Sizable suppression of magnon Hall effect by magnon damping in Cr2_2Ge2_2Te6_6

Two-dimensional (2D) Heisenberg honeycomb ferromagnets are expected to have interesting topological magnon effects as their magnon dispersion can have Dirac points. The Dirac points are gapped with finite second nearest neighbor Dzyaloshinskii-Moriya interaction, providing nontrivial Berry curvature with finite magnon Hall effect. Yet, it is unknown how the topological properties are affected by magnon damping. We report the thermal Hall effect in Cr$_2$Ge$_2$Te$_6$, an insulating 2D honeycomb ferromagnet with a large Dirac magnon gap and significant magnon damping. Interestingly, the thermal Hall conductivity in Cr$_2$Ge$_2$Te$_6$ shows the coexisting phonon and magnon contributions. Using an empirical two-component model, we successfully estimate the magnon contribution separate from the phonon part, revealing that the magnon Hall conductivity was 20 times smaller than the theoretical calculation. Finally, we suggest that such considerable suppression in the magnon Hall conductivity is due to the magnon damping effect in Cr$_2$Ge$_2$Te$_6$.Comment: 15 pages, 3 figures. Accepted for publication in Phys. Rev.

Significant thermal Hall effect in the 3d cobalt Kitaev system Na2Co2TeO6

Kitaev physics has recently attracted attention in condensed matter for its anticipated quantum spin liquid (QSL) state. The thermal transport measurement is crucial for probing the features of charge-neutral quasiparticles. In this letter, we report a significant thermal Hall effect in Na2Co2TeO6 (NCTO), a Kitaev QSL candidate, when the magnetic field is applied along the out-of-plane direction of the honeycomb plane. The thermal conductivity (??xx ) and thermal Hall conductivity (??xy) in NCTO reveal distinct magnetic field dependences below and above the Neel temperature (TN ) of 27 K. For T > TN, ??xx has a monotonic decrease in the field dependence, while ??xy persists up to T ??? = 150 K. On the other hand, both ??xx and ??xy exhibit complex field dependence for T < TN