Quantum Impurities and the Neutron Resonance Peak in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{O}_7$: Ni versus Zn

The influence of magnetic ( $S=1\mathrm{}$) and nonmagnetic ( $S=0\mathrm{}$) impurities on the spin dynamics of an optimally doped high temperature superconductor is compared in ${\mathrm{YBa}}_2({\mathrm{Cu}}_{0.97}{\mathrm{Ni}}_{0.03}{)}_3{\mathrm{O}}_7$ ( $T_c=80\mathrm{}\mathrm{K}$) and ${\mathrm{YBa}}_2({\mathrm{Cu}}_{0.99}{\mathrm{Zn}}_{0.01}{)}_3{\mathrm{O}}_7$ ( $T_c=78\mathrm{}\mathrm{K}$). In the Ni-substituted system, the magnetic resonance peak (which is observed at $E_r\simeq 40\mathrm{meV}$ in the pure system) shifts to lower energy with a preserved $E_r/T_c$ ratio while the shift is much smaller upon Zn substitution. By contrast Zn, but not Ni, restores significant spin fluctuations around 40 meV in the normal state. These observations are discussed in the light of models proposed for the magnetic resonance peak.