Large Phonon Thermal Hall Effect in the Antiferromagnetic Insulator Cu₃TeO₆

        Phonons are known to generate a thermal Hall effect in various insulators, including multiferroic materials [1] and cuprate Mott insulators [2,3,4], but the underlying mechanism is still unclear. Theoretical proposals for the phonon thermal Hall effect fall into two categories: intrinsic scenarios based on the coupling of phonons to their environment (e.g. [5]) and extrinsic scenarios based on the skew scattering of phonons by impurities or defects (e.g. [6]). 

        To shed new light on this question, we have studied an entirely different class of materials, the antiferromagnetic insulator Cu₃TeO₆. In this simple cubic material, we find the largest thermal Hall conductivity κ_xy of any insulator so far. The fact that κ_xy is ~ 50 times larger than in the cuprate Sr₂CuO₂Cl₂, for example, is strong evidence that the phonons are responsible for the Hall signal, since the phonon-dominated longitudinal thermal conductivity, κ_xx, is also ~ 50 times larger. So κ_xy seems to grow in proportion with the conductivity of phonons.

        We discuss the implications of the finding that the ratio κ_xy / κ_xx is roughly the same in these two very different materials, as well as in other insulating materials.

[1] Ideue et al., Nature Materials 16, 797 (2017). 

[2] Grissonnanche et al., Nature 571, 376 (2019). 

[3] Grissonnanche et al., Nature Physics 16, 1108 (2020).

[4] Boulanger et al., Nature Communications 11, 5325 (2020).

[5] Ye, Savary & Balents, arXiv:2103.04223 (2021).

[6] Sun, Chen & Kivelson, arXiv:2109.12117 (2021).