Positron-annihilation lifetime and Doppler broadening energy spectra have been measured in high-temperature superconductors $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+\delta }$ 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 ($T_c=90\mathrm{}$ 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-$T_c$ superconductivity.

• Received 29 May 1987

DOI:https://doi.org/10.1103/PhysRevB.36.3994