Thermal Hall effect in Sr₂IrO₄

Strontium iridate, Sr₂IrO₄, is a spin-orbit-induced Mott insulator that is isostructural to the cuprate La₂CuO₄, which is a charge-transfer Mott insulator. Various broken symmetries in the temperature-doping phase diagram of iridates point to similarities with the pseudogap phase of the cuprate superconductors [1]. 

To explore those possible similarities, we have measured the thermal conductivity, κ_xx , and the thermal Hall conductivity, κ_xy, of Sr₂IrO₄, in undoped crystals, hole-doped crystals with Rh substitution and electron-doped crystals with La substitution.

As in the cuprates [2,3], we observe a negative thermal Hall signal at all dopings. We find that the degree of chirality – the ratio | κ_xy / κ_xx | – increases with doping to a maximum value at x = 0.02 in (Sr_1-x La_x)₂IrO₄ and at x = 0.05 in Sr₂Ir_1-xRh_xO₄, then decreases to nearly vanish at the highest measured dopings, x = 0.04 and x = 0.15, respectively.

We attribute the negative thermal Hall signal to phonons, as in cuprates [4], and discuss the possible mechanisms by which phonons could acquire chirality in a magnetic field.

[1] Bertinshaw et al., Annu. Rev. Condens. Matter Phys. 10, 315 (2019).

[2] Grissonnanche et al., Nature 571, 376 (2019).

[3] Boulanger et al., Nature Communications 11, 5325 (2020).

[4] Grissonnanche et al., Nature Physics 16, 1108 (2020).