Superconductivity in Cu-V-Si alloys

The superconducting properties of copper ternary alloys containing small amounts of vanadium and silicon have been investigated. The wire samples of three alloys—Cu₉₀V_7.5Si_2.5, Cu₈₈V₉Si₃, and Cu₈₄V₁₂Si₄—show a resistive superconducting transition around 17 K, the characteristic transition temperature of the A15 compound V₃Si, followed by a plateau and a second transition between 5 and 10 K. The resistivity, however, does not drop to zero down to 2.5 K. The maximum drop in resistivity is observed for the alloy containing minimum amounts of vanadium and silicon. Complete super-conductivity is absent in these alloys for two reasons. First the effective superconducting volume fraction in these alloys is well below the threshold value obtained by either the effective medium theory or the site percolation theory for solid conduction. Second, the superconducting phase does not precipitate in a fine structure in the matrix and a strong proximity effect does not operate. Experimental evidence confirms the formation of brittle phase V₃Si in the cast alloys which does not get elongated to the same extent as the matrix when rolled or drawn. This reduces the effective volume fraction of V₃Si in the drawn wires. Annealing of these wires causes the superconducting particles to coalesce and grow in size, increasing the interparticle distances to the detriment of superconductivity in these alloys.