Tunneling results on gap anisotropy in niobium

Abstract

A recently developed theory to describe tunneling into the Meissner phase of clean superconductors is applied to a re-discussion of experimental data on (Nb single crystal)/Ag junctions taking band structure effects into account. Thereby several shortcomings are removed of the previous analysis which assumed an isotropic single gap. The analysis is based on independent band structure and non-tunneling experimental data. On this basis, quantitative account is obtained for our tunneling results concerning both broadening of characteristics in zero field and the effect of magnetic field for different crystal orientations. The results verify the theory which was based on solving the Bogoliubov–De Gennes equations for tunnel-injected quasiparticles including the Doppler shift imposed by the shielding current. Moreover, they represent realistic tunneling results on gap anisotropy in niobium.