High-Frequency Spin Waves in YBa₂Cu₃O\u3csub\u3e6.15\u3e/sub\u3e

Pulsed neutron spectroscopy is used to absolute measurements of the dynamic magnetic susceptibility of insulating YBa2Cu3O6.15. Acoustic and optical modes, derived from inand out-of-phase oscillation of spins in adjacent CuO2 planes, dominate the spectra and are observed up to 250 meV. The optical modes appear first at 74±5 meV. Linear-spin-wave theory gives an excellent description of the data and yields intralayer and interlayer exchange constants of J∥=125±5 meV and J⊥=11±2 meV, respectively, and a spin-wave intensity renormalization ZΧ=0.4±0.1