Preparation, tunneling, resistivity, and critical current measurements on homogeneous high<i>T</i> <i>c</i> A15 Nb3Ge thin films

Abstract

We have prepared homogeneous films of high Tc Nb3Ge as demonstrated by a total transition width of less than 1 K with a resistive Tc onset of 21.7 K, by paying particular attention to the constancy of substrate temperature. Xray diffraction analysis done both at Stanford and at Westinghouse shows no evidence of a second phase to the limits of the instruments (&amp;lt;1%). Tunneling as a function of thickness shows the material to be of good quality (high, sharp gaps; low excess conductance below the sum gap) at the surface for thicknesses of up to 1 μm and only slightly degraded at 5 μm. Further evidence of homogeneity is demonstrated by critical current measurements which give an extrapolated T*c of 20 K on material with resistive Tc onset of 21.9 K. Critical current densities at 14 K in a 0.5 T field are as high as 9×106 A/cm2, which are the highest ever reported for any material at this temperature. In an applied magnetic field of 7.5 tesla, Jc was 2.5×106 A/cm2 at 4.2 K. Tunneling as a function of thickness into Nb3Ge where the oxygen had been removed from the system after 1300 Å had been deposited but the deposition continued up to 1 μm showed only limited degradation (in terms of gap width, excess conductance below the gap as well as magnitude of gap). This gives some indication that oxygen may only be needed initially during the deposition rather than throughout the entire deposition. Tc correlates well with composition, as does resistivity, thus we see the correlation between increasing Tc and decreasing resistivity.