Temperature dependence of positron-annihilation parameters in YBa2Cu3O7-x above and below the superconducting transition.

Abstract

The temperature dependence of positron-annihilation parameters for $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ $x=0.7, 0.4, \mathrm{and} 0.0$ 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 $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ $x=0.4 \mathrm{and} 0.0$, 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\ifmmode\pm\else\textpm\fi{}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.