Thermal-transport studies of kagomé antiferromagnets

Abstract

Searching for the ground state of a kagomé Heisenberg antiferromagnet (KHA) has been one of the central issues of condensed-matter physics, because the KHA is expected to host spin-liquid phases with exotic elementary excitations. Here, we show our longitudinal () and transverse () thermal conductivities measurements of the two kagomé materials, volborthite and Ca kapellasite. Although magnetic orders appear at temperatures much lower than the antiferromagnetic energy scale in both materials, the nature of spin liquids can be captured above the transition temperatures. The temperature and field dependence of is analyzed by spin and phonon contributions, and large sample variations of the spin contribution are found in volborthite. Clear changes in are observed at the multiple magnetic transitions in volborthite, showing different magnetic thermal conduction in different magnetic structures. These magnetic contributions are not clearly observed in low- crystals of volborthite, and are almost absent in Ca kapellasite, showing the high sensitivity of the magnetic excitation in to the defects in crystals. On the other hand, a clear thermal Hall signal has been observed in the lowest- crystal of volborthite and in Ca kapellasite. Remarkably, both the temperature dependence and the magnitude of of volborthite are found to be very similar to those of Ca kapellasite, despite of about an order of magnitude difference in We find that of both compounds is well reproduced, both qualitatively and quantitatively, by spin excitations described by the Schwinger-boson mean-field theory applied to KHA with the Dzyaloshinskii–Moriya interaction. This excellent agreement demonstrates not only that the thermal Hall effect in these kagomé antiferromagnets is caused by spins in the spin liquid phase, but also that the elementary excitations of this spin liquid phase are well described by the bosonic spin excitations.