The Saga of $\alpha$-RuCl$_3$: Parameters, Models, and Phase Diagrams

TL;DR

This paper refines the microscopic spin model of $ ext{α}$-RuCl$_3$, constraining its parameters based on experimental observations, and explores its phase diagram using various theoretical methods to better understand its potential for realizing a Kitaev spin liquid.

Contribution

It provides new constraints on the effective spin model parameters of $ ext{α}$-RuCl$_3$, unifies different approaches, and maps its phase diagram with advanced computational techniques.

Findings

Constraints on exchange parameters narrow the phase space.

Unified view improves understanding of model derivations.

Phase diagrams reveal regions close to spin-liquid behavior.

Abstract

RuCl$_3$ was likely the first ever deliberately synthesized ruthenium compound, following the discovery of the $_{44}$Ru element in 1844. For a long time it was known as an oxidation catalyst, with its physical properties being discrepant and confusing, until a decade ago when its allotropic form $\alpha$-RuCl$_3$ rose to exceptional prominence. This "re-discovery" of $\alpha$-RuCl$_3$ has not only reshaped the hunt for a material manifestation of the Kitaev spin liquid, but it has opened the floodgates of theoretical and experimental research in the many unusual phases and excitations that the anisotropic-exchange magnets as a class of compounds have to offer. Given its importance for the field of Kitaev materials, it is astonishing that the low-energy spin model that describes this compound and its possible proximity to the much-desired spin-liquid state is still a subject of significant debate ten years later. In the present study, we argue that the existing key phenomenological observations put strong natural constraints on the effective microscopic spin model of $\alpha$-RuCl$_3$, and specifically on its spin-orbit-induced anisotropic-exchange parameters that are responsible for the non-trivial physical properties of this material. These constraints allow one to focus on the relevant region of the multi-dimensional phase diagram of the $\alpha$-RuCl$_3$ model, suggest an intuitive description of it via a different parametrization of the exchange matrix, offer a unifying view on the earlier assessments of its parameters, and bring closer together several approaches to the derivation of anisotropic-exchange models. We explore extended phase diagrams relevant to the $\alpha$-RuCl$_3$ parameter space using quasi-classical, Luttinger-Tisza, exact diagonalization, and density-matrix renormalization group methods, demonstrating a remarkably c... (arxiv cutoff; for the rest, see the paper)