ATOMIC SHORT-RANGE ORDER IN OXYGEN DEFICIENT YBA2CU3O7-DELTA

Low field magnetization measurements are presented for a set of oxygen deficient single crystals of YBa2Cu3O7-σ. It is demonstrated that the superconducting transition temperature Tc not only depends on the oxygen concentration but also on the degree of oxygen vacancy ordering of the samples. Various degrees of ordering were obtained by annealing crystals with reduced oxygen stoichiometries at various temperatures below 350°C and fast quenching them to cryogenic temperatures. Depending on oxygen stoichiometry and annealing temperature, changes in Tc of up to 27 K are observed. The observed annealing and quenching behavior is explained in terms of the kinetics of vacancy short-range order in the Cu-O chains. One major conclusion is that the so-called "60 K plateau" in the plot of Tc versus σ is completely absent when the crystals are in their most disordered state, i.e., with the smallest degree of vacancy short-range order. © 1990.