Die elektrischen Eigenschaften des Kondo-Supraleiters (La,Ce)Al2

In the ternary alloy (La, Ce)Al₂ the Kondo anomalies due to the Cerium impurities are studied in the normal and superconducting state. A vanishing of superconductivity in certain (La, Ce)Al₂ alloys below a second transition temperature has been observed for the narrow range of 0.6–0.7 at% Ce-additions. The transition back into the normal state has been observed by means of dc-conductivity and low frequency mutual inductance measurements down to 30 mK. Curves of the upper critical field versus temperature show a maximum at finite temperatures even for lower Ce-concentrations. The temperature dependence of the pair-breaking parameter has been deduced empirically from the upper critical field curves, assuming additive pair breaking theory. In contradiction to the theory no decrease of the pair-breaking parameter forT → 0 can be observed. Therefore the reentrance of superconductivity at ultra low temperatures cannot be expected.