Annealing study on single crystal Tl-2223 superconductors

Abstract

We have shown that crystals in the TlCaBaCuO system are robust to anneals (oxygen, vacuum or 10%H 2/90%N 2) to 500°C. Improvements in the superconducting properties, as determined from Meissner data, are observed for anneals above 300°C and suggest that as-grown crystals may not have fully equilibrated during growth despite an extended cool down cycle. There appears to be no systematic shift in T c with oxygen content in the TlO layer. Single crystal X-ray structure refinements have determined small atomic displacements which, in part, suggest the possibility of small amounts of Tl 1+ being present in the TlO layer of as-grown crystals which subsequently oxidizes during mild oxygen anneals to Tl 3+. Hence, changes in oxygen content may not result in altered carrier concentration in the CuO layers. Further, the nature of the oxygen position in the TlO layer, as determined by the usual least-squares refinement of X-ray data, appears to be related to the amount of oxygen present in the particular sample.