Charge Density Waves in YBa2Cu3O6.67 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 YBa2Cu3O6.67 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.