POSITRON-ANNIHILATION CHARACTERISTIC AND SUPERCONDUCTIVITY IN AL-SUBSTITUTED Y1 2/3 SYSTEMS/

In this paper we present for the first time the positron lifetime data for Al-substituted Y1 : 2 : 3 superconductors. The formula compositio n of experimental samples can be expressed in YBa2Cu3-xAlxO(y) with x = 0 similar to 0.4. The positron annihilation characteristics and Al-s ubstitution dependence of the lifetime parameters is given. Anomalous changes in these parameters were found near x = 0.15, which correspond s to the known structural transition from an orthorhombic to a tetrago nal phase. The results prove the presence of electronic weak localizat ion with increasing Al-substitution. The effect of the electronic weak localization on charge transfer and on superconductivity are also dis cussed.