Investigation by Electron Tunneling of the Superconducting Energy Gaps in Nb, Ta, Sn, and Pb

Electron tunneling between two superconductors separated by a thin potential barrier gives rise to nonlinear current vs voltage characteristics, from which a direct measurement of their energy gaps can be obtained. The temperature dependence of the $I:V$ characteristics for sandwiches of Ta/barrier/Pb and Nb/barrier/Sn has been studied and the energy gaps of tin and tantalum determined up to their transition temperatures. Good agreement has been obtained between the temperature dependence observed experimentally, and the predictions of the Bardeen, Cooper, and Schrieffer (BCS) theory of superconductivity.The full limiting values of the superconducting energy gaps obtained are 2εNb=(3.05±0.05)×10^-3 eV, 2εTa=(1.40±0.05)×10^-3 eV, and 2εSn=(1.15±0.06)×10^-3 eV. The lead apparently exhibited two prominent energy gaps with values of 2εPb⁽¹⁾=(2.67±0.05)×10^-3 eV and 2εPb⁽²⁾=(2.90±0.05)×10^-3 eV, at 0^∘{}K. This behavior is tentatively ascribed to anisotropy.Theoretical characteristics have been computed, using the BCS density-of-states expression, ρ=E(E²-ε²)1/2, and compared with the experimental results. Possible reasons for the deviations observed are discussed.