TEMPERATURE DEPENDENCE OF POSITRON ANNIHILATION PARAMETERS IN HIGH Tc SUPERCONDUCTOR YBa₂Cu₃O_7-x

The temperature dependences of the positron annihilation lifetime and the Doppler broadening shape parameters have been measured in high T_c superconductors YBa₂Cu₃O_7-x = between 77.2 and 300K. The measured parameters show an abrupt change across the range of superconducting transition temperature, indicating a change of the electronic structure. The structural instabilities have also been observed in three higher temperature ranges of115-135, 150-210 and 250-280K. This suggests that higher T_c superconductors might be found in these temperature ranges.