Enhanced transition temperature and upper critical field in V-Al/Cu-Ge composite superconductors

Superconducting V3(Ge,Al) composite tapes were produced by the solid-state diffusion between a V-Al alloy core containing 0-23at.%Al and a Cu-Ge alloy matrix containing 5 or 9at.%Ge. The transition temperature,Tc, and the upper critical field,Hc2(4.2K), especially the latter, are significantly increased with increasing aluminum concentration in the vanadium core from 7K and 2T at 0at.%Al to 12.5K and 18T at 23at.%Al, respectively. According to the Hc2versus temperature curve, H*c2(0) of ∼32.5T, Pauli-paramagnetic limitation parameter of ∼2.0 and spin-orbit scattering frequency parameter of ∼0.85 were obtained for the V-23at.%Al/Cu-9at.%Ge specimen. The composite specimen with thinner V-Al core shows more significant increase in Tcby the heat treatment. The Tc, the Hc2and the structures of the arc-melted V3(Ge,Al) specimens were also investigated to compare with those of the composite specimens.