One-dimensional nature of the magnetic fluctuations in YBa${}_2$Cu${}_3$O${}_{6.6}$

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.