Positron-annihilation studies of the superconductivity transition in YBa_{2}Cu_{3}O_{7-x}

Abstract

Positron-annihilation studies, by Doppler broadening, of the superconducting transition in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{x}}$(x\ensuremath{\approxeq}0.1) have been made. Below ${T}_{c}$ a large positive temperature dependence of the lineshape parameter is observed, while at (or near) ${T}_{c}$ an almost discontinuous increase in the lineshape parameter takes place. The behavior below ${T}_{c}$ may be consistent with a Bardeen-Cooper-Schrieffer-like theory if an energy band with a small dispersion crosses the Fermi level. The discontinuity is not clearly understood, but may be due to a major change in the electronic structure taking place with the onset of superconductivity.