Positron annihilation in the high-temperature superconductor YBa2Cu3O6+

Positron-annihilation lifetime and Doppler broadening energy spectra have been measured in high-temperature superconductors YBa2Cu3O6+ as a function of temperature between 10 and 293 K. The observed positron lifetime and Doppler boradening S parameter show an onset increase near the superconducting critical temperature (Tc=90 K). This variation does not exist in a similar nonsuperconducting sample that contains a saturated oxygen vacancy content. These results give evidence that the oxygen vacancy and electronic structure change play an important role for high-Tc superconductivity.