Determination of the Anisotropy of the Energy Gap in Superconducting Pb by Superconductive Tunneling

The anisotropy of the energy gap in superconducting Pb has been studied experimentally by the method of superconductive tunneling. Structure due to the Pb anisotropy has been observed in thick-Pb-versus-Al tunneling junctions at biases equal to the sum of the gaps. The theory has been analyzed and the experimental line shapes have been compared with theoretical predictions. Structure arising from the energy-gap anisotropy has also been observed in the subharmonic structure—below $2\Delta$—in Pb-versus-Pb tunneling junctions. These data have been interpreted in terms of the critical values—maxima, minima, and saddle points—of the energy-gap surface in $k$ space. The double energy gap $2\Delta$ has been observed to have a maximum at 2.98±0.03 meV and saddle points at 2.44±0.02 and 2.55±0.02 meV. Additional structure has been observed in the subharmonic region which yields leading terms in the subharmonic series at 2.11±0.02, 2.23±0.02, 2.33±0.02, 2.39±0.02, and 2.61±0.02 meV. These latter values represent either critical values of the gap or the average of two such critical values. The present status of the theory of the subharmonic structure is discussed briefly.