Giant thermal Hall conductivity in the pseudogap phase of cuprate superconductors

The nature of the pseudogap phase of cuprates remains a major puzzle. Although there are indications that this phase breaks various symmetries, there is no consensus on its fundamental nature [1].
Fermi-surface, transport and thermodynamic signatures of the pseudogap phase are reminiscent of a transition into a phase with antiferromagnetic order, but evidence for an associated long-range magnetic order is still lacking. Here we report measurements of the thermal Hall conductivity κ_xy in the normal state of four different cuprates and show that a large negative κ_xy signal is a property of the pseudogap phase, appearing with the onset of that phase at the critical doping p* [2]. It is also a property of the Mott insulator at p ≈ 0, where κ_xy has the largest reported magnitude of any insulator. Since this negative κ_xy signal grows as the system becomes increasingly insulating electrically, it cannot be attributed to conventional mobile charge carriers. Nor is it due to magnons, since it exists in the absence of magnetic order. Our observation is reminiscent of the thermal Hall conductivity of insulators with spin-liquid states [3, 4, 5] pointing to neutral excitations with spin chirality in the pseudogap phase of cuprates.

[1] Proust & Taillefer, Annu. Rev. Condens. Matter Phys. 10, 409 (2019).
[2] Grissonnanche et al., Nature 571, 376 (2019).
[3] Kasahara et al. Nature 559, 227 (2018).
[4] Lee et al., PRB 91, 125413 (2015).
[5] Samajdar et al. Nat. Phys. doi:10.1038/s41567-019-0669-3 (2019)