Spin-orbit-entangled nature of magnetic moments and Kitaev magnetism in layered halides $\alpha$-RuX$_3$ (X = Cl, Br, I)

TL;DR

This study compares the electronic structures of layered halide compounds $ ext{RuCl}_3$, $ ext{RuBr}_3$, and $ ext{RuI}_3$, highlighting how heavier halogens influence spin-orbit entanglement and Kitaev interactions, with $ ext{RuBr}_3$ showing potential for quantum spin liquid phases.

Contribution

It provides a comparative analysis of the electronic structures and magnetic interactions in $ ext{RuCl}_3$, $ ext{RuBr}_3$, and $ ext{RuI}_3$, emphasizing the role of heavier halogens in enhancing Kitaev interactions.

Findings

$ ext{RuBr}_3$ can host Kitaev spin liquid phases.

Heavier halogens promote stronger spin-orbit entanglement.

Electronic structures are similar but with notable differences in magnetic interactions.

Abstract

$\alpha$-RuCl$_3$ has been extensively studied recently because of potential bond-dependent Kitaev magnetic exchange interactions and the resulting quantum spin liquid phase that can be realized therein. It has been known that the covalency between Ru 4$d$- and Cl $p$-orbitals is crucial to induce large Kitaev interactions in this compound, therefore replacing Cl into heavier halogen elements such as Br or I can be a promising way to promote the Kitaev interaction even further. In a timely manner, there have been reports on synthesis of $\alpha$-RuBr$_3$ and $\alpha$-RuI$_3$, which are expected to host the same spin-orbit-entangled orbitals and Kitaev exchange interactions with $\alpha$-RuCl$_3$. Here in this work we investigate electronic structures of $\alpha$-RuCl$_3$, $\alpha$-RuBr$_3$, and $\alpha$-RuI$_3$ in a comparative fashion, focusing on the cooperation of the spin-orbit coupling and on-site Coulomb repulsions to realize the spin-orbit-entangled pseudospin-1/2 at Ru sites. We further estimate magnetic exchange interactions of all three compounds, showing that $\alpha$-RuBr$_3$ can be promising candidates to realize Kitaev spin liquid phases in solid-state systems.