Spin-orbit-enhanced magnetic surface second-harmonic generation in Sr 2 Ir O 4

An anomalous optical second-harmonic generation (SHG) signal was previously reported in ${\mathrm{Sr}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ 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 twofold 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 ${\mathrm{Sr}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$, our results altogether show that the anomalous SHG signal in parent ${\mathrm{Sr}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ originates instead from a surface-magnetization-induced electric-dipole process that is enhanced by strong spin-orbit coupling.