HIGH-FREQUENCY SPIN-WAVES IN YBA2CU3O6.15

Pulsed neutron spectroscopy is used to absolute measurements of the dy namic 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+/-5 meV. Linear-spin-wav e theory gives an excellent description of the data and yields intrala yer and interlayer exchange constants of J(parallel to) = 125+/-5 meV and J(perpendicular to) = 11+/-2 meV, respectively, and a spin-wave in tensity renormalization Z(chi) = 0.4+/-0.1.