A Kondo route to spin inhomogeneities in the honeycomb Kitaev model

TL;DR

This paper investigates how paramagnetic impurities affect the gapless spin-liquid phase of the honeycomb Kitaev model, revealing stable fixed points, topological changes, and similarities to the two-channel Kondo effect.

Contribution

It introduces a detailed analysis of impurity effects in the Kitaev model, including fixed points, topological flux changes, and a comparison to the two-channel Kondo problem, with new insights into impurity-induced phenomena.

Findings

Stable weak and strong coupling fixed points identified.

Strong coupling leads to nontrivial topology with Z2 flux.

Weak to strong coupling transition occurs near a topological phase transition.

Abstract

Paramagnetic impurities in a quantum spin-liquid can result in Kondo effects with highly unusual properties. We have studied the effect of locally exchange-coupling a paramagnetic impurity with the spin-1/2 honeycomb Kitaev model in its gapless spin-liquid phase. The (impurity) scaling equations are found to be insensitive to the sign of the coupling. The weak and strong coupling fixed points are stable, with the latter corresponding to a noninteracting vacancy and an interacting, spin-1 defect for the antiferromagnetic and ferromagnetic cases respectively. The ground state in the strong coupling limit in both cases has a nontrivial topology associated with a finite Z2 flux at the impurity site. For the antiferromagnetic case, this result can be obtained straightforwardly owing to the integrability of the Kitaev model with a vacancy. The strong-coupling limit of the ferromagnetic case is however nonintegrable, and we address this problem through exact-diagonalization calculations with finite Kitaev fragments. Our exact diagonalization calculations indicate that that the weak to strong coupling transition and the topological phase transition occur rather close to each other and are possibly coincident. We also find an intriguing similarity between the magnetic response of the defect and the impurity susceptibility in the two-channel Kondo problem.