Effect of Magnetic Impurities in Crystalline and Amorphous States of Superconductors

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It has been observed that the effect of magnetic impurities in a superconductor is drastically different depending on whether the host superconductor is in a crystalline or an amorphous state. Based on the recent theory of Kim and Overhauser, it is shown that as the system is getting disordered, the initial slope of the Tc 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 pure crystalline state the superconducting transition temperature Tc drops sharply from about 50% of Tc₀ (for a pure system) to zero near the critical impurity concentration. This pure limit behavior was indeed found by Roden and Zimmermeyer in crystalline Cd.