Investigation of superlattices and ultrathin layers made from HTS cuprates

The fabrication of epitaxially grown superlattices and ultrathin films made from high temperature superconductors has allowed a systematic study of certain properties of this class of materials. It is, for instance, possible to modify the anisotropy in a controlled manner and thus investigate the role of the reduced dimensionality of the high temperature superconductors on the superconducting properties. In particular, we have investigated the thermally activated flux flow as observed in the resistive tails of the superconducting transition. We find in YBa2Cu3O7/PrBa2Cu3O7 (YBCO/PrBCO) superlattices that the activation energy varies proportional to the thickness of the individual YBCO layers when the PrBCO thickness is large enough to decouple the superconducting layers (d-PrBCO > 36 Å). We thus find that the pancake vortices in the individual CuO2 layers in YBCO are strongly coupled and that the flux line lattice is 2D in the thin individual YBCO layers up to layer thicknesses of more than 200 Å. An analysis of the zero field resistive transition in terms of Coulomb gas scaling confirms this result