Break-junction tunneling on MgB2

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, Delta(L) similar to 7.2 meV, close to the expected BCS value, and a small gap, Delta(s) similar to 2.4 meV. The SIS break junctions show clearly that the small gap closes near the bulk critical temperature, T-c = 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 Delta(s) dominates the spectra. In these cases, a subtle feature is observed near Delta(s) + Delta(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. (C) 2002 Elsevier Science B.V. All rights reserved.