Charge density waves in YBa₂Cu₃O_6.67 probed by resonant x-ray scattering under uniaxial compression

We report a comprehensive Cu L₃-edge resonant x-ray scattering (RXS) study of two- and three-dimensional (2D and 3D) incommensurate charge correlations in single crystals of the underdoped high-temperature superconductor YBa₂Cu₃O_6.67 under uniaxial compression up to 1% along the two inequivalent Cu─O─Cu bond directions (a and b) in the CuO₂ planes. We confirm the strong in-plane anisotropy of the 2D charge correlations and observe their symmetric response to pressure: pressure along a enhances correlations along b, and vice versa. Our results imply that the underlying order parameter is uniaxial. In contrast, 3D long-range charge order is only observed along b in response to compression along a. Spectroscopic RXS measurements show that the 3D charge order resides exclusively in the CuO₂ planes and may thus be generic to the cuprates. We discuss implications of these results for models of electronic nematicity and for the interplay between charge order and superconductivity.