High temperature superconductors: Composition variation studies

Typically, high-Tc oxide superconductors are tolerant to a wide variety of cation substitutions and can sometimes be prepared with a wide range of oxygen stoichiometries (especially YB2Cu3O7-x). The composition variations provide control over superconducting properties and thus offer valuable insights into controlling mechanisms. In oxygen deficient (quenched) YBa2Cu3O7-x. The authors have observed room temperature annealing which they attribute to ordering the in chain region of the structure. Transition temperatures rise and orthorhombicity increases when samples are aged to 300 K. for samples with fixed oxygen content, this ordering is a reversible function of temperature in the range 0 < T , 150 C (an ordered condition can be preserved by quenching). Increasing the disorder strongly depresses Tc. These controlled disorder experiments (made possible by precise oxygen composition control), permit simultaneous variation of superconductivity and structure in a sample without varying its composition. Correspondence between these results and cation substitutions in YBa2Cu3O7-x, including Fe (on Cu sites) and rare earths (on Y sites), will be discussed