Quantum impurities and the neutron resonance peak in YBa2Cu3O7: Ni versus Zn.

The influence of magnetic (S=1) and nonmagnetic (S=0) impurities on the spin dynamics of an optimally doped high temperature superconductor is compared in YBa2(Cu0.97Ni0.03)3O7 (Tc=80 K) and YBa2(Cu0.99Zn0.01)3O7 (Tc=78 K). In the Ni-substituted system, the magnetic resonance peak (which is observed at Er approximately 40 meV in the pure system) shifts to lower energy with a preserved Er/Tc 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.