Susceptibility anisotropy and its disorder evolution in models for Kitaev materials

TL;DR

This paper models the anisotropic magnetic response of Kitaev materials like $ ext{RuCl}_3$ and $ ext{Na}_2 ext{IrO}_3$, explaining experimental susceptibility anisotropy and its temperature dependence, and predicts effects of magnetic doping.

Contribution

It introduces a detailed Heisenberg-Kitaev-$ ext{Gamma}$ model at elevated temperatures to explain susceptibility anisotropy in Kitaev materials, including effects of magnetic dilution.

Findings

Reproduces experimental susceptibility anisotropy in $ ext{RuCl}_3$.

Predicts doping effects on susceptibility evolution.

Highlights importance of off-diagonal $ ext{Gamma}_1$ coupling.

Abstract

Mott insulators with strong spin-orbit coupling display a strongly anisotropic response to applied magnetic fields. This applies in particular to Kitaev materials, with $\alpha$-RuCl$_3$ and Na$_2$IrO$_3$ representing two important examples. Both show a magnetically ordered zigzag state at low temperatures, and considerable effort has been devoted to properly modeling these systems in order to identify routes towards realizing a quantum spin liquid. Here, we investigate the relevant Heisenberg-Kitaev-$\Gamma$ model primarily at elevated temperatures, focusing on the characteristic anisotropy between the in-plane and out-of-plane uniform susceptibility. For $\alpha$-RuCl$_3$, we find that the experimentally observed anisotropy, including its temperature dependence, can be reproduced by combining a large off-diagonal $\Gamma_1$ coupling with a moderate $g$-factor anisotropy. Moreover, we study in detail the effect of magnetic dilution and provide predictions for the doping evolution of the temperature-dependent susceptibilities.