Role of paramagnetic inpurities in the barrier region for superconducting tunnelling

A microscopic theory for the tunnelling current through a superconductor-insulator-superconductor junction containing a pair of paramagnetic impurities is developed. A general interaction between the impurity spins is considered and the tunnelling current is calculated by considering both the elastic and inelastic scattering of conduction electrons with the impurity pair. It is found that the single particle tunnelling current exhibits discontinuities as a function of the bias voltage. The positions of discontinuities occur at voltages which depend on the energy gap and the strength of the impurity-impurity interaction.