Temperature dependence of the dynamic susceptibility χ’’(ω) in superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.6}$ (${\mathit{T}}_{\mathit{c}}$=53 K)

Temperature-dependent neutron-scattering measurements of the dynamic susceptibility χ’’(ω) in single-crystal ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.6}$ (${\mathit{T}}_{\mathit{c}}$=53 K) are reported. At high temperatures (T≳100 K) a universal dependence of χ’’(ω) on the ratio ω/T has been observed. A spin pseudogap ${\mathrm{\omega }}_{\mathit{g}}$≃10 meV results in a breakdown in this behavior at temperatures well above ${\mathit{T}}_{\mathit{c}}$. Below ∼40 K the scattering intensities at energy transfers of ΔE=5 and 8 meV demonstrate a surprising recovery with decreasing temperature. The behavior of χ’’(ω) over the range of energies (3–16 meV) and temperatures (1.5–300 K) studied presents a strong constraint on theoretical descriptions of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$.