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