1. Max-Planck-Institut für Festkörperforschung, 70569 Stuttgart, Germany
2. Laboratoire Léon Brillouin, CEA-CNRS, CE-Saclay, 91191 Gif-sur-Yvette, France
3. Institut Laue-Langevin, 156X, 38042 Grenoble cedex 9, France

Correspondence to: B. Keimer¹ Email: b.keimer@fkf.mpg.de

Abstract

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^{1, 2, 3}. This theory received strong support from experiments that indicated a one-dimensional character for the magnetic excitations in the high-T_c material YBa₂Cu₃O_6.6 (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 claim⁴, 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 order⁵. The debate about stripes has therefore been reopened.

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