Break-Junction Tunneling on MgB_2

TL;DR

This paper reviews tunneling data on MgB_2, revealing two distinct superconducting gaps and providing evidence that MgB_2 is a two-band superconductor with interband coupling effects.

Contribution

It demonstrates that MgB_2 exhibits two superconducting gaps and provides insights into the interband coupling mechanism through break-junction tunneling measurements.

Findings

Identification of two distinct energy gaps, Delta_L and Delta_S.

Small gap closes near the bulk critical temperature, T_c=39 K.

Observation of a feature near Delta_S+Delta_L indicating strong-coupling effects.

Abstract

Tunneling data on magnesium diboride, MgB_2, 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~7.2 meV, close to the expected BCS value, and a small gap, Delta_S~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 MgB_2 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 MgB_2.