Bond disorder in extended Heisenberg-Kitaev models: Spin textures and in-gap states in the high-field regime

TL;DR

This paper investigates how bond disorder affects spin textures and in-gap states in extended Heisenberg-Kitaev models under high magnetic fields, revealing impurity-induced phenomena relevant for materials like $ ext{RuCl}_3$.

Contribution

It introduces a detailed analysis of disorder effects on spin textures and in-gap states in the high-field regime of extended Heisenberg-Kitaev models, combining T-matrix and spin-wave methods.

Findings

Disorder smears the phase transition and creates impurity bands.

Impurity states fill the bulk spin gap at high fields.

Results align with heat-transport and NMR experimental data.

Abstract

We study the effect of bond disorder in extended Heisenberg-Kitaev models on the honeycomb lattice, relevant for materials such as $\alpha$-RuCl$_3$, in the semiclassical limit using a combination of T-matrix and real-space spin-wave approaches. Focusing on the regime of large applied magnetic field, we discuss two distinct but related disorder-induced phenomena, namely spin textures in the vicinity of isolated impurities and magnetic excitations below the bulk gap. A finite impurity concentration smears the field-tuned phase transition and turns the isolated in-gap states into impurity bands. As a result, there is a large field regime above the bulk transition into the high-field phase where impurity-induced states fill the bulk spin gap. We illustrate the field dependence of these in-gap states for parameters relevant for $\alpha$-RuCl$_3$, and we connect our results to heat-transport and NMR data which indicated their presence.