A structural distortion induced magneto-elastic locking in Sr$_2$IrO$_4$ revealed through nonlinear optical harmonic generation

TL;DR

This study reveals a structural distortion in Sr$_2$IrO$_4$ that causes a symmetry change, which explains previously observed forbidden Bragg peaks and supports a magneto-elastic locking mechanism despite local distortions.

Contribution

The paper uncovers a symmetry lowering in Sr$_2$IrO$_4$ through nonlinear optical measurements and explains its impact on magneto-elastic locking using an effective super-exchange model.

Findings

Symmetry lowering from I4_{1}/acd to I4_{1}/a observed.

Perfect magneto-elastic locking persists despite local distortions.

Reconciliation of tetragonal distortion with neutron diffraction results.

Abstract

We report a global structural distortion in Sr$_2$IrO$_4$ using spatially resolved optical second and third harmonic generation rotational anisotropy measurements. A symmetry lowering from an $I4_{1}/acd$ to $I4_{1}/a$ space group is observed both above and below the N\'{e}el temperature that arises from a staggered tetragonal distortion of the oxygen octahedra. By studying an effective super-exchange Hamiltonian that accounts for this lowered symmetry, we find that perfect locking between the octahedral rotation and magnetic moment canting angles can persist even in the presence of large non-cubic local distortions. Our results explain the origin of the forbidden Bragg peaks recently observed in neutron diffraction experiments and reconcile the observations of strong tetragonal distortion and perfect magneto-elastic locking in Sr$_2$IrO$_4$.