Superconductivity-induced phonon anomalies have been observed by neutron scattering for a c-axis polarized B2u mode with out-of-phase motions of O(2) and O(3) within a plane in YBa2Cu3O6+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 YBa2Cu3O6.7 the B2u mode also exhibits phonon anomalies associated with the so-called spin-gap phenomena.
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