Superconductivity-Induced Anomalies in c-Axis Polarized Phonon Modes of YBa₂Cu₃O₇ Studied by Neutron Inelastic Scattering

Superconductivity-induced phonon anomalies have been studied by inelastic neutron scattering for c-axis polarized modes in a single crystal of YBa₂Cu₃O₇. a mode with B_2u symmetry, which has out-of-phase motions of crystallographically distinct O(2) and O(3) atoms within a CuO₂ plane, apparently exhibits a frequency decrease and a narrowing of the profile width with onset of the superconductivity, while a mode which has in-phase motions of these oxygens with respect to Cu atoms within the CuO₂ plane does not show an appreciable anomaly. The (x²−y²)-like symmetry of the superconducting order parameter is suggested by considering recent theoretical arguments.