Proximate Kitaev Quantum Spin Liquid Behaviour in α-RuCl$_3$

Abstract: Topological states of matter such as quantum spin liquids (QSLs) are of great interest because of their remarkable predicted properties including protection of quantum information and the emergence of Majorana fermions. Such QSLs, however, have proven difficult to identify experimentally. The most promising approach is to study their exotic nature via the wave-vector and intensity dependence of their dynamical response in neutron scattering. A major search has centered on iridate materials which are proposed to realize the celebrated Kitaev model on a honeycomb lattice - a prototypical topological QSL system in two dimensions (2D). The difficulties of iridium for neutron measurements have, however, impeded progress significantly. Here we provide experimental evidence that a material based on ruthenium, {\alpha}-RuCl$_3$ realizes the same Kitaev physics but is highly amenable to neutron investigation. Our measurements confirm the requisite strong spin-orbit coupling, and a low temperature magnetic order that matches the predicted phase proximate to the QSL. We also show that stacking faults, inherent to the highly 2D nature of the material, readily explain some puzzling results to date. Measurements of the dynamical response functions, especially at energies and temperatures above that where interlayer effects are manifest, are naturally accounted for in terms of deconfinement physics expected for QSLs. Via a comparison to the recently calculated dynamics from gauge flux excitations and Majorana fermions of the pure Kitaev model we propose {\alpha}-RuCl$_3$ as the prime candidate for experimental realization of fractionalized Kitaev physics.

• The paper demonstrates that α-RuCl3 is a strong candidate for Kitaev quantum spin liquids via neutron scattering measurements.
• The study confirms a layered, 2D magnetic structure with stacking faults clarifying previous low-temperature discrepancies.
• The research reveals deviations from simple spin wave theory, suggesting the need for advanced quantum models for fractionalized excitations.

Overview of Proximate Kitaev Quantum Spin Liquid Behavior in α-RuCl<sub\>3</sub>

This paper addresses the identification of quantum spin liquids (QSLs) through experimental approaches, with a specific focus on the Kitaev model, previously considered prominent in iridate materials. The difficulty encountered in using iridium-based compounds for neutron measurements has propelled this study to examine α-RuCl<sub\>3</sub>, which exhibits similar Kitaev interactions on a honeycomb lattice and is more suitable for neutron scattering analysis.

Key Findings and Experimental Evidence

The study presents α-RuCl<sub\>3</sub> as a viable candidate for realizing Kitaev physics, evidenced by several critical observations:

1. Neutron Scattering Measurements: The analysis successfully demonstrates strong spin-orbit coupling in α-RuCl<sub\>3</sub>, which is consistent with the interactions expected from the Kitaev-Heisenberg (H-K) Hamiltonian. Neutron diffraction confirms the low-temperature magnetic order aligning with models proximal to QSLs.
2. Structural and Magnetic Properties: The study confirms that α-RuCl<sub\>3</sub> exhibits a layered, highly 2D nature, promoting unique magnetic interactions. Stacking faults are declared accountable for previously conflicting findings regarding low temperature magnetic properties.
3. Dynamical Behavior Analysis: The dynamical response of α-RuCl<sub\>3</sub> indicates deconfined phases characterized by excitations resembling those of the pure Kitaev model, such as Majorana fermions and gauge fluxes. Measurements show discrepancies with simple spin wave theory, suggesting a need for more complex quantum mechanical models to fully understand these phenomena.
4. Comparison to Theoretical Models: Upon juxtaposing α-RuCl<sub\>3</sub>'s dynamical behavior against the Kitaev model's predicted Majorana fermion dynamics, α-RuCl<sub\>3</sub>'s candidacy for realizing fractionalized excitations becomes meritable.

Implications and Potential Applications

The experimental evidence provided fortifies α-RuCl<sub\>3</sub>'s status as a prominent candidate for realizing the Kitaev QSL state, suggesting new directions for research into quantum computing technologies. QSLs exhibit properties advantageous for quantum information processing, owing to their robustness against decoherence and potential for non-Abelian anyon states conducive to topological quantum computation.

Speculation on Future Directions

Further investigations could focus on the impact of disorder and doping on the QSL phase in α-RuCl<sub\>3</sub> and related compounds. The non-linearities observed in the neutron scattering data propose the need for advanced computational models integrating beyond-linear spin wave theories or alternative theoretical frameworks. Continued research on single crystals could provide more granular insights into anisotropies and interlayer interactions that affect the QSL state.

In summary, this paper articulates a significant foundation for experimental evidence supporting the fractionalization of excitations in α-RuCl<sub\>3</sub>. These advancements in QSL physics could form the basis for innovative quantum computation technologies exemplifying the deep interconnection between theoretical models and practical materials science.