The pseudogap phenomenon in the cuprates is arguably the most mysterious puzzle in the field of high-temperature superconductivity. The tetragonal cuprate HgBa₂CuO_4+δ, with only one CuO₂ layer per primitive cell, is an ideal system to tackle this puzzle. Here, we measure the magnetic susceptibility anisotropy within the CuO₂ plane with exceptionally high-precision magnetic torque experiments. Our key finding is that a distinct two-fold in-plane anisotropy sets in below the pseudogap temperature T^*, which provides thermodynamic evidence for a nematic phase transition with broken four-fold symmetry. Surprisingly, the nematic director orients along the diagonal direction of the CuO₂ square lattice, in sharp contrast to the bond nematicity along the Cu-O-Cu direction. Another remarkable feature is that the enhancement of the diagonal nematicity with decreasing temperature is suppressed around the temperature at which short-range charge-density-wave formation occurs. Our result suggests a competing relationship between diagonal nematic and charge-density-wave order in HgBa₂CuO_4+δ.

铜酸盐中的假间隙现象可以说是高温超导领域中最神秘的难题。方形铜酸盐HgBa ₂ CuO _{4+ δ}，每个原始单元只有一层CuO ₂，是解决这个难题的理想系统。在这里，我们通过极高精度的磁转矩实验测量了CuO ₂平面内的磁化率各向异性。我们的主要发现是在伪能隙温度T ^*以下存在明显的两倍平面内各向异性，这为四向对称性破裂的向列相变提供了热力学证据。令人惊讶的是，向列导向器沿着CuO的对角线方向取向₂方形晶格，与沿Cu-O-Cu方向的键向列性形成鲜明对比。另一个显着的特征是，随着温度的降低，对角向列性的增强在发生短程电荷密度波形成的温度附近受到抑制。我们的结果表明，HgBa ₂ CuO _{4+ δ中}对角向列与电荷密度波阶之间存在竞争关系。