Temperature dependence of the dynamic susceptibility χ’’(ω) in superconducting YBa₂Cu₃O_6.6 (T_c=53 K)

Temperature-dependent neutron-scattering measurements of the dynamic susceptibility χ’’(ω) in single-crystal YBa₂Cu₃O_6.6 (T_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 ω_g≃10 meV results in a breakdown in this behavior at temperatures well above T_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 YBa₂Cu₃O_6+x.