Large negative thermal Hall conductivity in cuprate Mott insulators

A large negative thermal Hall signal k_xy has recently been observed to appear in the pseudogap phase of cuprates [1]. This signal persists to zero doping, in the antiferromagnetic Mott insulator La₂CuO₄. While it is clearly not due to electrons, this k_xy signal could come from magnons, or phonons — or more exotic excitations. To shed light on the underlying mechanism, we measured k_xy in two additional cuprate Mott insulators: Nd₂CuO₄ and Sr₂CuO₂Cl₂ . We observe a large negative k_xy signal in both materials. Because of the absence of spin canting in either of these materials without apical oxygens, and because of the absence of any change in k_xy upon crossing different magnetic states in Nd₂CuO₄, we infer that magnons are not the heat carriers responsible for k_xy. The similarity in the temperature dependence of k_xy and k_xx suggests a phonon scenario. If so, the mechanism whereby the pseudogap phase confers chirality to phonons remains to be elucidated.

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