Isotope Effect on the Superconducting Transition in Lead

Abstract

Observations of the difference in critical fields of superconducting Pb specimens of different isotopic mass are described. Measurements are made using the Meissner effect and a ballistic induction method in the range from 7.2 to about 1.4°K. The results near $T_c$ verify the theoretical prediction that $T_c=\mathrm{const}\times M^p$, where $M$ is the average isotopic mass, and yield a value of $p=-0.501\mathrm{}\pm 0.013$. The measurements at lower temperatures are inconclusive in their implications for the isotope effect, since all specimens measured exhibit large deviations from the predictions of the similarity principle. The deviations are apparently not related to the isotopic mass of the specimens. The agency responsible for the observed deviations has not been isolated, but it is believed that the anomalous $H_c$ values are characteristic of thermodynamically irreversible transitions. Thus the present results are not believed to constitute genuine evidence of a deviation from the similarity principle.