Zinc-induced modification of the dynamical magnetic susceptibility in the superconducting state of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ as revealed by inelastic neutron scattering

Inelastic-neutron-scattering measurements have been performed to determine the imaginary part of the dynamical susceptibility, χ″(Q,ω), of a ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathit{y}}$ ${\mathrm{Zn}}_{\mathit{y}}$ ${)}_3$ ${\mathrm{O}}_{6.97}$ sample exhibiting a superconducting transition at ${\mathit{T}}_{\mathit{c}}$=69 K. Zinc substitution induces striking modifications of the energy dependence of χ″(Q,ω) but magnetic fluctuations remain peaked at the antiferromagnetic wave vector, ${\mathit{Q}}_{\mathrm{AF}}$, at all investigated energies. In the superconducting state of the zinc-free compound, χ″(Q,ω) is restricted to a narrow energy range, ħω=33–47 meV, displaying a spin gap at ${\mathit{E}}_{\mathit{G}}$=33 meV and a resonant enhancement at ${\mathit{E}}_{\mathrm{\tau }}$=39 meV, both features vanishing upon heating up above ${\mathit{T}}_{\mathit{c}}$. In the y=0.02 substituted sample in the superconducting state, there is still an energy band in the range 32–47 meV but no clear resonance, and a signal is now observed in the low energy range, though the line shape of χ″(Q,ω) indicates some reminiscence of the spin gap of the pure compound. © 1996 The American Physical Society.