Phonons become chiral in the pseudogap phase of cuprates
The nature of the pseudogap phase of cuprates remains a major puzzle. One of its new signatures is a large negative thermal Hall conductivity κxy, which appears for dopings p below the pseudogap critical doping p∗, but whose origin is as yet unknown. Because this large κxy is observed even in the undoped Mott insulator La2CuO4, it cannot come from charge carriers, these being localized at p=0. Here we show that the thermal Hall conductivity of La2CuO4 is roughly isotropic, being nearly the same for heat transport parallel and normal to the CuO2 planes, i.e. κzy(T)≈κxy(T). This shows that the Hall response must come from phonons, these being the only heat carriers able to move as easily normal and parallel to the planes . At p>p∗, in both La1.6−xNd0.4SrxCuO4 and La1.8−xEu0.2SrxCuO4 with p=0.24, we observe no c-axis Hall signal, i.e. κzy(T)=0, showing that phonons have zero Hall response outside the pseudogap phase. The phonon Hall response appears immediately below p∗=0.23, as confirmed by the large κzy(T) signal we find in La1.6−xNd0.4SrxCuO4 with p=0.21. The microscopic mechanism by which phonons become chiral in cuprates remains to be identified. This mechanism 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 p∗. This intrinsic phonon Hall effect provides a new window on quantum materials and it may explain the thermal Hall signal observed in other topologically nontrivial insulators.