Anomalous Behavious of the Kondo Superconductor (La_1-xCe_x) Al₂

At low temperatures two phenomena of collective behavior of the conduction electrons in metals are known: superconductivity and the Kondo effect. With the system (La, Ce) Al2 we succeeded in demonstrating a strong interaction between these two phenomena as predicted by Müller-Hartmann and Zittartz in 1971.1 The most unusual behavior of this superconducting alloy is the vanishing of superconductivity below a second transition temperature at very low temperatures. This second transition back into the normal state was first observed by Riblet and Winzer2 in an adiabatic demagnetization cryostat with a standard ac mutual inductance technique. Figure 1 shows a typical example of these two transitions. The induction signal of the specimen compared to that of a clean LaAl2 probe of the same size is plotted vs. temperature. This specimen with 0.67 at. % Ce substitution of La in LaAl2 becomes superconducting between 1.5 and 1°K and normal again below 0.5°K. As a consequence of this second transition, the temperature dependence of all the other superconducting properties becomes strange. It is therefore proposed to call such an alloy a “Kondo superconductor.