Brightened Optical Transition as Indicator of Multiferroicity in a Layered Antiferromagnet

Two-dimensional van der Waals magnets show strong interconnection between their electrical, magnetic, and structural properties. Here we reveal the emergence of a luminescent transition upon crossing the N\'eel transition temperature of CrPS$_4$, a layered antiferromagnetic semiconductor. This luminescent transition occurs above the lowest absorption level. We attribute the optical transitions to excited states of the t$_{\rm 2g}$ orbitals of the Cr$^{3+}$ ions, which are influenced by the distortion of the octahedral crystal field. Specifically, we find at the crossing of the N\'eel temperature changes the distortion from an anti-polar to polar arrangement, thereby not only activating an additional luminescent pathway but also inducing a significant in-plane static dipole moment detected by a marked enhancement in the intensity of the second harmonic generation. Our findings suggest the presence of a multiferroic state in CrPS$_4$ below the N\'eel temperature.