Paramagnetic Impurity Effect in Dilute Superconducting La-Alloys

Abstract

The superconducting transition temperatures and the critical fields have been measured by the magnetic method in a number of dilute superconducting La-Ce and La-Gd alloys to study the magnetic impurity effect. The concentration dependence of the transition temperature shows a deviation from the Born approximation value in the Abrikosov-Gor'kov theory and this deviation is dependent upon the sign of the s - f exchange integral of the impurity. This result is regarded as an evidence for the Kondo effect in superconductivity. Magnetic behaviors characteristic of the inhomogeneous type-II superconductors have been observed in pure La and the alloys. Comparison of the upper critical field data with the theory of Fulde and Maki leads to a conclusion that the transport resistivity of La does not increase with impurity concentration. The Ginzburg-Landau parameter of α-La at T c is about 3.4 and decreases with increasing impurity concentration in accordance with Maki's theoretical prediction.