Spin anisotropy of the magnetic excitations in the normal and superconducting states of optimally doped YBa2Cu3O6.9 studied by polarized neutron spectroscopy

We use inelastic neutron scattering with spin polarization analysis to study the magnetic excitations in the normal and superconducting states of YBa2Cu3O6.9. Polarization analysis allows us to determine the spin polarization of the magnetic excitations and to separate them from phonon scattering. In the normal state, we find unambiguous evidence of magnetic excitations over the 10-60 meV range of the experiment with little polarization dependence to the excitations. In the superconducting state, the magnetic response is enhanced near the resonance energy and above. At lower energies, 10 less than or similar to E less than or similar to 30 meV, the local susceptibility becomes anisotropic, with the excitations polarized along the c axis being suppressed. We find evidence for a new diffuse anisotropic response polarized perpendicular to the c axis which may carry significant spectral weight.