Charge Density Waves in YBa2Cu3O6.67 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
YBa
2
Cu
3
O
6.67
under uniaxial compression up to 1% along the two inequivalent
Cu
─
O
─
Cu
bond directions (
a
and
b
) in the
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
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.