Magnetic anisotropy effects on quantum impurities in superconducting host

TL;DR

This paper investigates how magnetic anisotropy influences sub-gap excitations caused by quantum impurities in superconductors, revealing complex phase diagrams and potential spectroscopic detection methods for atomic-scale magnetism.

Contribution

It provides new insights into the interplay of Kondo screening, superconductivity, and magnetic anisotropy, including detailed phase diagrams and impurity coupling effects.

Findings

Complex ground-state phase diagrams for high-spin impurities.

Detection of Zeeman splitting as a spectroscopic tool.

Behavior of sub-gap states under impurity coupling.

Abstract

We study the magnetic anisotropy effects on the localized sub-gap excitations induced by quantum impurities coupled to a superconducting host. We establish the ground-state phase diagrams for single-channel and two-channel high-spin Kondo impurities; they unveil surprising complexity that results from the (multi-stage) Kondo screening in competition with the superconducting correlations and the magnetic anisotropy splitting of the spin multiplets. We discuss the possibility of detecting the Zeeman splitting of the sub-gap states, which would provide an interesting spectroscopic tool for studying the magnetism on the single-atom level. We also study the problem of two impurities coupled by the Heisenberg exchange interaction, and we follow the evolution of the sub-gap states for both antiferromagnetic and ferromagnetic coupling. For sufficiently strong antiferromagnetic coupling, the impurities bind into a singlet state that is non-magnetic, thus the sub-gap states move to the edge of the gap and can no longer be discerned. For ferromagnetic coupling, some excited states remain present inside the gap.