Anisotropic Energy-Gap Measurements on Superconducting Niobium Single Crystals by Tunneling

Abstract

Nb‐oxide‐In tunneling junctions were constructed on the as‐grown surfaces of Nb single crystals at controlled orientations. The energy gap was observed to be anisotropic for crystals with resistivity ratios of 178 or greater. The lowest gap, 2.84 mV, was observed for the $\text{〈100〉}$ direction. Local gap maxima were observed at the $\text{〈311〉}$ and $\text{〈111〉}$ directions, where the gap was about 3.19 mV. No anisotropy was observed in crystals having resistivity ratios of 65, presumably due to impurity scattering. Some coincidence of the energy gap anisotropy with main features of the proposed group V transition metal Fermi surface is noted.