The nature of the transition atH _c2 in low-κ type II lead-indium superconducting alloys

Abstract

The anomalous magnetic and calorimetric properties atH _c2, which we reported for a lead-indium sample, are in agreement with the prediction of a bulk first-order phase transition from recent theories for critical κ superconductors$\kappa \approx 1/\sqrt 2$. Other experimentalists, however, explain this behavior as an effect of the superconducting surface sheath. In an attempt to clarify the issue, magnetization and specific heat measurements are reported here on two samples with nominal concentrations of 1.9 and 1.4 at % indium in lead. The latter is type I above 6 K (T _c =7.12 K). It is shown that surface effects are relevant for the effect reported previously. However, after empirical surface treatments have been applied to eliminate or at least minimize such effects, most of the results for the sample, which fulfills the theoretical requirements, are in agreement with theoretical predictions of Fischer, Usadel, and Jacobs. A bulk first-order transition atH _c2 is thus, nevertheless, likely to occur for strong-coupling type II superconductors with critical κ values.