Six-state clock physics in an atomically thin antiferromagnet.

The study of collective behaviour driven by spontaneous symmetry breaking and topology is crucial for understanding phase transitions in quantum matter. The two-dimensional (2D) XY model, describing spins with continuous in-plane rotational symmetry, hosts the topological Berezinskii-Kosterlitz-Thouless (BKT) transition, where vortex-antivortex binding induces quasi-long-range order. This model was later extended to include anisotropy fields, leading to the six-state clock model, which predicts the instability of the BKT phase toward true long-range order at low temperatures. Here we investigate this physics in the van der Waals antiferromagnet NiPS3 using nonlinear optical micropolarimetry. As the material is thinned to a monolayer, its magnetic response switches abruptly from the 3D XXZ behaviour of multilayers to a distinct 2D regime consistent with a BKT state. Upon further cooling, the monolayer BKT phase becomes unstable and transforms into a pinned state with long-range order. These results, corroborated by Monte Carlo simulations, open pathways to explore spin vortices and topological dynamics in 2D antiferromagnets.