Magnetic impurities and superconductivity

Abstract

We study properties of Kondo alloys in which superconductivity is induced by the proximity effect. A review of these McMillan proximity composites in general is given with emphasis on the theoretical aspects of the physical models used to describe them. A comparison between theory and experiment is presented. We extend the existing theory of McMillan Kondo proximity systems by taking account of the energy dependence as well as the temperature dependence of pair-breaking using the phenomenological model of Müller-Hartmann, Schuh and Zittartz. This extended energy dependent theory is used in the calculation of the transition temperature of the composite, which has its re-entrance reduced when energy dependence is introduced. The specific heat jump of the proximity impurity composite is calculated using the extended theory. It is found that the transition temperature and specific heat jump in the proximity case behave in a similar qualitative fashion to that of bulk superconducting Kondo alloys. The maximum Josephson current of the composite, with energy dependence accounted for, is calculated, as are the thermal conductivity, ultrasonic attenuation and nuclear spin relaxation rate. The discussion of several uncertainties surrounding proximity Kondo composite theory is given throughout the text.