Microstructures of Cu- and Y-rich YBa₂Cu₃O_7−x thin films

Abstract

The defect microstructure of Cu- and Y-rich thin films of the high T c superconductor YBa 2 Cu 3 O 7−x (123) is of interest for several reasons. The highest T c ' reported for well-ordered stoichiometric films are typically 90-91 K. Post-annealed Cu- and Y-rich films may exhibit T c 's of 95-96 K. X-ray diffraction showed these non-stoichiometric films to be a highly faulted mixture of the 123 and 248 (Y 2 Ba 4 Cu 8 O 16 ,T c =80 K) phases. In situ films made by sputtering or electron beam evaporation typically show sharp but depressed T c 's (75-85 K); for our films these values approach 90 K when the films are Cu- and Y-rich. Both post annealed and in situ films exhibit high critical currents on and off stoichiometry; the pinning characteristics, which may be highly influenced by the defect microstructure, are superior for the in situ techniques. Transmission electron microscopy analysis shows that excess Cu and Y can be incorporated both as stacking fault (SF) defects and as second phase precipitates.