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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 Îșxy, which appears for dopings p below the pseudogap critical doping pâ, but whose origin is as yet unknown. Because this large Îșxy 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. Îșzy(T)âÎșxy(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>pâ, in both La1.6âxNd0.4SrxCuO4 and La1.8âxEu0.2SrxCuO4 with p=0.24, we observe no c-axis Hall signal, i.e. Îșzy(T)=0, showing that phonons have zero Hall response outside the pseudogap phase. The phonon Hall response appears immediately below pâ=0.23, as confirmed by the large Îșzy(T) signal we find in La1.6âxNd0.4SrxCuO4 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 pâ. 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.Center for Dynamics and Control of Material