  $\rm Sr_2IrO_4$ is an archetypal spin-orbit-coupled Mott insulator and has
been extensively studied in part because of a wide range of predicted novel
states. Limited experimental characterization of these states thus far brings
to light the extraordinary susceptibility of the physical properties to the
lattice, particularly, the Ir-O-Ir bond angle. Here, we report a newly observed
microscopic rotation of the IrO$_6$ octahedra below 50~K measured by single
crystal neutron diffraction. This sharp lattice anomaly provides keys to
understanding the anomalous low-temperature physics and a direct confirmation
of a crucial role that the Ir-O-Ir bond angle plays in determining the ground
state. Indeed, as also demonstrated in this study, applied electric current
readily weakens the antiferromagnetic order via the straightening of the
Ir-O-Ir bond angle, highlighting that even slight change in the local structure
can disproportionately affect the physical properties in the spin-orbit-coupled
system.
