Electron tunneling into Nb3Sn, Nb3Ge, and Nb3Al

Abstract

We have prepared high-quality tunnel junctions on thin films of ${\mathrm{Nb}}_3$Sn, ${\mathrm{Nb}}_3$Ge, and ${\mathrm{Nb}}_3$Al with artificial tunnel barriers of Al oxide and AlZr oxide. The ${\mathrm{Nb}}_3$Sn/AlZr-oxide/Pb tunnel junctions prepared on Sn-deficient material showed no measurable proximity effect. Whereas the tunnel spectra of Sn-deficient A15 NbSn could be satisfactorily analyzed by the standard Rowell-McMillan inversion procedure, the reduced density of states of the stoichiometric samples exhibited characteristic deformations in the form of an overswing at energies far above the highest phonon energy. These deformations can be fitted by proximity analysis, but it is shown that they can also be referred to the influence of an electronic density of states which is strongly structured in the range of phonon energies near the Fermi level. A very similar behavior is found for ${\mathrm{Nb}}_3$Ge and to a smaller extent for ${\mathrm{Nb}}_3$Al.