Excitations in the field-induced quantum spin liquid state of alpha-RuCl3 (1706.07003v1)

Abstract: The Kitaev model on a honeycomb lattice predicts a paradigmatic quantum spin liquid (QSL) exhibiting Majorana Fermion excitations. The insight that Kitaev physics might be realized in practice has stimulated investigations of candidate materials, recently including alpha-RuCl3. In all the systems studied to date, non-Kitaev interactions induce magnetic order at low temperature. However, in-plane magnetic fields of roughly 8 Tesla suppress the long-range magnetic order in alpha-RuCl3 raising the intriguing possibility of a field-induced QSL exhibiting non-Abelian quasiparticle excitations. Here we present inelastic neutron scattering in alpha-RuCl3 in an applied magnetic field. At a field of 8 Tesla, the spin waves characteristic of the ordered state vanish throughout the Brillouin zone. The remaining single dominant feature of the response is a broad continuum centered at the Gamma point, previously identified as a signature of fractionalized excitations. This provides compelling evidence that a field-induced QSL state has been achieved.

• The paper presents evidence that applied in-plane magnetic fields (~8T) suppress magnetic order and induce a quantum spin liquid state in α-RuCl₃.
• It reveals a broad continuum at the Γ point through inelastic neutron scattering, indicative of fractionalized excitations predicted by the Kitaev model.
• The work aligns experimental findings with theoretical models, highlighting its potential implications for topological quantum computing.

Field-Induced Quantum Spin Liquid State in α-RuCl₃

The paper investigates the field-induced quantum spin liquid (QSL) state in the material α-RuCl₃, providing significant evidence supporting the existence of fractionalized excitations, akin to those predicted by the Kitaev model on a honeycomb lattice. The Kitaev model, which predicts a unique QSL with Majorana fermion and gauge flux excitations, serves as the theoretical underpinning for this paper. While the material exhibits significant non-Kitaev interactions that lead to magnetic order at low temperatures, the application of in-plane magnetic fields of approximately 8 Tesla suppresses this order, revealing a field-induced QSL state.

Experimental Findings

Through inelastic neutron scattering (INS) experiments, the paper identifies the suppression of spin waves at 8 Tesla, showcasing a broad continuum centered at the Γ point, which is associated with fractionalized excitations. These findings indicate the transformation into a QSL state. The experiments conducted on high-quality single crystals of α-RuCl₃ utilized fields oriented along specific crystallographic directions. Notably, the disappearance of magnetic Bragg peaks associated with zigzag spin order corroborates this transition to a disordered state under field influence.

The excitation spectrum evolution, mapped up to 8 Tesla, indicates significant modifications with the spin-wave suppression at high fields and the presence of a gapped and intense scattering column at the Γ point. The paper finds a notable resemblance between the 8 T spectrum at 2 K and the zero-field response above T_N, indicating similar excitation dynamics across these parameter spaces.

Analytical Insights

The analysis suggests an intriguing correspondence between the observed high-energy continua and the theoretical predictions for a Kitaev QSL. While the full Hamiltonian descriptions remain uncertain, the paper compares observations to Kitaev QSL modeling, maintaining reasonable alignment with predictions of broad energy scattering for both ferromagnetic (FM) and antiferromagnetic (AFM) Kitaev scenarios. Furthermore, the shift in response continuity reflects time-reversal symmetry breaking under high fields.

Implications and Future Directions

The results presented here contribute significantly to the understanding of field-induced QSL states, aligning with theoretical predictions for non-Abelian quasiparticles carrying topological quantum computational potential. The measurement and observation of topologically protected edge states or related excitations are suggested as future research directions to verify and explore these phases' unique characteristics.

Overall, this paper enhances the exploration of quantum spin liquids, suggesting further inspection of α-RuCl₃ under various field orientations and thermal conditions could illuminate the presence of exotic states and transitions. Anticipated future work might include dissecting the contributions of non-Kitaev interactions and consolidating theoretical models aligning these emergent behaviors with specific field-induced excitations. Experimental observation of non-Abelian statistics in quasiparticle excitations, as hypothesized, could serve as a compelling affirmation of the model's predictions and pave the way for advancements in topological quantum computing mechanisms.