Positron annihilation study on Y-Ba-Cu-O high-Tcsuperconductors

Detailed positron annihilation measurements are reported on high-$T_c$ superconductors $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$. The lifetime and Doppler-broadening spectra show a significant change at about 240 K indicating a structural change in the sample far above 90 K where the resistance falls to zero. An analysis of the lifetime spectra shows positron trapping without any sign of saturation, indicating low specific trapping rates. On this basis, the charge states of Cu-O structures and a possible local charge decomposition are discussed.