Chiral phonons in cuprate superconductors

The nature of the pseudogap phase of cuprates remains a major puzzle. Although there are indications that this phase breaks various symmetries, there is no consensus on its fundamental nature [1].

Here we report measurements of the thermal Hall conductivity κ_xy in the normal state of four different cuprates and show that a large negative κ_xy signal is a property of the pseudogap phase, appearing with the onset of that phase at the critical doping p*[2]. It is also a property of the Mott insulator at p ≈ 0, where κ_xy has the largest reported magnitude of any insulator. Since this negative κ_xy signal grows as the system becomes increasingly insulating electrically, it cannot be attributed to conventional mobile charge carriers. Nor is it due to magnons, since it exists in the absence of magnetic order.

In order to identify the heat carriers responsible for this negative κ_xy, we turn to transverse heat transport along the c-axis: κ_zy. Here we show that the thermal Hall conductivity at p = 0 is roughly isotropic, being nearly the same for heat transport parallel and normal to the CuO₂ planes, i.e. κ_zy(T) ≈ κ_xy(T). This shows that the negative thermal 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*, we observe no c-axis Hall signal, i.e. κ_zy(T) = 0, showing that phonons have zero Hall response outside  the pseudogap phase. The microscopic mechanism by which phonons become chiral in cuprates  remains to be identified. This phonon Hall effect provides a new window on quantum materials and it may explain the thermal Hall signal observed in other topologically nontrivial insulators [4].

 

[1] Proust & Taillefer, Annu. Rev. Condens. Matter Phys. 10, 409 (2019).

[2] Grissonnanche et al., Nature 571, 376 (2019).

[3] Grissonnanche et al., Nature Physics 16, 1108 (2020).

[4] Kasahara et al., Nature 559, 227 (2018).