Field-Angle-Resolved Specific Heat in Na$_2$Co$_2$TeO$_6$: Evidence against Kitaev Quantum Spin Liquid

TL;DR

This study investigates the low-temperature specific heat of Na$_2$Co$_2$TeO$_6$ under magnetic field rotation, providing thermodynamic evidence that contradicts the presence of a Kitaev quantum spin liquid state in this material.

Contribution

The paper presents field-angle-dependent specific heat measurements of NCTO, showing results inconsistent with Kitaev model predictions and suggesting magnon excitations instead of Majorana quasiparticles.

Findings

Field-angle dependence of $C/T$ shows minima along bond directions

Results indicate nodeless, fully-gapped excitations

Evidence suggests gapped magnon excitations dominate, not Majorana quasiparticles

Abstract

Kitaev quantum spin liquids (KSLs) in layered honeycomb magnets are known to host Majorana quasiparticles, whose excitations depend strongly on the direction of the applied magnetic field. In the high-field phase of $\alpha$-RuCl$_3$, specific heat measurements have revealed characteristic field-angle dependence of low-energy excitations consistent with the Kitaev model, providing bulk evidence for the KSL state. Here we present low-temperature measurements of specific heat $C(T)$ for another KSL candidate $\textrm{Na}_2\textrm{Co}_2\textrm{Te}\textrm{O}_6$ (NCTO) under field rotation within the honeycomb plane. Above the critical field of antiferromagnetic order, the field-angle dependence of $C/T$ exhibits minima along the bond directions, contrasting with the maxima observed in the KSL state of $\alpha$-RuCl$_3$. Our analysis indicates nodeless, fully-gapped excitations, which are inconsistent with the angle-dependent Majorana excitations with gapless nodes predicted by the Kitaev model. These findings suggest that low-energy excitations in NCTO are governed by gapped magnon excitations rather than Majorana quasiparticles, providing thermodynamic evidence against a KSL state.