Phonon structure and proximity-effect tunnelling in strongly gapless superconductors

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Abstract
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 A thick for Ag-Pb sandwiches. For thin silver layers (∼200A), 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.