Theory of the field-revealed Kitaev spin liquid

Jacob S Gordon,Andrei Catuneanu,Erik S Sørensen,Hae-Young Kee

doi:10.1038/s41467-019-10405-8

Jacob S Gordon, Andrei Catuneanu + Show 2 more

Open Access

https://doi.org/10.1038/s41467-019-10405-8

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Abstract

Elementary excitations in entangled states such as quantum spin liquids may exhibit exotic statistics different from those obeyed by fundamental bosons and fermions. Non-Abelian anyons exist in a Kitaev spin liquid—the ground state of an exactly solvable model. A smoking-gun signature of these excitations, namely a half-integer quantized thermal Hall conductivity, was recently reported in α-RuCl3. While fascinating, a microscopic theory for this phenomenon remains elusive because the pure Kitaev model cannot display this effect in an intermediate magnetic field. Here we present a microscopic theory of the Kitaev spin liquid emerging between the low- and high-field states. Essential to this result is an antiferromagnetic off-diagonal symmetric interaction which allows the Kitaev spin liquid to protrude from the ferromagnetic Kitaev limit under a magnetic field. This generic model displays a strong field anisotropy, and we predict a wide spin liquid regime when the field is perpendicular to the honeycomb plane.

Highlights

Elementary excitations in entangled states such as quantum spin liquids may exhibit exotic statistics different from those obeyed by fundamental bosons and fermions
Several honeycomb materials have been suggested as Kitaev spin liquid (KSL) candidates, namely Mott insulators with strong spin-orbit coupling (SOC) featuring 4d or 5d transition metal elements[5,6,7,8,9]
The combination of crystal field splitting and strong spin-orbit coupling leads to a model based on pseudospin-12 local moments with bond-dependent interactions

Summary

Introduction

Elementary excitations in entangled states such as quantum spin liquids may exhibit exotic statistics different from those obeyed by fundamental bosons and fermions. While the observation of a half-integer quantized thermal Hall conductivity is the first experimental evidence of charge-neutral non-Abelian anyons in spin systems, a microscopic theory describing their appearance under a field in α-RuCl3 is missing. This is because, if the dominant interaction in α-RuCl3 is the ferromagnetic (FM) Kitaev term (as shown through ab-initio studies[25,26] and spin wave analysis36), the FM Kitaev phase is almost immediately destroyed, and the polarized state appears in an applied field[38,39,40] with no intervening phase. We introduce a microscopic theory with a brief review of the generic nearest neighbor spin model for spin-orbit coupled honeycomb materials, appropriate for α-RuCl3

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