Large negative thermal Hall response in the pseudogap phase of cuprates

The nature of the pseudogap phase of cuprate superconductors remains a mystery. In that phase, the Fermi surface is transformed even though translational symmetry is not broken [1]. A possible explanation is a spin-liquid-like state with topological order [2].
The thermal Hall conductivity κ_xy has recently emerged as a powerful probe of insulators with unusual forms of magnetism, such as quantum spin liquids [3] and quantum spin ice [4].
We report extensive measurements of the thermal Hall conductivity κ_xy in several families of cuprates across a wide range of dopings. We observe a large and negative thermal Hall response at temperatures below the pseudogap temperature T*, which appears immediately below the pseudogap critical doping p*. The negative κ_xy contrasts with the positive electrical Hall conductivity σ_xy and, moreover, the magnitude of κ_xy increases as doping is reduced towards p = 0, whereas σ_xy vanishes as the material becomes an insulator.
The negative κ_xy is therefore due to neutral heat carriers and it points to spin chirality [5], or perhaps topological excitations.

[1] Proust & Taillefer, ARCMP; arXiv:1804.08502 (2018).
[2] Scheurer et al., PNAS (2018).
[3] Kasahara et al., Nature (2018).
[4] Hirschberger et al., Science (2015).
[5] Lee et al., PRB (2015).