Further evidence for tunneling anisotropy in Nb crystals

Anisotropy of tunneling into single-crystal niobium is revealed by comparison of $d^2$V/${\mathrm{dI}}^2$ phonon spectra and ${\alpha }^{2F}$(ω) of (100) and (110) faces. This observation extends the recent report of Durbin, Buchanan, Cunningham, and Ginsberg of anisotropy between phonon spectra observed on (110) and (111) faces of Nb single-crystal films grown by molecular beam epitaxy.