Effect of Magnetic Impurities on the Density of States of Superconductors

Abstract

The technique of electron tunneling has been applied to a systematic study of the density of states in superconductors containing magnetic impurities. Vhereas earlier experiments on such superconductors examined the current-voltage tunneling characteristics at 1\ifmmode^\circ\else\textdegree\fi{}K, the present experiments have been refined so as to measure directly the tunneling conductance, and hence the density of states, down to 0.4\ifmmode^\circ\else\textdegree\fi{}K. The present data can therefore be compared with the predictions of the Abrikosov-Gorkov theory worked out in detail by Skalski, Betbeder-Matibet, and Weiss. Qualitatively, the addition of magnetic impurities causes a reduction of the energy gap and a broadening of the peak in the density of states. The detailed shape of the density-of-states curves agrees reasonably well with the theory, at least for a rare-earth impurity system like Gd in lead. The transition-element impurities Fe (in indium) and Mn (in lead) have a more pronounced effect than predicted, the energy gap being substantially more filled in and less well defined than in the Pb-Gd system. Experiments on pure superconducting films with superimposed layers of magnetic metal atoms yield similar marked effects on the density of states. The experiments indicate that it is the presence of strong correlations between electrons rather than the existence of a well defined energy gap which is the essential feature of the superconducting state.