Temperature dependence of positron-annihilation parameters in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ above and below the superconducting transition

The temperature dependence of positron-annihilation parameters for $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ $x=0.7, 0.4, \mathrm{and} 0.0$ are reported. It was found that a transition in the Doppler line-shape parameter $S$ was associated with the superconducting transition temperature ($T_c$) in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ $x=0.4\mathrm{} \mathrm{and} 0.0$, while no transition was observed in the nonsuperconducting Y${\mathrm{Ba}}_3$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.3}$. Positron lifetime parameters in Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ 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.