Scattering spectra for three types of parent insulators of high-${\mathit{T}}_{\mathit{c}}$ cuprates, ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_6$, ${\mathrm{Ca}}_{0.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_2$, and ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ due to phonons and magnons were investigated varying incident photon energies between 1.7 and 2.7 eV. The magnon scattering peaks of these materials were found to be suppressed when the incident photon energy approaches the charge-transfer gap energy. By contrast, peaks due to zone-boundary phonons appeared with strong second-order features at the same energies. Those peaks were ascribed to the bending and stretching vibrations of oxygens in the Cu-${\mathrm{O}}_2$ plane with anharmonic character.

• Received 21 February 1992

DOI:https://doi.org/10.1103/PhysRevB.46.6505