Strain tunable emergent magnetic state in Sr₂IrO₄

Iridates are one of the extensively studied transitional metal oxides because of the unique combination of the electron-electron and spin-orbit interaction. Sr₂IrO₄ is a notable example, which is a quasi-two-dimensional J_eff =1/2 canted antiferromagnetic (AF) Mott insulator with a layered structure that is remarkably similar to the parent phase of weakly spin-orbit-coupled high-Tc cuprates. However, due to the built-in spin-orbit entanglement, the J_eff = 1/2 moments can form significant inter-site quadrupoles in contrast to the S=1/2 moments of Cu ions. The resulting magnetoelastic coupling leads to spontaneous tetragonal symmetry breaking by the AF order in the B1g channel. In the experiment, we compared the elasto-responses of the AF order to in-situ B1g and B2g strains representing two orthogonal symmetry configurations. While the B1g strain efficiently detwins the spontaneous AF domains, new states that break the translational symmetry along the c-axis emerge with the B2g strain. Our model analysis shows that such an emergent state is driven by an unusual quartic interaction of B2g symmetry, competing with the intrinsic B1g anisotropy, and can be in situ tuned by the applied strain.