Thermal Transport of Fractionalized Antiferromagnetic and Field Induced States in the Kitaev Material Na${}_2$Co${}_2$TeO${}_6$

Abstract

We report an in-plane thermal transport study of the honeycomb Kitaev material Na${}_2$Co${}_2$TeO${}_6$ at subKelvin temperatures. In zero field, the $\hat{I}º(T)$ displays a rather weak $T$-dependence but has a non-zero residual term $\hat{I}{º}_0/T$, indicating strong phonon scattering by magnetic excitation and the possibility of itinerant spinon-like excitations coexisting with an antiferromagnetic order below 27 K. We propose the zero-field ground state is a novel fractionalized antiferromagnetic (AF*) state with both magnetic order and fractionalized excitations. With both the heat current and external field along the $a\ast$ (Co-Co bond) direction, the $\hat{I}{º}_{a\ast }$ exhibits two sharp minima at 7.5 T and 10 T, and its value at 8.5 T is almost the same as the pure phononic transport for the high-field polarized state. This confirms the phase boundaries of the reported field-induced intermediate state and suggest its gapless continuum excitations possibly transport heat. No such intermediate phase was found in the $\hat{I}{º}_a$ for the current and field along the $a$ (zigzag chain) direction. Finally, Na${}_2$Co${}_2$TeO${}_6$ displays a strongly anisotropic magneto-thermal conductivity since the in-plane (out-of-plane) field strongly enhances (suppresses) the $\hat{I}{º}_{a\ast }$ and $\hat{I}{º}_a$.

7 pages, 3 figures