Penetration of magnetic fields into superconductors III. Measurements on thin films of tin, lead and indium

The magnetic moments of thin superconducting films of tin, lead and indium, obtained by evaporation of the metal in vacuo, have been measured by a ballistic method. Values of the penetration depth, λ0, at absolute zero, derived from them by extrapolation, are estimated as 5.0 ± 0.1 x 10-6 cm. for tin, 3.9 ± 0.3 x 10-6 cm. for lead, and 6.4 ± 0.3 cm. for indium. Within the limits of experimental error the results agree with the penetration law of London & London. From the areas of the magnetization curves, estimates of the difference in surface energy (αn – α5) per unit area between free surfaces of the normal and superconducting phases respectively are derived, which are smaller than those previously assumed. No evidence is found for an increase in penetration depth at low temperatures and high fields such as that suggested by the theories of Heisenberg and Koppe.