Effect of Nonmagnetic Impurities on the Magnetic Resonance Peak in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{O}_7$

The magnetic excitation spectrum of a ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ 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 $E\simeq 40\mathrm{meV}$ is observed exclusively below the superconducting transition temperature $T_c$, the magnetic response in the Zn-substituted system is broadened significantly and vanishes at a temperature much higher than $T_c$. The energy-integrated spectral weight observed near $\mathbf{q}=(\pi ,\pi )$ increases with Zn substitution, and only about half of the spectral weight is removed at $T_c$.