Surface superconductivity in lowX Al–Ag alloys

Abstract

The magnetization of superconducting Al–4 at%; 5 at% Ag alloys with low xG (0.40 < xG < 1.30) are measured. The solid solutions are decomposed in silver rich Guinier‐Preston zones in a depleted matrix. During an isothermal ageing, the residual resistivity ϱ0 and xG decrease continuously. Due to proximity effects in the volume, these alloys behave like homogeneous superconducting solid solutions, and because of the great coherence length and the low xG value, the effects of surface superconductivity are important. Some experimental results are presented on the variation of (Hc 3/Hc, Hc 3/Hc 2) with xG, and on the magnetization of the superconducting surface sheath which agree with results issued from the G.L.A.G. theory. A qualitative explanation of the variation of the surface irreversibility, during the transition type II → type I, is given by the progressive disappearance of the superconducting sheath.