Different Behavior of Magnetic Impurities in Crystalline and Ammorphous States of Superconductors

TL;DR

This paper explains how magnetic impurities affect superconductors differently depending on whether they are crystalline or amorphous, supported by recent theoretical and experimental findings.

Contribution

It applies the Kim and Overhauser theory to describe the impact of disorder on magnetic impurity effects in superconductors, aligning with recent experimental results.

Findings

Decreased initial slope of Tc depression with increased disorder

Sharp Tc drop in crystalline superconductors near critical impurity concentration

Agreement of theory with experimental observations in Cd and superfluid He-3

Abstract

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 (KO), it is shown that as the system is getting disordered, the initial slope of the $T_{c}$ depression is decreasing by a factor $\sqrt{\ell/\xi_{0}}$, when the mean free path $\ell$ becomes smaller than the BCS coherence length $\xi_{0}$, 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 {\sl pure limit behavior} was indeed found by Roden and Zimmermeyer in crystalline Cd. Recently, Porto and Parpia have also found the same {\sl pure limit behavior} in superfluid He-3 in aerogel, which may be understood within the framework of the KO theory.