Effect of nonmagnetic impurities on the magnetic resonance peak in YBa₂Cu₃O₇

The magnetic excitation spectrum of a YBa₂Cu₃O₇ crystal containing 0.5% of nonmagnetic (Zn) impurities has been determined by inelastic neutron scattering. Whereas in the pure system a sharp resonance peak at E≃40meV is observed exclusively below the superconducting transition temperature T_cthe magnetic response in the Zn-substituted system is broadened significantly and vanishes at a temperature much higher than Tc. The energy-integrated spectral weight observed near q=(π,π) increases with Zn substitution, and only about half of the spectral weight is removed at T_c.