Diagonal nematicity in the pseudogap phase of HgBa₂CuO₄₊δ

The pseudogap phenomenon in cuprates is the most mysterious puzzle in the research of high-temperature superconductivity [1]. In particular, whether the pseudogap is associated with a crossover or phase transition has been a longstanding controversial issue. The tetragonal cuprate HgBa₂CuO₄₊δ, with only one CuO₂ layer per primitive cell, is an ideal system to tackle this puzzle. Here, we measure the anisotropy of magnetic susceptibility 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. Most surprisingly, the nematic director orients along the diagonal direction of the CuO₂ square lattice, in sharp contrast to the bond nematicity reported in various iron-based superconductors [2] and double-layer YBa₂Cu₃O_6+δ [3-5], where the anisotropy axis is along the Fe-Fe and Cu-O-Cu directions, respectively. 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 (CDW) formation occurs [6, 7]. This is in stark contrast to YBa₂Cu₃O₆₊δ, where the bond nematicity is not influenced by the CDW. Our result suggests a competing relationship between diagonal nematic and CDW order in HgBa₂CuO₄₊δ.