The layered 5d transition metal oxide Sr₂IrO₄ hosts a J_eff =1/2 antiferromagnetic Mott insulating state, which strikingly resembles to the parent state of high-T_c cuprate superconductors. Recently, great interest has been aroused in the series of Sr₂Ir_1-xRh_xO₄, especially on the proposed hidden order phase above the antiferromagnetic ordering, which has been highlighted by optical second-harmonic generation and polarized neutron diffraction experiments [1, 2]. Here, by using exceptionally precise in-plane torque magnetometry [3-5], we provide thermodynamic evidence that nematicity, a spontaneous breaking of rotational symmetry of the underlying lattice, develops well above the antiferromagnetic transition in pure and doped Sr₂Ir_1-xRh_xO₄. Our highly sensitive magnetic anisotropy measurements under in-plane field rotation reveal the growth of two-fold oscillations, which onsets at the hidden order temperature T^*. The present results demonstrate striking similarities between the hidden order phase in Sr₂Ir_1-xRh_xO₄ and the pseudogap phase in high-T_c cuprates.
[1] L. Zhao et al., 12, 32 (2016).
[2] J. Jeong et al., Nat. Commun. 8, 15119 (2017).
[3] S. Kasahara et al., Nature 486, 382 (2012).
[4] Y. Sato et al., Nat. Phys. 13, 1074 (2017).
[5] H. Murayama et al., arXiv:1805.00276.