Energy gaps of the A-15 superconductors Nb3Sn, V3Si, and Nb3Ge measured by tunneling

Tunnel junctions with good quasiparticle (Giaever) and pair (Josephson) tunneling characteristics have been made on electron-beam coevaporated thin films of some important high-temperature superconductors which have the A-15 crystalline structure. Using the thermally grown oxide of niobium tin to make Nb-SnoxidePb junctions we estimate the gap, Nb-Sn, as a function of composition from the current-voltage characteristics. Stoichiometric Nb3Sn is strong coupling with 2kBTc=4.2-4.4 whereas with less tin Nb-Sn has a low Tc and becomes weak coupling. On vanadium silicon we could only make good junctions by evaporating a thin silicon layer, with thickness in the range 1.6-16 nm, on the fresh A-15 film before junction fabrication. V-Si is essentially weak coupling for all compositions with 2kBTc=3.50.2. The characteristics of tunnel junctions on high-Tc niobium-germanium films always show evidence for multiple gaps and the data on this interesting material are more difficult to interpret. © 1979 The American Physical Society.