Nematic order in electronic systems breaks rotational symmetry without breaking translational symmetry — the system picks a direction but doesn't form a pattern. In many materials, nematicity is intertwined with magnetic order, making it hard to tell whether the nematic fluctuations are an independent degree of freedom or just a byproduct of the magnetic transition.
Lu et al. (arXiv:2603.12704) resolve this in CoTa₃S₆, a layered antiferromagnet, by measuring elastoresistivity — how the electrical resistance changes when the crystal is strained. They find a divergence-like antisymmetric elastoresistivity that develops rapidly below the stripe antiferromagnetic transition. The nematic signal diverges, while the magnetic transition temperatures barely shift under the same strain. The nematic and magnetic orders coexist but have separate origins.
The separation is demonstrated by their different responses to perturbation. Magnetic fields suppress the nematic fluctuations strongly but leave the antiferromagnetic ordering temperatures relatively unchanged. Strain shifts the nematic onset temperature dramatically but doesn't move the magnetic transitions. If the nematicity were simply a derivative of the magnetic order, it would track the magnetic response. It doesn't.
The hexagonal crystal structure adds a twist: three-state nematicity. In a square lattice, nematic order picks one of two directions (horizontal or vertical). In a hexagonal lattice, there are three equivalent directions, and the nematic order parameter has Z₃ symmetry instead of Z₂. The magnetoresistance data shows signatures consistent with this three-state structure — the nematic order doesn't just pick a direction; it chooses among three.