Effect of Nonmagnetic Impurities on the Magnetic Resonance Peak in   YBa2Cu3O7
  The magnetic excitation spectrum of a YBa_2 Cu_3 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 ~ 40 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 q = (pi,pi) increases with Zn substitution, and only about half of the spectral weight is removed at T_c. 
H.F. Fong, P. Bourges, Y.Sidis, L.P. Regnault, J. Bossy, A.Ivanov,   D.L. Milius, I.A. Aksay and B. Keimer