Epitaxial mismatch strain in YBa2Cu3O7-δ/PrBa2Cu3O7 superlattices

The effects of epitaxial strain in ultrathin YBa2Cu3O7-delta layers have been investigated by x-ray diffraction and transmission electron microscopy. The samples used were high quality [YBa2Cu3O7-delta (YBCO)N/ PrBa2Cu3O7, (PBCO)(M)](1000 Angstrom) superlattices, grown by high oxygen pressure sputtering, with N ranging between 1 and 12 unit cells and M = 5 unit cells. Superlattice structure is refined by fitting x-ray spectra to a structural model containing disorder related parameters. Epitaxial mismatch strain and the absence of step disorder are found for YBCO layer thickness below 4 unit cells. A surprising reorganization of interatomic distances results, which seems to correlate with the decrease in the critical temperature. For larger YBCO layer thickness, stress relaxes and step disorder builds up. Transmission electron microscopy observations show the presence of a-axis orientated microdomains, which seem to be correlated to the release of epitaxial strain: provided in plane mismatch is smaller in this orientation.