Two-Dimensional Geometry of Spin Excitations in the High Temperature   Superconductor YBa2Cu3O6+x
  The fundamental building block of the copper oxide superconductors is a Cu4O4 square plaquette. In most of these materials, the plaquettes are slightly distorted and form a rectangular lattice. An influential theory predicts that high-temperature superconductivity is nucleated in "stripes" aligned along one of the axes of this lattice. This theory had received strong support from experiments that appeared to indicate a one-dimensional character of the magnetic excitations in the high temperature superconductor YBa2Cu3O6.6. 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, 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. 
V. Hinkov, S. Pailhes, P. Bourges, Y. Sidis, A. Ivanov, A. Kulakov,   C.T. Lin, D.P. Chen, C. Bernhard, B. Keimer