Signatures of Three-State Potts Nematicity in Spin Excitations of the van der Waals Antiferromagnet FePSe ₃

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 TN ≈ 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.