Field dependent specific heat measurements of the Kitaev quantum spin liquid candidate Na₂Co₂TeO₆

The Kitaev quantum spin liquid has attracted much attention because the ground state is exactly solvable, and its elementary excitations can be described by Majorana quasiparticles. The candidates have been recently suggested for honeycomb magnets with 3d electrons such as cobalt-based materials. Recent studies of the layered honeycomb magnet Na₂Co₂TeO₆ with 3d electrons have shown that the antiferromagnetic phase is suppressed by an in-plane magnetic field application and changes to a spin-disorder phase above 7.5 T. Whether this field-induced spin-disorder phase is a Kitaev spin liquid or not is an unresolved issue in condensed-matter physics.

Here we study low-energy excitations in the field-induced phase of Na₂Co₂TeO₆ from the specific heat measurements using single crystals. In the Kitaev model, the Majorana quasiparticle excitations exhibit a strongly field-angle dependent gap. In our previous specific heat studies of α-RuCl₃, which is a 4d candidate of the Kitaev spin liquid, we have observed 6-fold oscillations of Majorana gap in the field-angle dependence within the honeycomb plane, fully consistent with the Kitaev spin liquid. In the present Na₂Co₂TeO₆, we find the gapped behavior at low temperatures, with significant anisotropy between two field directions; parallel and perpendicular to the Co-Co bond directions. However, the anisotropic behavior is quite different from that of α-RuCl₃. We will discuss the nature of the high-field phase of Na₂Co₂TeO₆ based on our results.