Superconducting Penetration Depth of Lead

The temperature dependence of the change in the superconducting penetration depth $\ensuremath{\lambda}$ from its value at absolute zero has been measured in polycrystalline and single-crystal specimens of lead, using radio-frequency fields at about 80 MHz. In the polycrystalline material, it has been found that the temperature dependence is the same as that of a weak coupling superconductor with $\frac{{\ensuremath{\Delta}}_{T}}{{\ensuremath{\Delta}}_{0}}$ given by the BCS theory and with an energy gap $2{\ensuremath{\Delta}}_{0}=3.75k{T}_{c}$, rather than $2{\ensuremath{\Delta}}_{0}=4.3k{T}_{c}$, the value found from tunneling measurements. Near the transition temperature ${T}_{c}$, $\frac{d\ensuremath{\lambda}}{dy=418\ifmmode\pm\else\textpm\fi{}9}$ \AA{}, where $y={(1\ensuremath{-}{t}^{4})}^{\ensuremath{-}\frac{1}{2}}$ and $t=\frac{T}{{T}_{c}}$. A full comparison with Nam's general formulation of the electromagnetic properties of superconductors (which encompasses strong coupling effects) awaits more detailed theoretical work on lead. Among the single crystals, there is a small anisotropy in the values of $\frac{d\ensuremath{\lambda}}{\mathrm{dy}}$ near ${T}_{c}$, and at lower temperatures a slight anisotropy in the form of the temperature dependence of $\ensuremath{\lambda}$. A review of all the relevant experiments on lead gives the following consistent values for the coherence length and London penetration depth of lead: ${\ensuremath{\xi}}_{0}=960$ \AA{}, ${\ensuremath{\lambda}}_{L}(0)=305$ \AA{}.