METASTABILITY-INDUCED CHARGE AND SPIN FLUCTUATIONS IN 123-TYPE SUPERCONDUCTORS

Various defects arising from lattice mismatch and coordination incompa tibility at the cation sites are known to lend the cuprates a metastab le state at temperatures and pressures where they exhibit superconduct ivity. The present paper considers the metastability induced by coordi nation incompatibility at Cu(1) sites in oxygen-deficient 123 type sys tems and looks for their possible effects on charge and spin degrees o f freedom. Depending upon the concentration of oxygen vacancies, the r esulting unstable charge state at the Cu(1) sites is shown to either t emporally fluctuate or spatially equilibrate, providing a mechanism fo r the 90 K and the 60 K plateaus, with a peak in the former observed i n the YBa2Cu3O7-y samples prepared under thermodynamic equilibrium con ditions.