Kondo "underscreening" cloud: spin-spin correlations around a partially screened magnetic impurity

TL;DR

This paper investigates the spatial spin correlations around a partially screened spin-1 magnetic impurity in a metal, revealing enhanced and sign-changing correlations due to the underscreened Kondo effect, with implications for understanding magnetic impurities.

Contribution

It provides the first detailed analysis of spatial spin correlations in the underscreened Kondo effect, highlighting differences from the fully screened case and confirming predictions from an effective ferromagnetic Kondo model.

Findings

Underscreening leads to more pronounced spin correlations.

Correlation decay is weaker with logarithmic corrections.

Sign change in spin correlator indicates ferromagnetic correlations.

Abstract

We consider the spatial spin correlations around a partially screened spin-1 magnetic moment in a metal exhibiting the underscreened Kondo effect. We find that the underscreening of the impurity spin results in spatial spin correlations that are more pronounced as compared to the fully screened Kondo effect; their power-law decay is weaker because of characteristic logarithmic corrections at large distances. The spin correlator also changes sign as a function of distance to the impurity allowing for ferromagnetic correlations between conduction electron spin density and the local moment. The numerical findings are shown to be in agreement with the predictions deriving from an effective ferromagnetic Kondo Hamiltonian.