Thermal Hall conductivity in the cuprate Mott insulators Nd$_2$CuO$_4$ and Sr$_2$CuO$_2$Cl$_2$

TL;DR

This study investigates the origin of thermal Hall conductivity in cuprate Mott insulators, revealing a phonon-based mechanism coupled to intrinsic excitations, independent of structural details or impurities.

Contribution

It demonstrates that phonons acquire chirality in cuprates through a mechanism linked to intrinsic CuO$_2$ plane excitations, not impurities or structural features.

Findings

Thermal Hall effect observed in Nd$_2$CuO$_4$ and Sr$_2$CuO$_2$Cl$_2$.

Phonon chirality is not due to impurities or structural domains.

Coupling to intrinsic CuO$_2$ excitations likely causes phonon chirality.

Abstract

The heat carriers responsible for the unexpectedly large thermal Hall conductivity of the cuprate Mott insulator La$_2$CuO$_4$ were recently shown to be phonons. However, the mechanism by which phonons in cuprates acquire chirality in a magnetic field is still unknown. Here, we report a similar thermal Hall conductivity in two cuprate Mott insulators with significantly different crystal structures and magnetic orders - Nd$_2$CuO$_4$ and Sr$_2$CuO$_2$Cl$_2$ - and show that two potential mechanisms can be excluded - the scattering of phonons by rare-earth impurities and by structural domains. Our comparative study further reveals that orthorhombicity, apical oxygens, the tilting of oxygen octahedra and the canting of spins out of the CuO$_2$ planes are not essential to the mechanism of chirality. Our findings point to a chiral mechanism coming from a coupling of acoustic phonons to the intrinsic excitations of the CuO$_2$ planes.