Metastability-induced charge and spin fluctuations in 123 type superconductors

Various defects arising from lattice mismatch and coordination incompatibility at the cation sites are known to lend the cuprates a metastable state at temperatures and pressures where they exhibit superconductivity. The present paper considers the metastability induced by coordination incompatibility at Cu(1) sites in oxygen-deficient 123 type systems and looks for their possible effects on charge and spin degrees of freedom. Depending upon the concentration of oxygen vacancies, the resulting unstable charge state at the Cu(1) sites is shown to either temporally fluctuate or spatially equilibrate, providing a mechanism for the 90 K and the 60 K plateaus, with a peak in the former observed in the YBa₂Cu₃O_7-y, samples prepared under thermodynamic equilibrium conditions.