Electron Tunneling Measurements on Pb near the Transition Temperature

The normalized conductance $\sigma =\frac{{\left(\frac{\mathrm{di}}{\mathrm{dv}}\right)}_s}{{\left(\frac{\mathrm{di}}{\mathrm{dv}}\right)}_n}$ of Pb-insulator-Al tunnel junctions and the gap at the gap edge, ${\Delta }_0\mathrm{}\left(t\right)\mathrm{}$, of Pb were determined for reduced temperatures in the range $0.92\le t\le 1$. The results are compared with a theory of strong-coupling superconductors at nonzero temperatures due to Scalapino, Swihart, and Wada. The normalized conductance is in good over-all agreement with theory. Predicted structure due to electron-quasiparticle recombination is much smaller than calculated and possibly entirely absent. Near $T_c$ we find a linear variation of ${{\Delta }_0}^2\mathrm{}\left(t\right)\mathrm{}$ with $t$; the slope of this line as measured is $\frac{d{\left[\frac{{\Delta }_0\mathrm{}\left(t\right)\mathrm{}}{k{T}_c}\right]}^2}{\mathrm{dt}}=14.6\mathrm{}\pm 0.3$, in excellent agreement with theory.