paper

Phonons become chiral in the pseudogap phase of cuprates

arXiv:2003.00111 · doi:10.1038/s41567-020-0965-y

Abstract

The nature of the pseudogap phase of cuprates remains a major puzzle. One of its new signatures is a large negative thermal Hall conductivity $I^º_{xy}$ , which appears for dopings $p$ below the pseudogap critical doping $p^{*}$ , but whose origin is as yet unknown. Because this large $I^º_{xy}$ is observed even in the undoped Mott insulator La $_{2}$ CuO $_{4}$ , it cannot come from charge carriers, these being localized at $p = 0$ . Here we show that the thermal Hall conductivity of La $_{2}$ CuO $_{4}$ is roughly isotropic, being nearly the same for heat transport parallel and normal to the CuO $_{2}$ planes, i.e. $I^º_{zy} (T) \approx I^º_{xy} (T)$ . 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 La $_{1.6 - x}$ Nd $_{0.4}$ Sr $_{x}$ CuO $_{4}$ and La $_{1.8 - x}$ Eu $_{0.2}$ Sr $_{x}$ CuO $_{4}$ with $p = 0.24$ , we observe no c-axis Hall signal, i.e. $I^º_{zy} (T) = 0$ , 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 $I^º_{zy} (T)$ signal we find in La $_{1.6 - x}$ Nd $_{0.4}$ Sr $_{x}$ CuO $_{4}$ 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.