The weights of various features in the magnetic spectra of cuprates

We review briefly the magnetic fluctuation spectra, obtained by neutron scattering, of the high temperature superconductor YBa₂Cu₃O_6.6 (T_c = 62.7 K). The resonance at 34 meV is the most prominent spectral feature in the superconducting state. Even so, its net weight accounts for a mere 1 % of the total squared moment expected for planar Cu²⁺ ions. For context, we also provide a discussion of the spectral weights associated with the magnetic order and spin waves in two-dimensional S = 1/2 Heisenberg antiferromagnets.