Identifying ordered phases and their underlying symmetries in materials that exhibit high-temperature superconductivity is an important step towards understanding the mechanism of that phenomenon. Indeed, the critical behaviour related to phase transitions of those ordered phases is expected to be correlated with the superconductivity. In cuprate materials, efforts to find such ordered phases have mainly focused on symmetry breaking in the pseudogap region whereas the Fermi-liquid-like metallic region beyond the so-called critical doping at which the pseudogap disappears has been regarded as a trivial disordered state. Here, we uncover a broken mirror symmetry in the Fermi-liquid-like phase in (Bi,Pb)₂Sr₂CaCu₂O_8+δ beyond the critical doping. We do this by tracking the temperature dependence of the rotational-anisotropy of second-harmonic generation for two different dopings. We observe behaviour reminiscent of an order parameter with an onset temperature that coincides with the strange metal to Fermi-liquid-like metal crossover. Angle-resolved photoemission spectroscopy shows that the quasiparticle coherence between CuO₂ bilayers is enhanced in proportion to the symmetry-breaking response as a function of temperature, suggesting that the change in metallicity and symmetry breaking are linked. These observations contradict the conventional quantum disordered scenario for over-critical-doped cuprates.

识别表现出高温超导性的材料中的有序相及其基本对称性是理解该现象机制的重要一步。事实上，与这些有序相的相变相关的关键行为预计与超导性相关。在铜酸盐材料中，寻找这种有序相的努力主要集中在赝能隙区域的对称性破缺，而超出所谓临界掺杂（赝能隙消失）的类费米液体金属区域被认为是微不足道的无序态。在这里，我们发现了超出临界掺杂的 (Bi,Pb) ₂ Sr ₂ CaCu ₂ O _{8+ δ}中费米液态相的破缺镜像对称性。我们通过跟踪两种不同掺杂的二次谐波产生的旋转各向异性的温度依赖性来做到这一点。我们观察到的行为让人想起有序参数，其起始温度与奇怪金属到费米液体金属的交叉一致。角分辨光电子能谱表明，CuO ₂双层之间的准粒子相干性与作为温度函数的对称破缺响应成比例地增强，这表明金属丰度的变化和对称破缺是相关的。这些观察结果与超临界掺杂铜酸盐的传统量子无序场景相矛盾。