Signatures of Three-State Potts Nematicity in Spin Excitations of the van der Waals Antiferromagnet FePSe₃

In two-dimensional (2D) nearly square-lattice quantum materials, electron correlations can induce an electronic nematic phase with 2-fold rotational (C₂) symmetry. For 2D materials with 3-fold rotational (C₃) symmetry, such as the honeycomb lattice, a vestigial three-state Potts nematic order has been observed in the van der Waals antiferromagnet FePSe₃ under uniaxial strain. Here, we use neutron scattering to probe the magnetic order and spin excitations of FePSe₃ under uniaxial strain. In the antiferromagnetic (AFM) ordered state, ∼0.6% tensile strain suppresses one zigzag domain and enhances the other two, reducing the AFM order and spin waves to C₂ symmetry. This broken C₃ symmetry in spin excitations persists slightly above T_N ≈ 108.6 K, where zigzag AFM order is absent. These results provide direct evidence of magnetoelastic coupling and suggest that three-state Potts nematicity in paramagnetic spin excitations originates as the vestigial order of the low-temperature zigzag AFM state.