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 form a 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. This theory received strong support from experiments that indicated a one-dimensional character for the magnetic excitations in the high-T<sub>c</sub> material YBa<sub>2</sub>Cu<sub>3</sub>O<sub>6.6</sub> (ref. 4). Here we report neutron scattering data on `untwinned' YBa₂Cu₃O_6+x crystals, in which the orientation of the rectangular lattice is maintained throughout the entire volume. Contrary to the earlier claim, 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 YBa₂Cu₃O_6+x, but the data may be consistent with liquid-crystalline stripe order. The debate about stripes has therefore been reopened.