inYBa2Cu3O6:6 A. Suchaneck, V. Hinkov, D. Haug, L. Schulz, C. Bernhard, A. Ivanov, K. Hradil, C. T. Lin, P. Bourges, B. Keimer,* and Y. Sidis Max Planck Institute for Solid State Research, D-70569 Stuttgart, Germany Physics Department and Fribourg Center for Nanomaterials, Fribourg University, CH-1700 Fribourg, Switzerland Institut Laue-Langevin, 156X, F-38042 Grenoble Cedex 9, France Institute for Physical Chemistry, University of Göttingen, D-37077 Göttingen, Germany Laboratoire Léon Brillouin, CEA-CNRS, CE-Saclay, F-91191 Gif-sur-Yvette, France

We report an inelastic-neutron-scattering and muon-spin-relaxation study of the effect of 2% spinless (Zn) impurities on the magnetic order and dynamics of YBa(2)Cu(3)O(6.6), an underdoped high-temperature superconductor that exhibits a prominent spin pseudogap in its normal state. Zn substitution induces static magnetic order at low temperatures and triggers a large-scale spectral-weight redistribution from the magnetic resonant mode at 38 meV into uniaxial, incommensurate spin excitations with energies well below the spin pseudogap. These observations indicate a competition between incommensurate magnetic order and superconductivity close to a quantum critical point. Comparison to prior data on La(2-x)Sr(x)CuO(4) suggests that this behavior is universal for the layered copper oxides and analogous to impurity-induced magnetic order in one-dimensional quantum magnets.

We present a neutron-scattering study of the static and dynamic spin correlations in the underdoped high-temperature superconductor YBa2Cu3O6.45 in magnetic fields up to 15 T. The field strongly enhances static incommensurate magnetic order at low temperatures and induces a spectral-weight shift in the magnetic-excitation spectrum. A reconstruction of the Fermi surface driven by the field-enhanced magnetic superstructure may thus be responsible for the unusual Fermi surface topology revealed by recent quantum-oscillation experiments.