High-Frequency Spin Waves in YBa2Cu3O6.15
  Pulsed neutron spectroscopy is used to make absolute measurements of the dynamic magnetic susceptibility of insulating YBa2Cu3O6.15. Acoustic and optical modes, derived from in- and 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 meV. Linear-spin-wave theory gives an excellent description of the data and yields intra- and inter-layer exchange constants of J_parallel =125 meV and J_perp = 11 meV respectively and a spin-wave intensity renormalization Z_chi = 0.4. 
S. M. Hayden (University of Bristol), G. Aeppli (AT&T Bell   Laboratories), T. G. Perring (Rutherford Laboratory), H. A. Moo...