Tunneling into a High -TcSuperconductor-Nb3Sn

Tunnel junctions were fabricated on ${\mathrm{Nb}}_3$Sn films reacted on Nb substrates. Derivative measurements were made from 0 to 50 meV and above 18 K. From these tunneling results, the phonon spectrum of ${\mathrm{Nb}}_3$Sn is shown to have a strong low-energy peak at 9 meV, a broad peak centered around 17 meV, and a small peak at 25 meV. Using this shape of the phonon spectrum and taking ${\mu }^{*}=0.11\mathrm{}$, a maximum $T_c$ of ∼ 21 K is obtained for this class of superconductors. This result suggests that crystal transformations arising from instabilities do not limit $T_c$ in this case.