Proximity effect in the thin-superconductor limit

The parallel nucleation field of N-S-layered samples was studied as a function of the ratio of the thickness of the superconductor d_s to its coherence length ${\xi }_s$. The ratio $\frac{d_s}{{\xi }_s}$ for each sample was varied typically from 1.2 to 2.5 by controlling the temperature. This allowed investigations of each sample in the thin limit ($d_s<1.84{\xi }_s$) as well as in the thick limit ($d_s>2{\xi }_s$). The experimental results are compared with the theories of Fink and Martinoli and Fink and Presson. The supposed limits of theoretical validity are carefully examined in terms of $\frac{d_s}{{\xi }_s}$. For $\frac{d_s}{{\xi }_s}<1.8$ the theory of Fink and Martinoli is satisfactory, while for $\frac{d_s}{{\xi }_s}>2.1$, the Fink-Presson theory works well.