Topological thermal Hall effect of strongly interacting bosons in a disordered quantum magnet SrCu₂(BO₃)₂

Topological phases of bosons, where topological edge states and dissipationless transport of bosons are expected, have attracted considerable interest. A suitable candidate is a disordered quantum magnet SrCu₂(BO₃)₂ [1] where the ground state is given by tiling quantum mechanical singlets on orthogonally arranged dimers of S = 1/2 Cu moments. The low energy excitation is triply degenerate S = 1 bosonic quasiparticles called triplon. It has been shown that the Dzyaloshinskii-Moriya interactions yield the triplon bands with nontrivial Chern numbers. Although topological modes associated with such a state are predicted to be detectable by the thermal Hall effect [2], no discernible thermal Hall conductivity κ_xy has been reported so far [3].
Here we report high resolution κ_xy measurements on SrCu₂(BO₃)₂. We observed finite κ_xy but its magnitude is much smaller than the theoretical prediction. Moreover, κ_xy is observed in a temperature range, which is much narrower than the prediction. We discuss the suppression of κ_xy in terms of damping of the triplon bands due to triplon-triplon interactions [4].
[1] H. Kageyama et al., PRL 82, 3168–3171.
[2] J. Romhányi et al., Nat. Commun 6, 6805.
[3] L. P. Cairns et al., JPS Conf. Proc. 30, 011089.
[4] M. E. Zayed et al., PRL 113, 067201.