Observations on Cooper-limit-like behavior in strong coupling: Normal-superconducting bilayers at critical fields

Tunneling measurements of clean Mg/Nb and Al/Nb (normal-superconducting, NS) bilayers, with N-metal thicknesses 100, reveal a superconducting order parameter at the N surface proportional in magnitude to that in S, even in fields closely approaching the critical values for bulk S (Nb). The parallel decay of N and S phonon features with field, as first observed here, implies an order parameter nearly constant over the NS structure even under strong local depairing conditions.