High-energy spin dynamics in La1.69Sr0.31NiO4

To test the prediction that the dispersion of the magnetic resonance in superconducting YBa2Cu3O(6+x) is similar to magnons in an incommensurate antiferromagnet, we have mapped out the spin dynamics in a stripe-ordered nickelate, La(2-x)SrxNiO4, with x approximately equal to 0.31, using inelastic neutron scattering. We observe spin-wave excitations up to 80 meV emerging from the incommensurate magnetic peaks with a surprisingly large and almost isotropic spin velocity: variant Planck's over 2 pi c(s) approximately 0.32 eV A. A comparison indicates that the inferred spin-excitation spectrum is not, by itself, an adequate model for the magnetic resonance feature of the superconductor.