Previously reported measurements of the ratio $\frac{H_{c3\mathrm{}}}{H_{c2\mathrm{}}}$ for tin-rich Sn-In alloys and indium-rich In-Bi alloys have been extended in further detail, and include variations in foil thickness (6 - 300 μ). The anomalous behavior in the ratio very near $T_c$ is found to be dependent on foil thickness. Each alloy system is characterized by a certain thickness, above which $\frac{H_{c3\mathrm{}}}{H_{c2\mathrm{}}}$ decreases markedly as $T_c$ is approached, and below which the opposite behavior takes place. The results can be interpreted qualitatively in terms of a sandwich model in which the strength of the pairing interaction is depressed in a layer at the surface of the sample. For the thickest samples (most nearly "semi-infinite"), there is good quantitative agreement with the recent theory of this proximity effect given by Fink and Presson.

• Received 29 April 1970

DOI:https://doi.org/10.1103/PhysRevB.2.2512