Temperature dependence of positron-annihilation parameters in YBa2Cu3O/sub 7-//sub x/ above and below the superconducting transition

The temperature dependence of positron-annihilation parameters for YBa2Cu3O/sub 7-//sub x/ x = 0.7, 0.4, and 0.0 are reported. It was found that a transition in the Doppler line-shape parameter S was associated with the superconducting transition temperature (T/sub c/) in YBa2Cu3O/sub 7-//sub x/ x = 0.4 and 0.0, while no transition was observed in the nonsuperconducting YBa3Cu3O/sub 6.3/. Positron lifetime parameters in YBa2Cu3O7 were found to be consistent with positrons localized at open volume regions (probably unoccupied crystallographic sites) in this material with a lifetime of 210 psec. The positron lifetime decreased by approximately 5% across the superconducting transition and subsequently by 4% to 193 +- 1 psec at 12 K. These results indicate that the electron density at this site increases approximately 9% between 100 and 12 K. Doppler line-shape results show that the average electron momentum at the annihilation site increases across the superconducting transition and from sample to sample as the oxygen concentration decreases