The positron annihilation anomaly near the superconducting transition temperature in single-phase YBa2Cu3O7-xcompounds

Abstract

The positron annihilation technique has been used for studying the high-T_c-superconductor YBa₂Cu₃O_7-x. The Doppler broadening lineshape parameter as a function of temperature between 75 and 300 K, and the positron lifetimes of the normal state and of the superconducting state have been measured. A sharp negative anomaly in the S-parameter was observed near the superconducting transition temperature. In addition, it was found that the S-parameter and mean lifetime decrease below the transition temperature T_c. These phenomena vanish for the samples annealed in vacuum at 800 degrees C. It is suggested that high-T_c superconductivity in the new oxide is probably accompanied by an adjustment in the positions of the oxygen vacancies and the occurrence of 'valence pairs', i.e. an enhancement of the covalent character of the electrons after the superconducting transition.