Magnetic Flux Penetration into Superconducting Thin Films

The absolute value of the penetration depth in Sn and Pb superconducting thin films was measured using a quantum-interference technique. The value of the penetration depth obtained for 1500-Å-thick films of Pb was 630 Å; the value for a 3000-Å-thick film of Sn was 770 Å, and for a 2000-Å-thick film was 730 Å. These results are probably consistent with the BCS microscopic theory within the experimental error, although a direct comparison between the theoretical and experimental values is made difficult by the uncertainty in the nonlocal correction for the film thickness. The temperature dependence of the penetration depth for Pb (measured for temperature less than 4.2 K) agrees with a previous measurement of Erbach et al. For Sn the temperature dependence deviates from the theoretically expected behavior near the transition temperature. Results are given to show that the critical current of a superconducting interferometer with two parallel junctions is not strictly periodic in the applied magnetic flux with a period equal to the flux quantum $\frac{\mathrm{hc}}{2e}$ because of the magnetic field dependence of the critical currents of the junctions.