  We have measured the spin fluctuations in the YBa2Cu3O6.5 (YBCO6.5, Tc=59 K)
superconductor at high-energy transfers above ~ 100 meV. Within experimental
error, the momentum dependence is isotropic at high-energies, similar to that
measured in the insulator for two dimensional spin waves, and the dispersion
extrapolates back to the incommensurate wave vector at the elastic position.
This result contrasts with previous expectations based on measurements around
50 meV which were suggestive of a softening of the spin-wave velocity with
increased hole doping. Unlike the insulator, we observe a significant reduction
in the intensity of the spin excitations for energy transfers above ~ 100 meV
similar to that observed above ~ 200 meV in the YBCO6.35 (Tc=18 K)
superconductor as the spin waves approach the zone boundary. We attribute this
high energy scale with a second gap and find agreement with measurements of the
pseudogap in the cuprates associated with electronic anomalies along the
antinodal positions. In addition, we observe a sharp peak at around 400 meV
whose energy softens with increased hole doping. We discuss possible origins of
this excitation including a hydrogen related molecular excitation and a
transition of electronic states between d levels.
