Spectral properties of superconductors with ferromagnetically ordered magnetic impurities

TL;DR

This paper theoretically investigates how ferromagnetically ordered magnetic impurities affect superconductors, revealing impurity band formation, quantum phase transitions, and changes in the nature of the superconducting phase transition.

Contribution

It provides a detailed analysis of the impact of ferromagnetically ordered magnetic impurities on superconducting properties, including impurity band effects and phase transition nature.

Findings

Impurity band formation within the superconducting gap influences superconductivity.

Magnetization exhibits a sudden drop at a critical impurity density or magnetic moment.

Superconducting phase transition shifts from second to first order at high impurity concentrations.

Abstract

We present a comprehensive theoretical study of thermodynamic properties of superconductors with a dilute concentration of magnetic impurities, with focus on how the properties of the superconducting host change if the magnetic moments of the impurities order ferromagnetically. Scattering off the magnetic impurities leads to the formation of a band of Yu-Shiba-Rusinov states within the superconducting energy gap that drastically influences superconductivity. In the magnetically ordered system, the magnetization displays a sudden drop as function of impurity density or magnetic moment amplitude. The drop occurs as the spin-polarized impurity band crosses the Fermi level and is associated with a quantum phase transition first put forward by Sakurai for the single impurity case. Taking into account that the background magnetic field created by the ordered impurity moments enters as a Zeeman shift, we find that the superconducting phase transition changes from second order to first order for high enough impurity concentration.