Magnetic Excitations in Superconducting YBa2Cu3O6+x Crystals Studied by Neutron Inelastic Scattering

Magnetic excitation spectra in single crystals of YBa 2 Cu 3 O 6+ x with superconducting transition temperature T c ≃66 K ( x ≃0.75) and 90 K ( x ≃0.9) are reported. For excitation energies in the range 25–45 meV, a scan at fixed energy yields a single peak with a large q -width centered on the antiferromagnetic wave vector, (π, π), in agreement with the work of Rossat-Mignod and coworkers. However, in contrast to the results of the latter group, we have not observed, despite significant effort, any q -dependent magnetic scattering in either crystal at energies below ∼ 20 meV, even at temperatures far above T c .