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

Positron-annihilation studies, by Doppler broadening, of the superconducting transition in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{x}}$(x≊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.