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 α-RuCl3, 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 Na2Co2TeO6 (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 α-RuCl3. 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.
@article{arxiv.2410.18449,
title = {Field-Angle-Resolved Specific Heat in Na$_2$Co$_2$TeO$_6$: Evidence against Kitaev Quantum Spin Liquid},
author = {Shengjie Fang and Kumpei Imamura and Yuta Mizukami and Ryuichi Namba and Kota Ishihara and Kenichiro Hashimoto and Takasada Shibauchi},
journal= {arXiv preprint arXiv:2410.18449},
year = {2025}
}