Unusual heat transport of the Kitaev material Na$_2$Co$_2$TeO$_6$: putative quantum spin liquid and low-energy spin excitations

TL;DR

This study investigates how magnetic fields influence heat transport in Na$_2$Co$_2$TeO$_6$, revealing field-tunable spin excitations and evidence for a quantum spin liquid state with dominant phononic heat conduction.

Contribution

It demonstrates that magnetic fields can significantly modify thermal conductivity and spin excitations in Na$_2$Co$_2$TeO$_6$, supporting its characterization as a Kitaev-like quantum spin liquid.

Findings

Magnetic fields beyond 10 T enhance thermal conductivity and create a double-peak structure.

Heat transport is mainly phononic and affected by magnetic scattering.

Evidence suggests the presence of a quantum spin liquid state with strong spin fluctuations.

Abstract

We studied the field dependent thermal conductivity ($\kappa$) of Na$_2$Co$_2$TeO$_6$, a compound considered as the manifestation of the Kitaev model based on the high-spin $d^7$ Co$^{2+}$ ions. We found that in-plane magnetic fields beyond a critical value $B_c \approx$~10 T are able to drastically enhance $\kappa$ at low temperatures, resulting in a double-peak structure of $\kappa(T)$ that closely resembles the behavior of $\alpha$-RuCl$_3$. This result suggests that heat transport in Na$_2$Co$_2$TeO$_6$ is primarily phononic, and it is strongly affected by scattering from magnetic excitations that are highly tunable by external fields. Interestingly, for magnetic fields $B // a$ (i.e., along the zigzag direction of the Co-Co bonds), there is an extended field range which separates the long-range magnetic order for $B\leq B_c\approx10$ T and the partially spin-polarized gapped high-field phase for $B\gtrsim 12$ T. The low-energy phonon scattering is particularly strong in this field range, consistent with the notion that the system becomes a quantum spin liquid with prominent spin fluctuations down to energies of no more than 2 meV.