Thermal Hall Effects of Spins and Phonons in Kagome Antiferromagnet Cd-Kapellasite
Abstract
We have investigated the thermal-transport properties of the kagome antiferromagnet Cd-kapellasite (Cd-K). We find that a field suppression effect on the longitudinal thermal conductivity kxx sets in below ~25 K, suggesting a large spin contribution kxxsp in kxx. We also find clear thermal Hall signals in the spin liquid phase in all Cd-K samples. The magnitude of the thermal Hall conductivity kxy shows a significant dependence on the sample's scattering time. On the other hand, the temperature dependence of kxy is similar in all Cd-K samples; kxy shows a peak at almost the same temperature of the peak of the phonon thermal conductivity kxyph which is estimated by kxx at 15 T. These results indicate the presence of a dominant phonon thermal Hall kxyph at 15 T. In addition to kxyph, we find that the field dependence of kxy at low fields turns out to be non-linear at low temperatures, concomitantly with the appearance of the field suppression of kxx, indicating the presence of a spin thermal Hall kxysp at low fields. Remarkably, by assembling the kxx dependene of kxysp data of other kagome antiferromagnets, we find that, whereas kxysp stays a constant in the low-kxx region, kxysp starts to increase as kxx does in the high-kxx region. This kxx dependence of kxysp indicates the presence of both intrinsic and extrinsic mechanisms in the spin thermal Hall effect in kagome antiferromagnets. Furthermore, both kxyph and kxysp disappear in the antiferromagnetic ordered phase at low fields, showing that phonons alone do not exhibit the thermal Hall effect. A high field above ~7 T induces kxyph, concomitantly with a field-induced increase of kxx and the specific heat, suggesting a coupling of the phonons to the field-induced spin excitations as the origin of kxyph.