Critical Field Measurements on Superconducting Lead Isotopes

Abstract

The measurement of the isotope effect upon the superconducting critical field of lead has been extended from ${T}_{c}$ to 1.28\ifmmode^\circ\else\textdegree\fi{}K, and an accurate critical-field curve for lead is reported for this temperature range. The measured critical-field curve is expanded as a function of ${T}^{2}$ and used in calculating the thermodynamic properties of lead. The values for the coefficient of the electronic specific heat in the normal state and for the latent heat of the superconducting transition are in good agreement with calorimetric measurements. The electronic specific heat in the superconducting state does not show an exponential $\frac{1}{T}$ dependence but instead a close resemblance to a ${T}^{4}$ dependence below 5\ifmmode^\circ\else\textdegree\fi{}K.The temperature dependence of the critical field of lead, like that of most superconductors, is not parabolic. However, the observed ${H}_{c}$ values lie above a parabola passing through ${H}_{0}$ and ${T}_{c}$ which is in the opposite direction from the deviations shown by all other superconductors for which precise data are available. It is shown that an empirical correlation exists for superconductors between the deviation from parabolic behavior and $\frac{{T}_{c}}{{\ensuremath{\theta}}_{0}}$, where ${\ensuremath{\theta}}_{0}$ is the Debye temperature at $T=0\ifmmode^\circ\else\textdegree\fi{}$K.Measurements of the isotope effect upon the critical field below ${T}_{c}$ show small differences in the coefficient of the normal electronic specific heat, $\ensuremath{\gamma}$, between specimens but in general give support to the principle of similarity in lead to about the same precision as has been reported for other superconductors.