Phonon chirality from impurity scattering in the antiferromagnetic phase of Sr₂IrO₄

A thermal Hall effect occurs in an increasing number of insulators and is often attributed to phonons, but the underlying mechanism is not known in most cases. Two main scenarios have been proposed: either a coupling of phonons to spins or scattering of phonons by impurities or defects, but there is no systematic evidence to support either of them. Here we present evidence for the phonon impurity scattering picture by studying the effect of adding rhodium impurities to the antiferromagnetic insulator Sr₂IrO₄, substituting for the spin-carrying iridium atoms. We find that adding small concentrations of rhodium impurities increases the thermal Hall conductivity, but adding enough rhodium to suppress the magnetic order eventually decreases it until it nearly vanishes. In contrast, introducing lanthanum impurities that substitute for the strontium atoms, which lie outside the IrO₂ planes that are the seat of magnetism, produces a much smaller enhancement of the thermal Hall conductivity. We conclude that the thermal Hall effect in this material is caused by the scattering of phonons by impurities embedded within a magnetic environment.