Two-dimensional geometry of spin excitations in the high-transition-temperature superconductor YBa2Cu3O6+x

The fundamental building block of the copper oxide superconductors is a Cu4O4 square plaquette. The plaquettes in most of these materials are slightly distorted to forma rectangular lattice, for which an influential theory predicts that high-transition-temperature (high-T-c) superconductivity is nucleated in 'stripes' aligned along one of the axes(1-3). This theory received strong support from experiments that indicated a one-dimensional character for the magnetic excitations in the high-T-c material YBa2Cu3O6.6 ( ref. 4). Here we report neutron scattering data on 'untwinned' YBa2Cu3O6+x crystals, in which the orientation of the rectangular lattice is maintained throughout the entire volume. Contrary to the earlier claim(4), we demonstrate that the geometry of the magnetic fluctuations is two-dimensional. Rigid stripe arrays therefore appear to be ruled out over a wide range of doping levels in YBa2Cu3O6+x, but the data may be consistent with liquid-crystalline stripe order(5). The debate about stripes has therefore been reopened.