Chiral phonons in the pseudogap phase of cuprates

The nature of the pseudogap phase of cuprates remains a major puzzle^1,2. One of its signatures is a large negative thermal Hall conductivity³, whose origin is as yet unknown. This is observed even in the undoped Mott insulator La₂CuO₄, in which the charge carriers are localized and therefore cannot be responsible. Here, we show that the thermal Hall conductivity of La₂CuO₄ is roughly isotropic; that is, for heat transport parallel and normal to the CuO₂ planes, it is nearly the same. 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 planes⁴. For doping levels higher than the critical doping level at which the pseudogap phase ends, both La_1.6−xNd_0.4Sr_xCuO₄ and La_1.8−xEu_0.2Sr_xCuO₄ show no thermal Hall signal for a heat current normal to the planes, which establishes that phonons have zero Hall response outside the pseudogap phase. Inside the pseudogap phase, the phonons must become chiral to generate the Hall response, but the mechanism by which this happens remains to be identified. It must be intrinsic (from a coupling of phonons to their electronic environment) rather than extrinsic (from structural defects or impurities), as these are the same on both sides of critical doping.