  Following the early discovery of stripe-like order in La-based copper-oxide
superconductors, charge ordering instabilities were observed in all cuprate
families. However, it has proven difficult to distinguish between uni-
(stripes) and bi-directional (checkerboard) charge order in Y- and Bi-based
materials. Here we use resonant x-ray scattering (RXS) to measure the
two-dimensional structure factor in YBCO, in reciprocal space. Our data reveal
the presence of charge stripe order, i.e. locally unidirectional density waves,
suggesting it as the true microscopic nature of charge modulations in cuprates.
At the same time, we find that the well-established competition between charge
order and superconductivity is stronger for charge correlations across than
along the stripes, which provides additional evidence for the intrinsic
unidirectional nature of the charge order.
