Pseudo Gap Formation Studied by B 2 u Phonon Measurements

Neutron scattering studies on a c -axis polarized B 2 u phonon have been carried out by using single crystals of YBa 2 Cu 3 O 7 , YBa 2 Cu 3 O 6.7 and YBa 2 Cu 2.9 Zn 0.1 O 6.75 . For YBa 2 Cu 3 O 7 , sharp decreases in the energy and width of the phonon mode have been observed with decreasing temperature T at around the superconducting transition temperature T c . For the underdoped samples, the gradual downward deviations of these quantities from the ordinary T -dependence begin far above T c with decreasing T , which is considered to be associated with the formation of the pseudo gap with the same symmetry as that of the superconducting order parameter, i.e., the ( x 2 - y 2 )-like one. These deviations become significant, with decreasing T , at temperatures higher than the so-called spin-gap temperature T SG of YBa 2 Cu 3 O 6.7 (∼150 K). Observed insensitive nature of the anomaly above T c to the Zn-doping can be explained by considering the wave vector ( q ) dependence of the pseudo gap and the coupl...