Size Effects in the Superconductivity of Cadmium

The magnetic threshold field of superconducting cadmium has been investigated as a function of specimen size. With the exception of the bulk material, the specimens were in the form of spheres. In all, five distinct groups were investigated ranging in size from the bulk specimen (irregularly shaped pieces of dimensions of the order of mm) down to spheres 44-62 microns in diameter. A ballistic mutual inductance technique was employed to detect the superconducting transitions. While all the specimens exhibited the same zero-field transition temperature ($T_c$), a marked increase in the magnetic threshold fields was observed for the three smallest groups. If this increase in the magnetic threshold curves is interpreted in light of existing theories as being due to the specimen size becoming comparable to the penetration depth, values of (10.4±0.4)×${10}^{-4\mathrm{}}$ cm and (8.8±0.3)×${10}^{-4\mathrm{}}$ cm are obtained for ${\lambda }_0$, the penetration depth at absolute zero. These two values for ${\lambda }_0$ are based on theoretical expressions presented by von Laue and Silin, respectively.