Thermal transport of fractionalized antiferromagnetic and field-induced states in the Kitaev material Na2Co2TeO6

Abstract

We report an in-plane thermal transport study of the honeycomb Kitaev material Na2Co2TeO6 at sub-Kelvin temperatures. In zero magnetic field, the kappa(T) displays a rather weak T dependence but has a nonzero residual term kappa 0/T, indicating strong phonon scattering by magnetic excitation and the possibility of itinerant spinonlike excitations coexisting with an antiferromagnetic order below 27 K. We propose the zero-field ground state is a fractionalized antiferromagnetic (AF*) state with both magnetic order and fractionalized excitations. With both the heat current and magnetic field along the a* (Co-Co bond) direction, the kappa a*(B) exhibits two sharp minima at 7.5 and 10 T, which confirms the phase boundaries of the reported field-induced intermediate state. No such intermediate phase was found in the kappa a(B) for the current and field along the a (zigzag chain) direction. Finally, Na2Co2TeO6 displays a strongly anisotropic magneto-thermal conductivity since the in-plane (out-of-plane) field strongly enhances (suppresses) kappa a* and kappa a.