Different behaviour of magnetic impurities in crystalline and amorphous states of superconductors

It has been observed that the effect of magnetic impurities in a superconductor is drastically different depending on whether the host superconductor is in the crystalline or the amorphous state. Based on the recent theory of Kim and Overhauser (KO), it is shown that as the system is getting disordered, the initial slope of the T_c depression is decreasing by a factor √ℓ/ξ₀, when the mean free path ℓ becomes smaller than the BCS coherence length ξ₀, which is in agreement with experimental findings. In addition, for a superconductor in a crystalline state in the presence of magnetic impurities the superconducting transition temperature T_c drops sharply from about 50% of T_c0 (for a pure system) to zero near the critical impurity concentration. This pure limit behaviour was indeed found by Roden and Zimmermeyer in crystalline Cd. Recently, Porto and Parpia have also found the same pure limit behaviour in superfluid He-3 in aerogel, which may be understood within the framework of the KO theory.