[Paper Review] Excitations in the ordered and paramagnetic states of honeycomb magnet Na2Co2TeO6

Weiliang Yao, Kazuki Iida+2 more|arXiv (Cornell University)|Mar 1, 2022

Advanced Condensed Matter Physics4 citations

TL;DR

This study uses inelastic neutron scattering on high-quality single crystals to map the full temperature-dependent magnetic excitation spectrum of the honeycomb magnet Na2Co2TeO6. It reveals a distinct third-nearest-neighbor spin interaction that acts as a 'soft link' in the ordered state, with six non-overlapping spin-wave branches that contradict all existing models. In the paramagnetic phase, short-range zigzag-type correlations persist, revealing key instabilities and constraining theoretical models for Kitaev-like magnets.

ABSTRACT

Na2Co2TeO6 is a proposed approximate Kitaev magnet, yet its actual magnetic interactions are elusive due to a lack of knowledge on the full excitation spectrum. Here, using inelastic neutron scattering and single crystals, we determine the system's temperature-dependent magnetic excitations over the entire Brillouin zone. Without committing to specific models, we unveil a distinct signature of the third-nearest-neighbor coupling in the spin waves, which signifies the associated distance as an emerging "soft link" in the ordered state. The presence of at least six non-overlapping spin-wave branches is at odds with all models proposed to date. Above the ordering temperature, persisting dynamic correlations can be described by equal-time magnetic structure factors of a hexagonal cluster, which reveal the leading instabilities. Our result sets definitive constraint on theoretical models for Na2Co2TeO6 and provides new insight for the materialization of the Kitaev model.

Motivation & Objective

• To determine the complete magnetic excitation spectrum of Na2Co2TeO6 across the entire 2D Brillouin zone.
• To resolve the nature of competing magnetic interactions in this proposed Kitaev magnet.
• To identify the origin of persistent short-range correlations above the ordering temperature.
• To provide definitive constraints for theoretical models of Kitaev-like magnetism in 3d transition metal systems.

Proposed method

• Performed inelastic neutron scattering (INS) on high-quality single crystals of Na2Co2TeO6 to map spin excitations over the full 2D Brillouin zone.
• Measured temperature-dependent spin wave dispersions from 5 K (ordered state) to above TN ∼26.5 K (paramagnetic state).
• Used powder-averaged INS data to validate consistency with prior powder measurements.
• Applied spin-wave theory with the H-K-Γ model including Heisenberg (J1, J2, J3), Kitaev (K), and off-diagonal (Γ, Γ′) interactions to fit the data.
• Calculated equal-time magnetic structure factors for various spin arrangements to compare with experimental paramagnetic correlations.
• Performed phonon background correction and quasielastic scattering analysis to isolate dynamic spin correlations.

Experimental results

Research questions

• RQ1 What is the full temperature-dependent spin excitation spectrum of Na2Co2TeO6 across the 2D Brillouin zone?
• RQ2 What is the origin of the six non-overlapping spin-wave branches observed experimentally?
• RQ3 How do dynamic correlations persist above TN, and what spin arrangement do they reflect?
• RQ4 Which magnetic interaction terms—especially beyond nearest-neighbor—best describe the observed spin waves?
• RQ5 How do the experimental results constrain existing theoretical models of the Kitaev magnetism in Na2Co2TeO6?

Key findings

• The third-nearest-neighbor Heisenberg interaction (J3) alone provides a highly accurate description of the low-energy spin waves, indicating it acts as an emerging 'soft link' in the ordered state.
• The presence of at least six non-overlapping spin-wave branches contradicts all previously proposed models, which predict fewer branches.
• Above TN ∼26.5 K, dynamic correlations persist and are best described by equal-time structure factors of a hexagonal cluster with zigzag-type spin arrangements.
• The paramagnetic state exhibits short-range magnetic order consistent with fluctuating zigzag-type correlations, indicating leading instabilities toward long-range order.
• The experimental data rule out all existing H-K-Γ model parameters reported in prior studies, including those from Songvilay et al., Lin et al., and Kim et al., due to poor spectral agreement.
• The results provide definitive constraints for theoretical models and deepen the understanding of competing interactions in Kitaev candidate materials.