Neutron-scattering study of chain-oxygen vibrations in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

The ab-plane-polarized transverse chain-oxygen vibrations in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ were investigated by inelastic neutron scattering on an untwinned single crystal at different temperatures. From the rather high frequencies of these modes (ν≊5 THz), the moderate phonon linewidth, and the absence of an appreciable frequency shift with temperature, we conclude that the chain-oxygen vibrations are not strongly anharmonic. Concurrently, diffraction data are incompatible with a wide double-well potential for the chain-oxygen atoms.