MAGNETIC EXCITATIONS IN SUPERCONDUCTING YBA2CU3O6+X CRYSTALS STUDIED BY NEUTRON INELASTIC-SCATTERING

Magnetic excitation spectra in single crystals of YBa2Cu3O6+x with superconducting transition temperature T(c) = 66 K (x congruent-to 0.75) and 90 K (x congruent-to 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, (pi, pi), 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 approximately 20 meV, even at temperatures far above T(c).