Crystal structure and magnetism in $\alpha$-RuCl3: an ab-initio study

TL;DR

This study uses ab-initio calculations to clarify the structure and magnetic interactions in $ ext{α-RuCl}_3$, revealing how spin-orbit coupling and electronic correlations influence its potential as a Kitaev spin liquid candidate.

Contribution

It provides a detailed 3D structural analysis of $ ext{α-RuCl}_3$ including SOC and correlations, and explores how these factors affect magnetic interactions and the realization of Kitaev physics.

Findings

SOC suppresses Ru atom dimerization, making the honeycomb structure more ideal.

Kitaev exchange depends strongly on Ru-Ru distance and Cl positions.

Electronic correlations alter the optimized 3D structure's space group symmetry.

Abstract

$\alpha$-RuCl$_3$ has been proposed recently as an excellent playground for exploring Kitaev physics on a two-dimensional (2D) honeycomb lattice. However, structural clarification of the compound has not been completed, which is crucial in understanding the physics of this system. Here, using {\it ab-initio} electronic structure calculations, we study a full three dimensional (3D) structure of $\alpha$-RuCl$_3$ including the effects of spin-orbit coupling (SOC) and electronic correlations. Three major results are as follows; i) SOC suppresses dimerization of Ru atoms, which exists in other Ru compounds such as isostructural Li$_2$RuO$_3$, and making the honeycomb closer to an ideal one. ii) The nearest-neighbor Kitaev exchange interaction between the $j_{\rm eff}$=1/2 pseudospin depends strongly on the Ru-Ru distance and the Cl position, originating from the nature of the edge-sharing geometry. iii) The optimized 3D structure without electronic correlations has $P{\bar 3}1m$ space group symmetry independent of SOC, but including electronic correlation changes the optimized 3D structure to either $C2/m$ or $Cmc2_1$ within 0.1 meV per formula unit (f.u.) energy difference. The reported $P3_112$ structure is also close in energy. The interlayer spin exchange coupling is a few percent of in-plane spin exchange terms, confirming $\alpha$-RuCl$_3$ is close to a 2D system. We further suggest how to increase the Kitaev term via tensile strain, which sheds new light in realizing Kitaev spin liquid phase in this system.