Charge Density Waves in YBa 2 Cu 3 O 6.67 Probed by Resonant X-Ray Scattering under Uniaxial Compression

We report a comprehensive Cu ${L}_{3}$-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 ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.67}$ under uniaxial compression up to 1% along the two inequivalent $\mathrm{Cu}─\mathrm{O}─\mathrm{Cu}$ bond directions ($a$ and $b$) in the ${\mathrm{CuO}}_{2}$ 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 ${\mathrm{CuO}}_{2}$ 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.