) junctions formed by a break–junction method. The collective tunneling literature reveals two distinct energy scales, a large gap, ∆L ∼ 7.2 meV, close to the expected BCS value, and a small gap, ∆S ∼ 2.4 meV. The SIS break junctions show clearly that the small gap closes near the bulk critical temperature, Tc = 39 K. The SIS spectra allow proximity effects to be ruled out as the cause for the small gap and therefore make a strong case that MgB2 is a coupled, two–band superconductor. While the break junctions sometimes reveal parallel contributions to the conductance from both bands, it is more often found that ∆S dominates the spectra. In these cases, a subtle feature is observed near ∆S +∆L that is reminiscent of strong–coupling effects. This feature is consistent with quasiparticle scattering contributions to the interband coupling which provides an important insight into the nature of two–band superconductivity in MgB2.