Spin-orbit-enhanced magnetic surface second-harmonic generation in Sr${}_2$IrO${}_4$

Abstract

An anomalous optical second-harmonic generation (SHG) signal was previously reported in Sr${}_2$IrO${}_4$ and attributed to a hidden odd-parity bulk magnetic state. Here we investigate the origin of this SHG signal using a combination of bulk magnetic susceptibility, magnetic-field-dependent SHG rotational anisotropy, and overlapping wide-field SHG imaging and atomic force microscopy measurements. We find that the anomalous SHG signal exhibits a two-fold rotational symmetry as a function of in-plane magnetic field orientation that is associated with a crystallographic distortion. We also show a change in SHG signal across step edges that tracks the bulk antiferromagnetic stacking pattern. While we do not rule out the existence of hidden order in Sr${}_2$IrO${}_4$, our results altogether show that the anomalous SHG signal in parent Sr${}_2$IrO${}_4$ originates instead from a surface-magnetization-induced electric-dipole process that is enhanced by strong spin-orbit coupling.

6 pages main text, 4 figures, 12 pages supplementary information

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• Anisotropy of Antiferromagnetic Domains in a Spin-orbit Mott Insulator
• Microscopic model realization of $\mathbf{d}$-wave pseudospin current order in Sr${}_2$IrO${}_4$
• Three-dimensional Stacking of Canted Antiferromagnetism and Pseudospin Current in Undoped Sr${}_2$IrO${}_4$: Symmetry Analysis and Microscopic Model Realization