Observation of the superconducting proximity effect from kinetic-inductance measurements

A method is described for observing the proximity effect in a thin-film-superconductor--normal-metal bilayer. The superconducting film must be thin compared to its penetration depth. The kinetic inductance of the superconducting film alone is then proportional to λ²(T)/d. It is found that in a bilayer the temperature dependence of the kinetic inductance deviates significantly from this dependence at low temperatures. The kinetic inductance may be measured in a nondestructive manner using the two-coil mutual-inductance technique. Bilayers of NbN/Al and Nb/Cu have both exhibited the effect, whereas a bilayer in which the proximity coupling was purposefully suppressed had the same temperature dependence as the superconducting film alone. The effect is analyzed in terms of a spatially varying penetration depth through the film thickness, and the observations are found to be in qualitative agreement with the predictions of the model. A more exact fitting will require improved theoretical models for the proximity effect.