Superconductivity-induced B_2u phonon anomalies of YBa₂Cu₃O_6+x and symmetry of the order parameter. Neutron inelastic scattering studies

Superconductivity-induced phonon anomalies have been observed by neutron scattering for a c-axis polarized B_2u mode with out-of-phase motions of O(2) and O(3) within a plane in YBa₂Cu₃O_6+x (x = 1.0 and 0.7). A mode which has in-phase motions of these oxygens with respect to Cu atoms within a Cu0₂-plane does not show any appreciable anomaly. Based on these results and recent theoretical arguments, the symmetry of the superconducting order parameter is pointed out to be (x² - y²)-like. In YBa₂Cu₃O_6.7 the B_2u mode also exhibits phonon anomalies associated with the so-called spin-gap phenomena.