Yu-Shiba-Rusinov states in real metals

TL;DR

This paper introduces a zero-bandwidth model to better understand Yu-Shiba-Rusinov states in real metals, accounting for complex impurity effects often neglected in classical models, and compares results with existing numerical methods.

Contribution

The paper presents a new zero-bandwidth model that includes higher spins, multiple channels, and anisotropy, providing analytical and numerical insights into YSR states beyond classical approximations.

Findings

Excellent agreement with numerical-renormalization-group results in certain regimes.

Phase diagrams as functions of pairing strength and anisotropy are obtained.

Model clarifies the behavior of YSR states in transition-metal impurities.

Abstract

Theoretical descriptions of Yu-Shiba-Rusinov (YSR) states induced by magnetic impurities inside the gap of a superconductor typically rely on a classical spin model or are restricted to spin-1/2 quantum spins. These models fail to account for important aspects of YSR states induced by transition-metal impurities, including the effects of higher quantum spins coupled to several conduction-electron channels, crystal or ligand-field effects, and magnetic anisotropy. We introduce and explore a zero-bandwidth model, which incorporates these aspects, is readily solved numerically, and analytically tractable in several limiting cases. The principal simplification of the model is to neglect Kondo renormalizations of the exchange couplings between impurity spin and conduction electrons. Nevertheless, we find excellent correspondence in those cases, in which we can compare our results to existing numerical-renormalization-group calculations. We apply the model to obtain and understand phase diagrams as a function of pairing strength and magnetic anisotropy as well as subgap excitation spectra. The single-channel case is most relevant for transition-metal impurities embedded into metallic coordination complexes on superconducting substrates, while the multi-channel case models transition-metal adatoms.