Neutron scattering studies on a c-axis polarized B2u phonon have been carried out by using single crystals of YBa2Cu3O7, YBa2Cu3O6.7 and YBa2Cu2.9Zn0.1O6.75. 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 Tc. For the underdoped samples, the gradual downward deviations of these quantities from the ordinary T-dependence begin far above Tc 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 TSG of YBa2Cu3O6.7 (∼150 K). Observed insensitive nature of the anomaly above Tc to the Zn-doping can be explained by considering the wave vector (q) dependence of the pseudo gap and the coupling strength of the B2u 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.
All Science Journal Classification (ASJC) codes
- Physics and Astronomy(all)