Pseudo Gap Formation Studied by B_2u Phonon Measurements

Neutron scattering studies on a c-axis polarized B_2u phonon have been carried out by using single crystals of YBa₂Cu₃O₇, YBa₂Cu₃O_6.7 and YBa₂Cu_2.9Zn_0.1O_6.75. For YBa₂Cu₃O₇, 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 (x²-y²)-like one. These deviations become significant, with decreasing T, at temperatures higher than the so-called spin-gap temperature T_SG of YBa₂Cu₃O_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 coupling strength of the B_2u mode to the gap, and also by considering that randomness effects introduced by the doping are more significant in the q-region with the smaller energy gap.