Thermal Hall conductivity in the cuprate Mott insulators Nd 2 CuO 4 and Sr 2 CuO 2 Cl 2 .

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.