Triple-q order in Na$_2$Co$_2$TeO$_6$ from proximity to hidden-SU(2)-symmetric point

TL;DR

This paper identifies a triple-q magnetic order in Na$_2$Co$_2$TeO$_6$ as arising from proximity to a hidden-SU(2)-symmetric point in an extended Heisenberg-Kitaev-Gamma model, explaining its magnetic excitation spectrum.

Contribution

It reveals the triple-q order in Na$_2$Co$_2$TeO$_6$ as a consequence of near proximity to a hidden-SU(2) point, linking experimental data to theoretical models.

Findings

Na$_2$Co$_2$TeO$_6$ exhibits a triple-q magnetic order.

The order arises from a Néel antiferromagnet via a duality transformation.

The magnetic excitation spectrum shows high symmetry and distinct dispersive features.

Abstract

In extended Heisenberg-Kitaev-Gamma-type spin models, hidden-SU(2)-symmetric points are isolated points in parameter space that can be mapped to pure Heisenberg models via nontrivial duality transformations. Such points generically feature quantum degeneracy between conventional single-q and exotic multi-q states. We argue that recent single-crystal inelastic neutron scattering data place the honeycomb magnet Na$_2$Co$_2$TeO$_6$ in proximity to such a hidden-SU(2)-symmetric point. The low-temperature order is identified as a triple-q state arising from the N\'eel antiferromagnet with staggered magnetization in the out-of-plane direction via a 4-sublattice duality transformation. This state naturally explains various distinctive features of the magnetic excitation spectrum, including its surprisingly high symmetry and the dispersive low-energy and flat high-energy bands. Our result demonstrates the importance of bond-dependent exchange interactions in cobaltates, and illustrates the intriguing magnetic behavior resulting from them.