Phonon structure and proximity-effect tunnelling in strongly gapless superconductors

The tunnelling characteristics of normal-metal-insulator-superconductor (NIS) and NINS junctions in the presence of a parallel magnetic field larger than the bulk critical field have been measured. The state that results from this surface superconductivity is gapless. By comparing data taken with no applied field and above HC₂, we are able to study the strong coupling effects in a superconductor and the interference effects in a proximity sandwich as the order parameter is varied. In the case of the proximity effect, we are also able to obtain gaplessness by the addition of magnetic impurities to the normal metal X. For tunnelling into a single superconducting film, the amplitude of the observed phonon structure scales with the order parameter squared. Although surface superconductivity is usually reduced by proximity with a normal metal, the surface sheath remains superconducting for a layer of silver up to - 800 Å thick for Ag-Pb sandwiches. For thin silver layers (-200 Å), the discrepancy between a simple interference model calculation and the experimental data becomes less as the order parameter is reduced, which implies that the discrepancy arises from multiple Andreev scatterings at the N-S boundary. A similar conclusion is reached from studies in which magnetic impurities are introduced into the normal metal.