Spin relaxation of a spin-1/2 Kondo impurity in a superconductor

Abstract

The behavior of a spin-1/2 Kondo impurity coupled to a superconducting metal by thes-d exchange interaction is analyzed. The impurity spin correlation function is expressed by the spin relaxation spectrum, which was calculated within the Tomonaga approximation, thereby taking care of the infrared singularities. The static spin correlation function χ 0 is self-consistently obtained from the relaxation spectrum. When the characteristic Kondo temperature is of the order of or larger than the superconducting gap, χ 0 is finite at zero temperature, showing that the ground state is a nonmagnetic complex with total spin 0. This result is related to previous calculations on bulk properties of the superconductor, such as the reduction of the phase transition temperature and of the specific heat jump due to magnetic impurities. The impurity relaxation rate is discussed in connection with retardation effects, and analogies to the relaxation of a spin in a normal metal with an applied magnetic field are pointed out.