The pseudogap phenomenon in cuprates is the most mysterious puzzle in the research of high-temperature superconductivity . 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  and double-layer YBa2Cu3O6+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.
|Publication status||Published - 2018 May 1|
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