  There is increasing evidence that inhomogeneous distributions of charge and
spin--so-called "striped phases"--play an important role in determining the
properties of the high-temperature superconductors. For example, recent
neutron-scattering measurements on the YBa$_2$Cu$_3$O$_{7-x}$ family of
materials show both spin and charge fluctuations that are consistent with the
striped-phase picture. But the fluctuations associated with a striped phase are
expected to be one-dimensional, whereas the magnetic fluctuations observed to
date appear to display two-dimensional symmetry. We show here that this
apparent two-dimensionality results from measurements on twinned crystals, and
that similar measurements on substantially detwinned crystals of
YBa$_2$Cu$_3$O$_{6.6}$ reveal the one-dimensional character of the magnetic
fluctuations, thus greatly strengthening the striped-phase interpretation.
Moreover, our results also suggest that superconductivity originates in charge
stripes that extend along the b crystal axis, where the superfluid density is
found to be substantially larger than for the a direction.
