Thermal Hall Effects of Spins and Phonons in Kagome Antiferromagnet Cd-Kapellasite

Abstract

We investigate the thermal-transport properties of the kagome antiferromagnet Cd-kapellasite (Cd-K). We find that a field-suppression effect on the longitudinal thermal conductivity κ_{xx} sets in below approximately 25 K. This field-suppression effect at 15 T becomes as large as 80% at low temperatures, suggesting a large spin contribution κ_{xx}^{sp} in κ_{xx}. We also find clear thermal Hall signals in the spin liquid phase in all Cd-K samples. The magnitude of the thermal Hall conductivity κ_{xy} shows a significant dependence on the sample’s scattering time, as seen in the rise of the peak κ_{xy} value in almost linear fashion with the magnitude of κ_{xx}. On the other hand, the temperature dependence of κ_{xy} is similar in all Cd-K samples; κ_{xy} shows a peak at almost the same temperature of the peak of the phonon thermal conductivity κ_{xx}^{ph} which is estimated by κ_{xx} at 15 T. These results indicate the presence of a dominant phonon thermal Hall κ_{xy}^{ph} at 15 T. In addition to κ_{xy}^{ph}, we find that the field dependence of κ_{xy} at low fields turns out to be nonlinear at low temperatures, concomitantly with the appearance of the field suppression of κ_{xx}, indicating the presence of a spin thermal Hall κ_{xy}^{sp} at low fields. Remarkably, by assembling the κ_{xx} dependence of κ_{xy}^{sp} data of other kagome antiferromagnets, we find that, whereas κ_{xy}^{sp} stays a constant in the low-κ_{xx} region, κ_{xy}^{sp} starts to increase as κ_{xx} does in the high-κ_{xx} region. This κ_{xx} dependence of κ_{xy}^{sp} indicates the presence of both intrinsic and extrinsic mechanisms in the spin thermal Hall effect in kagome antiferromagnets. Furthermore, both κ_{xy}^{ph} and κ_{xy}^{sp} disappear in the antiferromagnetic ordered phase at low fields, showing that phonons alone do not exhibit the thermal Hall effect. A high field above approximately 7 T induces κ_{xy}^{ph}, concomitantly with a field-induced increase of κ_{xx} and the specific heat, suggesting a coupling of the phonons to the field-induced spin excitations as the origin of κ_{xy}^{ph}.