The weights of various features in the magnetic spectra of cuprates

We review briefly the magnetic fluctuation spectra, obtained by neutron sca ttering, of the high temperature superconductor YBa2Cu3O6.6 (T-c = 62.7 K). The resonance at 34 meV is the most prominent spectral feature in the supe rconducting state. Even so, its net weight accounts for a mere 1% of the to tal squared moment expected for planar Cu2+ ions. For context, we also prov ide a discussion of the spectral weights associated with the magnetic order and spin waves in two-dimensional S = 1/2 Heisenberg antiferromagnets.