We report inelastic neutron scattering measurements at excitation energies $ħ\omega \sim 41$ meV in YB${\mathrm{a}}_2$C${\mathrm{u}}_3$${\mathrm{O}}_7$. We separate magnetic and phonon contributions to the scattering cross section by a detailed analysis of the momentum dependence of the scattered intensity. The previously reported broad peak around $\mathbf{q}=(\pi /a,\pi /a)$ in the normal state can be entirely accounted for by a phonon which primarily involves vibrations of the in-plane oxygen. Magnetic scattering centered around 41 meV and $\mathbf{q}=(\pi /a,\pi /a)$ appears in the superconducting state only. Theoretical implications of these findings are discussed.

• Received 22 December 1994

DOI:https://doi.org/10.1103/PhysRevLett.75.316