  Detailed structure of the magnetic excitation spectra x"(q,w) of the
superconducting oxide YBa2Cu3O6.5(the transition temperature Tc ~ 52 K) in the
wave vector (q)- and the energy(w)-space, and its temperature (T) dependence
have been studied. By adopting an effective energy dispersion of the quasi
particles which can reproduce the shape of the Fermi surface and by introducing
the exchange interaction between the Cu spins, a rather satisfactory agreement
between the calculation and the experimentally observed data can be obtained.
In the study, it has been found that the effects of the quasi particle- energy
broadening on the excitation spectra x"(q,w) is important. The sharp resonance
peak observed at w ~ 40 meV for the optimally doped system of YBa2Cu3Oy can be
naturally reproduced by the present model
