Thermal Hall Conductivity in Cuprates

Thermal Hall conductivity (k_xy) has been used to characterize the pseudogap phase of hole-doped cuprates [1]. A new and unexpected result was found: the presence of a negative thermal Hall signal appearing in the pseudogap phase, which grows as the sample becomes insulating, being maximal in the Mott insulator La₂CuO₄. The heat carriers responsible for that large thermal Hall conductivity were shown to be phonons [2].
The microscopic mechanism that confers chirality on phonons remains to be discovered. To shed light on this, we measured k_xy in two additional Mott insulators, Nd₂CuO₄ and Sr₂CuO₂Cl₂. This comparative study [3] shows that a large k_xy is still present in those Mott insulators, ruling out mechanisms such as apical oxygen, spin canting or structural domains. Strong similarities in the temperature dependence and relative magnitude of k_xy and k_xx provide further evidence that it is indeed the phonons that are responsible for k_xy in those materials. A possible mechanism of chirality could be the coupling of phonons to short-range antiferromagnetic correlations.

[1] G. Grissonnanche et al. Nature 571, 376 (2019).
[2] G. Grissonnanche et al. Nature Physics, arXiv:2003.00111 (2020).
[3] M-E. Boulanger et al. Nature Communications 11, 5325 (2020).