Phonon chirality from impurity scattering in the antiferromagnetic phase of Sr2IrO4

Overview & Synopsis

A thermal Hall effect is observed in an increasing number of insulators 1,2,3,4,5,6,7 , often attributed to phonons 1,5,7,8,9,10,11 , but the underlying mechanism is in most cases unknown. A coupling of phonons to spins has been invoked 12,13,14,15 and scattering of phonons by impurities or defects has been proposed 16,17,18,19,20 , but there is no systematic evidence to support either scenario. Here we present a study on the effect of adding Rh impurities to the antiferromagnetic insulator Sr2IrO4, substituting for the spin-carrying Ir atoms 21,22 . We find that adding small concentrations of Rh impurities increases the thermal Hall conductivity κxy dramatically, but adding enough Rh to suppress the magnetic order eventually decreases κxy until it nearly vanishes. We conclude that the thermal Hall effect in this material is caused by the scattering of phonons by impurities embedded within a magnetic environment.The thermal Hall effect is used increasingly to probe insulators 23,24 , materials with no mobile charge carriers. In the presence of a heat current J along the x axis and a magnetic field H along the z axis, a transverse temperature gradient T (along the y axis) can develop even if the carriers of heat are neutral (chargeless), provided they have chirality 25 or they acquire a handedness in the presence of a magnetic field. Of liquid candidates such as the layered antiferromagnet α-RuCl3 (refs. 3, 10).