Positron-annihilation studies of the superconductivity transition in YBa₂Cu₃O₇₋ₓ

Positron-annihilation studies, by Doppler broadening, of the superconducting transition in YBa₂{Cu}₃O_7-x(x{}0.1) have been made. Below Tc a large positive temperature dependence of the lineshape parameter is observed, while at (or near) Tc an almost discontinuous increase in the lineshape parameter takes place. The behavior below Tc 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.