Planar thermal Hall effect from phonons in cuprates

TL;DR

This study investigates the planar thermal Hall effect in cuprates, revealing a sizable phonon-induced response despite symmetry constraints, challenging existing understanding of phonon thermal Hall mechanisms.

Contribution

It demonstrates the presence of a significant planar phonon thermal Hall effect in cuprates, prompting a re-evaluation of phonon Hall transport theories in insulators.

Findings

Planar electronic thermal Hall effect is zero in cuprates.

Sizable planar phonon thermal Hall effect observed despite symmetry expectations.

Results challenge current theories of phonon thermal Hall mechanisms.

Abstract

A surprising "planar" thermal Hall effect, whereby the field is parallel to the current, has recently been observed in a few magnetic insulators, and this has been attributed to exotic excitations such as Majorana fermions or chiral magnons. Here we investigate the possibility of a planar thermal Hall effect in three different cuprate materials, in which the conventional thermal Hall conductivity $\kappa_{\rm {xy}}$ (with an out-of-plane field perpendicular to the current) is dominated by either electrons or phonons. Our measurements show that the planar $\kappa_{\rm {xy}}$ from electrons in cuprates is zero, as expected from the absence of a Lorentz force in the planar configuration. By contrast, we observe a sizable planar $\kappa_{\rm {xy}}$ in those samples where the thermal Hall response is due to phonons, even though it should in principle be forbidden by the high crystal symmetry. Our findings call for a careful re-examination of the mechanisms responsible for the phonon thermal Hall effect in insulators.