Charge Density Waves in YBa₂Cu₃O₆$_{.}$₆₇ Probed by Resonant X-Ray Scattering under Uniaxial Compression

We report a comprehensive CuL3-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 YBa2Cu3O6.67under uniaxial compression up to 1% along the twoinequivalent Cu─O─Cu bond directions (aandb) in the CuO2planes. We confirm the strong in-planeanisotropy of the 2D charge correlations and observe their symmetric response to pressure: pressure alongaenhances correlations alongb, and vice versa. Our results imply that the underlying order parameter isuniaxial. In contrast, 3D long-range charge order is only observed alongbin response to compression alonga. Spectroscopic RXS measurements show that the 3D charge order resides exclusively in the CuO2planesand may thus be generic to the cuprates. We discuss implications of these results for models of electronicnematicity and for the interplay between charge order and superconductivity