Tunneling study of localized bands in superconductors with magnetic impurities (normal Kondo alloys in the superconducting proximity)

Abstract

We present an experimental study of the magnetic behavior of dilute $\mathrm{CuX}$ ($X=\mathrm{Mn}, \mathrm{Cr}, \mathrm{Fe}$) and AuFe alloys using superconducting electron tunneling. Superconductivity is induced by overlaying the normal alloy film with a superconducting Pb one. The most important result of this study is the existence of localized states, within the superconducting energy gap of the sandwich, their position in energy being characteristic of the nature of the impurity and their amplitude being related to its concentration. This localized band had been predicted from the M\"uller-Hartmann and Zittartz (MHZ) theory which calculates, going beyond the first Born approximation, the scattering of conduction electrons by magnetic impurities. Initially this paper emphasizes the conditions in which a quasi-BCS approach, neglecting the spatial variation of the order parameter, can be used in the tunneling experiments. The characteristics of the impurity band are discussed systematically in the CuCr case. Systematic results on materials of various Kondo temperatures are presented. The agreement with the MHZ approach deteriorates as the Kondo temperature increases and strong-coupling effects should be considered. The possibility of using this technique as a new tool for sensitive spectroscopic analysis is suggested.