Energy gaps of theA−15superconductorsNb3Sn,V3Si, andNb3Ge measured by tunneling

Abstract

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 $A\ensuremath{-}15$ crystalline structure. Using the thermally grown oxide of niobium tin to make Nb-Sn---oxide---Pb junctions we estimate the gap, ${\ensuremath{\Delta}}_{\mathrm{N}\mathrm{b}\ensuremath{-}\mathrm{S}\mathrm{n}}$, as a function of composition from the current-voltage characteristics. Stoichiometric ${\mathrm{Nb}}_{3}$Sn is strong coupling with $\frac{2\ensuremath{\Delta}}{{k}_{B}{T}_{c}}=4.2\ensuremath{-}4.4$ whereas with less tin Nb-Sn has a low ${T}_{c}$ 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 $A\ensuremath{-}15$ film before junction fabrication. V-Si is essentially weak coupling for all compositions with $\frac{2\ensuremath{\Delta}}{{k}_{B}{T}_{c}}=3.5\ifmmode\pm\else\textpm\fi{}0.2$. The characteristics of tunnel junctions on high-${T}_{c}$ niobium-germanium films always show evidence for multiple gaps and the data on this interesting material are more difficult to interpret.