Observation of tunneling anisotropy in superconducting niobium crystals

Superconducting proximity-effect tunneling measurements have been performed on (110)- and (111)- oriented niobium crystals grown by molecular-beam epitaxy. Measurements of ac resistance yield information about the effective phonon density of states; different results were obtained for the two crystal orientations. In situ high-energy electron diffraction studies indicate atomically smooth interfaces in the proximity sandwich and also indicate amorphous-oxide barriers for these junctions. We therefore have a nearly ideal configuration, which may account for the sensitivity to crystal orientation. The failure of the McMillan inversion program to converge for the (111) data suggests the need for a $\overrightarrow{\mathrm{k}}$-dependent formulation of the equations.