Signatures of Z3 Vestigial Potts-nematic order in van der Waals antiferromagnets*

Layered van der Waals magnets have attracted much recent attention as a promising and versatile platform for exploring intrinsic two-dimensional magnetism. Within this broader class, the transition metal phosphorous trichalcogenides MPX3 stand out as particularly interesting, as they provide a realization of honeycomb lattice magnetism and are known to display a variety of magnetic ordering phenomena. Here, we report the observation of key signatures of vestigial Potts-nematic order in rhombohedral FePSe3. By performing linear dichroism imaging measurements—an ideal probe of rotational symmetry breaking—we find that the C3 symmetry is already broken above the Néel temperature. We show that these observations are explained by a general Ginzburg-Landau model of vestigial nematic order driven by magnetic fluctuations and coupled to residual strain. An analysis of the domain structure as temperature is lowered and a comparison with zigzag-ordered monoclinic FePS3 reveals a broader applicability of the Ginzburg-Landau model in the presence of external strain, and firmly establishes the MPX3 magnets as a new experimental venue for studying the interplay between Potts-nematicity, magnetism and superconductivity.