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-Sn--oxide--Pb junctions we estimate the gap, Δ/sub NbhyphenSn/, as a function of composition from the current-voltage characteristics. Stoichiometric Nb3Sn is strong coupling with 2Δ/k/sub B/T/sub c/=4.2--4.4, whereas with less tin, Nb-Sn has a low T/sub c/ 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 2Δ/k/sub B/T/sub c/=3.5 +- 0.2. The characteristics of tunnel junctions on high-T/sub c/ niobium-germanium films always show evidence for multiple gaps and the data on this interesting material are more difficult to interpret