Mirror symmetry breaking in a model insulating cuprate

Among the most actively studied issues in the cuprates are the natures of the pseudogap and strange metal states and their relationship to superconductivity¹. There is general agreement that the low-energy physics of the Mott-insulating parent state is well captured by a two-dimensional spin S = 1/2 antiferromagnetic Heisenberg model². However, recent observations of a large thermal Hall conductivity in several parent cuprates appear to defy this simple model and suggest proximity to a magneto-chiral state that breaks all mirror planes that are perpendicular to the CuO₂ layers^3–6. Here we use optical second harmonic generation to directly resolve the point group symmetries of the model parent cuprate Sr₂CuO₂Cl₂. We report evidence of an order parameter that breaks all perpendicular mirror planes and is consistent with a magneto-chiral state in zero magnetic field. Although this order is clearly coupled to the antiferromagnetism, we are unable to realize its time-reversed partner by thermal cycling through the antiferromagnetic transition temperature or by sampling different spatial locations. This suggests that the order onsets above the Néel temperature and may be relevant to the mechanism of pseudogap formation.