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 K-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 K-xx(sp) in K-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 K-xy shows a significant dependence on the sample's scattering time, as seen in the rise of the peak K-xy value in almost linear fashion with the magnitude of K-xx. On the other hand, the temperature dependence of K-xy is similar in all Cd-K samples; K-xy shows a peak at almost the same temperature of the peak of the phonon thermal conductivity ex which is estimated by K-xx, at 15 T. These results indicate the presence of a dominant phonon thermal Hall K-xy at 15 T. In addition to e y , we find that the field dependence of K-xy at low fields turns out to be nonlinear at low temperatures, concomitantly with the appearance of the field suppression of K-xx, indicating the presence of a spin thermal Hall K-xy(sp) at low fields. Remarkably, by assembling the K-xx dependence of ex K-xy(sp) data of other kagome antiferromagnets, we find that, whereas K-xy(sp) stays a constant in the low-K-xx region, K-xy(sp) starts to increase as K-xx does in the high-K-xx region. This K-xx dependence of K-xy(sp) indicates the presence of both intrinsic and extrinsic mechanisms in the spin thermal Hall effect in kagome antiferromagnets. Furthermore, both K-xy(ph) and K-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 K-xy(ph), concomitantly with a field-induced increase of K-xx and the specific heat, suggesting a coupling of the phonons to the field-induced spin excitations as the origin of K-xy(ph).