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^3,4,5,6. 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.

在铜酸盐中最积极研究的问题是赝能隙和奇异金属态的性质以及它们与超导性的关系¹。人们普遍认为，莫特绝缘母态的低能物理被二维自旋S  = 1/2 反铁磁海森堡模型²很好地捕捉到了。然而，最近在几个母铜酸盐中观察到大的热霍尔电导率似乎违背了这个简单的模型，并表明接近于破坏所有垂直于 CuO ₂层^3,4,5,6的镜面的磁手性状态。在这里，我们使用光学二次谐波生成来直接解决模型母铜酸盐 Sr 的点群对称性₂ CuO ₂ Cl ₂。我们报告了破坏所有垂直镜面并且与零磁场中的磁手性状态一致的有序参数的证据。尽管这个顺序显然与反铁磁性耦合，但我们无法通过反铁磁性转变温度的热循环或通过对不同空间位置的采样来实现其时间反转伙伴。这表明顺序开始于 Néel 温度以上，并且可能与赝隙形成的机制有关。