Spin waves in Na$_2$Co$_2$TeO$_6$ studied by high-frequency/high-field ESR: Successes and failures of the triple-$\mathbf{q}$ model

TL;DR

This study uses high-frequency ESR to investigate spin wave excitations in Na$_2$Co$_2$TeO$_6$, revealing complex behaviors that challenge existing triple-q models and suggest the importance of interlayer interactions.

Contribution

It provides detailed ESR measurements of Na$_2$Co$_2$TeO$_6$ and critically evaluates the triple-q model, highlighting its limitations and the need for considering interlayer effects.

Findings

Observation of multiple spin wave modes in different magnetic phases.

Identification of a field-induced phase transition at 4.7 T.

Failure of the triple-q model to fully explain the softening mode.

Abstract

The Kitaev candidate material Na$_2$Co$_2$TeO$_6$ is proposed to be proximate to a quantum spin liquid state but a suitable spin model and the nature of its ground states are still under debate. Our high-frequency/high-field electron spin resonance spectroscopy studies of Na$_2$Co$_2$TeO$_6$ single-crystals under in-plane and out-of-plane magnetic fields elucidate the ground state by investigating its low-energy spin wave excitations. Several excitation modes are observed in the low-field phase and in the phases induced by $B\parallel a^*$. In addition, the spectra exhibit a frequency-independent feature at the phase boundary connected to the putative quantum phase transition. For magnetic fields applied along the $c$ axis, the observation of three distinct spin wave modes in the antiferromagnetic (AFM) ground state reveals a previously unresolved splitting of the zero-field excitation gap into $\Delta = 211\,$GHz and $\Delta_2 = 237\,$GHz. The softening of one of these modes evidences a field-induced phase transition at $B_{\rm c1} = 4.7\,$T, which is corroborated by a clear anomaly in the isothermal magnetization. Spin wave calculations based on the extended Heisenberg-Kitaev model exclude a zigzag ground state of the AFM phase. A triple-q spin configuration correctly predicts two spin wave modes, but fails to reproduce the softening mode. Our analysis shows that the triple-q ground state model of Na$_2$Co$_2$TeO$_6$ is incomplete and suggests the relevance of interlayer interactions.