Nematicity in the pseudogap and charge order phases of the cuprate YBCO

Electronic nematicity is the spontaneous breaking of rotational symmetry in a metal, and it has been argued to potentially strengthen superconductivity. It is a key feature of iron-based high-temperature superconductors. In cuprate high-temperature superconductors, a number of experiments have revealed evidence of nematicity, but its origin remains unclear. We measured the in-plane anisotropy of the electrical resistivity ρ(T) and the Seebeck coefficient S(T) in a crystal of the cuprate YBa₂Cu₃O_y, at hole concentration p = 0.12. By using the same sample and contacts to measure transport along the a axis and the b axis of the orthorhombic crystal structure, by detwinning this sample twice, such that its length was first along a and then along b, we were able to precisely obtain the intrinsic in-plane anisotropy of these two longitudinal transport coefficients. We find no trace of any additional anisotropy in either ρ(T) or S(T) upon crossing below the pseudogap temperature T*. A large anisotropy appears only at much lower temperature, in tandem with the emergence of charge density wave correlations. We conclude that the pseudogap phase itself is not nematic, and the nematicity in YBa₂Cu₃O_y originates instead from the unidirectional charge order and its precursor correlations.