Chiral phonons in the pseudogap phase of cuprates

The nature of the pseudogap phase of cuprates remains a major puzzle. In the cuprate Nd-LSCO, a large negative thermal Hall conductivity κ_xy was seen to appear when the doping p is lowered below the critical doping of the pseudogap phase p* [1]. Its origin is still unknown. Here we show that the thermal Hall conductivity of the Mott insulator La₂CuO₄ is roughly the same for heat transport parallel and normal to the CuO₂ planes, i.e. κ_zy(T) ≈ κ_xy(T) [2]. This shows that the Hall response must come from phonons, as they are the only heat carriers that are able to move with the same ease both normal and parallel to the planes.
For p > p*, both Nd-LSCO and Eu-LSCO show no thermal Hall signal for a heat current normal to the planes, i.e. κ_zy(T) = 0 [2], which establishes that phonons have zero Hall response outside the pseudogap phase. The negative Hall response appears immediately below p* = 0.23, as shown by the large κ_zy signal observed in Nd-LSCO and Eu-LSCO with p = 0.21. We see that phonons become chiral inside the pseudogap phase, but the mechanism by which this happens remains to be identified.
[1] Grissonnanche et al., Nature 571, 376 (2019)
[2] Grissonnanche et al., Nat. Phys. (2020). https://doi.org/10.1038/s41567-020-0965-y