via pseudo Jahn-Teller Effect*

Emergent phenomenon cooperatively driven by electronic correlation and spin-orbit coupling is at the forefront of condensed matter physics. In 3d complex oxide systems, the orbital degeneracy can usually be lifted via the Jahn-Teller Effect. While in 5d systems even the orbital singlet can be highly sensitive to lattice distortion due to the presence of strong SOC. Here we show that, by applying anisotropic strain up to only 0.05%, we can in-situ modulate the metamagnetic transition field by almost 300% of the spin-orbit-coupled Mott insulator Sr₂IrO_4, which is an important representative of the 5d transition metal oxides. Our simultaneous measurements of resonant x-ray scattering and transport reveal that this drastic response originates from the complete strain-tuning of the transition between the spin-flop and spin-flip limits, and is always accompanied by large elasto- and magneto-conductance. This enables electrically controllable and electronically detectable metamagnetic switching. The resulted strain-magnetic field phase diagram reveals that the strain introduces C₄-symmetry-breaking magnetic anisotropy to the square lattice via pseudospin-lattice coupling, directly demonstrating the pseudo-Jahn-Teller effect of the spin-orbit-coupled correlated system. The extracted coupling strength is much weaker than the superexchange interactions, yet crucial for the spontaneous symmetry-breaking, affording the remarkably efficient strain-control. This work shows a promising avenue to effectively manipulate antiferromagnetic materials in the field of iridates, and more broadly functional 5d transition metal oxides by playing emergent phenomena with in situ strain.