Bond directional anapole state in a spin-orbit coupled Mott insulator Sr₂(Ir_1−xRh_x)O₄

In a spin-orbit coupled Mott insulator Sr₂(Ir_1−xRh_x)O₄, a possible hidden order phase has been reported distinctly above the Néel order phase, accompanying both inversion and time-reversal symmetry breakings. However, the nature of this mysterious phase remains largely elusive. We provide a detailed study of the hidden order phase through the combined study of in-plane magnetic torque and the nematic susceptibility measurements [1]. Our torque magnetometry provides thermodynamic evidence for an electronic nematic transition, breaking C₄ rotational symmetry of the underlying lattice. However, in contrast to ordinal even-parity nematic orders, the nematic susceptibility exhibits no divergent behavior towards the onset of the hidden order. These results imply that the nematicity is not a primary order parameter but a secondary one of an odd-parity order parameter. Moreover, our results demonstrate that the hidden order phase is consistent with a bond directional anapole order, in which the intra-unit-cell loop current flows along only one of the diagonal directions in the IrO₄ square.
[1] H. Murayama, et al., arXiv:2008.06380.